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Investigating photovoltaic-powered light-emitting diode baseddisinfection of water for point-of-use application

Author:Lui, Gough

Publication Date:2016

DOI:https://doi.org/10.26190/unsworks/3019

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Investigating photovoltaic-powered light-

emitting diode based disinfection of water for point-of-use application

Gough Yumu Lui

A thesis in fulfilment of the requirements for the degree of

Doctor of Philosophy

University of New South Wales

School of Civil and Environmental Engineering

Faculty of Engineering

March 2016

ORIGINALITY STATEMENT ‘I hereby declare that this submission is my own work and to the best of my knowledge it contains no materials previously published or written by another person, or substantial proportions of material which have been accepted for the award of any other degree or diploma at UNSW or any other educational institution, except where due acknowledgement is made in the thesis. Any contribution made to the research by others, with whom I have worked at UNSW or elsewhere, is explicitly acknowledged in the thesis. I also declare that the intellectual content of this thesis is the product of my own work, except to the extent that assistance from others in the project's design and conception or in style, presentation and linguistic expression is acknowledged.’ Signed …………………………………………….............. Date ……………………………………………..............

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Table of Contents

Acknowledgements ....................................................................................................... viii

Extended Abstract .......................................................................................................... ix

Research Highlights ...................................................................................................... xii

List of Publications and Presentations ....................................................................... xiv

List of Figures ................................................................................................................ xv

List of Tables .............................................................................................................. xviii

Glossary and List of Abbreviations ............................................................................. xx

1. Background and Introduction ................................................................................ 1

1.1. Thesis Structure .................................................................................................. 3

2. Literature Review .................................................................................................... 7

2.1. Introduction ........................................................................................................ 7

2.2. UV Disinfection ................................................................................................. 8

2.2.1. Mechanisms and Action Spectra ................................................................. 8

2.2.2. Standards and Guidelines ........................................................................... 9

2.2.3. Reactor Design .......................................................................................... 10

2.2.4. Photocatalysis, Advanced Oxidation Processes (AOPs) and

Photosensitisers ...................................................................................................... 11

2.2.5. UV-C Emission Technologies ................................................................... 12

2.2.6. POU and Other Challenges ...................................................................... 15

2.3. UV Light Emitting Diode (LED) Technology ................................................. 18

2.3.1. LED Operating Mechanism and History .................................................. 18

2.3.2. UV-C LED Research, Development and Commercialisation ................... 19

2.3.3. UV-C LED Disinfection ............................................................................ 22

2.3.4. UV-A, Visible Light and UV-B LED Disinfection ..................................... 23

2.4. Photovoltaics in POU Settings ......................................................................... 24

ii

2.4.1. History, Physics and Commercialisation .................................................. 24

2.4.2. Standalone PV System Design Issues and Reliability ............................... 25

2.4.3. Directly Coupled PV in Developing Regions ............................................ 27

2.4.4. Related Renewable Energy Based UV Disinfection .................................. 27

2.5. Integrating and Optimising POU PV/LED Disinfection .................................. 28

2.5.1. PV/LED Integration Challenges and Considerations ............................... 28

2.5.2. Design Case Studies .................................................................................. 33

2.5.3. POU Disinfection Setting .......................................................................... 34

2.6. Prioritising the Research Program .................................................................... 37

2.7. Conclusion ........................................................................................................ 43

2.8. Chapter Highlights ........................................................................................... 43

3. A Proof-of-Concept Bench Scale POU System.................................................... 45

3.1. Introduction ...................................................................................................... 45

3.2. Materials and Methods ..................................................................................... 47

3.2.1. LED Array Power Supply, Configuration and Characteristics ................ 47

3.2.2. Batch Reactor System ................................................................................ 50

3.2.3. Inactivation Experiments .......................................................................... 51

3.2.4. Action Spectra ........................................................................................... 53

3.3. Results and Discussion ..................................................................................... 53

3.3.1. Power Source and LED Characterisation ................................................ 53

3.3.2. Enhancement of irradiation by reactor internal reflection ....................... 57

3.3.3. Window Absorbance of Light .................................................................... 58

3.3.4. Inactivation Versus Time and Temperature .............................................. 59

3.3.5. Disinfection Lag-Phase and Repair Mechanisms ..................................... 64

3.3.6. S90s and Action Spectra ............................................................................. 65

3.3.7. LED Lifetime Versus System Cost ............................................................. 69

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3.3.8. Other Considerations - Target Dose, Light Absorbance, Viruses, Reactor

Material Compatibility, SODIS ............................................................................... 69

3.4. Conclusion ........................................................................................................ 71


4. Exploring Potential Routes for Enhancement..................................................... 74

4.1. Introduction ...................................................................................................... 74

4.2. Advanced Oxidation Processes ........................................................................ 74

4.2.1. Materials and Methods ............................................................................. 76

4.2.2. Results and Discussion .............................................................................. 79

4.3. Porphyrin Photosensitisers ............................................................................... 83

4.3.1. Materials and Methods ............................................................................. 83

4.3.2. Results and Discussion .............................................................................. 84

4.4. Synergies Through Wavelength Combinations ................................................ 90

4.5. Pulsed Irradiation ............................................................................................. 92

4.6. LED Overdriving .............................................................................................. 94

4.7. Conclusion ........................................................................................................ 95


5. UV-C LED Engineering Considerations ............................................................. 97

5.1. Introduction ...................................................................................................... 97

5.2. Materials and Methods ................................................................................... 100

5.2.1. Construction and Testing of UV Sensor .................................................. 100

5.2.2. UV-C LED Lifetime Testing Setup, Data Capture and Analysis ............ 102

5.2.3. Confirmation of Operating Thermal Conditions .................................... 105

5.2.4. Quartz Window Transmissivity ............................................................... 106

5.2.5. Post-Run UV-LED Emission Characteristics ......................................... 106

5.2.6. LED Emission Reduction Caused by Self-Heating ................................. 106

5.3. Results and Discussion ................................................................................... 108

iv

5.3.1. Review of UV-LED Lifetime Data ........................................................... 108

5.3.2. UV-Selective Electronic Sensor .............................................................. 111

5.3.3. UV-C LED Ageing .................................................................................. 113

5.3.4. Confirmation of Thermal Operating Conditions .................................... 116

5.3.5. Quartz Transmissivity ............................................................................. 118

5.3.6. Thermal Resistance and Self-Heating of LEDs ....................................... 120

5.4. Eye Safety ....................................................................................................... 127

5.5. Low-Cost UV Sensing .................................................................................... 128

5.6. Conclusion ...................................................................................................... 130

5.7. Chapter Highlights ......................................................................................... 131

6. Design, Construction and Testing of Three Prototype Deployment-Ready POU

Systems ......................................................................................................................... 134

6.1. Introduction .................................................................................................... 134

6.2. System Specifications and Design Rationale ................................................. 135

6.2.1. Design Variables and Rationale ............................................................. 135

6.2.2. Construction Features ............................................................................. 147

6.2.3. Device Schematics, Circuit Theory, Bill of Materials and Costing ........ 155



6.4.1. System A Performance ............................................................................ 164

6.4.2. System B Performance ............................................................................ 166

6.4.3. System C Performance ............................................................................ 168

6.5. Conclusion ...................................................................................................... 171


7. Comparing PV-LED Disinfection with SODIS Performance through Solar

Spectrum Based Modelling......................................................................................... 173

7.1. Introduction .................................................................................................... 173

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7.2.1. Solar Spectrum ........................................................................................ 175

7.2.2. Action Spectrum ...................................................................................... 179

7.2.3. PET Transmissivity ................................................................................. 180


7.3.1. Action Spectrum, PET Transmission and Solar Spectrum Inputs ........... 180

7.3.2. Diurnal Disinfection Profile ................................................................... 184

7.3.3. Contribution by Wavelength ................................................................... 186

7.3.4. Latitude and Seasonality ......................................................................... 187

7.3.5. Altitude Effects ........................................................................................ 189

7.3.6. Strengths and Caveats ............................................................................. 189

7.4. Comparison with PV-Powered LED Disinfection ......................................... 191

7.5. Conclusion ...................................................................................................... 193


8. Technical Developments, Market Forces and Disinfector Cost Modelling .... 195

8.1. Introduction .................................................................................................... 195

8.2. UV-LED Device and Pricing Evolution ......................................................... 195

8.2.1. UV-C/UV-B LED Market Developments ................................................ 196

8.2.2. UV-A and Visible LED Market Development ......................................... 201

8.2.3. Market Candidate LED Products for Disinfection ................................. 201

8.2.4. PV Power Developments ......................................................................... 204

8.3. Bulk Volume Discount ................................................................................... 205

8.3.1. UV-C LEDs ............................................................................................. 205

8.3.2. Non UV-C LEDs and Other Components ............................................... 206

8.4. Bayesian Network Modelling ......................................................................... 207

8.5. Scenario Analysis ........................................................................................... 217

vi

8.5.1. Modelling Various System Configurations ............................................. 217

8.5.2. Scenario A, B and C ................................................................................ 221

8.5.3. Scenario D1 and D2 ................................................................................ 222

8.5.4. Scenario E1 and E2 ................................................................................. 222

8.5.5. Scenario F1 and F2 ................................................................................. 222

8.5.6. Scenario G ............................................................................................... 223

8.5.7. Scenario H ............................................................................................... 223

8.5.8. Sensitivity to Findings Analysis .............................................................. 223

8.6. Comparison with Technical and Economic Analyses in Literature ............... 232

8.7. Conclusion ...................................................................................................... 234


9. Conclusions, Future Potential and Recommendations ..................................... 237

9.1. Potential Improvements to Existing Technology ........................................... 243

9.2. Further Research ............................................................................................. 244

9.3. Key Recommendations ................................................................................... 245

9.4. Is the PV+LED Concept Viable? ................................................................... 246

9.4.1. Technology and Market Forces .............................................................. 246

9.4.2. Training and Support Required .............................................................. 248

9.4.3. Possible Trial Locations ......................................................................... 249

9.4.4. Policy and Governance Issues ................................................................ 249

9.4.5. Funding and Cost Issues ......................................................................... 250

9.4.6. Patents and Intellectual Property ........................................................... 251

9.4.7. Socioeconomic Problems ........................................................................ 251

References .................................................................................................................... 252

Appendix A: SMARTS Input Cards ......................................................................... 285

Appendix B: Arduino Microcontroller Code ........................................................... 288

vii

Appendix C: System Configuration Cost Model Inputs .......................................... 295

Appendix D: Digital Appendix Listing ...................................................................... 297

viii

Acknowledgements

First of all, I would like to thank my supervisors, without whom this investigation

would not have been possible - Dr. Richard Corkish, for believing in me and arranging

this opportunity; to Prof. Richard Stuetz for keeping things together when there was a

difference of opinion and bringing years of experience to the table; and most of all, to

Dr. David Roser for persisting in educating me about water research (an area of which I

had absolutely no prior knowledge), for the late-night hypotheticals with new and open

directions, for pushing me to find the truth as best as possible by challenging

assumptions to "just do it anyway", and for convincing me that what I was doing was

worthwhile by reminding me of the bigger picture.

I would also like to thank Dr. Andrew Feitz, Prof. Nick Ashbolt and Dr. Paul Jagals for

providing their input and perspectives on my work and relating it back to reality; the lab

support staff for whom I had plenty of strange requests; and the facilities management

staff for entrusting me with access to the roof of the building - a privilege few have

obtained.

Thanks also go to my family, who have supported me throughout this long quest for

knowledge, and never doubted my ability to achieve.

Last, but not least, I would like to acknowledge the support of the Australian

Government's Australian Postgraduate Award (APA), the University of New South

Wales' Engineering Research Award (ERA) and the Australian Renewable Energy

Association (ARENA) for providing scholarship and project funds support.

Responsibility for the views, information or advice expressed herein is not accepted by

the Australian Government.

ix

Extended Abstract

Access to clean drinking water is difficult where centralised water distribution does not

exist. Water containers used to collect and transport water are likely to harbour biofilms

that can contaminate the water within leading to waterborne disease. Point-of-use

(POU) disinfection methods are one possible solution, however, existing POU

technologies often fall short of meeting user needs.

A literature review identified ultraviolet (UV) disinfection as potentially suitable for

POU situations with new UV-C light-emitting diodes (LEDs) overcoming the

limitations of mercury tube technology. However UV-C LEDs still possess drawbacks

e.g. uncertain lifetime, high costs, limited power output and limited availability. Further

opportunity was identified in using UV-A and visible LED technology whose maturity

has already led to lower LED costs, high light outputs and long device lifetimes. Due to

limited studies focussing on non-UV-C disinfection, the potential of other LEDs for

POU disinfection is relatively unclear. Several options for enhancing the performance of

LED technologies were identified, including the use of TiO2 photocatalysis, addition of

photosensitisers, synergies through wavelength combination, pulsed irradiation and

LED overdriving. Additionally, LED technologies are well suited to the use of

photovoltaic (PV) solar panels as a power source, as these are a proven low-voltage

renewable energy source featuring low-cost and high reliability. Further, PV has a good

track record of being deployed in remote areas for water pumping and lighting.

My interdisciplinary investigation addressed major knowledge gaps and engineering

considerations identified in the review and assessed the current and near future

feasibility of producing field-deployable POU disinfection devices utilising

commercially-available PV power and LED sources.

First, bench-scale POU-model experiments were conducted to determine the

effectiveness of commercial LED arrays (12 wavelengths spanning 270-740nm) for

inactivating E. coli K12 and E. faecalis ATCC 19433 in 0.05M NaCl solution. UV-C

LEDs (270nm) achieved >5 logs reduction in 3 min for E. coli and 18 min for E.

faecalis, with more mature UV-A (365-405nm) LEDs achieving ~5 log disinfection

x

within 3 hours for both test bacteria. Wavelengths of 430nm and 455nm showed

marginal performance with 3-logs reduction in 4-5 hours for E. coli and 2 to 3-logs in

six hours for E. faecalis. Other LEDs (310nm and >=525nm) did not produce

measurable levels of disinfection.

The combined inactivation rate data were used to construct action spectra, which proved

consistent with literature spectra. Consistent with solar radiation disinfection studies,

these data also showed the increase in dose requirement between E. faecalis and E. coli

was lower for UV-A (2-fold) than for UV-C (10-fold). Compared to UV-C, UV-A

disinfection showed a variable "lag-phase", followed by a rapid decline consistent with

literature reports. UV-A disinfection also suffered from slight photoreactivation with

late-growing colonies appearing in samples taken during log-phase inactivation.

Three prototype disinfection systems, representing different design options, were

constructed and trialled under simulated field conditions for disinfecting water

containing E. coli K12 at single-glass (250mL) (1 x 1mW 270nm LED [US$281]=> 4-

logs reduction in 15 min) and household (10L and 15 L) (8 x 1mW 270nm LEDs

[US$1799] => 2-logs in 1 h; 3 x 365nm 1.8W ‘quad’ LED arrays [US$956] => 5-logs

reduction in 3h, respectively) scales. Differences in performance from model reactors

were likely due to reactor hydrodynamics (270nm, < expectations) and path length

(365nm, > expectations).

Uncertainties surrounding the lifetime of commercially available UV-C LEDs,

solarisation, and the need for robust low-cost sensors were concurrently addressed in a

diode lifetime trial where the output of a UV-C LED was monitored through a quartz

window over a period of 5 000 hours. The UV-C LED retained 65% of its initial output

emission. No changes in the response characteristics of the sensor (US$34, SiC) or the

transmissivity of the quartz window were observed. This longevity combined with

shorter disinfection cycles with UV-C LEDs is likely to further favour their use over

UV-A LEDs, although further testing with a larger LED sample is needed.

Very low cost UV sensors (<US$6) were also successfully developed. It proved

possible to modify royal-blue LEDs to behave as photodiodes and use household items

xi

containing fluorochromes, e.g. tonic water, vitamin B and highlighter ink as simple UV-

C detectors. Eye safety was assessed using Australian Standard AS2243.5. Daily safe

exposure limits were exceeded for a distance of 30cm after 3, 43, 22 and 30 minutes for

the 270, 365, 385 and 405nm model arrays respectively.

Self-heating impacts on UV-C LED output were modelled using thermal resistance

calculations. These predicted ~18% loss of output power for operation without a

heatsink. This was experimentally confirmed, indicating thermal control is critical.

Contrary to expectations based on literature reports, TiO2 photocatalyst coated Raschig

rings did not enhance inactivation. However, the widely available porphyrin,

chlorophyllin, appeared to promote disinfection, achieving up to 5-logs reduction with

623nm LED irradiation in preliminary tests. Alone, 623nm is incapable of disinfection.

Other enhancement strategies identified in the literature (i.e. wavelength combinations,

LED pulsing and overdriving) were assessed to be of limited potential benefit.

For comparison, photoinactivation via the solar disinfection method (SODIS) was

modelled using NREL Simple Model of the Atmospheric Radiative Transfer of

Sunshine (SMARTS) software simulated sunlight spectra and bacterial action spectra.

High sensitivity of SODIS rates to seasonality, time-of-day, container material and

altitude, which would not affect PV-LED disinfection, was evident. It was also possible

to compensate for low sunlight intensity with larger PV modules and battery storage.

Finally, a market survey assessed present-day commercial LED and PV technology

advances and bulk pricing prospects. Costing models were developed to determine the

financial feasibility of various configurations. I concluded that while LED and PV

disinfection are realisable today, total system costs remain high. LED costs remain the

dominant factor with UV-C-based systems, although reductions in price are expected in

the near-term. UV-A systems exhibited higher costs in ancillary system components due

to higher energy demand. Variations in disinfector design and future prospects and areas

of consideration for PV-powered LED-based disinfection were identified.

xii

Research Highlights

Disinfection of E. coli K12 and E. faecalis ATCC 19433 was achieved with

emerging UV-C and mature UV-A, violet and blue LEDs permitting the

development of action spectra which showed disinfection doses required for the

two microorganisms were more similar for UV-A inactivation compared to UV-

C.

Deployment-ready POU prototype systems were constructed and proved the

LED and PV disinfection concept is realisable with present-day technology,

although costly.

UV-A disinfection appeared to have significant unrealized potential when

deployed in a long path-length reactor due to reduced absorbance by water. This

potentially allows UV-A disinfection to be applied to wastewaters with high

light absorbance in the UV-C range.

Commercial UV-C LED lifetime was determined to be significantly greater than

expected based on literature reports, which will likely improve the economic

feasibility of UV-C LEDs.

UV-C sensor options were successfully constructed and tested, which could

serve to improve process control and safety.

Eye safety was quantified for POU scale LED arrays. Accidental exposure under

several minutes was determined to be not immediately harmful.

Chlorophyllin as a photosensitiser was determined to have significant

disinfection potential.

Significant loss of output efficiency due to UV-C LED heating was modelled

and experimentally verified.

xiii

Photoinactivation using the SODIS method was modelled with simulated solar

spectra from SMARTS and bacterial action spectra. It appeared to be highly

sensitive to variations in spectral content of sunlight, with significant loss of

useful UV-B due to irradiation reductions by the absorbance of PET bottles and

thicker atmospheres associated with low sun elevation.

A market survey was undertaken to identify candidate LED products for POU

devices. Costing models developed showed promise of cheaper POU devices

due to new market entrants, mass-production of UV-C LEDs and steep volume

discounts.

xiv

List of Publications and Presentations

Publications

LUI, G. Y., ROSER, D., CORKISH, R., ASHBOLT, N., JAGALS, P. & STUETZ, R.

2014. Photovoltaic powered ultraviolet and visible light-emitting diodes for sustainable

point-of-use disinfection of drinking waters. Science of The Total Environment, 493,

185-196.

LUI, G. Y., ROSER, D., CORKISH, R. & STUETZ, R. A New Frontier: Photovoltaics

and Light-Emitting Diode Technology for Water Disinfection in Remote and

Developing Areas. 2014 Asia-Pacific Solar Research Conference, 2014.

LUI, G. Y., ROSER, D., CORKISH, R., ASHBOLT, N. J. & STUETZ, R. 2016. Point-

of-use water disinfection using ultraviolet and visible light-emitting diodes. Science of

The Total Environment, 553, 626-635.

Presentations

Photovoltaic Powered Ultraviolet Light-Emitting Diode Disinfection of Water, Three

Minute Thesis, 17 September 2012, UNSW Sydney, Australia.

Barriers to Deployment: Reviewing Photovoltaics and Light-Emitting diode technology

for Point-of-Use Water Disinfection, UNSW Water Research Center, 3 July 2014,

UNSW Sydney, Australia.

A New Frontier: Photovoltaics and Light-Emitting Diode Technology for Water

Disinfection in Remote and Developing Areas, Asia-Pacific Solar Research Conference

8-10 December 2014, UNSW Sydney, Australia.

Toward a Better Understanding of Solar Disinfection using Action Spectra and Solar

Modelling, IWA 3rd Water Research Conference, 11-14 January 2015, Shenzhen,

China.

xv

List of Figures

Figure 1 - Thesis structure overview diagram................................................................... 4

Figure 2 - LED efficiency improvement (Shur and Gaska, 2010) .................................. 18

Figure 3 - LED wavelength v. price and output efficiency (electrical power input

divided by optical output power) .................................................................................... 20

Figure 4 - Typical UV-LED lifetimes (Roithner Lasertechnik, 2011) ........................... 21

Figure 5 - Illustrative inactivation rates versus fluence for UV-C LED disinfection ..... 23

Figure 6 - Conceptual map of POU hardware within control and treatment modules

showing options and research and development needs ................................................... 36

Figure 7 - Bench-scale reactor experimental setup ......................................................... 53

Figure 8 - Spectral characteristics of the LEDs .............................................................. 55

Figure 9 - Measured carafe glass reflectance .................................................................. 58

Figure 10 - Quartz and UV-transmissive Perspex transmissivity ................................... 59

Figure 11 - Control plate log reduction vs. time for E. coli K12 and E. faecalis ATCC

19433 ............................................................................................................................... 60

Figure 12 - E. coli K12 and E. faecalis ATCC 19433 log reduction vs. time - 270nm .. 61

Figure 13 - E. coli K12 and E. faecalis ATCC 19433 log reduction by UV-Violet LEDs

- 365-405nm .................................................................................................................... 62

Figure 14 - E. coli K12 and E. faecalis ATCC 19433 log reduction vs. time - 430 and

455nm .............................................................................................................................. 63

Figure 15 - E. coli K12 and E. faecalis ATCC 19433 log reduction vs. time - 310 and

>=525nm ......................................................................................................................... 64

Figure 16 - Provisional batch reactor action spectra compared with normalised action

spectra data ...................................................................................................................... 67

Figure 17 - POU reactor action spectra vs. values in literature (UV-A/Violet) with

reflective enhancement.................................................................................................... 68

Figure 18 - Ceramic Raschig rings undergoing reconditioning ...................................... 77

Figure 19 - Suspended Raschig rings within reactor ...................................................... 78

Figure 20 - TiO2 experiment dark control plot ............................................................... 79

Figure 21 - TiO2 experiment inactivation plot for 365, 385 and 405nm (duplicate) ...... 80

Figure 22 - Gram stain of normal sized E. coli colony ................................................... 81

Figure 23 - Gram stain of late-growing E. coli colony ................................................... 82

xvi

Figure 24 - Chlorophyllin bleaching by 405, 430 and 455nm LEDs in 6 hours ............. 85

Figure 25 - Chlorophyllin bleaching by 525 and 623nm LEDs in 6 hours ..................... 86

Figure 26 - Chlorophyllin bleaching by 590, 660 and 740nm LEDs in 6 hours ............. 86

Figure 27 - Chlorophyllin experiment control data plot ................................................. 87

Figure 28 - Log reduction results for chlorophyllin plus 405nm LED irradiation.......... 87

Figure 29 - Log reduction results for chlorophyllin plus 623nm LED irradiation.......... 88

Figure 30 - Log reduction results for chlorophyllin plus 430, 455, 525, 590, 660 and

740nm LED irradiation ................................................................................................... 89

Figure 31 - UV photodiode sensor transimpedance amplifier schematic ..................... 101

Figure 32 - Photographs of experimental LED lifetime testing setup .......................... 104

Figure 33 - Self-heating and pulsing schematic (see also Figure 31) ........................... 107

Figure 34 - Self-heating and pulsing test device ........................................................... 107

Figure 35 - Comparison of LED operating lifetime trends ........................................... 114

Figure 36 - Non-temperature-compensated output and temperature trend ................... 116

Figure 37 - Ambient temperature thermocouple versus in-container thermocouple

temperature reading delta histogram ............................................................................. 117


temperature reading delta scatter plot ........................................................................... 118

Figure 39 - Quartz loss of transmission after 3 000 h exposure to 270nm UV-C LED 119

Figure 40 - Experimental warm-up power loss versus time trend plot at 10%, 50% and

100% duty cycle at 20mA drive current ....................................................................... 124

Figure 41 – Pulsed self-heating test duty cycle and current error due to turn-on transient,

turn-off microcontroller and MOSFET delay ............................................................... 126

Figure 42 - AS2243.5 daily exposure limits vs. wavelength and curve fits.................. 127

Figure 43 - Royal-blue LED based UV sensor ............................................................. 129

Figure 44 - Prototype System A internals ..................................................................... 148

Figure 45 - Commercial low-flow pumping maximiser circuit (System B) ................. 149

Figure 46 - Photograph of prototype System B controller ............................................ 150

Figure 47 - Labelled photograph of the three deployment-ready prototype systems ... 151

Figure 48 - Coffee tin used for System B reactor ......................................................... 152

Figure 49 - UV-C LEDs mounted to reactor without window...................................... 153

Figure 50 - System B reactor mechanical configuration ............................................... 153

Figure 51 - System A LED stem design and safety microswitch ................................. 154

xvii

Figure 52 - Photograph of System A in operation with LED indicators ....................... 154

Figure 53 - Photograph of System C components ........................................................ 155

Figure 54 - Schematic for prototype System A ............................................................. 156

Figure 55 - Schematic for prototype System B ............................................................. 157

Figure 56 - Schematic for prototype System C ............................................................. 158

Figure 57 - System A log10-reduction results (270nm, 1 LED, 250mL volume) ......... 164

Figure 58 - System B log10-reduction results (270nm, 8 LEDs, 1L reactor, 10L

disinfection volume)...................................................................................................... 166

Figure 59 - System C log10-reduction results (365nm, 3 LEDs, 10L reactor, 15L

disinfection volume)...................................................................................................... 168

Figure 60 - PVC pipe vertical reflectance ..................................................................... 169

Figure 61 - Diagrammatic representation of simulated tilt and zenith angle conditions

....................................................................................................................................... 176

Figure 62 - Block diagram of zenith-angle-based model .............................................. 178

Figure 63 - Block diagram of altitude simulation model .............................................. 179

Figure 64 - Action spectrum for E. coli WP2s (see also Figure 16) ............................. 181

Figure 65 - Measured PET bottle transmission ............................................................. 182

Figure 66 - Solar spectrum for selected conditions ....................................................... 183

Figure 67 - Daily disinfection profile for equator at equinox ....................................... 185

Figure 68 - Predicted disinfection power by sun zenith angle ...................................... 185

Figure 69 - Cumulative contribution to disinfection of E. coli WP2s by wavelength .. 186

Figure 70 - Latitude and seasonality effects ................................................................. 188

Figure 71 - Altitude effect on predicted disinfection rate at 0° latitude for 0° declination

at air-mass 1.5 ............................................................................................................... 189

Figure 72 - LED cost and wavelength optimisation by action spectra ......................... 204

Figure 73 - Discount factors for non-UV-C LED products and other components ...... 207

Figure 74 - Initial Bayes net concept ............................................................................ 210

Figure 75 – Final Bayes net design overview ............................................................... 212

Figure 76 – Revised Bayes network belief bars view ................................................... 216

xviii

List of Tables

Table 1 - UV light disinfection technologies .................................................................. 13

Table 2 - POU water disinfection technology - strengths and weaknesses..................... 16

Table 3 - PV powered UV-LED disinfection knowledge gaps ....................................... 30

Table 4 - Estimated costs for a theoretical point-of-use system ..................................... 32

Table 5 - Key knowledge gaps to concept realisation and justification .......................... 37

Table 6 - LED array quantities, model numbers, pricing, data and measured parameters

......................................................................................................................................... 49

Table 7 - Current driver measured current values ........................................................... 54

Table 8 - Average S90 inactivation constants .................................................................. 66

Table 9 - Provisional action spectra coefficients for batch reactor ................................. 67

Table 10 - Chlorophyllin bleaching by 405-740nm LED radiation in 6 hours ............... 84

Table 11 - Comparison of OPA336 and MCP6273 characteristics for transimpedance

amplifier ........................................................................................................................ 102

Table 12 - Summary of lifetime testing from literature ................................................ 109

Table 13 - UV sensor calibration values ....................................................................... 112

Table 14 - UV-C LED characteristics versus ageing .................................................... 116

Table 15 - Computed Tj rise and output power loss for different UV-C LED operating

conditions ...................................................................................................................... 123

Table 16 - Royal blue LED-based UV sensor output measurements............................ 130

Table 17 - Fluorescence of common fluorochrome containing materials exposed to one

TO-39 UV-C LED......................................................................................................... 133

Table 18 - Design considerations in POU device construction..................................... 136

Table 19 - Final characteristics of deployment-ready prototype systems ..................... 142

Table 20 - Prototype device E. coli K12 log10 reduction performance predictions and

achieved results ............................................................................................................. 146

Table 21 - Bill of materials and costs for System A ..................................................... 159

Table 22 - Bill of materials and costs for System B ..................................................... 160

Table 23 - Bill of materials and costs for System C ..................................................... 161

Table 24 - New UV-C LED products ........................................................................... 198

Table 25 - Present candidate LEDs for POU disinfectors a .......................................... 202

Table 26 - UV-C LED volume discount factor ............................................................. 206

xix

Table 27 - Bayes net model - nodes and algorithms ..................................................... 213

Table 28 - Cost modelling of different configurations of disinfector units .................. 218

Table 29 - Bayes net model sensitivity to findings analysis for 'Bulk Disinfector Cost

per Unit' ......................................................................................................................... 225

Table 30 - Bayes net model sensitivity to findings analysis for 'Project Treatment Cost'

....................................................................................................................................... 229

Table 31 - Projected UV-C LED parameters derived from Table 3, Ibrahim et al. (2013)

....................................................................................................................................... 233

Table 32 - Progress on identified issues ........................................................................ 239

xx

Glossary and List of Abbreviations

AC - Alternating Current

AlGaN - Aluminium Gallium Nitride, a semiconductor material used in UV-B/UV-C

LEDs.

AM - Air Mass, in regards to atmospheric effects on sunlight, represents the amount of

atmosphere traversed by rays from the sun and is a function of altitude/zenith

angle.

AOP - Advanced Oxidation Processes, are processes which generate and use various

reactive oxygen species to perform disinfection and chemical removal.

Band Gap - in a semiconductor junction, is the energy required for an electron to move

from the valence band into the conduction band, and represents the energy of a

photon generated by an LED, or the minimum photon energy required to

produce a current in the case of PV.

Bench Scale - initial experiments undertaken with a model POU reactor with SODIS-

like 1L volume intended to better understand wavelength and enhancement

technique impacts on disinfection.

BN – Bayes net, an acyclic graph probabilistic belief model based on Bayesian

mathematics.

CFD - Computational Fluid Dynamics, a modelling tool using algorithms and numerical

analysis to understand fluid flows and interactions with surfaces.

Current Crowding - the increase in electron/hole density in a semiconductor film near

an electrode contact due to sheet resistance.

CVD - Chemical Vapour Deposition

CW - Continuous Wave, a state of operation where power output is continuously

produced as opposed to produced intermittently (e.g. pulsing).

DC - Direct Current

Disinfection - the process of inactivating pathogens to a density safe for intended usage.

DWL - Dominant Wavelength, a characteristic of LED emission which represents the

wavelength corresponding to peak intensity.

Efficiency - the ratio between the desired output and the total input, e.g. LED electrical

efficiency is light output divided by electrical power in.

xxi

Flip-chip Package - a method of semiconductor mounting where the substrate faces the

outside of the package, with all generated light passing through the substrate to

escape the package.

Fluence - also known as dose, a measure of the amount of energy received over a unit

area, commonly expressed as mJ/cm2 or J/m

2 in the case of electromagnetic

radiation.

FWHM - Full-Width at Half-Maximum, a characteristic of LED emission which

describes the spectral width at half the maximum intensity.

GaAsP - Gallium Arsenide Phosphide, a semiconductor material used in red LEDs.

GaP - Gallium Phosphide, a semiconductor material used in green LEDs.

Heatsink - a component commonly made of aluminium or copper, designed to dissipate

heat from heat-sensitive electronic devices by increasing surface area for natural

or forced convection.

InGaN - Indium Gallium Nitride, a semiconductor material used in blue-violet LEDs.

Interlock - a safety device intended to prevent operation until the apparatus is

configured in such a way to be safe.

L50 - LED Lifetime to 50% of initial output



Lattice Mismatch - a difference in atomic spacing between deposited semiconductor

film and substrate that causes the development of grain boundaries and threading

dislocations.

LED - Light-Emitting Diode, a semiconductor based device that operates through

electroluminescence, producing light from a DC electrical current.

LED Array - a device consisting of multiple LED units interconnected primarily to

increase output power.

LP - Low Pressure, as pertaining to mercury tube UV technology, is a mercury vapour

lamp emitting a monochromatic line at 253.7nm.

LPHO - Low Pressure, High Output, a higher output version of a LP lamp.

MBE - Molecular Beam Epitaxy, a method of growing semiconductor films.

MEMOCVD - Migration-Enhanced Metal-Organic Chemical Vapour Deposition, a

more advanced technique over MOCVD for growing semiconductor films.

xxii

MOCVD - Metal-Organic Chemical Vapour Deposition, a manufacturing technique to

produce semiconductor films.

MOSFET - Metal Oxide Semiconductor Field-Effect Transistor, a semiconductor

electronic component which is voltage-controlled and is commonly employed as

an amplifier or solid-state switch.

MP - Medium Pressure, as pertaining to mercury tube UV technology, is a mercury

vapour lamp that has broadband polychromatic UV output.

MPPT - Maximum Power Point Tracker, an electronic device used to improve matching

loads to PV sources to maximise power extracted due to the current-voltage

characteristics of PV sources.

nm - Nanometer, when referring to wavelength.

Non-Radiative Recombination - alternative electron-hole recombination process that

does not result in the production of a photon within an LED, or production of an

external current flow in the case of a PV cell, and contributes to poor efficiency.

Op-Amp - Operational Amplifier, an integrated circuit differential amplifier which can

be configured via external feedback network (often comprised of resistors and

capacitors) to perform various operations (e.g. non-inverting amplifier, inverting

amplifier, voltage follower, integrator, differentiator, comparator, etc).

Overdriving - operating LED emitters above the manufacturer's continuous current

rating, in order to produce more output than rated at a cost of diminished

electrical efficiency and potential lifetime impacts.

PET - Polyethylene Terephthalate, a transparent plastic commonly used in disposable

water and soft-drink bottles and recommended for use in SODIS treatment.

PL - Photoluminescence, a method used for assessing the quality of semiconductor

materials.

POU - Point-of-Use, with regards to water disinfection technology refers to those

technologies which are of a sufficiently small scale and designed to be used at

the point of use (e.g. home, in the field) immediately prior to consumption.

Prototype Units - POU disinfection devices designed to operate standalone using PV

power to treat volumes of water at single-glass (250mL) and household (10-15L)

scales to prove the viability of the concept under simulated field conditions.

Pseudomorphic - in regards to a UV-C LED semiconductor growth technique which

uses a substrate with identical lattice structure to the semiconductor film grown

xxiii

above avoid mismatch and loss of efficiency, however, also suffers from

problems due to substrate absorbance.

PV - Photovoltaics is the use of the photovoltaic effect to generate electricity from light.

When employed with sunlight using solar panels, this is a form of renewable

energy that provides low-voltage DC with high reliability and no moving parts.

PVC - Polyvinyl Chloride, a plastic commonly used in plumbing.

Quad - with regards to LEDs, this denotes an LED package containing four individual

emitters combined into a single unit. By comparison, an array employs multiple

'units'.

Raschig Rings - a tubular ceramic form which is commonly used in chemical

engineering processes as a packed bed, offering high surface area to volume

ratio allowing for efficient chemical interactions.

ROS - Reactive Oxygen Species, chemically reactive molecules containing oxygen such

as hydroxyl radicals, singlet oxygen, superoxide and peroxides.

S.D. - Standard Deviation

Scale - used with respect to POU water volumes, namely single-glass (250mL), SODIS

(1-2L), or household (10-25L).

SETi - Sensor Electronic Technology Incorporated, a supplier of UV-C LEDs employed

and evaluated in this thesis.

Sheet Resistance - the resistance to current flow through a uniformly-thick

semiconductor film.

SiC - Silicon Carbide, a semiconductor material used in UV-selective sensors.

SLA - Sealed Lead-Acid Battery, a maintenance free rechargeable battery commonly

used in bulk energy storage due to low cost compared with alternative

chemistries.

SMARTS - The Simple Model of the Atmospheric Radiative Transfer of Sunshine, a

computer model developed by the National Renewable Energy Laboratory

(NREL), commonly used in the photovoltaics industry for estimation of the

intensity and spectral qualities of solar radiation at the Earth's surface.

SMD - Surface Mount Device, a component which does not have metallic leads (as a

through-hole device would) and instead relies on being soldered to the surface of

a printed circuit board. This style of device is more compact and is intended for

automatic manufacturing.

xxiv

SODIS - The Solar Disinfection Method, a POU disinfection technology typically

utilising PET bottles and sunlight to disinfect water.

Solarisation - a process whereby a material exposed to light loses transmissivity over

time due to damage to its crystalline structure.

Total Internal Refraction - a property which arises at interfaces between materials with

differing refractive indices due to Snell's Law, which causes an incident wave to

be bent back into the medium it came from rather than passing through the

interface into the underlying substratum e.g. PVC plastic or glass bottle. This

effect is commonly exploited in optical fibres.

Tunable - with regards to LEDs, the property of being able to have the emission

wavelength adjusted during manufacturing by changing the proportion of metals

within the semiconductor material.

UV - Ultraviolet, radiation of a wavelength shorter than the visible. Specific wavelength

ranges of interest for disinfection include UV-A (315-400nm), UV-B (280-

315nm) and UV-C (200-280nm).

Window - an optically transparent material used to protect LEDs from water while

allowing the transmission of generated radiation

Wp – in reference to PV panels, Watt-peak, the amount of power produced by a PV

panel under standard temperature and irradiance conditions of 25°C at

1000W/m2 with AM1.5 spectrum.

1

1. Background and Introduction

Water is an essential resource for all life on planet Earth. Only a small fraction of the

water on Earth is suitable for direct human consumption. Its proper management,

sustainable disinfection and efficient use is critical to the lives of all. In fact, it is so

important that access to clean drinking water has been recognised as an absolute human

right, one that is sadly not universally met in reality (Momba et al., 2006, United

Nations, 2010).

One does not have to venture far from the developed city areas in developed countries

to experience difficulties with accessing clean water. Many farms, for example, in

Australia's outback rely on collected rain and surface water from farm dams, creeks,

rivers and streams to supply their complete water needs. Various treatment methods,

often expensive and involving chemical consumables, are employed in such situations.

Those in developing countries are less fortunate, and are often remote from sources of

water, requiring the use of water container infrastructure to transport all household

water requirements (Jagals et al., 2013). This often involves making long dangerous

trips to the water source (Majuru et al., 2012, Ellis, 1997, Amnesty International, 2010).

The water itself varies significantly in quality and may be contaminated by surface

runoff from upstream users. The transportation mechanism introduces even greater risk

via recontamination, as containers are not often cleaned, resulting in biofilm

accumulation that serves to ensure any water contained within is contaminated (Majuru

et al., 2012).

Contaminated water contributes to over two million preventable infant deaths annually

due to waterborne diseases such as cholera, and otherwise results in diarrhoea and

associated reductions in quality of life through health loss in disability (World Health

Organization, 2012a, World Health Organization, 2012b). There is an urgent need for

effective water treatment at the household level if the United Nations' Millennium Goals

(United Nations, 2009) are to be met, with point-of-use (POU) technologies and water,

sanitation and hygiene (WASH) style interventions identified as being the most

effective (Bartram and Cairncross, 2010).

2

Present POU technologies are numerous (Sobsey, 2007), with some technologies

adapted from centralised disinfection roles in developed world areas. These

technologies may require consumables which are logistically difficult to supply, are

costly and require training to use safely. Other methods are energy intensive and

contribute to climate change. Many technologies are not capable of meeting the daily

household water requirements (~20L) leaving users at risk by using untreated water for

bathing, washing clothes, etc. Finally, even simple methods such as the Solar

Disinfection Method (SODIS) are subject to variable performance which can put users

at risk. It is clear that improvement in POU technologies are required.

This is reflected in some WASH style interventions which sometimes involved

providing clean supplies, which were not completely successful due to unreliability,

recontamination by water containers or lack of acceptance of POU technology on offer

(Hunter et al., 2009, Moabi, 2006, Jensen et al., 2002). Barriers to adoption, including a

lack of adequate information and effective training, can render technologies ineffective

in the long run. Where solutions required users to pay small amounts for chemicals,

long-term adoption rates remained below 30% (Luoto et al., 2011). Such interventions

are costly, and without the right solutions, it is likely that the money spent is not

achieving the expected results (Cameron et al., 2011).

It seems that affordable, high-tech, robust, simple, fool-proof solutions are required to

overcome this problem. Effective POU technologies that require minimal training and

that are likely to be culturally acceptable will be indispensable in the case of

humanitarian efforts and interventions in developing countries, and in disaster relief

where infrastructure has been rendered inoperable (Lantagne and Clasen, 2012).

Ultraviolet (UV) disinfection is a proven broad-spectrum disinfectant which possesses

attractive qualities for adaptation to POU (Bolton and Cotton, 2008). With emerging

UV-C and mature UV-A/visible light-emitting diode (LED) technology, and by

combining photovoltaic (PV) technology as a renewable energy source, truly

sustainable POU disinfection may be able to be realised.

3

However, this will require an integrated interdisciplinary approach, taking into account

areas of research in water disinfection, photovoltaics, electronics, optics, and

economics, to bring the concept first to the bench and then into reality. In this project it

was hoped that by pursuing this concept, I could capture the necessary knowledge in a

way which can benefit those who need it most. It was also hoped that this investigation

would one day make a lasting contribution to people's lives in the most basic way, by

ensuring the safety of their water supply and consequently improving the quality of life

of those who have no other alternatives.

1.1. Thesis Structure

This thesis documents an interdisciplinary investigation into the concept of PV-powered

LED-based disinfection. It encompasses a holistic approach by considering

opportunities and knowledge gaps in the fields of water research, photovoltaics and

electronics, and combines knowledge from these fields to realise deployment-prototype

systems. Determination of feasibility with consideration for the constantly developing

market was achieved through modelling and future directions identified. The structure

of the thesis is summarised in Figure 1.

Turquoise boxes indicate cross-disciplinary areas of knowledge synthesis, with green

boxes indicating the main line of investigation involving mostly water-research and

microbiology based outcomes. The grey box indicates areas mostly involving

engineering considerations, and explores various electronics related matters. Orange

boxes indicate secondary routes of investigation for comparison and enhancement

which provide supplementary decision-making information. Finally, the purple box

indicates the main synthesis of knowledge between economics, water disinfection

research, photovoltaics and electronics disciplines as inputs into cost modelling. The

boxes in bold indicate the most important facets of this study.

4

Figure 1 - Thesis structure overview diagram

Literature Review –

Background, Opportunities,

Knowledge Gaps

Engineering

Concerns –

UV-C LED

Lifetime &

Self-Heating

Losses,

Quartz

Solarisation,

UV Sensing,

Eye SafetyPrototype

Systems –

Configuration

Variables,

Construction

Strategies,

Performance,

Cost, Future

Opportunities

Bench Scale

Systems Design,

Experiments &

Experience –

LED

Characteristics,

Inactivation vs.

Time, Action

Spectra

SODIS

Performance

Comparison –

Solar Spectrum

Simulation &

Bacterial Action

Spectra

Market Progress

& Cost

Modelling –

LED & PV

Markets, Bulk

Discounts, Bayes

Net Cost Model

Enhancement

Potential –TiO2

Photocatalysts,

Photosensitizers,

Wavelength

Combinations,

LED Pulsing,

Overdriving

Conclusions, Future Potential

and Recommendations

5

The literature review in Chapter 2 identified important opportunities and knowledge

gaps, which included the need to better understand how different wavelengths can cause

inactivation and the need to address engineering considerations to bring bench-scale

results into the real-world.

Chapter 3 documents the initial experimental work in bench-scale SODIS-scale

disinfection systems to explore how different wavelengths of light produced by 12

different commercially-available LED arrays ranging from 270-740nm, perform in

inactivating E. coli K12 and E. faecalis ATCC 19433. This provided the basis to

develop extended action spectra and compare them with existing literature. The

resulting action spectra are instrumental in understanding how different LED

wavelengths with markedly different power levels will perform in disinfection roles.

Uncertainties about LED lifetime, reactor cost, water absorbance and different micro-

organisms were identified as needing resolution.

Opportunities for low-cost enhancement identified in the literature review were mostly

examined and explored in Chapter 4. Experiments involving the use of titanium dioxide

(TiO2) photocatalysts and chlorophyllin photosensitisers were undertaken, and a

discussion on the topics of wavelength combination, pulsed irradiation and LED

overdriving based on published literature is also presented. While enhancement still

appeared conceptually possible, consistent results were not replicated and each method

had its own constraints which rendered it unsuited for POU applications and inclusion

in prototype reactors.

Engineering issues and uncertainties identified in bench scale work and via the literature

review were addressed in detail in Chapter 5. Through LED lifetime testing, I

determined the realistic lifetime of presently commercially available UV-C LEDs,

which helped reduce uncertainty. I also determined that solarisation is not a likely

problem with UV-C LEDs with commercially available quartz windows. However,

thermal operating conditions of UV-C LEDs were identified as a significant factor

impacting negatively on LED lifetime and output power, based on thermal resistance

modelling and experimental LED warm-up trials. I identified a need to better measure

and define the UV-C output, and opportunities for improving safety, especially for

6

POU, through inclusion and implementation of various types of UV sensors by

exploring construction and testing of various types of sensors and fluorochromes. I also

assessed eye safety using standard guidelines to determine safe exposure thresholds.

The experience gained in the bench-scale experiments and engineering considerations

chapters informed construction of three flow-type prototype systems designed for

treatment of household-scale volumes of water. The design rationale, construction

strategies and bills of materials are provided in Chapter 6, where the construction of the

different prototypes is described and their performance assessed under field conditions.

Successful disinfection was achieved, and experience was gained during the process to

inform future designs discussed in Chapter 9.

Chapter 7 focuses on how SODIS performance varies due to different solar conditions

and considers how different wavelengths of sunlight contribute to the disinfection

process through photoinactivation. It also compares the spectral sensitivity of SODIS

with that of PV, and shows that, despite its increased complexity, a PV powered

disinfector offers critical advantages over SODIS, particularly during winter and outside

of the tropical latitudes.

Chapter 8 considers the financial feasibility of the devices by an examination of the

market, its progress in cost reduction and technology and consideration of bulk

discounts. This assessment employed a Bayes net and spreadsheet models to connect the

numerous variables in the design and costing of such a system, and undertook a

sensitivity analysis to determine the limiting factors and areas that need priority focus.

The findings and lessons learnt are summarised in Chapter 9, including a detailed

update of the initial knowledge gaps identified, the resulting knowledge obtained

through this interdisciplinary investigation and various reactor modifications and

configurations believed to be promising for future investigations.

7

2. Literature Review1

2.1. Introduction

Access to safe water is a human need and right. However, in developing regions,

contaminated water still claims 2.2 million lives each year due to waterborne pathogens

(World Health Organization, 2012b). Waterborne pathogens are particularly significant

in remote areas, and following natural disasters (Ratnayaka et al., 2009). This places

substantial burden on communities (Cameron et al., 2011), and reinforces the need to

improve water quality at household level through some form of disinfection technology

(World Health Organization, 2012a).

Existing disinfection was chiefly developed for large-scale centralised implementation,

involves complex processes, dangerous chemicals or costly consumables and is often

unsustainable in developing regions (Salazar-Lindo et al., 1993). Further, the high costs

associated with maintaining aging centralised drinking water systems (AWWA

Research Foundation, 2012) , is forcing a rethink and consideration of more sustainable

alternative system designs (Howe and Mitchell, 2012). In developing regions,

centralised disinfection to community stand pipes is also ineffective where household

water containers recontaminate the drinking water (Sobsey, 2007).

Consequently, point-of-use (POU) disinfection is increasingly promoted (World Health

Organization, 2012a) e.g. sand filters, solar disinfection (SODIS), boiling. Adoption

though has been limited (Mäusezahl et al., 2009, Rainey and Harding, 2005, Brown and

Sobsey, 2012) due to sub-optimal performance, unsuitability, acceptance limitations,

affordability, reliability and alignment with sustainable development principles. So,

further POU advances are urgently needed, with ultraviolet Light (UV) technology

emerging as a major small-scale disinfection candidate.

1 This chapter has been revised and expanded. It was substantially published as

LUI, G. Y., ROSER, D., CORKISH, R., ASHBOLT, N., JAGALS, P. & STUETZ, R. 2014. Photovoltaic

powered ultraviolet and visible light-emitting diodes for sustainable point-of-use disinfection of drinking

waters. Science of The Total Environment, 493, 185-196.

8

POU UV disinfection prospects have recently been boosted by Light Emitting Diode

(LED) semiconductor developments (materials with conductivity in between conductors

and insulators e.g. silicon). Aluminium-Gallium Nitride (AlGaN) LEDs can now emit

UV-C wavelengths comparable to mercury discharge tubes (Aoyagi et al., 2011,

Würtele et al., 2011, Vilhunen et al., 2009, Chatterley and Linden, 2009) and visible

light LED history suggests that within 10 to 20 years UV-LEDs could underpin

inexpensive, mass produced, long lived POU devices (Chatterley and Linden, 2009).

Concurrently, renewable energy technologies, especially photovoltaics (PV), have

matured sufficiently to reliably provide sustainable and grid independent energy (Parida

et al., 2011, Wenham et al., 2006). This UV-LED + PV combination promises a new era

in POU disinfection for drinking and waste waters (Crawford et al., 2005).

The rest of this review is divided into five sections. Section 2.2 outlines UV disinfection

concepts and methods, and compares UV-diodes’ potential with competing UV and

other POU disinfection technologies. Section 2.3 details LED development and

disinfection research. Section 2.4 describes features of PV technology pertinent to POU

application. Section 2.5 discusses the challenge of integrating LEDs and PV for POU

application. Section 2.6 concludes the review by recommending research needs.

2.2. UV Disinfection

Disinfection processes are systems which inactivate infectious microorganisms

(bacteria, viruses, parasitic protozoa, helminths) to densities where waters are safe for

intended uses (Ratnayaka et al., 2009). A popular method is exposure to intense UV

light i.e. electromagnetic radiation of 100-400nm wavelength further subdivided into

UV-A (315-400nm), UV-B (280-315nm), UV-C (200-280nm) and Vacuum UV (100-

200nm) (USEPA, 2006). Unlike chlorine, UV is a broad spectrum disinfectant and

leaves few residues (Qian et al., 2013, Choi and Choi, 2010). Histories of UV’s

development are provided by Bolton and Cotton (2008) and Whitby (2002).

2.2.1. Mechanisms and Action Spectra

Inactivation occurs through various photochemical processes. UV-B and UV-C photons

are absorbed by proteins, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA),

directly damaging their structure, primarily through the formation of pyrimidine dimers,

9

pyrimidine (6-4) pyrimidone photoproducts and protein-DNA cross-links. Typically

>99.9% of irradiated microorganisms lose infectivity and reproduction capacity when

exposed to 10-40 mW.cm-2

doses (USEPA, 2006).

Inactivation can also be induced by UV-A and visible light. The mechanisms are

incompletely understood but reactive oxygen species appear to be essential (Hamamoto

et al., 2007). Though higher doses are required than for UV-C this is offset by their

higher power, lower expense and higher efficiency. UV-A effectiveness can also be

enhanced by advanced oxidation photocatalysts e.g. TiO2 (Chatzisymeon et al., 2011).

Microbial sensitivity to different UV wavelengths is defined by ‘action spectra’ which

differs slightly from DNA absorbance, and between microorganisms. Inactivation is

commonly expressed as log-reduction:

where No and N are pre and post-exposure numbers (USEPA, 2006). Inactivation doses

are estimated from viable cell number versus UV dose plots, using collimated beam

apparatus measurements. There are now extensive databases of rate constants and

minimum doses (Hijnen et al., 2006, Bolton and Cotton, 2008). During commercial

reactor validation, rate constants are compared to the results of biodosimetry.

Microorganisms (and potentially host cells) can repair DNA via photoreactivation and

dark reactivation. These mechanisms are incompletely understood, but reactivation may

occur through enzymatic replacement of damaged nucleotides and recombination with

undamaged DNA (Bolton and Cotton, 2008, Shin et al., 2009). Fortunately, repair

mechanisms are not thought to severely degrade drinking water UV-C disinfection

performance (USEPA, 2006).

2.2.2. Standards and Guidelines

Standards governing UV-C water disinfection depend on target microorganisms. The

Austrian National Standard ÖNORM M5873-1 and M5873-2 and the German Guideline

DVGW W294 require a dose of 40 mJ.cm-2

to assure 3-log inactivation of

Cryptosporidium and Giardia oo/cysts based upon experiments conducted by

10

collimated beam apparatuses (USEPA, 2006). The “UV Design Guidance Manual for

the Long Term 2 Enhanced Surface Water Treatment Rule” (USEPA, 2006) provides

detailed guidance covering design and validation of reactors. Reactors are validated

through biodosimetry experiments to establish their log-inactivation credit. There are no

standards for UV-LED, UV-B, UV-A, or visible light based disinfection but UV-C

guides provide models, and the 'action spectra' concept seems applicable.

2.2.3. Reactor Design

Reactor designs aim to ensure that all treated water is exposed to a minimum desired

UV fluence. UV reactors can be separated into flow reactors and batch reactors. In flow

reactors, desired doses are achieved by matching water velocities and irradiation

intensity. Flow reactor efficiency is constrained by water travelling different paths

leading to point dose and fluence variance. In response, reactors must over-irradiate

water. Light intensity distribution modelling, e.g. multiple point source summation

models, is used to optimise reactor design. Lamps are treated as spaced point sources

allowing estimation of fluence and inactivation at any location inside a reactor.

Modelling also aids flow configuration optimisation. Such models often employ

Computational Fluid Dynamics (CFD), which breaks flow into many small finite

volume elements, can simulate particle flow paths, aggregate doses, and predict reactor

performance and UV-fluence distribution. Despite CFD’s value, experimental

validation is always required. Reticulation and pumping systems can also impact reactor

fluence e.g. via introducing air bubbles. Batch reactors, are typically used in bench-scale

experiments, e.g. collimated beam measurements, where a fixed volume of liquid is

exposed to varying doses (USEPA, 2006, Bolton and Cotton, 2008, Jamali et al., 2013).

To date the literature has reviewed the combination of PV, LED and POU disinfection

only briefly or as one aspect of a broader work. Crawford et al. (2005) looked at the

overall logistics as part of a feasibility study of portable PV/UV POU disinfection.

Vilhunen and Sillanpää (2010) compared LED use with Advanced Oxidation Processes

(AOP) treatment to other UV systems. Ibrahim et al. (2013) reviewed UV-C LEDs

within an economics perspective. Kneissl et al. (2010) and Wengraitis et al. (2013)

considered UV-C LED performance and bench-scale testing of prototype flow modules,

not alternative LED types or POU system fabrication. Nevertheless the knowledge

11

gained has been sufficient for prototype designs (Gaska et al., 2011, Würtele et al.,

2011, Crawford et al., 2005). Commercial, albeit expensive, units have subsequently

emerged (Aquionics, 2012).

2.2.4. Photocatalysis, Advanced Oxidation Processes (AOPs) and

Photosensitisers

UV disinfection can be combined with chemical oxidation of aqueous organic and

inorganic materials (Sarathy and Mohseni, 2010, Vilhunen and Sillanpää, 2010).

Applications include the control of halogenated disinfection by-products (DBPs) (Wang

et al., 2009, Jo et al., 2011), e.g. haloacetic acids, chemicals (Autin et al., 2013, Jamali

et al., 2013) and odours and biofilms (Lakretz et al., 2011).

‘Homogenous’ AOP reactions work by UV decomposing oxidants to reactive

intermediates including hydroxyl radicals (•OH) (Ikematsu et al., 2004). The two

predominant systems are ozone, and photo-Fenton which utilises iron or hydrogen

peroxide. Homogenous AOP is constrained by the need for an oxidant supply, their

hazardous nature and unintended secondary products e.g. trihalomethane (Metz et al.,

2011).

An alternative to chemical additives is 'heterogeneous' AOP, which generates hydroxyl

radicals in situ using photoactive metal-oxide catalysts, for example, titanium-dioxide

(TiO2) (Feitz, 1998). Dissolved oxygen hydrolyses water to form hydroxyl ions using

energy provided by TiO2 absorbing radiation <385nm and POU application has been

proposed (Pelaez et al., 2012, Lipovsky et al., 2011, Tsuyoshi and Akira, 2013,

Izadifard et al., 2013).

Improvements in inactivation can also occur through the use of porphyrin ring

containing photosensitisers, often termed photodynamic therapy or photodynamic

inactivation (Almeida et al., 2011). This technique has been used in medical and

wastewater treatment contexts and can allow for more effective use of light in the 420-

430nm region which exploit the Soret absorption bands (Jemli et al., 2002, Carvalho et

al., 2007). Application of this for POU usage appears possible with positive literature

12

reports on inactivation of bacteria and viruses (Chen et al., 2011a, Costa et al., 2012,

Rossi et al., 2012).

2.2.5. UV-C Emission Technologies

Several technologies can generate UV-C (Table 1), most notably UV mercury tube

technology, which includes units suited to POU disinfection. Crawford et al.(2005) and

Vilhunen and Sillanpää (2010) provide short reviews of recent technological

developments. Though highly refined, tube technology still suffers limitations at small

scale. For example, the ‘Steripen™’ is rated for 3000 to 8000 treatments (Steripen,

2013a, Steripen, 2013b, Steripen, 2012) of 48-90 seconds, an aggregate lifetime of only

200 hours. A possible reason is on/off switching with little filament preheating causes

electrode sputtering (Nachtrieb et al., 2005). By comparison LEDs may be pulsed

without damage, potentially improving small scale germicidal effectiveness (Li et al.,

2010, Mori et al., 2007a, Wengraitis et al., 2013) reducing energy usage and heat

production, and prolonging lifespan.

13

Table 1 - UV light disinfection technologies

Lamp Type Wave-length

(nm)

Lifetime

(h)

Germicidal

Efficiency

(%)a

POU Strengths POU Constraints References

Low-Pressure

Mercury (LP)

254.7 8000 –

10000

35–38 High germicidal

efficiency; long lamp

life; POU designs;

Proven.

Requires Hg; high voltages

and electrical ballasts;

sensitive to temperature

changes; fragile.

USEPA (2006)

Bolton and Cotton (2008)

Kowalski (2009)

Low-Pressure

High-Output

(LPHO)

254.7 8000 –

12000

30–35 Higher power output

than LP.

As for LP; reduced efficiency

compared to LP.

USEPA (2006)


Kowalski (2009)

Medium-

Pressure

Mercury (MP)

185 - 600 4000 –

8000

10–20 Higher output power;

fewer lamps

required; proven.

As for LP; shorter lamp life

and high operation

temperatures than LP.

USEPA (2006)


Kowalski (2009)

Xenon

Flashlamps

185 - 600 >108

(pulses)

Unknown Mercury free. Low efficiency as broadband

light source; short lifetimes.

high voltages; fragile.


Lamont et al. (2004)

Excimer

172, 222,

253, 259,

282, 308, 342

> 8000 7–40 Hg free; electrode-

less; high lifetimes;

diverse shapes.

Fragile; requires RF source. Oppenländer (2007)

Kogelschatz (2004)

14

Lamp Type Wave-length

(nm)

Lifetime

(h)

Germicidal

Efficiency

(%)a

POU Strengths POU Constraints References

Light-Emitting

Diode (LED)

Tunable from

240 to 415 in

10 to 15nm

increments)

1250 –

26000

Up to 11 Hg free; tunable

wavelengths; simple

power requirements;

robust; small; no

warm-up time;

pulsing possible.

Current high cost; low

efficiency; power output and

lifetime.


Sensor Electronic Technology

Inc (2012)

Seoul Optodevice (2012)

Roithner Lasertechnik (2011)

Crystal IS (2012)

Nichia Corporation (2012)

LED Engin (2012)

Gaska (2011)

Khan et al.(2005)

Chatterley and Linden (2010)

a: Germicidal Efficiency represents the efficiency of the conversion of electricity to light weighted by the DNA action spectrum.

15

2.2.6. POU and Other Challenges

Circumstances within poor communities (Sobsey, 2007) and experience with other

disinfection technologies introduce further, sometimes related, challenges (Table 2)

(Ibrahim et al., 2013, Crawford et al., 2005). For example where water containers are

used, a chlorine disinfected supply water can be re-contaminated (Jagals et al., 2013).

Low pressure UV may also be ineffective against some pathogens, notably Adenovirus

(Gerba et al., 2002, Rodríguez et al., 2013). Pathogen resistance may be resolved by the

use of polychromatic (medium pressure) UV-C (Eischeid and Linden, 2011) which is

noteworthy as diode tunability should make possible array sources with any desired

emission/intensity combination.

A useful source of lessons on what is, and is not, practical POU wise is likely the

SODIS (Arvidsson, 2013, Fisher et al., 2012, Graf et al., 2010, Davies et al., 2009,

Rainey and Harding, 2005, SODIS, 2012, Oates et al., 2003, Reed, 2004) literature. In

addition to SODIS being a widely evaluated competing POU technology this is because

it is also, like PV, constrained by sunlight availability, is a batch process, and there is

overlap in inactivation theory. Conversely PV/LED systems and experiments may aid

SODIS theory and experimentation.

16

Table 2 - POU water disinfection technology - strengths and weaknesses

Technology Strengths Limitations References

Ultraviolet

Light

Broad spectrum including

Cryptosporidium and

Giardia; minimal disinfection

by-products; short contact

times.

Energy intensivea; high voltages

a; Hg is hazardous

a;

consumes fragile tubesa.

Absorbed by organics and turbiditya; nitrite formation; no

residuala; Photoreactivation and dark repair

a.

Ratnayaka et al.(2009)


Chlorine-

based

Residuals inhibit

recontamination; easy to

monitor

Affected by metallic compounds, temperature, pH, contact

time; ineffective against Cryptosporidium; disinfection by-

products e.g. halomethanes; taste and odour; consumes

chemicals with short shelf lifea; requires careful dosing

and training; chlorine dangerousa.


Cheremisinoff (2002)


Ozonation

Does not produce traditional

DBPs, but chlorite &

bromate; more effective

against protozoa than

chlorine.

Poorly understood by-products; energy intensive and

inefficient (5-10%)a; toxic; must be decomposed before

releasea.



17

Technology Strengths Limitations References

Membrane

Filtration

Produces very high purity

water.

Energy intensivea; needs high pressures

a; membranes

consumablea and costly

a; require regular maintenance

a and

prefiltrationa; minerals removed

a; membrane fouling

a.

PSI Water Filters (2013)

Lee and Lueptow (2001)

Pandit and Kumar (2013)

Shah et al.(2012)

Boiling Unaffected by turbidity;

simple; suitable for

householdsa;

Energy intensivea; fuel expense

a; greenhouse gas emission;

scalding hazarda.



Solar

Disinfection

(SODIS)

Simple; suitable for

householdsa; Low cost.

>several hours sunlighta; seasonal (ex-tropics

a); requires

training for effective usea; affected by turbidity and UV

absorbancea; basic POU treats only small water volumes

a.

SODIS (2012)

Mäusezahl et al. (2009)

Fisher et al.(2012)

Sobsey (2007)

Gomez-Couso et al.(2010)

Sand/Ceramic/

Particle Matrix

Filtration

Reduces turbidity; simple

process suitable for

householdsa; useful pre-

treatment for other processes.

Ineffective for virusesa; variable effectiveness against

bacteria; bacteria growth in filtera; media consumable;

maintenancea.

Sobsey (2007)

Pandit and Kumar (2013)

a: Precise strengths and limitations vary between technologies. Issues identified need extra consideration where disinfection is

decentralised, infrastructure is poor, or technology is ‘Point-of-Use’.

18

2.3. UV Light Emitting Diode (LED) Technology

LEDs are solid-state semiconductor devices, which convert DC current into light with

wavelengths reflecting the band-gap of the materials used in manufacture. UV-C

technology only recently emerged following the development of blue, violet and UV-A

LEDs (Figure 2). The focus of researchers and manufacturers still includes increasing

performance (Ibrahim et al., 2013), thus disinfection UV LED technology is still a

‘work in progress’ facing diverse challenges before optimum commercialisation.

Figure 2 - LED efficiency improvement (Shur and Gaska, 2010)

2.3.1. LED Operating Mechanism and History

Practical long wavelength visible LEDs were first commercially introduced in the mid

1960’s. LEDs use electroluminescence, electrically excited electrons generating photons

at semiconductor material junctions e.g. GaAsP (Red LEDs). Over time, semiconductor

systems were developed emitting Green (e.g. GaP) and Blue/violet (e.g. InGaN).

Improved theory, manufacturing techniques, and refined material systems reduced

defects and non-radiative recombination processes which in turn improved electrical

efficiencies and reduced heat losses. With visible LED technology near maturity,

0

10

20

30

40

50

60

70

1960 1970 1980 1990 2000 2010 2020

Ele

ctr

ical E

ffic

iency

(%

)

Year

UV-C AlGaN Green

Blue SiC Blue InGaN

Red/Orange

19

research has shifted to UV and associated applications e.g. disinfection, fluorescence

induction (Khan et al., 2005). A key development has been the Al/Ga/N/In material

systems that generate 350nm to 210nm wavelengths by varying (tuning) the proportions

of gallium, aluminium, nitride and indium (Khan, 2006). Further efficiency gains have

been made through better understanding multiple quantum well behaviour (Verma et al.,

2010, Tamulaitis et al., 2008).

2.3.2. UV-C LED Research, Development and Commercialisation

Manufacturing high performance UV-C LEDs is still challenging. AlGaN film is

difficult to grow without cracking along grain boundaries and threading dislocations

formed by lattice mismatch with substrate. These defects allow non-radiative

combination, producing heat instead. Strain differences in the film create electric fields

leading to the quantum confined Stark effect (Murotani et al., 2008) further reducing

efficiency. ‘Flip-chip’ packaging (deposited film faces downward) also generates losses

due to substrate absorption. Separately, achieving high molar fractions of aluminium

incorporation, necessary for shorter wavelength UV-C diodes, is difficult due to gas

phase reactions (Khan et al., 2005). Levels of aluminium needed lead to high sheet

resistance, current crowding, inefficient semiconductor use and heating (Reed et al.,

2008b), and poor ohmic contacts to semiconductor film. The latter combine to reduce

LED efficiency and lifespan (Shur and Gaska, 2010). As a result efficiency decreases

significantly with shorter wavelengths, and the shortest commercially manufactured are

>240nm. Compared to visible LEDs, devices <365nm have short lifespans, limited

availability and high cost.

The results of studies utilising UV-LEDs are consistent with these limitations.

Meneghini et al.(2008b) reported UV-C LEDs losing 30% of initial output power within

200 hours. Ohmic contact on the semiconductor film was not degraded, implying that

operation generated stress and defects within the active material. They also reported

catastrophic failures following sudden output power decreases, and behaviour similar to

short circuits. Reed et al.(2008a) similarly reported their 25mA UV-LEDs had a half-

life of only 250 hours.

20

To assess the impact of these constraints, commercial specifications and prices were

collected. This showed a 10,000 fold difference between the efficiency and price of UV-

C and UV-A/Blue LEDs (Figure 3) and > 100 fold difference in lifetime (Figure 4).

These differences probably reflect the relative maturity of near visible wavelength

technology (e.g. InGaN), versus current AlGaN systems, market volume and industrial

demand for high power UV curing.

Figure 3 - LED wavelength v. price and output efficiency (electrical power input

divided by optical output power)

In response, UV-C LED manufacturers are refining fabrication techniques, producing

thicker films (fewer defects), and packaging to reduce heating and improve performance

and lifetime. In place of suboptimal Metal-Organic Chemical Vapour Deposition,

Molecular Beam Epitaxy (MBE) or reactive MBE (Shur and Gaska, 2010), low

temperature growth techniques were trialled to reduce biaxial tensile strain (Khan et al.,

2005). Recent production uses pulsed atomic layer epitaxy, subsequently developed into

Migration-Enhanced Metal-Organic Chemical Vapour Deposition (MEMOCVD),

featuring flexible precursor pulse optimisation (Shur and Gaska, 2010).

y = 2.50E+07e-4.76E-02x

R² = 5.48E-01

y = 8.95E-05e2.96E-02x

R² = 6.91E-01

0.01

0.1

1

10

100

0.001

0.01

0.1

1

10

100

1000

240 260 280 300 320 340 360 380 400 420 440 460

LE

D e

lectr

ical eff

icie

ncy (%

)

Bulk

Pri

ce p

er

opti

cal outp

ut pow

er

($/m

W)

Wavelength (nm)

Bulk US$ 2012 per mW

Efficiency

21

Figure 4 - Typical UV-LED lifetimes (Roithner Lasertechnik, 2011)

MEMOCVD allows high rate growth of buffer layers, and slower growth rate of active

layers, resulting in better charge carrier mobility, atomic incorporation, improved

surface coverage, enhanced surface migration, longer lifetimes, narrower

photoluminescence (PL) lines and reduced noise. Superlattice buffer layers also help

reduce thin film strain, film cracking and dislocation density. Improvement in

performance and lifetime has also been obtained via new device geometries (Khan et

al., 2009) and better device packaging enhancing heat dissipation. To avoid mismatches

between substrate lattice constant and film, pseudomorphic growth may be used with

AlN substrates. This promises higher powered diodes less affected by temperature with

more consistent performance. Nevertheless, diode efficiencies are still constrained by

substrate UV absorption and matrix refractive index forcing narrow extraction angles.

Internal quantum efficiencies (70%) approach visible LEDs, implying excellent photon

generation but a need to improve photon extraction. A pseudomorphic LED reportedly

achieved >60mW output power at 270nm (Grandusky et al., 2013).

As a result, UV-LEDs should eventually compete with mercury tube technology. Recent

research reports indicate External Quantum Efficiencies up to 1.48% at 260nm

(Grandusky et al., 2011), and 11% at 278nm (Shatalov et al., 2012), and lifetimes

50

100

200250

350

350

1000

1

10

100

1,000

10,000

240 260 280 300 320 340

Typic

al L

ifeti

me (h)

Wavelength (nm)

>5000 hours

22

potentially in the 10 000 h (continuous 270- 280nm) to 26 000 h (250–260nm) range

(Gaska, 2011). An advantage of UV-LEDs over similar lifetime LPs is capacity for on-

off cycling without ‘lamp’ damage, improving heat dissipation and lifespan, and

supporting pulsed mode inactivation (Wengraitis et al., 2013, Wang et al., 2005, Li et

al., 2010). Consequently the UV-LED market is projected to reach $150m by 2016

(Optics.Org, 2012) and through to $369.58 million to $520 million by 2019-2020

(Markets and Markets, 2016, Semiconductor Today, 2015). Concurrently mass

production and increasing demand should reduce unit cost as with visible LEDs

(Kneissl et al., 2011).

2.3.3. UV-C LED Disinfection

Several disinfection studies have confirmed LED effectiveness (Figure 5) with UV-C

LEDs performing similarly to LP tubes (Würtele et al., 2011, Vilhunen et al., 2009,

Chatterley and Linden, 2010, Chatterley and Linden, 2009, Oguma et al., 2013, Nelson

et al., 2013). These authors concurred that limited power output, disinfection volume,

long exposures, and high costs, still made UV-LED disinfection impractical but were

still optimistic. This view is supported by new UV-LED disinfection devices appearing

e.g. Sensor Electronic Technology Inc. demonstration of prototype water disinfection

LED units (Shur and Gaska, 2010, Gaska et al., 2006); and Aquionics’ (2012)

commercial disinfection system, UV-Pearl, targeted at medical industry low-flow

applications. Bench-scale experiments with POU-oriented aims have shown similar

results (Nelson et al., 2013, Chatterley, 2009).

23

Figure 5 - Illustrative inactivation rates versus fluence for UV-C LED disinfection

a: BO2011: Bowker et al. (2011); WU2011: Würtele et al. (2011); AO2011: Aoyagi et

al. (2011); BC2008: Bolton and Cotton (2008).

2.3.4. UV-A, Visible Light and UV-B LED Disinfection

UV-A LED based disinfection has been demonstrated (Yagi et al., 2007, Mori et al.,

2007b, Hamamoto et al., 2007) alone and in combination with UV-C (Chevremont et

al., 2012b, Chevremont et al., 2012a). Hamamoto et al. (2007) concluded that UV-A

disinfection is influenced by reactive oxygen species based on hydroxyl scavenger (e.g.

mannitol) experiments. UV-A demonstrated less direct DNA damage than UV-C.

Separately UV-C/UV-A synergism was reported by Chevremont et al.(2012b) .

Despite requiring much higher doses (54-504J.cm-2

), UV-A (365nm) LED systems may

still achieve useful disinfection (3.4-5.7-log10 reduction). Compared to UV-C LEDs,

UV-A LEDs are much more efficient (5-30%), long lived (>20 000 hours),

commercially available in high powered (>1W) modules, and can be combined with

photocatalysts (Shannon et al., 2008). Importantly UV-A is less impacted by UV

absorbing organics (Yagi et al., 2007) e.g. tannins.

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70 80

Dec

imal

Red

uct

ion (

log

10

N0/N

)

Dose (mJ.cm-2)

E. coli 255nm BO2011 E. coli 275nm BO2011 E. coli ATCC 11229 BC2008

B. subtilis spores 269nm WU2011 B. subtilis spores 282nm WU2011 B. subtilis spores BC2008

MS2 255nm AO2011 MS2 280nm AO2011 MS2 BC2008

MS2 275nm BO2011 MS2 255nm BO2011

24

Visible light inactivation is also possible and not confined to Violet/Blue wavelengths

like SODIS (Sinton et al., 2002). Inactivation mechanisms are not fully understood but

likely include activation of endogenous ‘photosensitive’ compounds especially

porphyrins (Hamblin et al., 2005). Recent interest reflects Violet/blue LEDs’ (405nm to

470nm) ability to inactivate wound bacteria with little tissue damage (Guffey and

Wilborn, 2006), offering a safer alternative, or complement, to UV-C technology.

Exogenously supplied photocatalysts and “photosensitisers” can also enhance UV-A

and visible light disinfection (Lipovsky et al., 2011). Exogenous porphyrins and other

organic photosensitisers combined with blue, green, yellow and red light (Kharkwal et

al., 2011) are capable of dermal, biofilm and water disinfection (Rossi et al., 2012,

Jančula et al., 2010, Brovko, 2010).

Despite the importance of UV-B in SODIS (Sinton et al., 2002), UV-B diode potential

remains unexplored. It is suggested that despite potential advantages in higher water and

substrate transmissivity, it is currently constrained by the need to use expensive AlGaN

technology (as in UV-C LEDs) while being less effective against DNA (Webb and

Brown, 1979).

2.4. Photovoltaics in POU Settings

PV modules are semiconductor devices which convert sunlight into direct-current (DC)

through the photovoltaic effect (Green, 1998). They have proved a reliable, quiet, low-

cost source of off-grid renewable energy wherever sunlight is available. Commercial PV

modules come in many sizes and technologies, with efficiency of 10-22% and lifetimes

> 20 years. Their main limitation is intermittent electricity generation.

2.4.1. History, Physics and Commercialisation

The PV effect was discovered in 1839, and has seen major commercial use following

development of silicon cells in 1953 by Bell Labs. Key historic milestones are detailed

by Green (1990) and Honsberg and Bowden (2012). Improvements continue with the

single junction PERL cell achieving 25% efficiency (Green, 2009, Green et al., 2012).

Novel technologies, e.g. multiple-junction cells, promise greater solar spectrum

utilisation and higher efficiencies due to the Shockley-Queisser limit. Complementing

25

this has been improved mass production and manufacturing techniques, and lower cell

cost.

In the photovoltaic effect, bulk semiconductor cell materials absorb photons, generating

electron-hole pairs which diffuse through the material as minority carriers until reaching

the p-n cell junction where they are swept up by the electric field as majority carriers,

creating a unidirectional current. Current flows out through metallised contacts

providing electric power. Some electrons and holes are lost to recombination, before

reaching the junction due to defects in the crystal lattice and impurities (Corkish, 2004).

Photon absorption depends on device band-gap. Provided photons have greater energy,

they contribute to photo-current production. Excess energy is wasted as heat (Green,

1998). Individual silicon cells produce ≈0.5V. Connected in series and parallel they

achieve higher voltages and current respectively (Wenham et al., 2006).

Ten to 22% efficiency is obtainable with current commercial cell technologies

(monocrystalline, multicrystalline, quasi-monocrystalline silicon wafers, amorphous and

microcrystalline silicon thin films, cadmium-telluride thin film, copper indium gallium

diselenide thin film). Crystalline cell costs have fallen from US$76 per watt-peak

(1976) to US$0.93 per watt-peak (2013), competitive with mains electricity (Branker et

al., 2011). Screen-printed silicon wafer solar cells currently dominate production

(Honsberg and Bowden, 2012). Thin film technologies may reduce future prices

because of lower semiconductor demand (Wenham et al., 2006).

2.4.2. Standalone PV System Design Issues and Reliability

PV electricity supplied system design requires matching between cells, modules,

shading, blocking diodes, fuse protection, and temperature effects, to usage loads. Poor

matching produces system failures, and reduces available power and affordability

(Wenham et al., 2006).

Cells connected in string series must operate at the same current. Mismatched currents

occur through varying operating conditions, cell damage, incorrect connections, shading

and dirt on modules potentially causing hot-spot heating damage, less current,

permanent cell damage and cover glass cracking (Wenham et al., 2006). Bypass diodes

26

improve performance by limiting reverse current potential and overheating likelihood.

Circuit design can be tested pre-construction using Single Diode photovoltaic source

models. Experience shows the latter can achieve high agreement with measured module

behaviour (Chatterjee et al., 2011). Power generated varies as a function of current

produced, panel voltage, and internal series and shunt resistances, insolation and

temperature. To maximise power production, operating points should dynamically

match voltage and current, potentially yielding efficiency increases of 2.8 to 18.5% over

fixed voltage/current operation (Salas et al., 2005).

Controlling PV module operating points, employs a Maximum Power Point Tracker

(MPPT) circuit which matches loads to a module (Salas et al., 2006). MPPT algorithm

design principles include “Perturb, Observe and Check” (Alonso et al., 2009), “Sample

and Hold” (Weddell et al., 2012), “Predictive Control through Incremental

Conductance” (Kakosimos and Kladas, 2011), “Current Pulse” (Noguchi et al., 2002),

“Current Mode” (Tan et al., 2007), “Root-Finding” (Chun and Kwasinski, 2011) and

hybrid methods (Moradi and Reisi, 2011). Interest is growing in genetic and artificial

intelligence algorithms (Mellit and Kalogirou, 2008). Algorithms are implemented

using microcontrollers and flexible Field Programmable Gate Arrays.

A standalone power supply system commonly uses PV modules as the energy source, a

battery to store charge, a charge controller, and optionally, MPPT. Appropriate sizing of

panels and battery banks, allow autonomous PV based supply while components are

linked to mains power use inverters (Wenham et al., 2006).

Through mass production, all system components can be readily purchased and system

design guidance is available e.g. Australian Standards AS4509 (Australian Standards,

2010), (McLean et al., 2003). For off-grid POU, direct PV powering of disinfection

devices could be undertaken, provided the circuit design tolerates periods of instability

and unavailability. Regional/remote directly-coupled application PV water pumping

provides an existing model (Odeh et al., 2006, Meah et al., 2008a, Mokeddem et al.,

2011).

27

Commercial PV are commonly warranted for >20 years and field data shows panels are

one of the most reliable system components (Collins et al., 2009). Commercial panels

are validated through qualification tests such as IEC 61215 (IEC, 2005). The limitation

is that testing only considers known failure modes (Wohlgemuth and Kurtz, 2011).

Operationally, modules fail for various reasons (Ferrara and Philipp, 2012) including

yellowing or browning of encapsulants and back sheets, delamination, bubble

formation, oxidation and discolouration of busbars, corrosion of connections, cracking

of back sheet, hot spots, cell breakage and micro cracks. Underlying causes include

local climate stresses, corrosion and intrinsic failures.

2.4.3. Directly Coupled PV in Developing Regions

PV has short energy payback periods (<2 y) (Fthenakis et al., 2009) compared to

operational lifetime (>20 y) (Kaldellis et al., 2010). Energy prices are comparable to or

below retail grid energy prices, making PV environmentally and economically sensible

(Branker et al., 2011, Solarbuzz, 2013). PV technology already meets POU water

pumping and lighting needs in developing communities (Meah et al., 2008a, Pode and

Diouf, 2011). Optimisation principles are well understood (Corrêa et al., 2012). Systems

can be direct-connected (Mokeddem et al., 2011) or include batteries (Kaldellis et al.,

2011) and incorporate MPPT.

While direct-connected systems sacrifice efficiency for simplicity, battery-based

systems can be seasonally inefficient e.g. excess storage in summer versus insufficient

storage during winter. Thus sizing for loads is essential to maximise system cost benefit

(Odeh et al., 2006) without compromising disinfection. PV water pumping is

sufficiently inexpensive, judging by grid and diesel generator comparisons (Meah et al.,

2008a, Kaldellis et al., 2011), that additional to providing greenhouse gas emission

benefits, over-sizing PV systems may be a secondary concern compared to maintenance

and design policy (Meah et al., 2008b).

2.4.4. Related Renewable Energy Based UV Disinfection

Crawford et al. (2005) conceptually compared a range of potentially PV powered POU

disinfection technologies including conceptually LEDs. Vitello et al. (2011) described a

mobile drinking water disinfection unit powered by renewable energy for natural

28

disasters. The system was powered by PV and/or a wind turbine, with ultra-capacitor

storage. It treated water using paper and activated carbon pre-filters, and a mains and

inverter powered LP disinfection unit, was transportable and supplied water for 240

people. A prototype successfully disinfected E. coli and total coliforms in lake water.

Limitations included reservoir size, low flow, the inverter and ultra-capacitor storage

(Vitello et al., 2011). Close et al. (2006) reported a PV-powered UV-A LED wastewater

disinfection trial supplementing conventional decentralised sewage treatment without an

AC disinfection unit or inverter but provide no data. Subsequently, Close et al. (2006)

reported effective UV disinfection but using LP tubes. They concluded UV-LED

disinfection is its infancy and required further research, presumably based on

unsuccessful trials. Chatterley and Linden (2009) undertook more realistic UV-LED

trials, but similarly concluded that LEDs are a new uncompetitive technology while

agreeing great advances are expected soon.

A recent review of using UV-LEDs for water disinfection was identified (Song et al.,

2016). It appeared to focus on the reported mechanistic disinfection abilities of UV-

LEDs by cataloguing dose-response and time-response data from published results. It

identified differences in results and the need for standardized test protocols to build

action spectra. Implicitly, it confirmed the need to look at wavelength combinations,

pulsed irradiation and reactor design for practical applications. This directly mirrors my

understanding which motivated this study (Lui et al., 2014), the need to increasingly

focus on engineering requirements to bring such bench-scale results into reality, and

reconfirms its relevance and timeliness.

2.5. Integrating and Optimising POU PV/LED Disinfection

2.5.1. PV/LED Integration Challenges and Considerations

LEDs driven by DC voltage supplied using photovoltaic cells can conceptually disinfect

water without DC to AC conversion, be self-sustaining (Aoyagi et al., 2011) (i.e. does

not require supply of consumable chemicals, filters) and hence be suited to POU

situations (developing communities, disaster relief). Based on considerations presented

in prior sections, a list of likely barriers to LED disinfection realizing its POU promise

were tabulated (Table 3). Significant barriers included LED technology immaturity, as

29

current UV-LED POU systems are still very expensive, and bench-scale models often

lack critical hardware and design features required in situ. Thus the PV/UV-LED POU

concept appears valid but devices necessary for full prototypes mainly fall short on

diode lifetime, price and power output (Figure 3, Figure 4).

To realise field-deployable systems, further research also appears necessary to address

constraints similar to those applying to existing UV systems (de Silva and Fu, 2006,

Newsome, 1989) e.g. user safety, dosage monitoring, field validation, control of scale

and solarisation of quartz sleeves or shields. Commercial PV availability simplifies

power supply needs, but introduces new issues: suboptimal power efficiency through

multiple power conversions (charge controller DC-AC inverter, within AC UV

systems), controller reliability uncertainty, current matching, and systems for optimally

sizing and balancing PV panels, LEDs and batteries (McLean et al., 2003).

Progress on these challenges might be partly obtained by simulating mature UV-LED

technology using existing PV and visible light diode knowledge. For example analysing

the characteristics of likely PV/UV-LED components could accelerate the addressing of

questions about system complexity and prototype costs (Ibrahim et al., 2013) and

efficiency, and accelerate mature UV-LED technology roll out. Examples in this thesis

are the market survey of appropriate products at retail (Appendix D), and tabulating

ranges of prices for various essential components of systems (Table 4). Complementing

this, the conceptual attributes of LEDs (tolerance to intermittent power supply, lack of

warm-up time and on-off cycle penalties) (Bolton and Cotton, 2008) need field

validation.

30

Table 3 - PV powered UV-LED disinfection knowledge gaps

Theme Challenges, Opportunities

and Research Priorities

Elements/Significance/Comments

UV-C-

LEDs

Emission Efficiency

Output Power

Lifetime

Unit Cost

Multiple challenges e.g. AlGaN film, cost, few product markets.

Pulsing Promising. Opportunities exist to over-drive LEDs without compromising LED life though

disinfection takes longer. Needs further investigation into different wavelengths and

pathogens.

Other Disinfection

Technologies

Quantitative comparison needed including with miniature LP tubes.

UV-A,

violet, blue

(UV-B?)

LEDs

Disinfection Mechanisms Effectiveness against viruses and protozoa unclear. Role of Porphyrin peak (420nm) unclear.

Potential of exogenous photosensitisers need exploration.

Advanced Oxidation Durability of catalysts unclear. Potential of catalysts other than TiO2 unclear. POU Cost

benefit analysis required.

UV-B Potential should be explored if cost effective diodes/semiconductors become available.

Oxygenation and mixing Possibly critical to assess.

Wavelength combination Promising synergy e.g. UV-A, UV-C.

31

Theme Challenges, Opportunities

and Research Priorities

Elements/Significance/Comments

PV Power

Supply

Current and voltage balancing

and matching

Modelling possible, depending on configuration and power demands of different LEDs.

Battery Cost v. Benefit Compile lessons from PV use analogues e.g. water pumping, lighting. Cost benefit of over-

sizing needed. Alternate storage e.g. ultracapacitors.

PV-LED

Integration

Scale-up/large units Explore module approach issues (current drivers, high voltages, series string units).

Cost drivers Explore seasonality and environmental impacts on operation.

POU

Suitability

Safety Address eye hazard issues (UV-C, very bright UV-A/Violet emissions especially).

Maintenance PV lessons need transfer. Overheating a concern for both PV and LEDs.

Training/Local Construction Needs scoping.

Particles and Organics Impacts need scoping.

Reactor and Module Design Need to understand water container disinfection need and transferability of developed world

modules.

Cost effective production and

Use

PV mature and provides LED model. POU reactor specification not yet scoped or quantified.

UV-A/visible/photocatalysis disinfection dosages not quantified.

32

Table 4 - Estimated costs for a theoretical point-of-use system

Component Costa for

UV-C

Costa for

UV-A

Factors affecting

LEDs $200 – 1000+

(0.6mW to

3mW output)

$60 – 800+

(120mW to

6000mW

output)

Wavelength, power

requirement, number of

LEDs, disinfection time

requirement, lifetime

requirement

Heatsink $0 – 15 $10 – 100 Power dissipated by LEDs,

Climate condition

Optics (Windows) $15 – 100 $1 – 100 Wavelength, size, material,

thickness, quality of finish

PV Panels $40 – 85 (5 to

40Wp)

$130 – 500

(60 to

400Wp)

PV Market Prices, choice of

branded panel,

power/availability

requirements, degree of over-

sizing.

Pump (if flow

system)

$0 - 200 $0 - 200 Power output, germicidal

efficiency of wavelength,

head requirement

Control Electronics

(Charge, MPPT,

Matching, Safety,

Current-Drivers)

$60 - 120 $120 - 400 LED array configuration,

sensors, in house design

versus off the shelf solution

Battery (if any-time

operation is required)

$0 – 50 $0 – 500 Availability, hours of

operation without sunlight

Enclosure, Mounting

Hardware, Cables

$30 – 80 $50 – 120 Size of system, size of PV

array, current levels involved

Approximate

Estimated Present-

Day Total

$345 -

$1650+

$371 -

$2720+

Choice of flow system,

choice of battery backup,

degree of over-sizing of PV.

33

Component Costa for

UV-C

Costa for

UV-A

Factors affecting

Caveats and

Considerations

UV-C

requires less

electrical

power,

offsetting

some of the

initial costs in

lower

material

requirements.

UV-A

solution

requires

much more

electrical

power, and

a larger

number of

diodes,

dissipating

more heat

and

introducing

higher

levels of

complexity.

The two variations presented

do not provide equivalent

disinfection speed or

capacity, nor do they provide

equivalent service lifetime.

Both variations, amongst

others, deserve further

research and testing to

determine which systems will

be most suitable given

technological constraints.

a: Data sourced by sampling products on the retail market (in 2013) for each category

(Appendix D).

2.5.2. Design Case Studies

POU designs still require optimisation of safety, heat dissipation, electrical efficiency,

cost and energy budgets for different treatment volumes, dosages and source water

characteristics. These needs are illustrated by consideration of Yagi et al.’s (2007)

study. Disinfection was achieved but it required an array of 140 high powered 3W UV-

A LEDs, dangerous voltages (ca 100V) to drive LEDs, 420W of power and a cost of

approximately US$6000 (based on 2013 products). Assuming 20% efficiency, 340W of

heat was produced, requiring thermal management to avoid LED damage, premature

failure and burns. The normal solution, small heatsinks with fans, is failure prone,

creates noise and seems unsuited to POU environments. Such high fluence may pose an

eye hazard. Another vulnerability was the connection of multiple diode arrays in series,

making open circuits more likely. Mitigation by segmenting arrays into shorter series

34

strings would introduce a different complication/cost of requiring multiple current-

drivers. Finally the numbers of diodes employed indicated a need to maximise batch

reactor optical efficiency. This example illustrates that while small POU devices are

possible, constructing durable community scale disinfection devices may prove more

challenging.

One emerging approach for better exploiting UV-A LED attributes may be pulsing. Li

et al. (2010) reported that pulsed 365nm (100Hz) had "significantly greater germicidal

ability than continuous irradiation" on Candida albicans and E. coli biofilms consistent

with high intensity pulsing effectively decontaminating food (Elmnasser et al., 2007).

Mori et al. (2007b) noted pulsing irradiation reduced heat generation and improved

radiation penetration into samples. LEDs might also be overdriven in bursts without

reducing lifetime. Wengraitis et al. (2013) reported that E. coli were significantly more

sensitive to pulsed energy (10% duty cycle) compared to continuous operation. Though

pulsing increases the time required for delivering a given dose, pulsing together with

overdriving could be combined to improve disinfection efficiency along with designing

reactors to optimise fluence distribution. Further enhancement might be obtained using

diode wavelength mixtures as direct disinfection by violet/blue light is possible (Webb

and Brown, 1979) and non UV-C disinfection maybe enhanced using photocatalysts and

photosensitisers (Cadet et al., 1986).

2.5.3. POU Disinfection Setting

Mostly critically, PV powered LED disinfection needs to be integrated into the realities

of remote, rural and inadequately serviced communities, at an individual household

scale (Luoto et al., 2011, Brownell et al., 2008). Inactivation of pathogens must be

sufficiently reliable and sustainable to satisfy service delivery agencies and fool-proof

for use where operation is poorly understood. Devices will need to decontaminate both

water and biofilms harbouring pathogens in 20-25L water containers, even where

primary supplies are clean. Disinfection will need to be sufficiently robust to minimise

recontamination during handling e.g. extended storage when deliveries are sporadic.

Devices may need to cope with variable water demands up to 50L.d-1

.family-1

and

secondary uses promoting hygiene e.g. bathing (Jagals et al., 2013).

35

From Paul Jagal's personal experience in Southern Africa (Jagals, 2012, pers. comm.,

19th July), a LED disinfector must also be affordable by people living on <$1.25 per

day, compatible with container use, require infrequent parts replacement beyond

rudimentary servicing, be matched to PV power sources, and be safe and rugged enough

for handling by inexperienced individuals especially children. Battery use could be

problematic due to supply constraints and domestic economics. Even if durable devices

prove feasible, support would still be needed e.g. cleaning tools for connectors and LED

windows, funding agency subsidies, organised training and maintenance and possibly

local level servicepersons.

A complement to individual household units might be larger community sterilisers

operating at water distribution points. Professional/entrepreneurial management of LED

sterilisers might be feasible, but then benefits would depend on willingness to pay and

the poorest could be disadvantaged. Importantly, centralisation cannot completely

replace POU treatment because containers are used wherever water is not piped in-

house and protected. In urban environments with some centralised electricity but no

reticulated water a disinfection unit alone might be sufficient. In this case the LED

sterilisers would need to be robust enough to endure intermittent electricity availability,

surges and brown-outs. Depending on the community, LED technology may need to

spawn a family of devices tailored to different living situations.

36

Figure 6 - Conceptual map of POU hardware within control and treatment modules showing options and research and

development needs

PV Panels

POU Pump

Heatsink

Current Driver

System Controller

POU Reactor

Battery

Transparent window

Cost (UV-A/Violet), Optimization/ enhancement

(Photocatalysis, pulsing)

Manufacture, Mass production,

Intellectual Property UV-A/Vis LEDUV-C/UV-B LED

System Sizing for

Operational Availability,

Load Matching

Regulation, Guidelines,Education,

Communication

MPPT/Charge Controller

Power Supply

Unit

Cost Reduction, Efficiency Increases

Algorithm Optimization

Chemistry, Energy Density, Cycle Life

Hardware,Software, Reliability,

Safety

Design

Design Calculations

Dose/Turbidity Sensor

Safety Interlock

Quartz window

User Interface

Configuration, CFD, Maintenance,

Catalysts

Information & energy flows

UV/Visible Radiation

Research Inputs

Modules

Water

R&DHardware

Mains

OR

OR OR

OR

Container alone or Distinct Reactor?

Evaluate Effectiveness

POU Water Container

Efficiency, Lifetime, Power/LED, Cost

Optimization

37

2.6. Prioritising the Research Program

While a large number of knowledge gaps were identified, this investigation mainly

concerns itself with the key challenges which stand in the way of realising a first-

generation prototype PV-powered, LED-based POU water disinfector. Disinfection

technology is a broad research area. As conventional UV-C based disinfection is well

understood, and dosage requirements for numerous pathogens already widely published

in literature, re-examination and comprehensive comparison with conventional UV-C

technology was not deemed necessary. The area of solar disinfection through SODIS

has also been widely trialled and proven, and a comprehensive reassessment was not

deemed necessary.

Instead, the investigation was an interdisciplinary project focused more towards

engineering challenges related to the conception of the device with a focus towards its

feasibility, safety and practicality. This is an area lacking active investigation, but

vitally essential to transferring bench-scale results into real-world results. This required

a combination of both microbiology and photovoltaic engineering skills to best assess

the trade-offs and opportunities presented by the combination of photoinactivation with

photovoltaics. This would be a novel contribution by overcoming the challenges of

translating bench-scale results into simple, robust, sustainable, fully-specified real-life

prototypes which could be further developed upon.

A list of what was deemed critical knowledge gaps, and associated justifications are

presented in Table 5.

Table 5 - Key knowledge gaps to concept realisation and justification

Knowledge

Gap

Justification Addressed?

Wavelength

effects on

inactivation by

LED irradiation

High cost sensitivity of LEDs to output wavelength

means significant cost-reduction may be possible

using longer-wavelength LEDs.

Immaturity of commercial development of LEDs with

Yes

38

UV-C wavelengths best suited for disinfection results

in high costs, uncertain lifetimes, limited supply and

limited output power.

Longer output wavelengths may be amenable to

reflective enhancement and reduced systems material

costs due to less possibility for solarisation-based

material damage.

Action spectra detailing inactivation dose

requirements versus a wide range of wavelengths

outside the UV-C region are not widely found in

published literature.

Wavelength

effects on

different classes

of pathogens

Inactivation rates at certain wavelengths known for a

wide number of pathogens (e.g. 254nm), however,

this may not hold true for different wavelengths due to

different inactivation mechanisms.

No

Enhancement

possibilities

with TiO2,

pulsed

irradiation, LED

overdriving and

photosensitisers

Enhancement techniques can potentially reduce

required UV dose to achieve the same amount of

inactivation, directly reducing LED demands and

price of the unit, which improves feasibility of the

concept.

Results for TiO2 inactivation tend to be highly

variable and are rarely implemented for drinking

water contexts but have had some promising results.

Photosensitisers may also have significant results with

royal-blue (455nm) LEDs due to their absorbance

characteristics and are very inexpensive, although

their suitability for POU usage may be limited due to

the requirement for consumables.

Pulsed irradiation and LED overdriving may also

assist in cost reduction, but have not been considered

thoroughly in present literature.

Partially

UV-C LED Due to the expense of UV-C LEDs, their lifetime Yes

39

Lifetime poses a critical question when considering financial

feasibility. However, if they have long lifetimes, their

high upfront costs may be inconsequential.

Existing literature on UV-C LEDs is relatively

pessimistic with regards to lifetime, placing realistic

lifetime bounds at 250h to 1 000h.

Most published literature is for prototype devices, and

not commercially available products.

Literature makes extensive use of extrapolation which

is not representative of actual degradation, especially

if other failure modes, such as catastrophic failure,

were to occur. This has been evidenced by Meneghini

et al. (2008a).

UV-C LED self-

heating effects

Self-heating is acknowledged as a key factor in output

reduction of UV-C LEDs below the manufacturer

stated figures. However, a systematic examination of

the magnitude of the reduction and its causes have not

been undertaken.

For mass manufacturing and product design targeting

specific UV doses, understanding this is necessary to

ensure that dosage requirements are met throughout

the operating envelope.

Yes

Quartz

solarisation

As windows are used to separate liquid from the

LEDs, these are vulnerable to degradation in

transmission in a process known as solarisation. The

process by which this occurs is understood, however,

the magnitude of this effect is not well known and

varies depending on material quality.

Quartz is an expensive component of the reactor

system, thus understanding the characteristics of a

low-cost quartz disc may serve to reduce reactor costs.

Yes

User safety Exposure to UV radiation is known to be hazardous, Yes

40

concerns and

sensing

technologies

however, the research presented to date does not

attempt to quantify this.

Implementation of effective UV-C sensing

technologies provides a route which allows for

confirmation that the system is working without

putting the user at risk.

Lower cost methods may need to be developed to

meet the needs of developing countries.

Safety of the units is paramount if they are to be

adopted.

Integration

between PV and

LED

disinfection

systems and

physical

construction

Careful design is required to match load to energy

source.

Control systems are required to ensure user safety and

provide feedback.

Such integration is seen to be a key missing step, as

UV-C LED disinfection has been demonstrated, and

standalone PV-power is widely used in developing

countries already.

Physical construction of such reactors and material

choice is a key consideration where parts need to be

easily accessible and end-user serviceable.

Yes

Performance of

unit with a

variety of source

waters

Performance is likely to vary depending on the water

used due to the presence of UV absorbing compounds.

Pathogen mix is likely to be diverse in comparison.

Wide variation in water quality in terms of turbidity,

dissolved solids, etc.

No

Understanding

of and

comparison with

SODIS

Improvement in wavelength effects developed

initially may help us to better understand SODIS from

a more systematic approach.

Comparison with SODIS will help better understand

the energy efficacy of direct solar compared with

indirect solar (i.e. through PV and LED) and the

Partially

41

relative benefits and drawbacks of each.

Financial

feasibility

Costings of whole systems have not been published in

the past, and an understanding of how LED prices and

volume production affects the cost has not been

previously considered.

Financial feasibility is necessary if the concept is to be

successful and widely adopted.

Hypothetical situations can be simulated to predict

future feasibility.

Yes

Further

optimisation of

the designs for

field trial

deployment

units

Improved performance is desirable.

Use of computational fluid dynamics (CFD)

techniques considered necessary for optimised

designs.

No

Regulatory and

governance

issues, mass

manufacturing

optimization,

intellectual

property

Issues may be a barrier to the adoption of the systems

if not addressed.

Communication with end users for training and safety

is necessary to ensure full benefits.

Costs optimization could be achieved with mass

manufacturing.

Patents could serve to protect designs, or become a

barrier to mass manufacturing.

No

Microbiological assays were performed using E. coli K12 as the primary indicator,

consistent with conventional practice and US EPA water treatment biodosimetry

experiment guidelines (Bolton and Cotton, 2008, USEPA, 2006). This allows for direct

comparison with the large amount of existing literature, and is an appropriate first-test

as the lack of success with E. coli generally rules out any further prospects as E. coli is

comparatively weak compared with protozoa. Further assays were performed with E.

faecalis ATCC 19433 to allow for dose requirement comparisons, and to provide new

data at the bench scale.

42

Testing of other pathogens, including viruses and spores was considered. However, due

to the complexity of our experiments which aimed to simulate POU conditions even at

bench scale, it was deemed impractical. Due to assessment at a large number of

wavelengths at duplicate or triplicate scale, the combinatorial explosion that would

occur would have prevented the assessment of engineering issues which were deemed

more important owing to the lack of information. The logistics of handling different

assays would have only compounded the problem. Fortunately, literature comparing the

dose response of pathogens is widely published, and can be taken as a reference to

predict performance with other pathogens. Validation of the reactor with other

pathogens remains a key knowledge gap which is suitable for further research.

The choice of bacteria alone is supported by risk assessment of waterborne disease in

the field. Enger et al. (2012) found that the greatest risk to health in the water supply in

developing countries was posed from enteric bacteria which were more prevalent than

any other class of pathogen by 4-orders of magnitude. This supports my focus on E. coli

and E. faecalis as the primary indicators, however, further research should also focus on

other pathogens, viruses and protozoa.

Similarly, addressing other types of source waters through both simulated and actual

waters was considered, but rejected for similar logistical reasons especially due to the

diversity of source waters. The choice of clean water as the test medium avoided

variability from source waters which may otherwise affect the results. In the case that

adequate disinfection performance could not be obtained in clean water circumstances,

it would be expected that even lower performance would be obtained with real waters.

The effects of the water matrix would also make a good candidate for further research.

A variety of regulatory and governance issues were also identified that were not

addressed directly by this project, but were considered briefly in the conclusion (Section

9.4). This is because the scope was constrained to creating prototype devices. The

further optimization of these devices was not explored, and would be the subject of

future research. Full consideration of the regulatory and governance issues is likely to

fit in with the production of optimised field trial deployment units.

43

2.7. Conclusion

In his review of POU water disinfection in rural and remote regions of developing

regions, Sobsey (2007) concluded that current disinfection technologies are

inapplicable, sub-optimal or have significant uncertainties. They may be unsustainable

in rural settings, or require consumables, large energy inputs, and chemicals or training.

They may be constrained by costs, scale, safety, effectiveness, or performance issues

(Table 2). Of those available, UV disinfection appears the most promising efficacy-wise

and technically. Its main drawbacks identified were availability and cost.

Combining LED and PV technologies together has the potential to sustainably

overcome many of these barriers. UV-LED technology progress appears sufficiently

rapid judging by visible LED evolution that high quality POU disinfection should be

feasible within 10 years as suggested by Ibrahim et al. (2013). But to achieve practical

disinfection, diverse priority research areas (Table 3, Figure 6) also need addressing.

Current research focuses on UV-C LED technology. My review shows much else can

be done on complementary technologies (e.g. UV-A, photocatalysis), support (e.g.

addressing POU realities) and integration (e.g. PV control, current driver circuits). For

LED disinfection to resolve the clean water problem in a timely fashion, these issues

should be researched and resolved using prototype UV-LED technologies concurrently

with the UV-C LED investigations.

2.8. Chapter Highlights

With the recent advent of UV-C LEDs, UV has become more suitable for use in

POU applications as a proven broad-spectrum disinfection technology.

UV-C LEDs are currently still expensive, have uncertain lifetimes, low-

efficiencies, low per-unit output powers and limited availability.

Opportunities were identified to leverage UV-A and visible mass-market LEDs

for disinfection, with enhancement through photocatalysis, photosensitisers,

wavelength combination, pulsing and overdriving.

A better understanding of how different wavelengths contribute to inactivation

for various pathogens is required.

44

PV is a proven, reliable, low-cost energy source with a track record in remote

deployments. Integration with PV can conceptually realise a sustainable POU

device which is simple, robust and sustainable.

Engineering challenges in PV, LED, pump and battery integration, configuration

and control, user safety, dose sensing, reactor design, thermal management, cost

optimisation and material compatibility need to be addressed.

Current high costs are likely to reduce as LED technology matures.

45

3. A Proof-of-Concept Bench Scale POU System2

3.1. Introduction

Existing point-of-use (POU) technologies such as chlorination, ozonation, conventional

mercury tube based ultraviolet (UV), filtration and boiling generally fall short of

meeting the needs of those in rural, developing areas as they are constrained by

logistical problems in supplying consumables with limited lifetime and high costs,

performance limitations in throughput and volume, have high energy intensity, limited

efficacy, safety and reliability concerns and acceptance limitations (Chapter 2). UV and

visible radiation are attractive agents for POU drinking water disinfection (Bolton and

Cotton, 2008). They are broad spectrum, use no chemical consumables, generate few

disinfection by-products and leave no residual odour and taste.

It is now possible to generate a full range of UV and shorter visible wavelengths using

light-emitting diodes (LEDs). LEDs address disadvantages of current low-pressure (LP)

mercury tube technology (Shur and Gaska, 2010) such as power-cycling, warm-up time,

high voltages, fragility, loss of lamp output (Heath et al., 2013) and mercury content.

They have a lower power rating than most UV-C tubes, and run on safe low-voltage

direct-current (Brownell et al., 2008, Chatterley and Linden, 2010, Chatterley and

Linden, 2009). Consequently, in combination with photovoltaic (PV) solar panels, they

promise reliable cost effective low maintenance water disinfection for those

communities which most need it e.g. in rural Africa, Asia and Latin America.

Despite this promise, much developmental work is still needed. In a review of UV LED

technology, with colleagues, I identified how commercial deep-UV LEDs continue to

be expensive, exhibit low electrical efficiencies and have uncertain operational lifespans

(Lui et al., 2014). Despite positive market growth projections and manufacturer

2 This chapter has been revised and expanded. It was substantially published as

LUI, G. Y., ROSER, D., CORKISH, R. & STUETZ, R. A New Frontier: Photovoltaics and Light-

Emitting Diode Technology for Water Disinfection in Remote and Developing Areas. 2014 Asia-Pacific

Solar Research Conference, 2014.

and

LUI, G. Y., ROSER, D., CORKISH, R., ASHBOLT, N. J. & STUETZ, R. 2016. Point-of-use water

disinfection using ultraviolet and visible light-emitting diodes. Science of The Total Environment, 553,

626-635.

46

statements indicating impending availability, low-cost high-powered deep-UV LEDs

are yet to eventuate. Advancement is also constrained by the limited number of

manufacturers, each having a different technological approach (Hirayama et al., 2015).

A separate question was which development road is preferable, one based on lower

powered expensive UV-C based units or one based on more mature, inexpensive UV-A

(315-400nm) and visible range (400-740nm) LEDs which are also capable of substantial

disinfection (Maclean et al., 2009, Mori et al., 2007b) and are already commercially

viable. In contrast to UV-C/B emitters, these LEDs already have large markets e.g.

lighting, signage; possess long lifetimes, and are relatively inexpensive, mass produced

and widely available. Their effectiveness should also be less impacted by absorption by

waterborne organics (e.g. tannins), than UV-C (Cantwell and Hofmann, 2011).

From these considerations, two main questions emerge. Firstly, what is the optimum

mix of wavelength, disinfection power, LED cost, lifetime, and emission efficiency?

Secondly, how well can engineering constraints and costs be harmonised with electrical

power supply and the realities of regional community and household clean water needs?

Central to answering these questions is detailed knowledge of i) LED inactivation

variability, i.e. disinfection action spectra analogous to those for UV-C (Izadifard et al.,

2013), and ii) engineering challenges e.g. ensuring eye safety, preventing overheating.

I concluded that to answer these questions a series of pathogen inactivation experiments

should first be undertaken using a realistic model (POU) reactor comparable to that

which could be used in a regional or remote community. This chapter firstly quantifies

E. coli K12 and E. faecalis inactivation by a range of wavelengths produced by

commercially available LEDs spanning UV-C (270nm) to deep red (740nm). Then the

inactivation rates, in turn, are used to estimate action spectra analogous to those

developed for UV-C disinfection. Finally, device construction experience is used to

develop an understanding of water-related disinfection issues and refine the list of

engineering considerations.

47

3.2. Materials and Methods

3.2.1. LED Array Power Supply, Configuration and Characteristics

Commercially available LEDs, spanning UV-C to deep red, were surveyed for selection

criteria such as suitable wavelength, affordable price, engineering convenience and

stock availability. LEDs in mass-produced wavelength bins, preferably with a "star

base" or hermetically sealed “through-hole” packages, were chosen for their

availability, ease of array construction and robustness. In view of potential use with PV,

low power demand and consumption was viewed as essential, and kept under 50W to

minimise risk of overheating and incompatibility with model reactor size. Multi-emitter

LED arrays were preferred and purchase costs were kept as low as practicable. The high

costs of UV-B and UV-C LEDs reflects the absence of mass-production at present. The

units finally selected are shown in Table 6. The rationale behind LED selection

stemmed from a desire to cover the full spectrum of commercially produced visible and

UV-A LEDs and also compare their performance to UV-C and UV-B LEDs and assess

their viability.

To minimise heating and maximise output and lifetime, UV-A and visible wavelength

arrays were mounted to a Fischer Elektronik SK 584/50 SA 1K/W heatsink using

thermally conductive paste. The 270nm and 310nm arrays were mounted on strip-board

in a series configuration, with rear cooling of the TO-39 packages (Sensor Electronic

Technology Inc, 2012). All arrays were cooled by a 120mm computer fan to minimise

risk of overheating.

Current driver units were selected to satisfy LED array power requirements. For 270nm

and 310nm arrays, an On Semiconductor NSI45020AT1G 20mA linear regulator was

used. The power supply was set at 36V to overcome the voltage drop of the LEDs and

the regulator. For the 430nm array, an XP Power LDU2430S1000 DC-DC 1000mA

LED Current Driver Module was used. The supply was set to provide 28V, to ensure

supplied power remained within the module operating specifications. For other

wavelength arrays, an XP Power LDU2430S700 DC-DC 700mA LED Current Driver

Module was used. Power to the current driver units was supplied by a pair of Manson

HCS-3102 switch-mode bench-top power supplies. Multiple current driver modules

48

were run in parallel from each power supply with loading < 50% to ensure output

stability. Current drivers (On Semiconductor, 2014; XP Power, 2014) were tested using

a multimeter (Agilent Technologies U1241B) to confirm their current output

specifications.

LED spectra, Dominant Wavelength (DWL) and Full-Width Half Maximum (FWHM)

were measured (21°C, <1s) using an Ocean Optics S2000 fibre-optic spectrometer, UV-

rated SMA-905 thick fibre-optic cable and OOIBase32 software.

Estimates of array power output were obtained where possible by direct measurement,

and from manufacturer data sheets. Firstly array light output power range and typical

emission value, and electrical input power were obtained from test report data (270 and

310nm arrays) or product datasheets, except for the 430nm array, where no data were

available and efficiency was assumed to be the same as 405nm LEDs. Visible range

LEDs’ outputs which were reported in lumens were converted to mW using a photoptic

to radiometric conversion chart (Labsphere, 2008). Electrical efficiency was calculated

as estimated typical optical output power divided by electrical input power.

Array emission power for wavelengths ≥365nm was also measured radiometrically.

Ideal radiometry of LEDs requires the use of very large, >2m integrating spheres and

calibrated detectors, to which I lacked access. So as an alternative, verification of LED

output powers was undertaken using a Perkin-Elmer 150 mm Integrating Sphere and an

Ocean Optics S2000 Fibre-optic Spectrometer with an ND3 neutral density filter (0.1%

transmission). Each LED source was placed on the top port of the integrating sphere,

while the ND3 filter and probe were positioned at the orthogonal port. Other sphere

ports were closed with Spectralon discs. LEDs were energised for <10s. Spectrometer

signals were then integrated for periods selected to avoid detector saturation. The

spectra were recorded using OOIBase32.

49

Table 6 - LED array quantities, model numbers, pricing, data and measured parameters

Nomi

nal λ

(nm)

Array

Qty.

Model Vendora Cost

(US$)

DWL

(nm ±

S.D.)

FWHM

(nm ±

S.D.)

Nominal Optical

Output Power

(mW, Typical &

Range)

Measured

Optical

Output Power

(mW)

Input

Power

(W,

Typical)

Electrical

Efficiency

(%)

270 5 UVTOP270TO39FW SETI 1168 272b 10

b 3.07

c (2.4-4.0)

g 0.55

c 0.6

310 3 UVTOP310TO39FW SETI 768 311b 10

b 1.50

c (1.08-1.80)

g 0.30

c 0.5

365 2+1 LZ1-10U600 + LZ4-

40U600-0000

LEI 246 368 ± 2 10 ± 0.4 2120 (1068-3420) 1613 17 12

385 3 LZ4-40UA00-00U4 LEI 356 389 ± 0.2 11 ± 0.4 8700 (4800-14400) 7174 33 27

405 3 LZ4-40UA00-00U8 LEI 356 406 ± 2 13 ± 0.5 11400 (4800-14400) 8169 33 35

430 9 EP-U4545K-A3 ET 41 418b 18

b 3654

d,f 2480 11 35

d

455 7 XTEARY-02-0000-

000000N04

Cree 33 448b 20

b 6545

f 6906 15 44

525 3 LZ4-40G100-0000 LEI 102 521 ± 0.7 35 ± 0.6 3719e (2757-5379) 2489 30 12

590 3 LZ4-40A100-0000 LEI 104 598 ± 0.7 16 ± 0.2 1890e (1326-2067) 1219 19 10

623 3 LZ4-40R100-0000 LEI 104 634 ± 0.5 18 ± 2 5077e (3288-6414) 3785 19 27

660 3 LZ4-40R200-0000 LEI 104 659 ± 0.5 19 ± 0.2 7800 (6000-9000) 4536 35 35

740 3 LZ4-40R300-0000 LEI 100 733 ± 0.8 30 ± 2 3630 (3000-7200) 2308 19 19

50

a: SETI = Sensor Electronic Technology Inc., LEI = LED Engin Inc.; ET = Epileds

Technologies, b: Single measurement taken covering the whole array, independent

testing of individual diodes not possible, c: Manufacturer tested value, d: Estimated, e:

Converted from Lumens, f: Range not specified, g: Not Measured

The routine method for radiometric calibration, used in photovoltaics, uses simulated

solar spectra data generated by SMARTS v.2.9.5 (National Renewable Energy

Laboratory, 2013). The latter are based on inputs described in IEC60904-3 Ed.2 (IEC,

2008), adjusted for local date, time, elevation and solar geometry. On clear days this

program predicts spectral intensity within 1.7%, comparable to current instrumentation

limits of 1-3% (Gueymard, 2004). The Ocean Optics spectrometer was calibrated

radiometrically by pointing the light conducting fibre with a ND3 filter at the solar-noon

sun (Sydney, 33.86°S, 151.2094°E 31st October 2014 11:39am EST, 70° altitude,

elevation 100m) on a clear day, recording the maximum spectrum and correlating the

spectrum with the output from SMARTS. This avoided the problem of detector

saturation and allowed the ND3 filter's wavelength-dependent effects on the

measurements from the integrating sphere to be measured and included in calibration

calculations.

The effective sphere multiplier, a dimensionless value representing the increased

radiation measured at the surface of the sphere due to an infinite series of internal

reflections, was computed from measurements of the sphere apertures. Reference data

for Spectralon were obtained from the manufacturers, and geometry calculations were

used to compute the sphere multiplier. This allowed the absolute power output from

LEDs over all angles to be determined experimentally (Labsphere, 2008) and compared

with manufacturer specifications.

3.2.2. Batch Reactor System

Alfi Avanti 1.3L vacuum carafes with double-wall reflective glass inserts were used as

model batch disinfection POU reactors (Figure 7). Their largest horizontal internal cross

section and mouth diameters were 13cm and 5.2cm respectively. These containers were

chosen due to their availability, wide opening, low cost, UV-resistant material (glass),

51

reflectivity enhancement potential, volume similar to SODIS PET bottles, and ease of

cleaning.

Glass wall reflectivity, and hence disinfection enhancement potential, was determined

destructively by fragmenting one carafe and measuring the reflectivity of 20cm2

fragments (Perkin-Elmer Lambda 1050 Spectrophotometer with integrating sphere

attachment). Wavelength specific enhancement was estimated using the Ocean Optics

S2000, fibre probe and ND3 filter to measure green LED light impinging from different

angles.

During inactivation experiments, flask water was stirred using Teflon coated magnetic

stirrer bars to maintain oxygen saturation. Each measurement run employed one control

and three irradiated flasks. Protection for the LEDs against splash was provided by

75mm diameter x 6mm windows (quartz rounds for 270nm and 310nm LEDs, UV-

transmissive Perspex® windows for other wavelengths, Figure 10) (Davies et al., 2009).

3.2.3. Inactivation Experiments

Stationary phase E. coli K12 ATCC W3110 or E. faecalis ATCC 19433 cultures were

prepared by inoculating Tryptone Soy agar slopes (Oxoid CM0131) and incubating for

24h at 35°C. Sterile 1L volumes of 0.05M NaCl solution were adjusted to pH 7.6 ± 0.1

and inoculated with approximately 106 colony forming units (CFU)/mL. This was done

by washing the slope with 2mL of NaCl solution into a glass test tube with a further

3mL of NaCl solution. This suspension was vortexed using a vortex mixer until an

evenly cloudy solution was formed. From this suspension, 0.35mL was put into the

sterile 1L volumes of NaCl to form a suspension with 106

CFU/mL.

Continuous culture broth was not used due to a desire to simplify the experiment and

minimise handling.

Experiments were run for up to six hours in an air-conditioned room with the ambient

(room) temperature regulated to 21°C. During this time half-mL aliquots were extracted

and diluted in pH 7.3 phosphate-buffered saline (PBS, Oxoid BR0014) in 10-fold steps.

Drop plating in triplicate was used for initial runs to determine optimal sampling

intervals (Miles et al., 1938), while subsequent runs used spread-plating (0.5mL per

52

plate). Water temperatures were observed to increase from 21±0.3°C initially to maxima

of 21 (270/310nm), 23 (365nm), 24 (525nm), 25 (385/430/455/590nm), 26 (623nm), 31

(405nm), 32 (660nm), and 36°C (740nm), at experiment end.

Inoculated plates were initially incubated for 24h at 35°C before initial counting. They

were then re-incubated and checked for further colony growth and morphology changes

after 48, 72 and 96h. For the drop plate and spread plate assays, counts were made of

plates with <75 CFUs per drop, and 30 to 300 CFUs per plate respectively. Dark

controls (no irradiation) were sampled at the beginning, middle and end of runs e.g. 0,

180 and 360 minutes for a 6h run. Inactivation was determined as log base-10 reduction.

53

Figure 7 - Bench-scale reactor experimental setup

a: 405nm UV-A/Violet (left) and 270nm (right) arrays, b: internal reflective glass

(Dewar) container, c: one reactor in experimental configuration

3.2.4. Action Spectra

The inactivation data collected for each wavelength were used to estimate wavelength

specific decimal inactivation doses (S90s) for E.coli K12 and E. faecalis. These were in

turn used to construct action spectra. Inactivation rates were calculated assuming: 1)

The continuous stirring of the suspension caused the whole volume of water to be

subjected to the average LED fluence; 2) The average fluence was the output light

power divided by the maximum cross section of the carafe; 3) The LED output power

was completely transmitted into the water column. Since pure water absorbs only small

amounts of light relative to the flask path length (12 cm) at the spectrum extremes.

Minimum transmittances at 300 and 700nm are ≈ 95 - 99% per 10 cm (Litjens et al.,

1999). No adjustments were made for reflection so S90 values necessarily apply to the

model reactors. However, the magnitude of reflection was estimated to assess its impact

and how well non-reflective reactors would perform. For comparison, literature action

spectra and bacterial rate constant data, especially for E. coli, were extracted from tables

in Webb and Brown (1979), Peak et al. (1982), Kelland et al. (1983), Chen et al.

(2009b) as mJ.cm-2

per 1-log reduction or by digitising and unit-converting plot data.

3.3. Results and Discussion

3.3.1. Power Source and LED Characterisation

LEDs were assessed to determine how closely they met their specifications. Current

drivers (On Semiconductor, 2014, XP Power, 2014) were tested to confirm their

specifications. Compared to the specified 8-10% range, the maximum and mean errors

were 3.2%, and 1.2% respectively, close to measurement errors. The measured results

for each driver are shown in Table 7.

54

Table 7 - Current driver measured current values

Unit

Nominal Current

(mA)

Measured

Current (mA)

Percentage

Error (%)

LDU2430S700 700 718 2.57

LDU2430S700 700 709 1.29

LDU2430S700 700 708 1.14

LDU2430S700 700 703 0.43

LDU2430S700 700 700 0.00

LDU2430S700 700 699 0.14

LDU2030S700 700 705 0.71

LDU2430S1000 1000 1014 1.40

NSI4520AT1G 20 19.36 3.20

NSI4520AT1G 20 19.79 1.05

LED characteristics, measured in this study or supplied by the manufacturers, are shown

in Table 6. The 270 and 310nm diodes closely matched their emission wavelength

specification values (FWHM ≈10nm), while the commercial mass produced diodes

displayed broader peaks. This accounted for several diode arrays having wavelengths

longer than specified (365, 385, 623nm) and others exhibiting asymmetric tails (430,

455, 740nm) as seen in Figure 8. This was unsurprising as mass produced LEDs are

"binned" after processing and advertised wavelengths typically refer to the shortest

wavelength of the bin and reflect manufacturer tolerances in their measurements.

55

Figure 8 - Spectral characteristics of the LEDs

Thus prior to field POU application, it may be desirable to validate the characteristics of

mass-produced LEDs, in case actual wavelengths differ markedly from advertised

values. Further, when multiple LEDs are combined, the FWHMs will likely be wider

due to component differences. These spectra highlight that LEDs are not strictly

monochromatic, with half the peak intensity being spread in a band up to 35nm wide

(Table 6).

Experimentally determining LED output power, efficiency and dose, for comparison

with data sheets, proved challenging. Emissions from the non-mass-produced 270nm

and 310nm LEDs were too small to measure with the integrating sphere so the supplied

test report data were used. In the case of the longer wavelength mass-produced LEDs,

the specification optical output "range" and typical value reflected the manufacturer's

binning process and differed substantially from measured values. Manufacturer

specifications, though, were still useful for design and measurement comparison

purposes as they indicated the general output order of magnitude. Overall, measured

UV-A and visible light LED optical power estimates (Table 6) were 73 ± 13 (S.D.) % of

0

10

20

30

40

50

60

70

80

90

100

250 350 450 550 650 750

Inte

nsi

ty r

ela

tiv

e to

pea

k (

per

cen

t)

Wavelength (nm)

Nominal Wavelength (nm)

270 310 365 385 405 430 455 525 590 623 660 740

56

the typical datasheet values and in all cases, except 455nm LEDs, the measured value

was lower than the datasheet value. This may have been due to errors in calculating the

sphere multiplier value, as the integrating sphere was not designed for use outside a

spectrophotometer and included ports whose area could not be satisfactorily measured.

Other possible reasons for lower than expected power output were the use of simulated

sunlight spectra for calibration and elevated LED operating temperature.

In the case of mass production, validating the characteristics of the LEDs may be more

easily achieved where investments in purpose-built test apparatus can allow for quality-

control checking which can measure the relevant parameters in seconds, just as LED

manufacturers do during manufacture. However, a much better result can be achieved in

partnership with LED manufacturers, by specifying tighter binning tolerances in volume

ordering and requesting guarantees of specification, which would reduce or eliminate

the need for the downstream manufacturer to test the incoming LEDs. However, in

return, a higher cost is expected as fewer products would be expected to meet the

requirements for the narrower bins and supply may become insufficient to meet the

demands, especially if the narrower binning becomes commercially popular for other

markets.

A simpler and more reliable approach may be to assume worst-case output from the

LED based on the provided data and engineer over-irradiation to ensure that systems

meet minimum dose requirements. However, this approach may be unnecessarily costly

and inefficient. Employing calibrated UV sensors may allow for precise dosage

determination to enhance time/energy efficiency by changing disinfection time to

compensate for actual output, or drive current to reach a target output, however, will

involve complex control systems and additional costs in sensors and calibration which

might not be justified.

Obtaining more accurate results, would be difficult as calibration sources for integrating

spheres with sufficient power for this application were not available, and generally carry

an uncertainty of 7.0-9.1% (S.D.) depending on wavelength (Ocean Optics, 2015),

which is comparable to the differences observed. To avoid introducing additional

57

uncertainty, the ‘typical’ manufacturer supplied optical (or derived where units were in

lumens) power value, where available, was opted for when calculating irradiation doses.

3.3.2. Enhancement of irradiation by reactor internal reflection

The carafe’s silvered glass samples exhibited >85% reflectance for wavelengths

>400nm (peak 92% at 550nm) but negligible reflectance below 330nm (Figure 9). This

indicated enhancement of inactivation was only possible for ≥365nm LEDs. The

reflection measurements also showed that light from the top and bottom of the pot were

of similar magnitude. The intensity of light measured at the flask centre and reflected

from the bottom of the flask was 98-105% of that measured when the spectrometer

probe was faced directly toward the LED source. This variation reflected the angle of

acceptance of the optical fibre probe and repeated reflection. By comparison side

reflected light was a minor component (8-10% of direct light readings). This variation

was probably due to the container’s flat-bottom (Figure 7), which made reflection in the

vertical axis dominant. It was concluded that enhanced irradiance due to reflection was

around two fold for UV-A and visible wavelength LEDs and negligible for UV-B and

UV-C wavelengths where the carafe glass strongly absorbs. Overall reflection

enhancement was small compared to the impact of wavelength on the UV dose required

for a different S90s which ranged over several orders of magnitude.

58

Figure 9 - Measured carafe glass reflectance

3.3.3. Window Absorbance of Light

The transmittance of the quartz round and Perspex discs were also measured for

reference (Figure 10). The transmission reached around 92-93% which indicates the

material was optically transparent to the UV-A and visible wavelengths, with the loss in

optical transmission most likely due to surface reflection on the front and rear sides as

no anti-reflection coatings were used.

0

10

20

30

40

50

60

70

80

90

100

250 350 450 550 650 750

Refl

ecta

nce (%

)

Wavelength (nm)

Glass Reflectance

59

Figure 10 - Quartz and UV-transmissive Perspex transmissivity

3.3.4. Inactivation Versus Time and Temperature

A plot of controls accompanying inactivation trials (Figure 11) showed a mean change

of 0.037 ± 0.233 (S.D.) logs over 6 h. In addition reactor waters which showed the

greatest heating (660/740nm, 20-32/36°C, 0-6h) showed only small average declines

0.28/0.41-logs). It was concluded that the reactor bacterial populations were essentially

unchanged over the 6 h experiments in the absence of irradiation, increased water

temperature had little effect and log reductions above 0.5 could be generally considered

statistically significant (P< 0.05) given the 0.233 standard deviation.

0

10

20

30

40

50

60

70

80

90

100

200 300 400 500 600 700 800

Tra

nsm

issi

vit

y (

%)

Wavelength (nm)

Quartz

UV-Transmissive Perspex

60

Figure 11 - Control plate log reduction vs. time for E. coli K12 and E. faecalis

ATCC 19433

The 270nm array (Figure 12) achieved ≥5-log reductions after 3 min of exposure with

E. coli K12 without any evident "lag" phase. This was the fastest inactivation achieved

and supports continued focus on UV-C LEDs, which, though expensive, consumed little

power and generate no significant waste heat. With 270nm LEDs, E. faecalis ATCC

19433 (Figure 12) required 5 times longer for ca. 5-log inactivation (15-18 min). In

addition a 5 min lag phase was seen.

By contrast, the 310nm LED array achieved only 1-log reduction over 6h with E. coli

(Figure 15). No significant inactivation at all was recorded for 310nm LEDs and E.

faecalis in two further trials. The poor performance of the 310nm diodes was consistent

with the 50% lower LED array output power and much lower cell sensitivity to UV-B

(Webb and Brown, 1979). As these reductions were barely significant, no further

310nm LED trials were undertaken.

E. coli K12

y = 0.0001x + 0.0718R² = 0.0057

E. faecalis ATCC 19433

y = -0.0006x + 0.0607R² = 0.1992

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 60 120 180 240 300 360

Lo

g R

edu

ctio

n

Time (minutes)

E. coli K12E. faecalis ATCC 19433E. coli K12E. faecalis ATCC 19433

61

Figure 12 - E. coli K12 and E. faecalis ATCC 19433 log reduction vs. time - 270nm

With E. coli, UV-A LED arrays in the range of 365-405nm performed comparably

(Figure 13) requiring 60 to 120 min to achieve 5-log reduction. The 365nm LEDs were

somewhat less effective probably reflecting lower array power (two single LED

packages and a quad LED package v. three quad 385 and 405nm LED packages).

Nevertheless, a minimum, 3-log reduction was seen after 120 min exposure.

In contrast with 270nm diodes, E. coli and E. faecalis inactivation occurred at more

comparable rates with the 365-405nm LEDs except for one outlier 365nm run requiring

6h to achieve 4-log reduction.

0

1

2

3

4

5

6

0 3 6 9 12 15 18

Lo

g R

edu

ctio

n

Time (minutes)

E. coli 270nm (n=4)E. faecalis 270nm (n=5)E. coli 270 nm (n=4)E. faecalis 270 nm (n=5)

62

Figure 13 - E. coli K12 and E. faecalis ATCC 19433 log reduction by UV-Violet

LEDs - 365-405nm

The blue LEDs achieved marginal inactivation rates with E. coli. Both 430 and 455nm

arrays achieved inactivation sufficient for 4-log in six hours in 5 of 6 trials, though the

455nm LEDs emitted 2.7 times the estimated output of the 430nm array (Figure 14,

Table 6). Though these LEDs were the best on a cost per output watt basis, they

required 4-5 h to achieve a 3-log reduction. This put them at the limit of practical POU

usage, as the time required was similar to the low cost SODIS process used in many

localities (Mbonimpa et al., 2012).

With E. faecalis, 430nm and 455nm LEDs achieved similar marginal disinfection (2 to

3-logs in 6 h). Again inactivation was only ≈2 times slower than for E. coli. As with

E. coli, it was concluded that these LEDs did not offer a viable POU disinfection

solution.

0

1

2

3

4

5

6

0 60 120 180 240 300 360

Lo

g R

edu

ctio

n

Time (minutes)

E. coli 365nm (n=3)E. coli 385nm (n=4)E. coli 405nm (n=4)E. faecalis 365nm (n=2)E. faecalis 385nm (n=5)E. faecalis 405nm (n=6)

E. coli 365 nm (n=3)E. coli 385 nm (n=4)E. coli 405 nm (n=4)E. faecalis 365 nm (n=2)E. faecalis 385 nm (n=5)E. faecalis 405 nm (n=6)

63

Figure 14 - E. coli K12 and E. faecalis ATCC 19433 log reduction vs. time - 430

and 455nm

Finally, visible-light LEDs between 525nm and 740nm failed to significantly inactivate

E. coli or E. faecalis (Figure 15). The 525nm green LED even appeared to stimulate a

≈1-log increase in the bacterial population. The increase in population for 525nm may

have been a result of reading error which would have contributed approximately 0.5-log

to the reading. From an examination of the control readings, it may have been an

outlier, as a control movement of above 1-log had been recorded. Other reasons may

include cell de-clumping, growth stimulation (Karu et al., 1983), changes in gene

expression (Drepper et al., 2011), nutrient carry-over from the agar slope and final

division of the cells through stimulation of the respiratory chain (Tiphlova and Karu,

1988). One 740nm LED trial achieved ≈0.8-log decrease in bacterial population but this

was not replicated. Overall, it was concluded that E. coli K12 was effectively insensitive

to LED light in the green to deep red region even though temperature rises of 14°C and

17°C in 6h were recorded for 660 and 740nm runs, presumably due to red to infrared

water absorbance and heat containment by the carafe. Usefully these tests in effect

provided radiation positive controls and confirmed that wavelength, not water heating,

was the critical inactivation factor.

-1

0

1

2

3

4

5

6

0 60 120 180 240 300 360

Lo

g R

edu

ctio

n

Time (minutes)

E. coli 430nm (n=4)E. coli 455nm (n=4)E. faecalis 430nm (n=6)E. faecalis 455nm (n=5)

E. coli 430 nm (n=4)E. coli 455 nm (n=4)E. faecalis 430 nm (n=6)E. faecalis 455 nm (n=5)

64

For statistical confirmation, the 525-740nm LED log inactivation data were compared

with controls using paired t-tests. The 180 and 360 minute data indicated borderline

significance with E. coli at 525nm (p=0.054), and for two marginally significant runs

with E. faecalis at 660nm (p=0.037) and 740nm (p=0.009). However, repeat E. faecalis

trials did not show any significant differences (p=0.835, 0.219).

Figure 15 - E. coli K12 and E. faecalis ATCC 19433 log reduction vs. time - 310

and >=525nm

3.3.5. Disinfection Lag-Phase and Repair Mechanisms

Disinfection plots for UV-A and violet-blue LEDs indicated a lag phase of up to 60

min, followed by acceleration to >2-fold the average inactivation rate. This possibly

reflected accumulation of damage due to harmful radicals, or DNA/enzyme damage

(Maclean et al., 2009, Murdoch et al., 2012). This delay was not observed with UV-C

irradiation and E. coli but, judging from the E. faecalis plot, this may have been due to

sampling frequency.

-1

0

1

2

0 60 120 180 240 300 360

Lo

g R

edu

ctio

n

Time (minutes)

E. coli 310nm (n=1)E. coli 525nm (n=1)E. coli 590nm (n=2)E. coli 623nm (n=1)E. coli 660nm (n=2)E. coli 740nm (n=2)E. faecalis 310nm (n=2)E. faecalis 525nm (n=1)E. faecalis 590nm (n=2)E. faecalis 623nm (n=2)E. faecalis 660nm (n=2)E. faecalis 740nm (n=2)

E. coli 310 nm (n=1)E. coli 525 nm (n=1)E. coli 590 nm (n=2)E. coli 623 nm (n=1)E. coli 660 nm (n=2)E. coli 740 nm (n=2)E. faecalis 310 nm (n=2)E. faecalis 525 nm (n=1)E. faecalis 590 nm (n=2)E. faecalis 623 nm (n=2)E. faecalis 660 nm (n=2)E. faecalis 740 nm (n=2)

65

The lag-phase observation is consistent with observations of disinfection at different

wavelengths by Vermeulen et al. (2008) and Jagger (1981), which is sometimes termed

as "thresholding" and is likely related to the types of bonds which can be broken by

photons of specific energies.

Re-incubation of plates for up to four days revealed substantial numbers of late

appearing E. coli colonies from UV-A/violet exposed waters. The colonies were

identical in colour and texture but often smaller in size, while Gram-stained cells had

identical morphology. This delayed growth was mainly seen with samples from early in

the log inactivation phase. The post 24 hour count increase factor averaged 2.3 ± 1.4

(S.D.). However, all plates from the UV-A run endpoints remained clean. Plate counts

of water exposed to 270nm did not exhibit any change, indicating UV-A and violet to

blue LED damage to E. coli K12 was more amenable to repair. Also, unlike waters

irradiated by UV-A and violet to blue LEDs, colonies from the UV-C runs did not vary

in maximum size initially or upon extended incubation. Clean un-inoculated control

plates (agar only) showed no growth, indicating that contamination did not occur.

These prolonged incubation results were consistent with different damage and photo-

repair and dark repair capacity and mechanisms by which UV-A/blue to violet

disinfection occurs (Sommer et al., 2000, Zenoff et al., 2006). The results suggested a

need to ensure UV-A dosage is sufficient to counter photo-reactivation effects

(Rodriguez et al., 2014). Alternatively, viable, but non-culturable (or difficult to culture)

cell and growth-dependent inactivation (Maraccini et al., 2015) could account for the

observations.

3.3.6. S90s and Action Spectra

Because of their potential use in disinfection modelling, action spectra for the model

reactor were estimated for the two test bacteria. Firstly, average inactivation rates were

transformed into S90s (Table 8). Then action spectrum curve coefficients were estimated

via regression of S90s against wavelength and plotted along with literature inactivation

data and action spectra for comparison (Figure 16). Where no significant inactivation

was observed, the limiting S90 value is indicated by an arrow.

66

The literature indicated marked inflections in action spectra around 320nm, reflecting

the changing DNA sensitivity. Assuming such biphasic action spectra also occurred in

the strains tested here, log-linear fits (365-455nm data, extrapolated to 320 and 740nm)

and log-quadratic fits (250-320nm interval) were derived (Table 9). From the linear fits,

S90s for E. coli and E. faecalis at 320nm were estimated to be 1.0 × 104 and 1.6 x 10

4

mJ/cm2, respectively. Due to the limited data for the UV-C and UV-B region, the 270

and 310nm (E. coli only) measurements and 320nm estimates were supplemented with

254nm rate data for E. coli and E. faecium from Bolton and Cotton (2008).

Table 8 - Average S90 inactivation constants

Nominal

Wavelength

(nm)

Average

Irradiation

Power

(mW/cm2)

S90 Inactivation Constant

(mJ/cm2

for 1 log-reduction)

E. coli K12 E. faecalis ATCC

19433

Mean ± S.D. n Mean ± S.D. n

270 0.017 5.8 ± 0.5 × 10-1

4 4.7 ± 2.4 × 100 5

310 0.0085 1.6 × 102 b

1 -4.2 ± 5.5 × 104 a 2

365 12 2.5 ± 1.1 × 104 3 4.2 ± 2.1 × 10

4 2

385 49 6.1 ± 1.3 × 104 4 8.1 ± 1.2 × 10

4 5

405 65 8.6 ± 1.9 × 104 4 1.3 ± 0.23 × 10

5 6

430 21 c 1.0 ± 0.16 × 10

5 4 1.9 ± 0.38 × 10

5 6

455 37 3.0 ± 1.5 × 105 4 4.1 ± 0.90 × 10

5 5

525 21 -4.7 × 105 a,b

1 -2.3 × 106 a,b

1

590 10 3.1 ± 3.0 × 106 a 2 1.9 ± 1.9 × 10

6 a 2

623 29 3.2 × 107 a,b

1 -3.0 ± 24 × 106 a 2

660 44 3.4 ± 0.14 × 106 a 2 -3.3 ± 10 × 10

6 a 2

740 21 1.3 ± 1.8 × 107 a 2 -6.9 ± 5.0 × 10

6 a 2

a: Not significant by paired t-test, b: Single result, hence no S.D. available, c:

Estimated.

67

Figure 16 - Provisional batch reactor action spectra compared with normalised

action spectra data

a: Webb and Brown (1979), b: Peak et al. (1982), c: Kelland et al. (1983), d: Chen et

al. (2009b)

Table 9 - Provisional action spectra coefficients for batch reactor

Wavelength

(X) range

(nm)

E. coli K12 R² E. faecalis ATCC 19433 R²

270≤X<320 log(Y) = 1.3260 × 10-3

X2 -

0.70029 X + 92.128

0.99 log(Y) = 1.1427 × 10-3

X2 -

0.60513 X + 80.691

0.98

X≥320 log(Y) = 1.028 × 10-2

X +

0.7097

0.81 log(Y) = 1.013 × 10-2

X +

0.9718

0.91

a: Y = inactivation constant (mJ / cm2 / 1-log reduction)

The inactivation rates and provisional action spectra generally showed good agreement

with other studies using E. coli or a comparable group to E. faecalis, Bacillus subtilis

spores. It was noteworthy that the reported rate constants varied over a ≈1-log range at

270nm and the power required to inactivate E. faecalis was approximately ten times

1E-1

1E+0

1E+1

1E+2

1E+3

1E+4

1E+5

1E+6

1E+7

1E+8

250 300 350 400 450 500 550 600 650 700 750

Ina

ctiv

ati

on

Co

nst

an

t (m

J/c

m2/l

og)

Wavelength (nm)

Webb E. coli WP2sPeak E. coli B/rKelland E. coli SR385Chen B. subtilisE. coli K12E. faecalis ATCC 19433E. coli K12 FittedE. faecalis ATCC 19433 Fitted

Arrows Indicate Limit of Detection

Data Not Significant

at 95th Percentile

Webb E. coli WP2s a

Peak E. coli B/r b

Kelland E. coli SR385 c

Chen B. subtilis d

E. coli K12E. faecalis ATCC 19433E. coli K12 FittedE. faecalis ATCC 19433 Fitted

68

higher at 270nm than for E. coli but closer at longer wavelengths (see Figure 17),

suggesting a change in the relative importance of inactivation mechanisms depending

on wavelength.

Based on the difficulties encountered in estimating exposure dose, it is speculated that

some variation in non UV-C literature rates may reflect experimental factors such as

fluence or wavelength determination, container volumes and media, and as in the

present experiments, reactor design. More accurate fluence levels can be obtained using

ferrioxalate actinometry but it is also subject to calibration and reproducibility

problems, high cost, and complications in reflective reactors (Harris et al., 1987, Kirk

and Namasivayam, 1983).

Figure 17 - POU reactor action spectra vs. values in literature (UV-A/Violet) with

reflective enhancement

It was concluded that more precise measurement of dosage versus inactivation rate

calibration is desirable. That said, the primary aim of assessing how effective different

LED wavelengths were with the model POU LED steriliser was achieved. Real world

deployable disinfection units would likely still need final validation using microbial

1E+4

1E+5

1E+6

320 340 360 380 400 420 440 460 480

Ina

ctiv

ati

on

Co

nst

an

t (m

J/c

m2/l

og)

Wavelength (nm)

Webb E. coli WP2s

Peak E. coli B/r

Kelland E. coli SR385

E. coli K12


E. coli K12 Fitted

E. faecalis ATCC 19433 Fitted

Webb E. coli WP2s a

Peak E. coli B/r b

Kelland E. coli SR385 c

E. coli K12E. faecalis ATCC 19433E. coli K12 FittedE. faecalis ATCC 19433 Fitted

Arrows indicates

inactivation constant

decrease by factor of 2 due to reflection

69

indicators as is normal practice with UV-C reactors (USEPA, 2006). Nevertheless the

action spectra developed appeared to be sufficient for order of magnitude costing of

reactors using LEDs operating at different wavelengths and assessment of LED

potential more generally.

3.3.7. LED Lifetime Versus System Cost

Currently, UV-C and UV-B LED technologies are relative costly per Watt output,

limited in supply, have relatively short reported lifespans, and are still under

development. Despite this, in the model reactors, these factors were relatively less

constraining than expected, with E. coli and E. faecalis reduced by 5-log in 3 and 18

min respectively for the UV-C system. Given a claimed 250h lifetime, this still

corresponded to between 5 000 and 833 batches of water disinfected. To reach the same

level of disinfection, UV-A LED based systems needed to run for 2-3h. Given a UV-A

diode lifetime of 32 000 h, a UV-A array could disinfect 10 000 to 16 000 batches of

water (>29 years of service with one batch per day). But UV-A LED efficiency is

approaching maturity (Mukai et al., 2006) whereas projections point to much greater

relative UV-C LED cost reductions, improved availability and emission efficiency.

Presently, LEDs appear to dominate reactor costs (US$356 to US$1158 per UV array)

but other inputs were not trivial and were likely to increasingly affect reactor costs if

LED costs fall. Provisionally, it was estimated that additional major electronic

components costs, depending on sophistication and current required, would range from

US$20 to US$100. Separately PV costs are expected to reduce slowly from

≈US$1/Watt-peak, yielding a maximum of US$50. It was estimated that battery storage,

depending on stand-alone requirements, to range from US$20 to US$200. All these

factors appear to favour UV-C arrays.

3.3.8. Other Considerations - Target Dose, Light Absorbance, Viruses,

Reactor Material Compatibility, SODIS

Standards target a dose of 40mJ/cm2 to ensure 3-log inactivation of Cryptosporidium

and Giardia. This level applies to traditional LP UV-C disinfection, which is

approximated by the UV-C LEDs. Based on the geometry of the bench-scale model

reactor and the irradiance from the 5-LED array, 29 minutes of exposure would be

70

required to achieve this level of fluence or approximately 144 minutes of exposure from

a single UV-C LED. While not entirely prohibitive, the extended irradiation time is

likely to affect the feasibility of the reactor due to limited UV-C LED lifetime. The

40mJ/cm2 dose is approximately 68 times greater than the actual dose needed to

inactivate E. coli K12 in the bench-scale reactor, and 8 times greater than the actual

dose needed to inactivate E. faecalis ATCC 19433 in the bench-scale reactor.

Water absorbs strongly in the UV wavelengths, with increasing absorbance towards

shorter UV-C wavelengths. Further, raw waters often contain dissolved humic acid

compounds, which markedly reduce delivered UV-C dosage (Bolton and Cotton, 2008).

Water absorption impacts (Cantwell and Hofmann, 2011) were not investigated

experimentally because possible water types are so diverse. However, the average

reactor water depth/path length at 12cm was much greater than in commercial UV-C

disinfection reactors and hence UV-C LED based reactors would likely be vulnerable to

UV absorbing compounds. In the case of 20L water containers absorption would have a

still greater impact. By comparison, longer wavelengths can penetrate deep source water

columns and achieve respectable disinfection as indicated by solar radiation

experiments (Davies et al., 2009). So while disinfection of water with elevated colour

and organic matter by UV-C reactors seems problematic, with >1L volumes UV-A

systems might work. Testing of this was not performed with the bench-scale reactors for

similar reasons as to why ferrioxalate actinometry could not be employed, namely that

the incorporation of wavelength-absorbing compounds alters the light distribution inside

a reflective reactor, which produces results which cannot be directly compared to the

non-coloured result. Water matrices are also extremely diverse, and their absorption

varies significantly, making a generalised result difficult to achieve.

As pathogen models, two bacterial species were used to assess performance as they

represent standardized indicator bacteria, and it appears that the primary contributor to

waterborne disease is often likely enteric bacteria in poorly serviced communities

(Enger et al., 2012). However, viruses, especially Adenovirus, are notoriously more

resistant to UV-C and other wavelengths than faecal indicator bacteria (Eischeid et al.,

2011, Hijnen et al., 2005, Oguma et al., 2015). Further challenge testing validation is

needed before such systems can achieve wider acceptance and regulatory approval.

71

Conversely, in POU situations, present solutions may be non-existent and, thus, any

level of risk reduction can be of use. Research has shown that bacteria pose the greatest

risk to those in developing countries (Enger et al., 2012), so simple reactors

configurations and relatively low radiation doses may still be useful. Nevertheless,

future work should address enteric viral (e.g. Heaselgrave and Kilvington (2012)), and

possibly parasitic protozoan disinfection (Johnson et al., 2005).

In POU settings, the dominant water storage infrastructure is plastic containers.

Reportedly they cannot tolerate UV exposure, which depending on wavelength, causes

cracking and crazing (Rånby, 1989, Singh and Sharma, 2008). Also, the size of the

containers and the presence of contaminated biofilms (Jagals et al., 2013) could

compromise in situ disinfection. However, glass containers may be a reasonable

alternative for directly consumed water.

Finally given the complexity and cost of the simple POU models it must be asked

whether SODIS (or SODIS with part-pasteurisation) might be superior. The benefits of

UV-A may prove marginal given comparable disinfection of large water volumes can

also be achieved in a few hours using solar radiation reactors (Davies et al., 2009).

Electronics might be better used to optimise and automate large volume SODIS and

resolving whether this would be more cost effective and sociologically successful

(Mäusezahl et al., 2009) than pursuing the UV “magic-bullet”? Conversely LED use

conceptually provides greater control and the ability to disinfect water year round even

outside of the tropics.

3.4. Conclusion

The potential for utilising various LED technologies for drinking water disinfection in

regional and remote communities was explored using a model batch reactor. UV-C

LEDs achieved >5-log reduction in 3 min with E. coli K12 and in 18 min with E.

faecalis. UV-A (365-405nm) LEDs achieved ~5-log disinfection within 3 h for both test

bacteria. Violet-blue (430-455nm) LEDs achieved ~4-log reduction with E. coli K12

and ~3-log reduction with E. faecalis within 6h. Wavelengths of 525-740nm showed no

appreciable inactivation. Action spectra constructed for E. coli K12 and E. faecalis

corresponded well with those in existing literature.

72

Both UV-C and UV-A LEDs were capable of exceeding the >4-log reduction target

recommended by WHO for household POU (World Health Organization, 2014, World

Health Organization, 2015) making them practical candidates for POU applications at

least for bacterial inactivation.

UV-C LEDs have a lower power consumption (~0.5W), which could easily be supplied

by a small PV system. However, these LEDs appear to still be constrained by expense,

limited power output, availability, efficiencies, lifetime and safety issues. LED lifetime

may be a less significant factor than initially expected given the higher speed of

disinfection and the future expectation of lower cost, longer lived UV-C LEDs.

UV-A LEDs are cheaper than UV-C diodes but disinfection was much slower, and they

had higher energy demand (17-33W) leading to potential thermal constraints on reactor

design.

The process of designing the model reactors highlighted other important factors needing

consideration including water absorbance, secondary component costs, complexity, and

uncertain pathogen type and sensitivity.


Practical LED arrays were constructed with commercially available components

with consideration for cost, physical size, thermal dissipation and power

consumption.

Time vs. inactivation was determined, with good disinfection levels (>5-log)

achieved by 270, 365, 385, and 405nm within 3 minutes to 6 hours for both E.

coli K12 and E. faecalis ATCC 19433. LEDs of 430nm and 455nm were

capable of marginal 2-4-log inactivation in 6 hours, whereas wavelengths

≥525nm did not show inactivation.

Action spectra for both model bacteria were developed and corresponded well

with literature.

73

An initial "lag-phase", increased photoreactivation and slow-growing colonies

during log-phase disinfection were observed with UV-A disinfection but less so

with UV-C (E. coli only).

Differences in dose requirement between E. coli and E. faecalis were 10-fold in

UV-C and approximately 2-fold in UV-A, suggestive of differences in

inactivation mechanism.

74

4. Exploring Potential Routes for Enhancement

4.1. Introduction

In Chapter 3, disinfection of water at bench-scale was achieved for wavelengths of

270nm (UV-C) and 365-455nm (UV-A/Violet/Blue). While on its own, the level of

disinfection was highly satisfactory in the UV-C region, the UV-A disinfection required

significantly higher doses. Even with greatly increased output compared to UV-C

LEDs, UV-A LEDs still required much longer exposure times to attain the same level of

inactivation, with the violet-blue end of the spectrum only achieving marginally

satisfactory results after a full six hours exposure.

From the literature review (Chapter 2), several prospective routes were identified which

could enhance the prospects of LEDs in the UV-A, violet and visible spectrum region

that deserved further investigation. This chapter examines two enhancement techniques

through practical experiments, and analyses the prospects of a further three based on

theory and other literature reports.

Advanced Oxidation Processes (AOP) using photocatalysts were explored through

bench scale experiments identical to those performed earlier with the exception of the

addition of titanium dioxide (TiO2). Investigation into photosensitisers was also

performed in a similar manner, using commercially available chlorophyllin dietary

supplement at various concentrations to trial its potential. Additionally, an analysis of

the existing research surrounding multiple-wavelength synergies, pulsed irradiation and

LED overdriving is also presented.

4.2. Advanced Oxidation Processes

The term AOP encompasses a wide range of oxidative processes that rely on the

generation of hydroxyl (•OH) radicals for their effects (Dalrymple et al., 2010). These

methods have applicability in controlling both emerging chemical contaminants and for

disinfection. Two classes of methods are used, termed homogenous and heterogeneous.

Homogenous methods use the addition of oxidants, e.g. hydrogen peroxide (H2O2),

which is broken down by UV to form hydroxyl radicals directly. While this method is

75

straightforward and can be suitable for industrial processes, it is not suitable for POU

usage because of the risk of remnant H2O2 in the case of incomplete degradation or

overdosing, and the need to supply H2O2 which is consumed in the process and has a

short shelf-life (IJpelaar et al., 2007).

Heterogeneous methods rely on the addition of photocatalysts, commonly TiO2 or a

similar metal-oxide catalyst (Ibhadon and Fitzpatrick, 2013). TiO2 can absorb light of

approximately 380nm and shorter due to its band gap and produce an electron-hole pair.

This then reacts with the water to create hydroxyl species which can cause oxidative

damage to pathogens and degrade chemicals. The catalyst is not consumed in the

process, which makes it simpler to provide. It is possible to use it dispersed in liquid as

a suspension or in immobilised form. Modified titania catalysts have been produced for

use at different wavelengths even down into the visible spectrum (Likodimos et al.,

2010, Pelaez et al., 2012, Sakthivel and Kisch, 2003, Byrne et al., 2011), however, their

quantum efficiency is lower.

Many studies have been published on the success of employing photocatalysts for use in

degrading chemical substances and biofilm control (as identified in the literature

review), with fewer focusing on disinfection. However, the publications that do focus

on disinfection have shown promising results:

Paleologou et al. (2007) focused on the addition of Degussa P25 TiO2 at 0.25-

0.75g/L showing maximal benefit in combination of both TiO2 and H2O2, but

around a 1.5-log increase in inactivation of E. coli over UV-A alone after 20

minutes exposure to a 9W 350-400nm lamp.

Xiong et al. (2011) used modified mesoporous TiO2 with Ag and demonstrated

a 3-3.5-log improvement in E. coli disinfection over 240 minutes exposure to

two 8W Backlight Blue (BLB) lamps.

Sichel et al. (2007) investigated solar photocatalytic enhancement of

disinfection of E. coli in a compound parabolic reflector using immobilised

Degussa P25 on a synthetic fibre support and demonstrated an improvement

over solar radiation of 2-log in 90 minutes exposure.

Chen et al. (2009a) demonstrated clear benefits in the use of 6nm anatase phase

TiO2 immobilised on clear glass plates, with disinfection of E. coli achieving

76

over 6-log improvements over 60 minute exposure to UV-A light of 1, 5 and

10W/m2 intensity.

Chatzisymeon et al. (2011) showed E. coli inactivation improvement of 6-7 logs

within 90 minutes exposure to a 9W UV-A lamp using Degussa P25 TiO2 at a

concentration of 100mg/L.

While Degussa P25 has been used in many investigations, its application in POU

context is complicated by the need to remove the suspended TiO2 for both cost (a desire

to reuse the catalyst) and health reasons, which requires complex filtering which in turn

requires both consumable filters and energy input. Some other studies with immobilised

catalysts appear to show reduced enhancement levels, likely due to limitations in

surface area, lifetime of hydroxyl radicals and their ability to diffuse toward pathogens

before being lost to recombination (Izadifard et al., 2013, Tsuyoshi and Akira, 2013).

A review of the mechanisms (Dalrymple et al., 2010) suggests that photocatalytic

disinfection is the product of numerous possible pathways, including extracellular

targets in the membrane wall (the peptidoglycan layer, the lipopolysaccharide layer and

the poshpholipid bilayer), and intracellular targets (enzymes and coenzymes and DNA).

As LED light sources can emit reasonably efficiently at UV-A compared to UV-C, there

is potential to combine them with photocatalysis for enhancement of disinfection

especially as high power outputs are available (Izadifard et al., 2013, Jamali et al., 2013,

Natarajan et al., 2011).

4.2.1. Materials and Methods

Ceramic Raschig rings coated with powdered TiO2, as previously used by Feitz (1998)

and Davies et al. (2009), were reconditioned by submerging them in a 4% hydrogen

peroxide (H2O2) solution and exposing them to natural sunlight for 48 hours (Feitz,

2012, pers. comm., 16th November). The rings were observed to effervesce in sunlight

which suggested that reactions were occurring and the catalyst had been activated

(Figure 18).

77

Figure 18 - Ceramic Raschig rings undergoing reconditioning

The rings were then washed under ultrapure water five times to ensure all traces of

H2O2 were removed, to prevent interference with the bench-scale experiment.

The ability of the rings to enhance disinfection was tested using the same bench-scale

testing protocol at wavelengths of 365, 385 and 405nm in duplicate as described in

Chapter 3. It was expected, due to the band-gap of TiO2 that enhancement would only

be possible for 365nm and marginally for 385nm. As 270nm LEDs performed well

enough on their own, 310nm LEDs had very limited power and both were very

inefficient at generating UV light (Autin et al., 2013), they were not tested with the

Raschig rings. As the TiO2 material can only generate at most one radical for each

absorbed photon with energy greater than the band gap, LEDs with shorter wavelengths

than the band gap are at a distinct disadvantage. As the second law of photochemistry

(Stark-Einstein law) states that each photon can only cause a single reaction to occur,

the "excess" energy above the band gap is wasted. Further to this, the LEDs with shorter

78

wavelengths produce less output due to their lower efficiency and do so at higher cost

by orders of magnitude (as detailed in Chapter 2), and thus it was not economically

sensible to consider combination with TiO2 mainly due to the increased cost of

supplying the material and additional logistical challenges of immobilising it and

ensuring it remains active."

Each ring measured approximately 25mm in outer diameter, 19mm in inner diameter

and 26mm in length for a surface area of 40cm2

area per ring. A total of six Raschig

rings, in two rows of three rings, were suspended inside each reactor using polymer

fishing line (Figure 19). This arrangement ensured that the rings would not interfere

with the magnetic stirrer bar used to maintain a completely mixed volume and complete

oxygenation. As the rings were suspended in the reactor and the reactor operated as a

batch reactor, the water was in continuous contact with the TiO2 covered rings and thus

achieved the maximum contact time possible, maximising the likelihood of seeing an

effect.

Figure 19 - Suspended Raschig rings within reactor

79

4.2.2. Results and Discussion

An examination of the control data (Figure 20) showed a mean deviation of -0.032 ±

0.085 (S.D.) logs within the 240 minute runs. On this basis, it was likely that log

reductions above 0.2 were statistically significant.

Figure 20 - TiO2 experiment dark control plot

The data for the disinfection runs (duplicate) is presented in Figure 21.

y = -0.0006x + 0.0206

R² = 0.3886

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.2

0 60 120 180 240

Lo

g1

0R

edu

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n

Time (minutes)

80

Figure 21 - TiO2 experiment inactivation plot for 365, 385 and 405nm (duplicate)

The anomaly with the first 365nm run where disinfection was not achieved even in 120

minutes may have been a result of a transient connection fault with the LED array,

however, in my experience, the "lag" phase at the beginning of disinfection is somewhat

variable and thus the result is not clearly anomalous. Compared with the bench scale

disinfection without the presence of Raschig rings (Figure 13) where 5-log inactivation

was achieved between 60-120 minutes for these wavelengths, the disinfection

performance was similar or even marginally worse. The lack of clear enhancement

could have been due to the limited surface area of TiO2 available, and the loss of light

from a change in reactor optics due to the absorption of the rings. But irrespective, the

immobilized TiO2 did not provide any measurable benefit.

Solar irradiation studies with Raschig rings performed by Davies et al. (2009) reported

an optimal configuration inactivation rate of 1.60-logs/MJ.m2. By comparison, unit

converting the S90 bench-scale reference figures from Table 8, the better-performing

non-TiO2 reactors achieved inactivation rates of 4-logs/MJ.m2 at 365nm, 1.64-

logs.MJ.m2 at 385nm and 1.16 logs.MJ

-1.m

2 at 405nm. This indicated that LED

irradiation provided better inactivation than the optimised TiO2 solar reactor, with the

0

1

2

3

4

5

6

0 60 120 180

Lo

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Exposure Time (minutes)

365nm Reference365nm-1365nm-2385nm Reference385nm-1385nm-2405nm Reference405nm-1405nm-2

81

exception of 405nm. This was likely to due to differences in the spectral distribution of

the input energy and reactor geometry. It also suggests that 385nm and longer LED

disinfection have similar energy requirements to sunlight driven visible-wavelength

disinfection.

Late growing colonies were observed consistent with experiments on UV-A

wavelengths performed earlier. The colonies exhibited identical shape and texture. A

Gram stain was performed to confirm their type and morphology, and showed identical

rod shape of approximately 1.5µm length and Gram negative result (Figure 22 and

Figure 23).

Figure 22 - Gram stain of normal sized E. coli colony

82

Figure 23 - Gram stain of late-growing E. coli colony

I also considered trialling Viridian self-cleaning window glass samples (approximately

30cm2 area) with TiO2 coating, but these were not tested due to difficulties in mounting

them within the reactor. However, it appears unlikely that the samples would have been

able to produce positive results as their surface area was significantly less than that of

the Raschig rings (as only four pieces would fit), and the TiO2 coating's morphology

and thickness were optimised for visible light transmission rather than disinfection

purposes.

As positive results with TiO2 photocatalytic enhancement were not replicated, it was

decided not to pursue this with the full scale prototype systems. It is possible that the

opaque nature of the rings may have affected the light distribution within the reactor,

and that the surface area of the immobilised TiO2 was not sufficient compared to that of

slurry or suspension, especially when the short lifetime and diffusion of hydroxyl

radicals is considered. However, for POU applications, recovery of the TiO2

photocatalyst is necessary, thus immobilisation appears a key requirement. Despite the

inability to replicate the results, further investigation into the use of immobilised TiO2

photocatalysts for POU drinking applications may still be warranted based on other

83

positive results in the literature. Modified metal-oxide photocatalysts may show promise

for visible light disinfection as well, although attention is called upon the immobilising

substrate as it can become an area where bacteria may adsorb and cause cross-

contamination (Shi et al., 2016).

4.3. Porphyrin Photosensitisers

A common class of photosensitiser are porphyrins. The literature review identified the

application of porphyrins as antimicrobial photosensitising agents in medical contexts

for antimicrobial photodynamic therapy (Almeida et al., 2011). Application testing on

microbes suggests this could potentially be extended to POU water disinfection (Kumar

et al., 2016, Rossi et al., 2012, Kim et al., 2013, Costa et al., 2012, Chen et al., 2011a,

Carvalho et al., 2007, Jemli et al., 2002).

Porphyrins are a class of heterocyclic compounds, and their addition as photosensitisers

can allow more effective use of the wavelength components in sunlight in microbial

disinfection presumably due to the Soret absorption band in the 420-430nm region.

There has been some published literature showing porphyrin-assisted inactivation with

visible (Hamblin et al., 2005), blue and ultraviolet light (Kjeldstad and Johnsson, 1986,

Nitzan et al., 1992).

In light of this, it was seen as worthwhile exploring the potential for porphyrin

photosensitisation by experimenting with a widely commercially available porphyrin -

namely chlorophyllin dietary supplement.

4.3.1. Materials and Methods

The same apparatus and procedure (see Section 3.2.3) was used to determine the

disinfection potential of various wavelengths in the presence of chlorophyllin. Instead

of other more commonly trialled porphyrins, such as methylene blue and rose bengal, I

chose chlorophyllin due to its non-toxic nature, wide availability, water solubility and

low-cost. The chlorophyllin was obtained from a commercially produced dietary

supplement drink (Nature's Way) which consisted of an active ingredient reconstituted

from sodium magnesium chlorophyllin powder (2%). A small amount of the

commercial preparation (5mL) was added to each 1L batch of the 0.05M NaCl solution

84

sufficient to achieve a high absorbance of 0.5 at 405nm. Dark controls were used to

confirm that the presence of chlorophyllin without light did not affect the survival of E.

coli K12.

Determination of the absorbance of the chlorophyllin was made by using a Varian Cary

50 UV-Vis spectrophotometer and 1cm polystyrene cuvette to perform scanned

measurement of absorbance from 350-700nm after six hours of exposure to LED

irradiation. This showed the extent that the light had photo-bleached the porphyrin,

potentially revealing the potential inactivation enhancement by each LED wavelength.

In line with other porphyrin photodynamic inactivation experiments, only wavelengths

of 405nm and longer were explored, as porphyrin absorption bands are typically in this

range.

4.3.2. Results and Discussion

Examining the absorbance spectrum of the media and chlorophyllin mixture showed

that the chlorophyllin had significant absorbance peaks around 405nm and 623nm. The

photobleaching effect of different LED wavelengths was compared after six hours of

irradiation (Figure 24, Figure 25 and Figure 26). The absorbance at 405nm was

measured and the effectiveness of each LED at bleaching chlorophyllin was determined

by taking the ratio of the absorbance of the photobleached liquid versus the control

(Table 10).

Table 10 - Chlorophyllin bleaching by 405-740nm LED radiation in 6 hours

Wavelength (nm) Percentage of control absorbance after 6h exposure (%) a

405 44

430 69

455 65

525 66

590 68

623 56

660 82

740 67

a: Lower percentages indicate greater photobleaching has occurred

85

It was found that 405nm LEDs were most effective at bleaching, followed by 623nm

LEDs. Most other tested LED wavelengths (455, 430, 525, 590 and 740nm) performed

similarly, except for 660nm LEDs which resulted in the least bleaching.

Overall, photobleaching occurred at all tested wavelengths as compared to the dark

control, possibly implying the generation of radicals or byproducts which may enhance

disinfection.

Figure 24 - Chlorophyllin bleaching by 405, 430 and 455nm LEDs in 6 hours

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

350 400 450 500 550 600 650 700

Ab

sorb

an

ce (

1 c

m p

ath

len

gth

)

Wavelength (nm)

Control

405nm

430nm

455nm

86

Figure 25 - Chlorophyllin bleaching by 525 and 623nm LEDs in 6 hours

Figure 26 - Chlorophyllin bleaching by 590, 660 and 740nm LEDs in 6 hours

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

350 400 450 500 550 600 650 700

Ab

sorb

an

ce (

1 c

m p

ath

len

gth

)

Wavelength (nm)

Control

525nm

623nm

0

0.1

0.2

0.3

0.4

0.5

0.6

350 400 450 500 550 600 650 700

Ab

sorb

an

ce (

1 c

m p

ath

len

gth

)

Wavelength (nm)

Control

590nm

660nm

740nm

87

Figure 27 - Chlorophyllin experiment control data plot

Figure 28 - Log reduction results for chlorophyllin plus 405nm LED irradiation

y = 0.0002x + 0.004

R² = 0.2299

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0 60 120 180 240 300 360

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Time (minutes)

-1

0

1

2

3

4

5

6

0 30 60 90 120 150 180 210

Lo

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405nm-1

405nm-2

405nm-3

Reference

88

Guided by this, preliminary disinfection testing was performed. Mean dark control

movement was 0.04 ± 0.06 (S.D.) logs throughout a six hour run. Log reduction figures

greater than 0.2 are likely to represent statistically significant outcomes.

Initial log-reduction experiments (Figure 28) focused on 405nm as a candidate

wavelength for chlorophyllin due to the high absorption peak, however, even with the

observed bleaching, it did not demonstrate any improved disinfection rates. In fact, the

disinfection took marginally longer with the chlorophyllin (5-log reduction in

approximately 165 minutes vs. 120 minutes for no chlorophyllin), likely due to the

absorption of UV energy and the reduction in reflective enhancement that accompanies

that.

Figure 29 - Log reduction results for chlorophyllin plus 623nm LED irradiation

The following series of experiments (Figure 29) focused on the secondary absorption

peak, with 623nm red LED selected as the candidate wavelength. From prior bench-

scale experiments without the presence of chlorophyllin, no worthwhile disinfection

was achieved with this wavelength. Addition of chlorophyllin resulted in a change

where earlier runs were able to show significant levels of disinfection of around 5-log in

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290 minutes. Unfortunately, repeat trials saw the performance degrade run upon run,

which suggests that some changes within the dietary-supplement grade reagent may

have occurred (e.g. oxidation).

Figure 30 - Log reduction results for chlorophyllin plus 430, 455, 525, 590, 660 and

740nm LED irradiation

Experiments focusing on other wavelengths (single runs except 660 and 740nm) were

not able to show distinct benefits (Figure 30), with similar disinfection times for 430nm.

Enhancement was seen with 455nm achieving quicker disinfection by approximately

two hours, but this was the exception with no meaningful level of disinfection for 525,

590, 660 and 740nm, although this may have been due to the fact they were later runs

which were likely impacted by a loss of effectiveness of the reagent.

Due to the mostly sub-optimal results obtained with E. coli K12, experiments with E.

faecalis ATCC 19433 were not conducted. It is unlikely that even this level of

inactivation would be duplicated with E. faecalis owing to the difference in cell wall

structure in Gram positive bacteria.

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455nm Reference

90

Further experiments using laboratory grade reagents may be warranted, although this

method is of limited applicability to POU usage due to the need to supply consumables

and the effect it has on the quality of the water, which is likely to have altered colour,

taste and odour making it less palatable. For these reasons, it is considered a less

desirable form of enhancement that needs further research. The desire to potentially

remove the chlorophyllin from the water post-treatment adds additional complications

and the safety of remaining byproducts in the water remains unknown.

However, the experiments do serve to illustrate and highlight the fact that wavelengths

which were not considered useful from a direct photoinactivation standpoint can

become effective in the presence of photosensitisers. This could occur with natural

organic matter in water which is classically problematic for UV-C, but could prove to

be advantageous for violet/blue as well as visible red LEDs allowing them to perform

more rapid disinfection than would otherwise be expected.

4.4. Synergies Through Wavelength Combinations

Literature on multiple wavelength disinfection has suggested that there could be

enhancement of disinfection through the combination of wavelengths. This possibility is

supported by the bench-scale experiments which suggest different mechanisms and

effects with different wavelengths and by the determination of wavelength contributions

to disinfection with SODIS which suggests that SODIS may be an example of

synergism through exposure to broad spectrum UV and visible light.

Research by Webb et al. (1978) exposed E. coli K12 to 254nm and 365nm radiation. It

was found that prior exposure to large doses of 365nm radiation increased sensitivity of

the bacteria to 254nm, whereas exposure to small doses of 365nm exhibited the reverse

effect of reducing sensitivity to 254nm in a phenomenon termed photoprotection. The

authors claim that this supports the hypothesis that 365nm radiation damaged the

recombination and excision repair mechanisms within the bacteria and is the cause of

the observed synergism.

Further research of theirs also uncovered synergistic effects of 405nm and 365nm

radiation (Webb et al., 1982) with key observations including the requirement for

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365nm radiation to be under aerobic conditions to be effective and that 405nm caused

single strand breaks but no formation of pyrimidine dimers. They concluded similarly

that 365nm can serve to disable repair mechanisms which can repair damage created by

405nm. This observation is somewhat contradicted by Jiang et al. (2009) who detected

pyrimidine dimers in DNA generated directly due to UV-A irradiation.

As a result, the mechanism by which UV-A based disinfection occurs is still under

debate but is likely a combination of several effects depending on the conditions under

which disinfection takes place. One other feature of note is that these studies considered

synergistic effects with sequential exposure, rather than simultaneous exposure which

limits its usefulness in POU situations where LED power and disinfection time are

constrained for practicality reasons.

More recently, Chevremont et al. (2012b) also considered coupling UV-C (255nm,

280nm) and UV-A (365nm, 405nm) LEDs for simultaneous operation in disinfecting

E. coli and E. faecalis in a broth medium. It was concluded that coupled wavelengths of

280/365, 280/405 and 255/365nm produced the maximum inactivation. However, from

my examination of their data, it appears that the log reductions only showed at most, a

50% synergy enhancement over the effect of summing the results for the individual

diodes.

Overall, from the literature, it would seem that the prospect of useful synergisms with

wavelengths are still limited, despite the numerous combinations which can be achieved

with LEDs. It is important to remember that LEDs are not strictly monochromatic

sources, and they do have a spread of output wavelengths commonly around 10-30nm

FWHM. As a result, the most meaningful synergisms are likely to be cross-UV band

(i.e. UV-C and UV-A). Based on the above experiments, my conclusion is that, UV-C

was sufficient on its own without requirement for supplementing with UV-A, and the

motivation behind employing UV-A was primarily some small potential for reductions

in LED costs. Combining UV-A with UV-C would most likely eliminate the cost

benefits of UV-A and concurrently eliminate the low-energy and low-heat advantages

of UV-C. Also, using one type of LED simplifies design and power optimisation.

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4.5. Pulsed Irradiation

The proposal to use pulsed irradiation for UV disinfection appears to have evolved from

the use of Xenon flash-lamps as a broad-band source of UV light which inherently

releases its radiation in the form of pulses. This type of lamp has special applications in

food decontamination, where extended continuous exposure to light risks altering the

properties of the product. This type of lamp can also be applied to water disinfection

(Lee et al., 2009, Huffman et al., 2000).

According to a review of pulsed light disinfection by Elmnasser et al. (2007), the

mechanism of pulsed irradiation extends beyond the photochemical mechanisms

through direct disruption of the DNA. It is theorised that additional damage occurs via:

the broad spectrum of xenon flashlamps causing synergisms in damaging DNA

repair mechanisms and cell enzymes;

photothermal mechanisms due to temporary overheating of bacteria through

absorption of UV light by water within the cells resulting in cell rupture and

membrane disruption;

and physical mechanisms in damaging cell membranes through cyclic stressing.

The log reductions achievable with pulsed systems could not be duplicated by

continuous exposure to an equivalent dose (Wang et al., 2005), suggesting there are

merits to pulsed irradiation. Lamont et al. (2004) observed photoreactivation occurs in

samples treated by pulsed irradiation, although at levels which are unlikely to severely

degrade disinfection. Similar benefits appear to also apply to viruses, which suggests

that the knowledge of pulsed irradiation mechanisms is incomplete (Jean et al., 2011).

Pulsed irradiation from a Xenon lamp source has been applied to commercial

applications and found to be capable of inactivation of bacteria, viruses and

Cryptosporidium (Huffman et al., 2000). To date, the response benefits of pulsing in

terms of frequency, duty cycle, pulse widths and sensitivity have not been explained

theoretically making it difficult to optimise the combination of pulse settings and select

appropriate wavelength combinations.

93

A perceived advantage of LEDs, in this regard, over mercury tubes is the ability to

instantaneously switch on and off without the need for warm-up and without cycle life

penalties (Gaska, 2011). Further, operation of LEDs under pulsed conditions could lead

to longer lifetimes (Chen et al., 2012). These properties make LEDs particularly

attractive for experiments with pulsed irradiation. In fact, pulsed operation of visible

light LEDs is regularly used by consumer lighting for dimming purposes through

strategies such as pulse-width modulation (PWM) and pulse density modulation (PDM)

which achieve more energy efficient dimming than running at a lower forward current

(Cypress Technology, 2008). Pulsed irradiation could work in part by achieving better

energy efficiency by reducing junction temperature rise and average operating current

while still retaining high peak emission power. Encouragingly, while LEDs do not

achieve the high intensities that Xenon flashlamps do, several publications have shown

enhanced sensitivity still occurs under pulsed operation.

A study by Wengraitis et al. (2013) systematically explored the effects of various

repetition rates and duty cycles on disinfection of E. coli. Their results appear to show

increased sensitivity to radiation at duty cycles of 10% (around twice as sensitive) and

25% (around 25% more sensitive) with only slight sensitivity to pulse repetition

frequency (decreasing sensitivity with increased frequency). Duty cycles of 50%, 75%

and 90% resulted in lower sensitivity than continuous operation. With regards to energy

efficiency, duty cycles of 25% offered similar results to continuous operation, with a

duty cycle of 10% offering around twice as much energy efficiency. Duty cycles of

50%, 75% and 90% were less energy efficient than continuous wave (CW) operation.

Outside of UV-C applications, it has been reported that pulsed irradiation germicidal

effects on biofilms utilising UV-A LEDs were also enhanced, maximising at a pulse

frequency of 100Hz at 25% duty cycle (Li et al., 2010).

In conclusion, though pulsing may have conferred some efficiencies, the results

obtained by the above studies suggest that continuous wave operation (as per the bench

scale models) still offers the best results. As LEDs remain the dominant cost within the

systems, and surplus LED power is rarely available, pulsed irradiation using UV-

A/violet/blue wavelengths at the low duty cycles which produced increased sensitivity

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and efficiency unfortunately significantly lengthened disinfection times to the point

where the real disinfection rate was slower at the cost of more control circuitry. Pulsed

irradiation with UV-C LEDs could be more acceptable as these offer very rapid

disinfection without pulsing and consume significantly less power than the UV-

A/violet/blue arrays. However, for these wavelengths the benefits of pulsing did not

seem significant compared to their inherent performance. It was concluded that pulsing

alone did not as yet confer sufficient enhancement for POU application at present to

justify incorporation in prototype reactors.

4.6. LED Overdriving

LEDs are generally tested and specified by the manufacturer at a certain current termed

"rated current" but can in fact be operated at levels above this through to the absolute

maximum rated current with some restrictions in order to attain additional light output

at potentially little-to-no additional expense (CREE Inc., 2012b).

In order to achieve this, overdriving LEDs must be performed while taking into account

whether the operation is pulsed or continuous, and whether the thermal configuration of

the LED arrays permits for safe overdriving without major compromise of the lifetime.

For high-current, high-intensity mass produced LEDs, CREE Inc. (2012b) provides

engineering support data and guidance as to what level of overdriving will not degrade

LEDs. One disadvantage of overdriving is that the efficiency of the device reduces as

higher currents are used - this means that driving at twice the maximum rated current

will only yield approximately 38% increase in output for an XP-E LED (as an example).

The cause of this is due to the increase of internal temperature and current crowding

based resistive losses within the semiconductor. Continual high-current operation is also

likely to cause shortened lifetime due to higher temperature operation and due to metal

ion migration (Chang et al., 2012).

For pulsed applications, Cree recommends not exceeding 300% of maximum rated

current for duty cycles below 10%, 200% for duty cycles between 10-50% and 100%

otherwise. Even following these guidelines, proper thermal management is necessary,

and lifetime testing is needed for all application scenarios.

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The current potential for overdriving UV-C LEDs is even less due to their higher

package thermal resistance. Even operating at their present ratings, their UV output and

lifetime is likely to be constrained and continuous overdriving is inadvisable (see

Section 5.3.6 for full consideration of this issue). Due to the good performance of UV-

C, this is unlikely to be necessary in practice. If overdriving was used in concert with

pulsing, this might serve to reduce the impact of duty cycle on lengthening disinfection

times and improve the time efficiency. But the prospective benefits still appeared

marginal at this point and requiring extensive research, so it was decided not to

incorporate overdriving and pulsing into the UV-C based prototype disinfectors.

For higher powered UV-A devices, continuous overdriving may similarly not be

advisable due to the large amount of heat that needs to be dissipated, possible lifetime

impacts and increased power demand costs. Operating at twice the maximum current

will result in more than twice the amount of heat being generated, necessitating more

expensive larger heatsinks or the use of fans which are mechanical and prone to failure.

Compared with the 38% increase in output, it appears unlikely on initial consideration

to be cost effective due to the additional balance-of-systems costs incurred in more PV

panels, battery storage, thicker wiring, higher capacity current drivers, larger heatsinks,

etc.

In conclusion, the use of pulsing and overdriving in combination appears to be

potentially promising for overcoming time efficiency problems with low duty cycle

operation, however, a full evaluation appeared to require additional primary research

and was beyond the primary aim of my study. To realize a pulsed and overdriven

system would also require specialised current drivers that are capable of providing the

peak current, and the production of duty-cycle waveforms to drive the current drivers to

produce the pulsed output. CW operation of such a configuration would not be possible.

4.7. Conclusion

Five routes to enhance the disinfection prospects of LEDs were explored experimentally

and through theoretical examination of published literature. In the experiments, no

significant enhancement with TiO2 based photocatalysts was seen. Separately, the use of

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commercial chlorophyllin as a photosensitiser resulted in significant, albeit variable,

enhancement. This was not further pursued due to its drawback of requiring the supply

of consumables, its impacts upon the colour, taste and odour of water, and its unknown

breakdown products which may have health effects. As other literature has shown

repeated reports of positive results with TiO2, its application in immobilised form for

POU drinking water applications may deserve further exploration. Overall, it appears

the use of photocatalyst and photosensitisers with LEDs merits further research but the

technology is still insufficiently mature for incorporation into POU reactors at this

point.

From a consideration of wavelength combination, pulsed irradiation and overdriving, it

was concluded that they were likely of limited benefit in POU applications where rapid

disinfection utilising the full capabilities of the LEDs was required.

For all methods of enhancement, specific disadvantages in the POU drinking water

context were identified that reduced their appeal. These included the increased system

complexity and increased costs of LED combination, poor time efficiency in pulsed

irradiation and lifetime, heat and balance of systems costs increases with overdriving.


Photocatalytic enhancement using TiO2 was not reproduced despite positive

reports in literature, although this avenue still deserves further attention.

Photosensitiser based enhancement using chlorophyllin dietary supplement

achieved significant, albeit variable, enhancement with 623nm LEDs (incapable

of inactivation alone) of >5-log reduction, rapidly declining on repeat runs.

Other LEDs did not exhibit any clear benefits. Their key drawback is the

requirement for consumable chemicals.

Wavelength combinations, pulsing and LED overdriving were considered

through examining literature, with limited benefits and clear drawbacks in high

cost, low time-efficiency and thermal problems limiting their usefulness in POU

settings.

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5. UV-C LED Engineering Considerations

5.1. Introduction

Over the past decade, UV-C LEDs have attracted high levels of interest for use in

disinfection roles since they possess numerous advantages over conventional mercury-

tube type systems (Gaska, 2011). Many proof-of-concept papers were able to

demonstrate that UV-C LEDs were capable of taking on the task of water disinfection,

and concluded that the technology was promising (Würtele et al., 2011, Chatterley and

Linden, 2009, Vilhunen et al., 2009).

Nevertheless, UV-C LEDs continue to have high initial cost, low unit emission power,

low electrical efficiency and uncertain lifetimes (Lui et al., 2014). When economic

feasibility is considered, in order to compete with the established mercury-tube

technology, these drawbacks often lead to the conclusion that UV-C LED technology is

not presently viable except in niche applications, but will be viable in the ‘near future’

(Chatterley and Linden, 2010, Ibrahim et al., 2013).

Making the transition from bench-scale to full-scale requires addressing numerous

engineering considerations, as previously identified in Figure 6 in the literature review.

The bench-scale systems (Chapter 3) practically addressed LED selection, power supply

requirements and demands and, to some degree, thermal considerations but also raised

concerns in regards to eye safety, lifetime and window solarisation. Furthermore, a

desire for sensor technologies to detect UV radiation was identified as necessary to

safeguard against LED failure and determine and monitor dosage. This chapter

addresses what I viewed as the concerns experimentally and theoretically to better

understand the larger economic implications of UV-C LED based disinfection systems.

Research into UV-C LEDs is a dynamic area with rapid advances, but product cycles

have been long in recent years, leading to uncertainty as to whether progress is being

made with commercially available devices. The most popular products available in the

UV-C LED market appear to have identical datasheet specifications to those released

over four years ago e.g. Sensor Electronic Technology Inc (2011). This uncertainty is

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compounded by the limited lifetime data on datasheets, which put UV-C LED lifetime

between 50-1000 hours for degradation to 50% output for wavelengths of 240-300nm

respectively, as of 2011. Further openly published details about the L50 lifetime of LEDs

is sparse, with much existing literature focusing on prototype LED products which are

not commercially available, and operating conditions not necessarily relevant to POU

disinfection applications.

As disinfection is a radiation dose-dependent process, the sustainable output of the UV-

C LEDs is also paramount. However, the output of UV-C LEDs is not solely impacted

by ageing. Temperature is another major factor with datasheet figures suggesting losses

of 15% for every 10°C rise (Sensor Electronic Technology Inc, 2012). Also, literature

reports of pulsed operation being beneficial in reducing self-heating induced output

degradation are at odds with the need to maximise the delivered dosage to optimise

disinfection. Nor do proposals for pulsing seem to recognise how output declines with

continuous use due to heating. Rather, emissions in short pulses is assumed to occur at

the same rate as occurs in continuous wave (CW) mode. A thorough examination into

cause and effects of LED self-heating is necessary.

A further practical engineering consideration is the need to protect LEDs from liquid

ingress while maintaining radiation flux into the water being disinfected. Ordinary silica

glass and many plastics transmit light in the 365-405nm range with > 80% efficiency

(Davies-Colley et al., 1997), however, few materials are both transmissive at UV-C and

compatible with an aqueous environment. Window material selection can also be

complicated by long term exposure to high intensity light, leading to declining

transmissivity over time, a process known as solarisation (Schreiber et al., 2005).

Solarisation occurs in quartz, glass and plastic due to absorption of high energy light

causing covalent bond breakage leading to declining transmission. The degree is

dependent on the crystalline structure, chemical composition and purity of the window

material.

Bearing in mind the need for safety given the potentially damaging nature of UV

radiation and to ensure the dose delivered matches expectations, another engineering

issue is the type and suitability of UV-C sensors which could be used for on-line

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monitoring of radiation dosages and to guard against failure of the LEDs. Related to

eye safety protection, monitoring is also essential to avoid accidental exposure to the

output from LED arrays.

The issue of UV-C longevity first was addressed through a review of the literature.

Then, to confirm and validate manufacturer claims and expectations, an experimental

trial was conducted involving long term operation of a commercially available UV-C

LED in CW. The trial was conducted at room temperature at its rated current, which

simulates the full power demand of a POU disinfection system. Concerns regarding

solarisation were explored simultaneously by placing one of the quartz windows

obtained for the bench-scale model inactivation experiments in the light path between

the LED undergoing the longevity test and a UV sensor. The UV-selective sensor was

also designed and constructed to record the UV-C output of the LED over time and to

provide cost and specification data for POU devices.

Thermal self-heating losses were first explored theoretically using thermal resistance

models. Predicted losses were then experimentally validated with a second UV-C LED

and UV-selective sensor system used in both pulsed and CW operation.

To complement the UV sensor trial, other alternative low-cost sensor technologies were

investigated. These included using modified royal-blue LEDs and common

fluorochromes. Eye safety was addressed through comparison of LED emission

intensity with Australian Standards daily safe exposure thresholds for non-ionizing

radiation.

These experiments were also seen as supplementing literature based understanding of

technological progress in UV-C LED manufacturing, and provide better certainty

around the lifetime of commercial devices. This work also provided an input into

costing models, e.g. allowing more accurate cost estimates for LEDs and sensors and

power output estimates under various thermal operating conditions. In future, these data

should inform designers in regards to safety requirements around UV LED arrays and

potential solarisation issues.

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5.2.1. Construction and Testing of UV Sensor

For long term monitoring of the output of the UV-C LED, a stable, reliable and accurate

method of output measurement was necessary. Use of fibre-coupled spectrometers is

complicated by high cost, potential problems with software logging over long periods

and changes in transmission of the fibre over time.

To overcome these issues, a low-cost (~US$30) SGlux GmbH (2012) AG38S-TO

broadband UV photodiode sensor was used. This photodiode component is similar to

the photodiodes used in commercial UV disinfection units for monitoring of fluence,

and is sensitive only to wavelengths in the 220-370nm range, which helped to simplify

the experimental set-up as the exclusion of visible light was not necessary. The sensor is

hermetically sealed for stability, with a quartz glass window, and is directly oriented

towards the UV-LED source without any coupling optics.

The photodiode current signal is small (~0.1-700nA), and so requires amplification. As

recommended by the manufacturer, a simple transimpedance amplifier utilising an

operational amplifier (op-amp) was used (SGlux GmbH, 2012). This amplifier provides

the zero-bias condition required to use the photodiode in photovoltaic mode. Capacitor

and resistor values were chosen from commercially produced components to produce a

~0.1s R-C time constant to filter out high frequency noise and a full-range gain within

0-5V (TTL logic voltage) for easier interfacing with instrumentation and

microcontrollers. These values were from the E12 standard value series, a standard set

of values used by manufacturers to standardise component values into 12 values per

decade. Further components were added to regulate voltage, smooth voltage fluctuations

and protect against incorrect connection. The detector schematic is shown in Figure 31.

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Figure 31 - UV photodiode sensor transimpedance amplifier schematic

Higher gains necessitate higher resistances which are difficult to maintain and may be

subject to disturbances from stray voltages, atmospheric humidity and dirt, or a second

stage non-inverting op-amp amplifier, but this was not necessary due to the sensitivity

of the data-logging equipment. Due to the difficulty of construction with the

manufacturer's recommended op-amp (OPA336), which is only available in surface

mount packaging, a substitute from Microchip (MCP6273), which exhibits more

favourable characteristics across most metrics (Table 11) as noted by Pease (2001) and

Blake and Bible (2004), was used. The circuit was constructed on vero strip-board and

fully insulated using electrical tape.

As the sensor itself may degrade through the course of the LED lifetime testing from

UV exposure, it was calibrated prior to use by measuring the sensor output when

exposed to a rarely-used 36W germicidal lamp after 15 minutes warm-up time as

standard reference. An Agilent Technologies (2014) U1241B 4-digit handheld digital

multimeter was used, with the sensor placed directly on, and at distances of 30cm and

50cm from the lamp to obtain reference readings. The dark current reading was also

measured by placing the sensor in a dark container. The values were recorded, and the

experiment repeated at the conclusion of the lifetime testing to check the stability of the

MCP6273

-

+ AG38S

LM7805

In GND Out

1N4004

Vss EN

Vdd

Output

8.2MΩ

10nF

1N4148

100nF

1kΩ

Signal Output

- +

DC

Input

10µF

+

1N4004

+

-

102

sensor. Sensor use also provided a useful assessment of whether the lifetime of such

sensors is an issue in POU applications.

Table 11 - Comparison of OPA336 and MCP6273 characteristics for

transimpedance amplifier

Characteristic OPA336 a MCP6273

b

Gain Bandwidth Product (Mhz) 0.1 2.0

Voltage Noise (nV/√ Hz) 40 20

Current Noise (fA/√ Hz) 30 3

Input Offset Voltage (µV) -125 to 125 -3000 to 3000

Input Bias Current (pA) 1 1

a Texas Instruments (2005),

b Microchip Technology Inc. (2008)

5.2.2. UV-C LED Lifetime Testing Setup, Data Capture and Analysis

The set-up was designed to evaluate a UV-C LED operated continuously at rated

current without a heatsink as an individual unit under room temperature conditions. This

setup also allowed exploration of the potential for solarisation of one of the 7.6cm

diameter x 6.4mm thick (US$39) polished quartz discs used as a window in the bench-

scale POU reactor tests.

As in Chapter 3, the UV-C LED tested was a new commercially available Sensor

Electronic Technology Inc. UVTOP270TO39FW 270nm LED supplied in September,

2014 (unit code F57, manufacturer-tested dominant wavelength 274.9nm, output power

1.18mW at 20mA). The LED was powered with an On Semiconductor (2014)

NSI45020AT1G 20mA linear current regulator integrated-circuit in series.

In order to stabilize alignment of the components of the test system, a small size IKEA

Pruta food storage container, termed the "inner container", was appropriately modified.

A hole was drilled into the top lid to mount the LED, and a corresponding hole was

drilled in the bottom to mount the photodiode sensor. A section of the sidewall of the

container was removed to allow the quartz disc to be slotted into place in the optical

path between the LED and the sensor.

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This set-up was, in turn, sealed inside a tall large-sized IKEA Pruta food storage

container, termed the "outer container", to reduce the possibility of atmospheric

contaminants affecting the experiment. Two 3mm holes were drilled to allow the wires

which power the LED and sensor, and the analog signal voltage to pass through the

outer container (Figure 32). Due to concerns about the possibility of the metal-bead

thermocouple causing inadvertent short circuiting, the TM Electronics (2012) KA01

thermocouple was instead mounted near the exterior of the outer storage container.

Power was supplied in parallel from a single Manson HCS-3102 switch-mode bench-

top power supply, set to provide 12V DC.

The signal from the transimpedance amplifier and external thermocouple was connected

to a Keithley (2013) Model 2110 5.5 digit bench-top digital multimeter, connected to a

computer running KI-TOOL v.2.03 software. This allowed long term auto-ranging dual-

measure operation, allowing for simultaneous measurements of voltage and

temperature, recorded at an interval of 2.5s (1 440 readings per hour). Data were

exported periodically for safekeeping against unexpected power outages and software

failures. Data losses amounted to less than one-hour of data loss in the first 3 000h of

monitoring (<0.03%) and a total of 54h lost in the whole 5 000h of observation (1.08%)

due to mains power quality problems and disruptions.

The raw data were processed to determine the percentage decline in the output over time

using the following multi-stage process:

1. Average every 1 440 readings to produce an hourly result.

2. Subtract the dark current voltage reading.

3. Compensate for temperature difference from 25°C based on the recorded

thermocouple temperature and LED manufacturer compensation equation.

4. Divide by the corrected 15-minute "warmed-up" reading (the initial reading).

5. Transform into a percentage (multiply by 100).

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Experimental configuration mounted within containers

LED emitter front-side Current driver and connections on rear

UV-selective sensor and transimpedance board wrapped in insulating tape

Figure 32 - Photographs of experimental LED lifetime testing setup

105

This processing was required due to high recording frequency, and the temperature

sensitivity of the UV-C LED. On power-up from room temperature, a sharp drop in the

output intensity occurred due to initial self-heating of the LED junction which was not

representative of degradation. This occurred at each point where power was interrupted,

a total of six times. To exclude this, a 15-minute warm-up period was allowed for and

the reading at 15-minutes was taken as the initial value. This initial self-heating effect is

detailed in Section 5.3.6.

Compensation for ambient temperature variations was performed by using the

manufacturer's temperature co-efficient line (Sensor Electronic Technology Inc, 2011)

of the form:

y = 0.015 x + 1.375

where x is the temperature in degrees Celsius, and y is the relative output power at a

reference value of 25°C.

5.2.3. Confirmation of Operating Thermal Conditions

UV-C LEDs are highly sensitive to temperature and their output power varies

significantly in response to the ambient temperature. During a preliminary trial, the

ambient temperature was recorded by a thermocouple located outside the outer

container. As the temperature inside the container was likely higher due to the

insulating effect of the container itself, I determined whether a significant temperature

difference existed, by concurrently collecting measurements in parallel using a Keysight

Technologies (2014) U1461A and Agilent Technologies (2014) U1241B handheld

digital multimeter, each equipped with a TM Electronics (2012) KA01 thermocouple

and Bluetooth radio interface to the Keysight Mobile Logger application running on an

Android tablet computer in order to log the temperature inside and outside the outer

container over a period of 26 hours.

The paired dataset was examined for the temperature difference, compared with the

accuracy specifications for the equipment and analysed statistically to determine

significance.

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5.2.4. Quartz Window Transmissivity

In order to attempt to quantify the sensitivity of the low-cost commercially available

quartz windows to solarisation by UV-C wavelengths, the quartz round was placed in

the optical path between the UV-C LED and detector. As the window was only partially

exposed to the UV-C radiation in the region immediately facing the LED (120° half-

power beam spread), it was possible to determine the solarisation of the window by

comparing the transmissivity of the material in the exposed segment versus the

unexposed segment using a Perkin Elmer Lambda 1050 spectrophotometer.

5.2.5. Post-Run UV-LED Emission Characteristics

In order to determine whether the LED's characteristics changed as a result of ageing,

measurements of the output spectrum were made using an Ocean Optics S2000 fibre

spectrometer, initially, and after 3 000h, to determine the full-width at half-maximum

(FWHM) and dominant wavelength (DWL).

5.2.6. LED Emission Reduction Caused by Self-Heating

The self-heating losses that occur in LEDs represent a loss of power in CW operation

mode as compared with the manufacturer's reported test power output values. These

values are often short "pulse" measurements with the junction temperature kept at 25°C.

Calculations, based on industry-standard typical package thermal resistance values and

dissipated power of the UV-C LED, were used to estimate the power loss under various

operational scenarios. This assessment employed a second UVTOP270TO39FW 270nm

LED (supplied in September 2014, unit code O59, manufacturer-tested DWL 276.3nm,

output power 1.20mW at 20mA), fitted with a second UV sensor unit built to the design

described above and coupled directly to the LED. The LED was operated with the same

power supply and current driver component, but used a modified circuit incorporating a

logic-level N-channel MOSFET to allow pulsed as well as continuous operation (Figure

33, Figure 34). Pulse-width modulation signals were generated using the

microcontroller program (Appendix B). This program was loaded onto a Freetronics

(2015) Leostick, an Arduino (2016) open-source compatible microcontroller, to control

the LEDs duty cycle, with the recording rate of the Keithley multimeter set to its

minimum value to provide maximum temporal resolution.

107

Figure 33 - Self-heating and pulsing schematic (see also Figure 31)

Figure 34 - Self-heating and pulsing test device

The UV-C LED was operated at its rated 20mA (CW) and also at 50% and 10% duty

cycles at 976.5Hz for 1-hour intervals with a 1-hour rest period between intervals. The

Sensor

as per

prior

figure

Freetronics

Leostick

PWM

Signal

+

-

NSI45020AT1G

UVTOP270

1kΩ

1kΩ 1.1Ω Bypass

STP22NF03L +

-

1N4004

50µF +

+

+

-

-

Shunt Output

Sensor

Output

12V

Power

Input

108

recorded values for each run were normalised to the 1-hour warmed-up output for

comparison.

Pulsing and steady state current was measured with a 1.1Ω shunt resistor connected to a

Pico Technology (2014) Picoscope 2205A USB-connected Digital Oscilloscope. This

shunt was bypassed during regular operation by closure of the bypass switch.


The findings have been grouped into five main sections - a review of UV-LED lifetime

data, and results relating to the UV sensor, UV-C LED, quartz window and thermal

aspects.

5.3.1. Review of UV-LED Lifetime Data

A general review of LED reliability by Chang et al. (2012) focused on failure

mechanisms of mass-produced LEDs. It classed failures into three main categories:

Semiconductor-related failures (defect/dislocation generation and movement, die

cracking, dopant diffusion, electromigration).

Interconnect-related failure (contact metal interdiffusion, electrostatic

discharge).

Package-related failure (carbonisation and yellowing of the encapsulant,

delamination, lens cracking, phosphor thermal quenching, solder joint fatigue).

The cause of failure can be both intrinsic to the LED due to manufacturing defects and

choice of materials, as well as extrinsic to the LED from choice of drive currents,

heatsinking and mounting (CREE Inc., 2012a).

Literature reports regarding lifetime and testing specific to UV-C LEDs are limited in

number, vary in their testing conditions and are often conducted by the research teams

of UV-C LED manufacturers on non-commercially available prototype LEDs. A

summary of their results is provided in Table 12.

109

Table 12 - Summary of lifetime testing from literature

Study λ

(nm)

Drive

Current

(mA)

Summary of

Result (% of

initial power in

number of

hours)

Comment

Shatalov et al.

(2006) and

Khan (2006)

280 20 50% in 100h

(100µm2)

70% in 100h

(200µm2)

90% in 200h

(10µm2

micropixel)

Demonstrated geometry

impacts on lifetime.

Reed et al.

(2008a)

280 25 50% in 250h Linear extrapolation used.

Reed et al.

(2008b)

280 100 50% in 20h Linear extrapolation used.

Reed et al.

(2008b)

280 100 50% in 1 400h Linear extrapolation,

operating at 1% duty cycle

(100uS pulse),

approximately 14h of on-

time.

Reed et al.

(2008b)

280 20 50% in 1 420h

at 25°C

50% in 370h at

50°C

50% at 100h at

75°C

Accelerated degradation

due to increased operating

temperature. Linear

extrapolation used.

Meneghini et

al. (2008a)

295 20 80% in 100h

70% in 250h

then negligible

to 1050h

Output measurements

made at 10mA which

might increase apparent

rate of degradation.

110

Study λ

(nm)

Drive

Current

(mA)

Summary of

Result (% of

initial power in

number of

hours)

Comment

Khan et al.

(2009)

280 20 50% in 1 250h

Shatalov et al.

(2009)

235-

310

20 61% in 81h Likely short wavelength,

actual wavelength not

specified.

Sun et al.

(2009)

255 2 >97% in 15

000h in nitrogen

>97% in 8 000h

in vacuum

Verification for space

conditions for

instrumentation, LED

output is significantly less

than rated (~1%), due to

10% duty cycle and 10%

rated current,

corresponding to 150h and

80h of on-time

respectively.

Commercially available

LED.

Vinod et al.

(2009)

280 400 90% in 600h Large size micropixel

array.

In addition to these studies, an abstract by Gaska (2011) implies the potential for 270-

280nm diodes with lifetime exceeding 10 000h, and 250-260nm space-qualified diodes

with up to 26 000h continuous operation, but without details as to the measurement

protocol, operating current, temperature and remaining output. Two other publications

showed that "a small number of devices" suffered catastrophic failure by internal short

circuit due to poor device geometry and crystal lattice defects (Meneghini et al., 2008b,

Shatalov et al., 2006).

111

With such a large range of reported values, the actual lifetime of UV-LEDs in

disinfection applications was unclear. The uncertainty stems from several factors,

including:

The use of diodes with different wavelengths from different manufacturers with

different emitter size and differing levels of manufacturing expertise.

The majority of studies utilising non-commercially available prototype LED

emitters.

The use of different operating conditions in terms of drive current, duty cycle,

temperature, atmosphere.

Differences in output determination method (i.e. projected versus actual

measurement).

In some cases, operating at lower currents likely reduced the current density and hence

strain in the film, and also reduced the semiconductor junction/film temperature rise,

reducing the rate of degradation and likelihood of catastrophic failure. Operating at low

duty cycles (e.g. 10%) will result in an expectation that the diode will be able to operate

at least proportionally longer (i.e. 10 times), with an added benefit of lower

temperatures. Few of these tests exceeded 1 000 hours of real-time testing and rely on

regression line based extrapolations which may not be fully indicative of the

degradation patterns occurring under real operating conditions, especially if

catastrophic failure (as opposed to degradation) and other failure modes emerge.

As a result, some of the longer lifetimes claimed for UV-C LED operation are not

applicable to POU applications, which requires high power continuous current operation

and can experience high operating temperatures. Assessment of the quality of the LEDs

is paramount if reliable long-term operation is to be expected (Bürmen et al., 2008).

5.3.2. UV-Selective Electronic Sensor

The UV-selective sensor construction proved straightforward and the device was

determined to be sufficiently sensitive. Its total cost was about US$32 which is

significantly cheaper than available commercial sensors which commonly retail for

US$200 and upwards. The readings obtained at different distances are summarised in

Table 13. Slightly positive change readings up to 5.7% were recorded, and were likely

112

due to errors in positioning and the use of inductive control gear on the reference lamp,

resulting in output variations due to line voltage which can vary up to +10%. It was

concluded that no significant degradation of the UV sensor sensitivity occurred over the

LED lifetime experiment.

Table 13 - UV sensor calibration values

Distance (cm) Pre-Experiment

(mV) at 0h

Post-Experiment

(mV) at 3 000h

Change (%)

Dark 1.5 1.4 - 0.1

0 2957 3045 + 3.0

30 244.9 258.8 + 5.7

50 47.3 47.9 + 1.4

The calibration experiment also revealed that the sensor had a wide dynamic range

when illuminated with a relatively intense source such as the germicidal lamp. Warm-up

of the lamp and variations in output due to fluctuations in line voltage were observed in

the recorded data from the sensor. The gain-setting resistor (Figure 31, 8.2MΩ) was a

carbon film type with a stability of, at worst, 500ppm/°C (or 0.05%/°C). This resulted in

an expected temperature-related gain variance throughout the experiment of 0.8%.

Other components in the sensor were not value-critical or had no documented

temperature-related coefficients.

When the sensor was employed for UV-C LED measurement through quartz at close

range (1cm), the output voltages were in the order of 50mV due to the low UV output.

In the future, a more expensive sensor with a larger area could be employed, that

produces a proportionally larger signal for a given intensity of UV, improving the signal

to noise ratio, or additional amplification added at the cost of increased noise.

Dark current readings could have been elevated due to the use of 0V as the ground

reference, operating the rail-to-rail op-amp in its non-linear zone (below 0.015V).

Improvement in linearity might be achieved by using a second amplifier to produce a

virtual-ground, elevating signal levels by a fixed offset and keeping the operation within

the amplifier's linear region (Baker, 2004).

113

5.3.3. UV-C LED Ageing

The ambient temperature for the LED lifetime experiment was maintained at 16 to

32°C, for the first 1 550 hours. Subsequently, the room was air-conditioned to maintain

a temperature as close to 25°C as practicable and avoid extreme elevated summer

temperatures. At 3 000 hours, the experiment was temporarily dismantled to test the

UV-sensor and quartz disc for solarisation. The experiment was originally to have been

terminated at this point. However, it was decided to continue through to 5 000 hours to

see if any other failure mode should exhibit itself. As a result, the alignment of the

sensor was slightly altered past the 3 000h mark, and the data were scaled to

compensate for this. Due to the minimal loss of output between 3 000 hours and 5 000

hours, it was judged unlikely that any detectable levels of solarisation or loss of

sensitivity in the UV sensor occurred in this period. At the conclusion after 5 000h of

virtually continuous operation, the UV-C LED was found to be still operational.

Between 3 000h and 5 000h, the output was only reduced from 68% to 65% of its initial

output, far surpassing expectations based on datasheet and published literature (250h to

50% output).

For comparison, emission trend data points without extrapolation from papers

referenced in Table 12 with similar temperature conditions were plotted together with

the temperature-compensated hourly-average data from this study to place the results

into context (Figure 35).

These results appear to represent the longest measured continuous full-rated-current

operating time for a UV-C LED reported, and is the one of only two published tests of

commercially available UV-C LED of the literature surveyed (Sun et al., 2009). The

LED exhibited an approximately logarithmic decay of the output power, which is

sometimes described as biphasic in other literature reports. Despite temperature

compensation, diurnal variations in the output occurred. These, however, reflected the

response of the thermocouple to ambient temperature changes and were unlikely to

represent inherent LED variance.

114

Figure 35 - Comparison of LED operating lifetime trends

The visible LED lighting industry uses L80, L70 and L50 as their metrics of lifetime,

corresponding to a lumen maintenance of 80%, 70% and 50% respectively (ASSIST,

2005). However, as lumens are not applicable to LEDs outside the visible wavelength

range, the radiometric power was used for comparison instead. The output power fell to

80% of the initial power at about 562h, and reached 70% by about 2 320h. If the

logarithmic trend continued, the L50 would be expected to lie at around 39 600h

provided other failure modes past 5 000h did not emerge. It is likely that even longer-

term use of UV-C LEDs may be possible provided the reduction in efficiency and

output is acceptable.

The output decline curve followed a similar path to that reported in literature where an

initial "break-in" phase with a rapid decline of power output, is succeeded by a much

shallower and lengthier period of slower decline. This pattern has been postulated to

occur due to an increase in non-radiative recombination in areas of the semiconductor

which have partially failed due to stress, leaving only the better quality areas as an

active emitter with a profile suggestive of a diffusion process (Shatalov et al., 2006,

y = -7.052ln(x) + 124.65

R² = 0.9696

0

10

20

30

40

50

60

70

80

90

100

0 1000 2000 3000 4000 5000

Percen

tage o

f In

itia

l O

utp

ut

(%)

Operating Time (h)

280nm, 20mA, Unpackaged, 25 C (Khan, 2006)280nm, 20mA, Packaged, 25 C, microPixel (Khan, 2006)280nm, 20mA, Packaged, 25 C, 100um² (Khan, 2006)280nm, 20mA, Packaged, 25 C, 200um² (Khan, 2006)295nm, 20mA stress, 10mA measure (Meneghini, 2008)280nm, 25mA (Reed, 2008a)280nm, 100mA (Reed, 2008b)280nm, 20mA, 25 C (Reed, 2008b)280nm, 100A/cm² (Khan, 2009)Unspecified, 20mA (Shatalov, 2009)280nm, 400mA, 880um² (Shatalov, 2009)270nm, 20mA (This Study)

This Study

115

Meneghini et al., 2008a). Overall, the emission decline appeared to follow a logarithmic

decrease, especially after the initial break-in phase (R2 = 0.9696).

As the LED had not experienced catastrophic failure at the termination of the

experiment, this suggests that the quality of the film and the geometry of the

manufacturing process have greatly improved since the earlier literature was published.

Should this prove true generally, there is a potential that the UV-C LED could remain

useful for a much longer operational period than initially expected, albeit at a lower

emission power and with reduced efficiency especially at low currents. In disinfection

systems, inactivation of bacteria generally correlates with the dose, which means that

such aged LEDs will require longer times to achieve equivalent disinfection. However,

the LED was clearly not rendered ineffective after the nominal lifetime of 250h had

expired.

The raw reading data with dark current voltage subtracted was also plotted without any

temperature compensation, with the temperature plotted on a secondary axis (Figure

36). This illustrated the high sensitivity of UV-C LEDs to their operating temperature,

as increases in the ambient temperature correlate with dips in the measured UV output.

Significant change points in the experiment data have been marked on the chart

indicating the first few hours without thermocouple readings, the use of air-

conditioning, the dismantling and re-assembly of the system and data loss events.

Data-loss events generally correlated with unexpected power outages due to severe

weather and resulted in the LED test apparatus being powered down. When restarted,

the LED is in a cool state, resulting in higher than expected output readings when

powered on. The LED settled back into its regular degradation regime soon-after, thus

indicating that turning the LED on and off does not appear to have any significant

impact on LED lifetime.

A sizeable drop in the raw intensity (voltage) outputs is seen after 3 000h due to the

dismantling and reassembly of the system. As the sensor does not contain a cosine

corrector, its alignment and angle relative to the source affects the absolute reading.

This change in reading is believed to be solely due to a slightly different alignment, and

116

was compensated for in Figure 35 by deriving a compensation factor for the 3 002h

reading versus the 3 000h reading assuming no degradation had taken place.

Figure 36 - Non-temperature-compensated output and temperature trend

Measurement of the dominant wavelength (DWL) and full-width half-maximum

(FWHM) were undertaken, with the pre and post 3 000h experiment results showing a

slight reduction in peak wavelength and a minor widening of FWHM presumably due to

ageing (Table 14).

Table 14 - UV-C LED characteristics versus ageing

Characteristic Pre-Experiment (nm) at

0h

Post-Experiment (nm)

at 3 000h

DWL 277.3 275.92

FWHM 9.88 10.94

5.3.4. Confirmation of Thermal Operating Conditions

To determine whether the externally mounted thermocouple was providing

representative ambient temperature readings of the experiment assembly, a total of

0

10

20

30

40

50

60

70

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 1000 2000 3000 4000 5000

Tem

peratu

re ( C

)

Raw

Volt

age -

Dark

Cu

rren

t V

olt

age (

V)

Operating Time (h)

Voltage

Temperature

System dismantled

and reassembled

Air-conditioning

in use

Significant data

loss events

Thermocouple in-use

117

14 009 paired temperature readings were taken over a period of 26 hours on a

representative day (13th September 2015) with no air conditioning and compared. The

mean difference between the temperature in the outer container versus the room was

1.12 ± 0.93 (S.D.) °C, with a range of 4.40°C. Due to the thermocouple error

specification of 1.5°C ± 0.25% and the meter error specification of 1°C ± 1%, only

0.98% of the readings showed any difference outside that of instrumentation error, to a

maximum of 0.55°C. I thus concluded the difference in the temperature measured by a

thermocouple placed outside the container in the room compared to one inside the outer

container was trivial and the ambient temperature readings could be validly used for

correction purposes.

The distribution of temperature differences was examined (Figure 37) and was found to

be a bimodal distribution with most readings lying in a range of -1°C to +3°C, (mode

+1.8°C). The asymmetry may be explained by calibration differences between the

multimeters used for recording, and the dual peaks reflecting a slight time lag between

the temperature inside and outside the container and equilibration in response to

significant temperature swings during the morning and evening (Figure 38).


temperature reading delta histogram

-2 -1 0 1 2 3 40

200

400

600

800

1000

1200

Temperature Difference (°C, Inside outer container - Ambient)

Num

ber

of

Readin

gs

(out

of

14,0

09)

118


temperature reading delta scatter plot

5.3.5. Quartz Transmissivity

Solarisation is a potential concern as quartz windows are required for UV-C operation,

and lie in the optical path of sensors, LEDs and windows. The percentage difference in

transmission in exposed versus unexposed areas of quartz after 3 000h is shown in

Figure 39. For wavelengths longer than 270nm, the transmission was essentially

unchanged. It was also evident that there was a slight perturbation in transmission

resulting in a 2.2% gain of transmission peaking at 245nm and a 1% loss in

transmission peaking at 215nm. This may be attributed to measurement errors due to

surface damage, light source instability or contamination. As the window tested was

6mm thick, it was expected that it would be more vulnerable to solarisation than the

<1mm thick windows incorporated in most LED packages. The degradation was

assessed as small compared to the decline in UV-C LED output due to age and variation

due to temperature fluctuation.

y = 0.9126x + 1.1588

R² = 0.671222

23

24

25

26

27

28

29

22 23 24 25 26 27 28 29

Am

bie

nt

Tem

per

atu

re R

ead

ing

( C

)

Temperature Reading Inside Outer Container ( C)

119

Figure 39 - Quartz loss of transmission after 3 000 h exposure to 270nm UV-C

LED

Where mercury based UV lamps are employed for disinfection, solarisation is a major

cause of declining lamp output (Heath et al., 2013). Datasheets for common 36W LP

tubes, indicate the 14.6W of UV-C output generates an envelope flux of 15mW/cm2 by

calculation (GE Lighting, 2006).

Thus where glass or plastics are used with UV-A, attention to spectral absorption may

be necessary to avoid severe impacts on those materials (Singh and Sharma, 2008). For

UV-C, quartz is most commonly used, with a cost that is relatively high at US$39 per

75mm diameter x 6mm round, and hence a POU cost consideration. Further, quartz is

produced in varying purities, with highly solarisation resistant synthetic quartz/fused

silica having higher initial cost, but being potentially more cost effective being more

durable and solarisation resistant. Lamp sleeves of domestic under-sink LP tube based

disinfectors, where lamps continually operate, are known to have UV-C transmissivity

levels of around 60-68% (Shah, 2009).

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

200 250 300 350

Lo

ss o

f T

ran

smis

sio

n (

%)

Wavelength (nm)

120

No information on UV-C LED lens solarisation was identified from our literature

review. But chip size (≈1mm2) and 0.8mW output power indicate a flux at the LED

surface of ≈ 80mW/cm2 (Sensor Electronic Technology Inc, 2012). Using the spread

angle (120°) and depth (3.5mm), the front window of the TO-39 package would be

subject to an average flux of 1.2mW/cm2, an order of magnitude less than the LP tube.

However, this problem could resurface as efficiencies rise. The procured LEDs had an

efficiency of 1%, but the most efficient UV-C device reportedly has 11%, with further

improvements expected. Also, because of non-uniform emission, the LED chip’s central

area is likely to experience higher than average flux level and be more vulnerable.

Research into new LED encapsulation materials that are compatible with deep-UV

emissions is an ongoing area, with new materials offering potential benefits in better

UV resistance (Bae et al., 2016).

A different concern is damage to the LED substrate itself. Construction of deep-UV

LEDs involves flip-chip packaging on aluminium nitrate and sapphire substrates, which

results in the emitted light escaping through the substrate. This arrangement exposes the

transmission substrate to high UV flux. Currently it is unclear if UV absorbance may

also lead to bond breakages in the substrate and alter transmission characteristics over

time. If UV-C LEDs are to become comparably affordable, efficient and long lived to

UV-A LEDs the window and substrate component may influence their field deployment

in POU reactors.

5.3.6. Thermal Resistance and Self-Heating of LEDs

While LEDs are normally small (<1 cm2), practical emission chip densities are limited

by thermal and power constraints. LEDs are vulnerable to high temperatures, which

accelerate failure rates and reduce efficiency. Heat is normally dissipated by copper or

aluminium heatsinks. Passive cooling is reliable but dissipation is limited in many

applications. Performance can be improved using fans, but such systems are more prone

to mechanical failure.

The lifetime of semiconductor devices is impacted by their operating temperature. It is

clear from Reed et al. (2008b) that the operating temperature of an LED has a

121

significant bearing on the observed lifetime, with a linear regression lifetime to 50% in

1 420h at 25°C, 370h at 50°C and 100h at 75°C. Operation at continuous wave is simple

and maximises power output, but also maximises the self-heating of the device more

than intermittent pulsed-operation would.

Current UV-C LEDs also have high temperature co-efficient of power, resulting in

significant reduction of output power when operated at higher temperatures, namely a

15% power reduction for 10°C rise (Sensor Electronic Technology Inc, 2012). The

LEDs are supplied with a power output specification at 25°C, despite likely operating at

higher temperatures due to high ambient temperatures and difficulties with thermal

management.

The key temperature of importance is not the ambient temperature but the junction

temperature (Tj), though the two do interact. This is the temperature of the

semiconductor junction itself, and is not a directly measurable quantity. Depending on

how this film is packaged, the junction will operate at a higher temperature than its

surroundings due to thermal resistance from the junction to the case. In the case of low-

powered LEDs, often they are operated without heatsinks as they do not dissipate

significant amounts of power and a heatsink is not absolutely necessary to prevent

thermal run-away and destruction of the device. However, this adds a larger thermal

resistance from case to ambient to the equation. Where higher powered LEDs are

involved, a heatsink is necessary and the thermal resistance of the heatsink to ambient

plus the resistance between the case to heatsink through the thermal interface material

needs to be considered (Ohm, 1827).

Indicative values for thermal resistance for TO-18 and TO-39 (my devices) style

packages, as used by the UV-C LED industry to date, were obtained from Dokić and

Blanuša (2015) and ST Microelectronics (2003). Indicative values for thermal interface

resistance were calculated from the area of contact from dimensional diagrams and a

thermal interface material conductance figure of 200mm2°C/W (Narumanchi et al.,

2008). Heatsink thermal resistance was obtained from popular commercially available

units (Avnet Inc., 2015, RS Components, 2015). The projected temperature rise of the

junction and loss of output power with and without heatsink at different operating

122

currents for both case styles are shown in Table 15. This table was computed on the

assumption that heat production scales proportionally to drive current noting this

slightly overestimates the heating at lower drive currents due to a slight reduction in

forward voltage normally experienced when operating at lower currents.

Table 15 also computes the expected losses in power with an "infinite heatsink" where

all losses are solely internal to the package.

The smaller TO-18 package can be seen to have poorer performance due to its high

thermal resistance, with a projected full-rated current loss of output of almost 44% due

to accumulated self-heating when operated without a heatsink. The larger TO-39

package suffers about 18% loss in comparison. As a result, CW operation can be seen to

be especially difficult with best results obtained with larger packages and heatsinking.

Even when a heatsink is added, the TO-18 package is still at a distinct disadvantage due

to the high internal thermal resistance and limited contact area. Using the infinite heat-

sink case, the TO-18 package operating at full rated current loses 12.2% of rated power

due to internal thermal resistance alone, compared to just 2.93% for the TO-39 package.

Operating at very low current/duty cycles (1%) results in negligible (sub 1%) loss for

both styles of package.

123

Table 15 - Computed Tj rise and output power loss for different UV-C LED

operating conditions

LED Attribute a

Drive Current (mA)

0.2 2 c 5 10

c 15 20

b,c

Approx. Heat (W) 0.0010 0.0098 0.0244 0.0488 0.0732 0.0976

Package Type TO-18

Tj Rise w/o HS (°C) 0.29 2.93 7.32 14.64 21.96 29.28

Power Loss w/o HS (%) 0.44 4.39 10.98 21.96 32.94 43.92

Tj Rise w/HS (°C) 0.18 1.82 4.54 9.08 13.62 18.16

Power Loss w/HS (%) 0.27 2.72 6.81 13.62 20.43 27.24

Tj Rise inf. HS (°C) 0.08 0.81 2.03 4.07 6.10 8.13

Power Loss inf. HS (%) 0.12 1.22 3.05 6.10 9.15 12.20

Package Type TO-39

Tj Rise w/o HS (°C) 0.12 1.17 2.93 5.86 8.78 11.71

Power Loss w/o HS (%) 0.18 1.76 4.39 8.78 13.18 17.57

Tj Rise w/HS (°C) 0.06 0.65 1.62 3.25 4.87 6.50

Power Loss w/HS (%) 0.10 0.97 2.44 4.87 7.31 9.74

Tj Rise inf. HS (°C) 0.02 0.20 0.49 0.98 1.46 1.95

Power Loss inf. HS (%) 0.03 0.29 0.73 1.46 2.20 2.93

a: Calculations based on TO-18 package Rj-a 300°C/W, Rj-c 83.3°C/W, Rint 2.78°C/W,

Rc-a 100°C/W, and TO-39 package Rj-a 120°C/W, Rj-c 20°C/W, Rint 1.35°C/W, Rc-a

45.2°C/W, where Rj-a is the thermal resistance from junction to ambient for the package,

Rj-c is the thermal resistance from junction to casing for the package, Rint is the thermal

resistance contribution from the thermal interfacing material and Rc-a is the thermal

resistance from case to ambient of the attached heatsink. Infinite heatsink calculations

include only the heat rise as a result of Rj-c.

b: Drive current used in bench-scale UV-C and UV-B LED experiments.

c: Drive currents tested in warm-up loss experiments.

124

Experimental validation of these predicted power losses due to self heating was

performed and resulted in the findings shown in Figure 40. With my TO-39 package, at

20mA, the LED began at 118-131% of its 1-hour stabilised output, resulting in a power

loss range of 15-24% (expected 17.57%). Running at 50% duty cycle for a nominal

10mA average forward current, the LED output began at 110-112%, with a power loss

range of 9-11% (expected 8.78%). Finally, when running at 10% duty cycle for a

nominal 2mA, the LED exhibited slower-than-normal current regulation, resulting in a

delayed peak output of 102-106%, with a power loss range of 2-6% (expected 1.76%).

The measured figures were in close agreement with the calculated values, noting that

the losses were greatest for the first run of the day when the LEDs had longest to cool

down to ambient temperature.

Figure 40 - Experimental warm-up power loss versus time trend plot at 10%, 50%

and 100% duty cycle at 20mA drive current

100

105

110

115

120

125

130

135

1 10 100 1000

Percen

tage o

f 1-h

ou

r O

utp

ut

Pow

er (

%)

Time (s)

20mA 10mA 2mA

5 runs per current level

First Run of the Day

125

Another reason for the between-run deviation from the expected figures may also be

variations in average drive current during pulsed operation. Actual measured drive

currents were 3.029, 10.92 and 20.17mA as opposed to the 2, 10, 20mA expected. This

was due to errors in the duty cycle output from the microcontroller, coupled with turn

on overshoot and turn-off delay in the MOSFET and current regulator resulting in

excess current reaching the LED (Figure 41).

These findings show it is imperative that designers of POU UV-C LED based systems

take measures to minimise the temperature of the LED as far as practicable, and more-

so than is necessary with visible LEDs. There is also a need to account for all the loss of

UV power in ageing, optics, operating at higher temperatures, variations in current

supplied and from variations from LED to LED. These factors can easily compound to

require significant overprovision to meet required dosage under all conditions. For

example, operating at 45°C ambient means 30% loss of output (TO-18), adding 18%

loss due to no heatsink with up to 50% loss due to LED ageing. This may result in just

28.7% of the initial UV-C output being available at end of life (~40 000h). One

improvement would be the use of packaging better suited for interfacing to heatsinks to

ensure operating temperature is maintained as low as practicable to maximise lifetime

and output.

The problem of heating with UV-A LEDs, though less severe, still needs consideration.

To illustrate, a 100-fold more powerful (10W) LED Engin 365nm LZ4-00U600 UV-A

LED has an Rj-c of 1.1°C/W which reduces the case contribution to heating. Combined

with a large heatsink of Rc-a 1°C/W and a layer of thermal paste with contact area of

20mm diameter (314mm2) results in a total thermal resistance of 2.7°C/W and Tj rise of

27.2°C. This shows the difficulty in maintaining acceptable temperature rises at high

powers, even with proprietary customised packaging, and the difficulty in having high

LED density, pointing to a need to maximise the efficiency of the LEDs to minimise

excess heat generation at the emission source.

126

Figure 41 – Pulsed self-heating test duty cycle and current error due to turn-on transient, turn-off microcontroller and MOSFET delay

Intended Current Level (20mA)

Turn-on Transient Overshoot and Ringing

Intended Turn-Off Time (100μs) Microcontroller Duty-Cycle Error (+4.7μs)

MOSFET Turn-off Delay (+2μS)

Intended Turn-On Time (0μs)

Time (μs)

Curren

t (mA

)

127

5.4. Eye Safety

Intense light, especially UV-C, poses a potential eye and skin hazard. Visible light is

less harmful not only physiologically but also due to the blink reflex, which limits

exposure. The key exposure limit is defined in Australia by AS2243.5 (Australian

Standards, 2004) as ca. 3mJ/cm2 of 270nm radiation over a 24 h period. Relative risk

posed by other wavelengths is given as a “spectral effectiveness" scaling factor relative

to this wavelength. AS2243.5 was used to construct a plot of safe exposure doses as a

function of wavelength (Figure 42).

Figure 42 - AS2243.5 daily exposure limits vs. wavelength and curve fits

Tolerable exposure times for the bench-scale UV LED arrays (Table 6) were calculated,

assuming a point source with a 30° spread and distance of 30cm. The exposure limits

were 3, 43, 22 and 30 min for 270, 365, 385 and 405nm (400nm) respectively. These

are probably conservative, as these LEDs had wider spread angles and prolonged direct

viewing of the LED emissions is unlikely, further reducing damage potential from

accidental exposure. It does, however, illustrate that even small UV-C LED POU

y = 0.0006x2 - 0.356x + 49.422

R² = 0.9755

y = 0.1193x - 34.676

R² = 0.9953

y = 0.0162x - 1.4815

R² = 0.9997

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

250 270 290 310 330 350 370 390

log

10

Exp

osu

re L

imit

(m

J/c

m2)

Wavelength (nm)

≈UVC

≈UVB

≈UVA

128

devices need to incorporate safety features such as sensor-based kill-switches, despite

their low optical power.

5.5. Low-Cost UV Sensing

Due to the likely variation in UV-C output over the lifetime of the diodes, the need to

promote safety and the potential for catastrophic failure, there is a need to monitor the

UV dose simply and efficiently in order to ensure appropriate disinfection is taking

place and that the diodes have not silently failed. As human vision is not sensitive to

UV-C, and direct exposure to the LEDs is an identified safety hazard (Figure 42), other

methods of sensing must be explored.

The traditional method utilising SiC-based UV sensor photodiodes, as in Section 5.2.1,

is a reliable method which is simple, repeatable and offers a quantifiable reading which

directly relates to the amount of UV-C reaching the sensor. As the sensor is spectrally

selective, it is unaffected by sunlight. While it has many advantages, it represents a

significant cost compared with the total cost of the disinfection unit (Table 22) and

lower cost techniques may be useful for once-off checks.

It was theorised that inexpensive royal-blue LEDs could potentially be used as UV-C

photodiodes, though with less sensitivity and spectral sensitivity. This would exploit the

photovoltaic effect of semiconductor junctions, running an LED in reverse similar to a

solar cell, generating a photocurrent when a photon with larger energy than the

semiconductor band gap is absorbed. The only impediment identified was the plastic

lens encapsulation which prevents the transmission of UV-C to the semiconductor

junction. It was found that this plastic lens could be removed by simply disregarding the

manufacturer's advice to not apply any pressure to the lens during soldering. As a result,

a variant of the sensor circuit using a royal-blue LED as the sensing element (mounted

on the underside) was produced (Figure 43) for a component cost of US$6.

129

Figure 43 - Royal-blue LED based UV sensor

The circuit was modified to have a secondary op-amp as a voltage comparator, driving a

red output LED with trimpot level adjustment. This permitted the circuit to operate as a

low-cost qualitative sensor independent of expensive measurement equipment.

Operation entailed adjusting the threshold until the LED turned off, ideally in a dark

room, and then exposing the sensor to UV-C. If the LED turned on, then UV-C

radiation was assessed as being detected.

It was determined that this sensor had sufficient sensitivity in the UV-C region when

tested with the LP UV tube source, and with 270nm UV-C LEDs (Table 16). However,

it was not as spectrally selective, with voltage output being developed for light from

UV-C through to green wavelengths. Furthermore, with the bare package without

encapsulation, it is likely that atmospheric contaminants may affect the performance of

the sensor over time and it is not suitable for contact with liquid. However, as it is

suitable for detection of UV-C in a darkened room and has a cost of around US$6

compared to US$34 for the UV-selective sensor described earlier, it still offers a

potentially large POU cost saving.

130

Table 16 - Royal blue LED-based UV sensor output measurements

Distance from LP tube (cm) Sensor Output

(mV)

Comment

Dark 10 Dependent on ambient light

0 4994 Sensor saturated

30 293.9 Dependent on alignment

50 191.3

Single 270nm UV-C LED 26.6 <1cm distance

Even lower cost LED emission detection could be achieved with fluorochromes which

fluoresce under exposure to UV-C. Due to the limited output of the fluorescence, this

technique worked best under darkened room conditions. It was determined that

commercially produced tonic water (Schweppes, containing quinine), Vitamin B2

(Cenovis Vitamin B2 complex, containing riboflavin) and highlighter ink (Stabilo Boss,

multiple colours) all responded visibly to UV-C LED radiation at 270nm (Table 17).

This opens the potential for extremely low cost qualitative checking of UV-C LED

based POU disinfection device operation. A more permanent solution could be the use

of silica bead encapsulated fluorochromes (Li et al., 2011).

5.6. Conclusion

The UV-C LED lifetime experiments successfully characterized key operational and

engineering characteristics of a single commercially available 270nm UV-C LED

operating in a simulated POU environment. Most strikingly the LED achieved

successful CW operation through 5 000h, with output that diminished to 80% of the

initial power at about 562h, and 70% by about 2 320h. The emission decline followed a

logarithmic function, and the LED still retained 64.96% of its initial stabilised output

power after 5 000h. This result suggests, despite diminished power, current UV-C LEDs

might already offer useful disinfection capacity when operated for longer periods. This

longevity suggests that, if shown to be general, is a major consideration when assessing

the financial feasibility of POU applications (see Section 3.3.7). Other UV-C LED

operational characteristics had little change, e.g. a shortening of dominant wavelength

by 1.4nm and widening of full width at half maximum of 1.1nm. Overall, the data

131

suggests short term experimental results are a good guide to long term operation.

Theory and experiments aimed at understanding LED self-heating showed that

designers of POU UV-C LED systems should minimise the LED operating temperature

as much as practicable, in order to avoid loss of output from high junction temperatures

during CW operation, and to reduce likely loss of lifespan since degradation rates are

known to increase with temperature. Loss of output can be predicted using temperature

co-efficient of power equations and thermal resistance calculations. UV-C LED

manufacturers should endeavour to use packages with lower thermal resistance to

minimise self-heating losses from junction temperature rise.

The use of UV sensors appears to be highly desirable, and degradation of UV-C

selective sensors was not a significant issue. Low cost UV sensors are also a possibility

based on royal blue LEDs or common fluorochromes. However, such low-cost

approaches are only suitable for qualitative monitoring and on-line monitoring is still

best performed using traditional SiC based UV-C selective photodiodes. Additionally,

solarisation does not appear to be a significant issue with current UV-C LEDs, with a

maximum recorded transmission loss of 1% occurring at wavelengths other than those

used. Higher powered UV-C LEDs with longer lifetimes may see solarisation re-emerge

as a problem.

An assessment of UV-A LED lifetime and solarisation was not attempted in this study

owing to their lower costs and longer specification lifetimes which exceed the length of

a PhD program. It is known though that the lens on UV-A LEDs is predominantly glass,

which makes this a potential candidate material for UV-A windows.


A review of literature on UV-C LED lifetime revealed much uncertainty. Few

publications focused on commercially-available LEDs and long-term operation.

Papers did not address the impacts of solarisation of window material, thermal

configuration in self-heating losses, or safety and sensing requirements for POU

applications.

132

Continuous wave operation of a commercially-available UVTOP270 UV-C

LED at its full rated current for 5 000h was achieved with 65% of initial output

remaining, far surpassing expectations of 250-1 000h based on literature.

Solarisation was not observed at 270nm from 3 000h exposure to the UV-C

LED.

Thermal losses due to self-heating in UV-C LEDs were modelled using thermal

resistance theory and validated experimentally, demonstrating they are a major

consideration for prototype reactors.

Sensors based on UV-selective photodiodes, modified LEDs and fluorochromes

were successfully trialled allowing for low-cost UV sensing.

Eye safety factors were estimated using guidance from Australian Standards. It

was found that short accidental exposures were not harmful, although, more

powerful diodes will make effective control measures necessary, primarily in the

case of UV-C based units.

133

Table 17 - Fluorescence of common fluorochrome containing materials exposed to one TO-39 UV-C LED

Substance Unexposed Exposed

Tonic Water (Quinine)

Vitamin B2 (Riboflavin)

Stabilo Boss Highlighter

Ink

134

6. Design, Construction and Testing of Three Prototype

Deployment-Ready POU Systems

6.1. Introduction

In prior bench-scale experiments, the concept of LED-based disinfection was

established as technically feasible and better understanding of its reactor inactivation

characteristics (Chapter 3), enhancement possibilities (Chapter 4) and engineering

considerations (Chapter 5) was developed.

In order to bring the PV+LED concept into the real-world and realise functional POU

devices, this chapter directly addresses the practical engineering aspects of designing

and constructing a deployment-ready disinfection system by describing the construction

and performance of three possible designs.

The most promising candidate LED wavelengths of 270nm and 365nm were selected

based on bench-scale trials, and consideration of configuration variables along with

their trade-offs was tabulated. From this, three different systems reflecting illustrative

subsets of reactor configuration possibilities were conceived. Construction strategies,

schematics, circuit theory and bills-of-materials costing were developed, and the

disinfection performance tested under simulated field conditions with E. coli K12 and

compared to that predicted from the model reactor experiments. The choice of E. coli

K12 was due to its wide use as a standard indicator micro-organism. For logistical

reasons, experiments were not conducted with E. faecalis as the relationship between E.

coli K12 and E. faecalis ATCC 19433 dose requirements were previously established in

Chapter 3.

135

6.2. System Specifications and Design Rationale

This section details the rationale behind the selected prototype disinfection units, and

outlines their key specifications, schematics and construction strategies.

6.2.1. Design Variables and Rationale

The first stage of designing the prototype unit involved developing the specifications for

the units themselves. Through the literature review (Chapter 2 and Lui et al. (2014)) and

experience garnered through the construction of the bench-scale prototype unit

(Chapters 3, 4 and Lui et al. (2016)), a variety of design variables were documented as

needing specification when building a system. The variables and their impacts are

itemised in Table 18.

136

Table 18 - Design considerations in POU device construction

Design

Variable

Design Aspects Key Considerations Constraints

LED

Wavelength

and Optical

Power

Disinfection

performance -

disinfection

volume and time.

Efficiency of the

device.

Lifetime of the

device.

Path length

achievable.

Cost of the LEDs.

UV-C LEDs provide more rapid

disinfection and at low power

input, minimising PV and wiring

costs.

UV-A LEDs are less affected by

absorbance of water and natural

organic matter and can achieve

higher path lengths. They also

generally have longer lifetimes and

are commercially mass produced.

Wavelengths of 365nm and shorter

may allow for enhancement by

photocatalysts (e.g. TiO2).

Pathogen inactivation sensitivity

described by action spectra may

show less difference at UV-A

UV-C LEDs have potentially short lifetimes, high costs,

low electrical efficiency, low per-unit power and high

sensitivity to operating temperature. Because of the

harmful wavelength, it should be used with safety

devices which can increase cost. Use of quartz

windowing is necessary due to most materials being

opaque at UV-C wavelengths and is an additional

expense.

UV-A LEDs require significantly higher input power

and/or longer exposure times to achieve comparable

level of disinfection. This leads to higher costs of PV,

possible need for battery and increased wiring costs.

Large arrays of LEDs can suffer from reliability

problems when LEDs fail open or short circuit.

Both types of LEDs require good thermal management

to maximise output and lifetime. Higher levels of heat

137

Design

Variable


compared to UV-C. are generated with larger UV-A arrays. Using heatsinks

with fans pose reliability problems due to mechanical

parts.

Current

Driver Type

Regulates current

to LEDs.

Efficiency of

device.

Switching converters are highly

efficient, commonly 85-94%

efficient in power conversion

resulting in more flexibility in

voltages and lower power

demands.

Linear regulators are

straightforward to implement and

are robust against a wide range of

voltages, and are quite low-cost.

Resistor based drivers are very

cheap and simple.

Switching converters are more complex to make, with

modules being expensive and potentially prone to

failure as capacitors fail. They can also introduce radio

frequency interference which can interfere with

reception. They can be vulnerable to surges in voltage.

Linear regulators often result in poor efficiency

depending on the difference in input and output

voltages, as the difference is dissipated as heat, making

them unsuitable for high current/high-power LED

applications.

Resistor based current regulation is not suitable where

input voltage is not fixed as the output current will vary

depending on input voltage. It also shares similar

efficiency and suitability problems with linear

regulators.

138

Design

Variable


Disinfection

Volume &

Pumping

Optical power

requirement.

Reactor geometry

and materials.

Pump or agitator

requirement.

Pumped systems generally achieve

better results due to better

intermixing and oxygenation,

resulting in more even UV dosing.

Pumps consume significant amounts of energy and may

require their own controller/maximiser to get started

under load conditions.

Pumps are mechanical and are likely to lead to

reliability problems in the long term.

Pumps add cost to the system.

System

Controller

Allows the system

to make more

sophisticated

control decisions.

Improved versatility with

possibility for timed, UV-sensing

control regimes.

Better feedback could be provided

to the user (e.g. LEDs, LCDs,

buzzers).

Enhancement due to pulsing can be

performed.

Modifications to the operating

regime can be added after initial

construction.

Reduction in reliability through introduction of

complexity in both more sensitive hardware and

program code which can be prone to bugs.

Additional costs which may not be justified where

similar results can be had with analogue or simple

digital logic electronics.

139

Design

Variable


Sensor Allows for

verification of safe

positioning of

device.

Allows for

verification of

actual output and

detection of dose

as additional

safety against

turbid water or

LED failure.

A microswitch safety switch is

simple, low-cost and does not

require contact with water.

When wired in series with the

power supply, it does not require

additional control logic to detect its

position value.

Liquid sensor less easily defeated

and moderate cost.

UV-selective sensor allows

continuous monitoring and

verification of UV-LED output.

Microswitch safety switches is easily defeated, does not

verify actual output of LEDs and is mechanical.

Liquid sensor requires specific electrodes to ensure they

will not corrode, can be affected by biofilm or dirt

causing false negatives and user frustration and requires

contact with water and could cause cross-

contamination.

UV-selective sensors more costly than other types of

light sensors and require more electronics to produce

readable signals.

140

Design

Variable


PV Panel

Sizing

Provides main

source of power.

No moving parts, silent, low-cost,

proven reliability, widely

commercially available.

Can be oversized to provide margin

against some weather, although

small panels may suffice for low-

wattage UV-C LEDs.

Energy production dependent on weather conditions.

Limited efficiency, can require large areas to collect

sufficient energy needs.

Larger panels can be difficult to transport.

Can be damaged by impact force due to containing

glass.

Battery

Requirement

Provides

alternative source

of power from

stored energy.

Ability to use unit at any time

including cloudy periods and at

night.

Potential to use year round and

outside of tropical latitudes unlike

SODIS systems.

Significant additional costs in the batteries which have a

finite lifetime and requirement for charge controller

electronics to maximise lifetime.

Requirement for additional PV panels to supply energy

to charge batteries.

Environmental hazard in disposal.

Increased bulkiness and weight making transportation

more difficult.

141

Design

Variable


Reactor

Structural

Materials

Used to construct

the reactor where

disinfection will

take place.

Affects lifetime of unit where

materials (especially plastics)

degrade over time.

Construction difficulty as some

materials require special tools to

work with.

Enhancement potentials - e.g.

reflection, total internal refraction,

path length/size, LED direct

mounting.

LED window optics need to remain

transmissive at desired wavelength

over long periods.

Some of the more suitable materials are costly and

difficult to work with.

Consideration needs to be made as to availability in

remote areas.

LED windows may solarize depending on the purity of

the material.

LED windows may represent significant cost where

made from quartz, and could be fragile.

142

With so many variables, the space of possibilities for PV-LED based POU disinfectors

is necessarily large. Instead of attempting to address all possible variations, three

distinctly different prototype systems were specified in order to illustrate the flexibility

of such systems. The specifications of these systems are summarised in Table 19.

Table 19 - Final characteristics of deployment-ready prototype systems

Parameter System A System B System C

Wavelength (nm) 270 270 365

Optical Power (mW) 1 8 5400

Number of LEDs 1 8 3

Input Electrical Power (W) 0.3 3 71

PV Power (W) 2 10 80

Hydrodynamics/Mixing Hand-

agitated

Pumped

0.3L/min

Pumped

7L/min

Microcontroller Yes Yes No

UV Sensor No Yes No

Battery Backup No No Yes (432Wh,

SLA)

Volume Treated (L) 0.25 10 15

Reactor Volume (L) 0.25 1 10

Approximate Path Length (cm) 15 20 150

System A was designed for rapid disinfection of small volumes for direct consumption.

The concept mirrored the Steripen™, a hand-held battery-operated single-bottle 90-

second cycle disinfection device intended for occasional recreational use, with the

advantage of not requiring consumable batteries (Power-Sonic Corporation, 2009). It

also emulates the low-volume throughput seen in SODIS-based systems. Systems B and

C were designed to serve household needs (e.g. drinking, vegetable washing, etc.), with

processing volume similar to the water containers used in developing nations.

The choice of wavelengths was made based on results obtained during bench-scale

experiments. UV-C at 270nm was favoured for its quick disinfection ability and low

143

power consumption. Maintaining the choice of 270nm allowed for reuse of the existing

five-LED 270nm array from the bench-scale experiments in order to reduce costs and

allowed exploration of its performance both when scaled up to 10L volumes (System

B), and when operated as a single-LED disinfector for single-glass 250mL volumes

(System A).

A UV-A system (System C) was also explored despite its drawbacks of greater heat

production and energy consumption, as the LEDs are widely commercially available,

more mature LED technology, pose less safety hazard and can better penetrate through

water (Morris et al., 1995, Davies-Colley and Vant, 1987). The choice of 365nm

resulted in lower optical power than possible with 385nm or 405nm. However, as such

mass produced LEDs are often sold on the shortest wavelength of the bin (Figure 8), it

was necessary to choose a shorter wavelength to ensure that potential for photocatalytic

enhancement is not excluded (although it was not used in this experiment). An

additional benefit emerged after the construction of System C. The latest generation of

365nm LEDs were found to have an array power of approximately double of those used

in the bench-scale experiments and were available at half the price.

To demonstrate the use of a microcontroller based system to monitor the disinfection

process, control the LEDs/pump and provide feedback to the user, System A and B both

utilised Freetronics (2015) Leostick Arduino (2016) compatible microcontrollers,

complete with buzzer and indicator LED feedback. These were chosen due to their low

power consumption, robustness, low-cost, compact size and flexibility in configuration.

The design and programming software are free, open-source and based on standardised

Wiring programming language, a C-derivative with software code provided in Appendix

B.

Additional safeties were incorporated into System A through the inclusion of a

hardware microswitch which interrupted the power supply, rendering the unit

inoperable when not placed on top of a suitable container. Such a system is also more

robust than using a switch input to a microcontroller, which could potentially fail to

interrupt the UV output due to microcontroller hangs or software bugs. Safeties in

System B included a UV sensor (SGLux AG38S-TO SiC photodiode, MCP6273 rail-to-

144

rail operational amplifier configured as a transimpedance amplifier - for detailed

schematic, see Section 5.2.1). This was interfaced to the microcontroller in a dual-stage

non-inverting op-amp amplifier configuration (Sedra and Smith, 1982).

System A was operated using a timed disinfection principle, where each disinfection

cycle lasted three minutes. The status of the disinfection was indicated by LEDs on the

side of the unit, and a buzzer which would indicate when sufficient power was

available, when disinfection was taking place, and when the cycle was completed and

the container needed to be manually agitated. The PV input voltage was continually

monitored, and if it fell out of specifications due to cloud cover or shadowing, the unit

was designed to cease to operate and signal an error condition. A button was provided

for the user to start (or restart in the case of an error condition) the disinfection cycle or

perform further disinfection.

System B operated continuously, with LED and buzzer alert to warn users when

measured UV or input voltage was insufficient to maintain safe disinfection. This was

essential as the system contained a low-powered low-flow pump which shared power

with the LEDs.

System C was conceived without microcontroller feedback as it contained a battery and

was much less likely to need any supervisory control as the battery provided a more

stable source of power. The battery also enabled the possibility for any-time

disinfection. A charge controller protected the battery and indicated problems to the

user (e.g. battery over-discharge, output short circuit, etc). The increased safety of UV-

A and its visibility obviated the need for a discrete sensor. Instead, System C relied

solely on a hardware power switch for control.

Both System A and System B did not include a battery, and thus they were more

vulnerable to weather influences. In order to alleviate this to some extent, the size of the

panels used to power the units were designed to be several times larger than actually

necessary, so it was possible to operate them under partly cloudy conditions or at low

solar elevations.

145

While both System B and System C were targeted at household needs, System C

utilised a much more powerful high-flow pump to ensure more thorough mixing of its

full reactor and storage container volume, via higher flow rates, and was designed to

exploit UV-A's potential to penetrate deeper into water by incorporating a long path

length. As the pump required full immersion for safe operation, it was necessary to have

a larger 15L volume as opposed to a 10L volume for System B. System B did not

employ such a large reactor, as UV-C is much less able to penetrate deeply into water.

All UV LEDs and their associated electronic components were chosen based on the

bench-scale models detailed in Section 3.2.1.

Prior to the construction of the devices, a prediction of the disinfection performance

based on bench-scale model performance and action spectra were made (Table 20). The

predictions were made using total energy per volume basis only, while considering the

optical energy that a volume of liquid was exposed to for the time stated in the

condition, and did not take into account reactor geometry, reflective enhancement, path

length or absorbance. On the whole, it predicted potentially very good performance of

UV-C based systems, surpassing the required levels of disinfection with marginal

performance at UV-A. Another prediction could be made using the doses determined by

the action spectra, and average fluence levels based on reactor geometry without taking

absorbance, reflection and scattering into account. These figures provide a performance

upper bound.

The total dose in the case of the UV-C systems approached the USEPA target of

40mJ/cm2.

The total dose provided by the UV-A LEDs was approximately 6 MJ/m2. This is

comparable energy-wise to S90 doses of sunlight (Table 2, Davies et al., 2009) with the

difference that it is in the form of UV-A which is a more potent (S90 = 0.25MJ/m2)

disinfectant of E. coli than natural sunlight on average (ca 3-20 MJ/m2) .

Calculations for System A used a 250mL tall beaker with dimensions of 6cm diameter

and 10cm height. The maximum cross sectional area was 28cm2 with a total irradiance

146

of 1mW resulting in a flux of 0.0354mW/cm2. After 12 minutes of exposure, the total

theoretical delivered dose was 25.5mJ/cm2. Using the S90 figures from Table 8, this

resulted in a predicted 44-log10s reduction.

System B used a reactor with a volume of 2L and a dimension of 12.5cm diameter and

17cm height. The maximum cross-sectional area was 212.5cm2 with a total irradiance of

8mW resulting in an estimated flux of 0.0376mW/cm2. After 60 minutes of exposure,

the delivered dose is 135.36mJ/cm2 into 1/5th of the total 10L treated volume. The

equivalent dose was therefore 27.1mJ/cm2 which potentially should have achieved

approximately 47-log10s reduction (assuming full penetration of UV through the whole

water column).

Finally, the calculations for System C focused on the reactor with a 9.5L volume and

dimensions of 9cm diameter and 150cm height. The maximum cross-sectional area was

63.6cm2 with a total irradiance of 5400mW resulting in a flux of 84.9mW/cm

2. After

180 minutes of exposure, the delivered dose was 917J/cm2 into 9.5L, with equivalent

dose of 581J/cm2 into 15L. The predicted log reduction was thus 23-log10.

Table 20 - Prototype device E. coli K12 log10 reduction performance predictions

and achieved results

System Condition On Total Energy

per Volume

Basis (logs) a

By Action Spectrum

(logs) b

Measured in

Trial (logs)

A 250mL/12m 20 44 4

B 10L/1h 25 47 2.25

C 15L/3h 1.27 23 5

a: Based on applying LED optical input energy per volume rates determined from the

bench-scale experiments in Chapter 3, b: Based on action spectrum determined in

Section 3.3.6.

The wide range of predicted log-reduction values stems from the simplifying

assumptions used in their calculation. It is assumed that the total input energy is coupled

into the system, resulting in an average fluence that remains constant over the largest

147

surface area of the reactor. This assumption is unlikely to hold true in reality due to the

layout of the LEDs, the absorbance of the water, the dissipation of energy at the reactor

walls, the fact that the water is intermixed and the flow hydrodynamics of the reactor

which may provide areas of lower fluence which lower net inactivation rates. These

unrealistically high values are again seen when using action spectra to determine

SODIS inactivation in Chapter 7.

6.2.2. Construction Features

The systems were designed to be robust and simple to construct with components that

are widely commercially available and mostly non-specific allowing for component

substitutions, easing the construction challenge for rural and remote communities.

All electrical components and batteries were sourced from multinational electronic

component distributors. Electrical circuits were constructed on vero (strip) board

intended for rapid soldered prototype construction, without the need to produce a

printed circuit board (PCB) first. The circuitry was enclosed in weatherproof plastic

boxes, with either PVC-coated wire wrapped in UV-resistant electrical tape or UV-

resistant PV-rated wire used for external connections. Silicone sealant was used to

secure boards into their enclosures, seal wire entrances to the enclosures, and to provide

additional support for wire connections to boards (Figure 44).

148

Figure 44 - Prototype System A internals

Electrical power for the low-flow pump (System B) employed a commercially-supplied

pumping maximiser circuit. The components were of unknown origin and configuration

due to being potted in a solid waterproof compound (Figure 45), which was retained.

The Primus-branded high-flow camping shower pump required 12V DC at 3A, which

was supplied from the 24V DC PV system via a commercially available voltage

converter intended for usage in a truck. The necessity to use a 24V DC PV system was

due to the buck-type current drivers which necessitated an input voltage at least 2V

greater than the LED voltage (16.5V). This provided enough margin to ensure operation

through to a completely discharged battery (21V).

149

Figure 45 - Commercial low-flow pumping maximiser circuit (System B)

To overcome potential reliability problems from using a large LED array in System B,

the LED array was powered by eight independent current driver channels run in parallel

(Figure 46). This permits any diode to fail either open or short circuit without any

consequence to the operation of the other LEDs. By contrast where LEDs are arranged

in a series configuration to save current driver costs, or in the unsafe parallel

combination this can shorten LED lifetime, and cause all other diodes to fail to operate

at once given an open or shorted condition. The downside is slightly increased cost and

complexity. The current regulators were also chosen to be ON Semiconductor

NSI45020AT1G 20mA linear regulators as opposed to the less costly and commonly

used resistors to ensure dosage consistency even with variations in input voltage and to

ensure safety of the LEDs in case of higher transient voltages being applied (e.g. by

users modifying their devices to run off truck batteries). More efficient switching

converters were rejected on the grounds of increased cost and complexity, sensitivity to

voltage transients, production of unstable "ripple" current outputs and potential radio

interference. As LEDs represent the largest cost in the system, it appeared prudent to

take all efforts to protect them.

150

Figure 46 - Photograph of prototype System B controller

The physical reactors (Figure 47) were constructed with commercial off-the-shelf

supplies available at hardware stores including screws, brass plumbing connectors,

flexible plumbing hose, food-grade vinyl hose, 90mm diameter PVC stormwater pipe,

solvent glue, Teflon tape, solder and potable water grade silicone sealant. The pumps

used were obtained from commercial solar fountain pumping kits and camping showers.

The body of UV-C reactor (System B) was made from a recycled coffee tin (Figure 48).

151

Figure 47 - Labelled photograph of the three deployment-ready prototype systems

152

Figure 48 - Coffee tin used for System B reactor

Only basic hand-tools and power tools such as a hacksaw, screwdriver, tapered reamer,

caulking gun, Stanley knife, soldering iron and a cordless drill were used in the

construction process.

During the design process, it was envisaged that UV-C systems could avoid the need for

a quartz window and save cost through exploiting the fact that SETi UVTOP packages

are hermetically sealed. Alternatively, letting the LEDs directly contact the water may

provide further benefits in reducing the operating temperature of the LED, increasing its

output and improving its lifetime without any additional costs, especially when the high

specific heat of water is considered which results in a near "infinite heatsink". Likewise,

mounting them onto a metal-based reactor can further help to increase the effect,

allowing for the reactor to perform as a heatsink. This was exploited by drilling holes,

reaming them to size and sealing the LEDs in place with silicone (Figure 49). A similar

effect could be exploited for non-hermetically-sealed LEDs through the use of heat-

pipes to carry heat from the LEDs to the reactor for its eventual disposal.

153

Figure 49 - UV-C LEDs mounted to reactor without window

The LEDs were arranged in System B to try and improve the distribution of flux within

the reactor by staggering sets of LEDs. The arrangement of the LEDs and sensors is

shown in Figure 50.

Figure 50 - System B reactor mechanical configuration

A similar effect was achieved for System A by mounting the LED at the end of a 7cm

piece of 6mm food-grade vinyl hosing using silicone sealant (Figure 51). This permitted

Water In

Water Out

UV-C LED (in front)

UV-C LED (behind)

UV-C Sensor

154

the LED to be submerged in the water, while remaining waterproofed, thus using the

water as a heatsink (Figure 52).

Figure 51 - System A LED stem design and safety microswitch

Figure 52 - Photograph of System A in operation with LED indicators

155

A potential synergy exists, where raising the temperature of the water could also help

improve disinfection through pasteurisation effects, although this is unlikely to be

useful with UV-C LEDs due to their low power.

As UV-A LEDs employed with System C were not hermetically sealed and were not

capable of being submerged, the design instead employed an end-cap which was drilled

out. The opening was covered with a piece of UV-transparent Perspex and sealed with

silicone to provide a window. The LEDs were mounted on a conventional heatsink

using 3mm screws and thermal grease with no fan cooling, and wiring was covered in

silicone to provide additional splash-protection.

Figure 53 - Photograph of System C components

6.2.3. Device Schematics, Circuit Theory, Bill of Materials and Costing

The schematics detailing the electrical connections for the three prototype units are

shown in Figure 54, Figure 55 and Figure 56. The systems vary significantly in

complexity, with System B containing more discrete parts than System A, whereas

System C relies more on commercially available modular components.

The length of this thesis precludes a full discussion and explanation of electronic circuit

theory. This is adequately explained by numerous electronics textbooks, including

Microelectronic Circuits by Sedra and Smith (1982) and The Art of Electronics by

156

Horowitz et al. (1989). Instead, a brief overview of significant components and their

role within the system is presented below.

Figure 54 - Schematic for prototype System A

System A can be thought of as a simplified version of System B. Safety is achieved by

having the microswitch interrupt power to the whole apparatus when lifted away from a

suitable container. A 1N4004 diode is used to protect against reverse polarity

connections, to prevent damage to the power supply, LED, MOSFET and

microcontroller. Two 1000μF capacitors provide short-term energy storage to ensure

proper microcontroller operation. The LM7805 is a robust linear regulator used to

derive 5V for the microcontroller from a variable PV input. In order to gauge whether

the solar input is sufficient to run the apparatus, power monitoring is achieved using

analog input A0 connected to a voltage divider consisting of a 6.8kΩ and 1kΩ resistor

to scale the voltage down to a safe level for the microcontroller. A MOSFET

(STP22NF03L) is used as a switch, controlled by digital output 6 on the

microcontroller, with the NSI45020AT1G regulating the current to the LED. Two 1kΩ

resistors help to protect the microcontroller in case of MOSFET failure, and bleed

excess gate charge to ground during switching off of the MOSFET. Microcontroller

digital inputs 2 and 3 are used to sense the push-button state, and digital output 4 and 5

are used to drive two indicator status LEDs directly, with two 560Ω resistors to regulate

the current to these LEDs as their exact operating current is not as important as

compared with the UV-C LED. The Leostick also has an internal piezoelectric buzzer

LM7805

In GND Out

1N4004

10µF + 12V +

-

+ 1000µF 1000µF

+

Freetronics

Leostick

Vcc

GND

2

A0 3

4

5

6

UVTOP270

NSI45020AT1G

STP22NF03L

Start Button

Microswitch

Red LED Green LED

560Ω 560Ω 1kΩ 1kΩ

1kΩ

6.8kΩ

157

which is connected internally to digital output 11, serving as an audio feedback to the

user.

Figure 55 - Schematic for prototype System B

System B follows a very similar configuration to System A, with the exception that the

number of LEDs has increased with all LEDs connected to their own current drivers, all

connected in parallel with a pump unit. The number of status output LEDs has also been

doubled, allowing for independent indication of voltage status from the PV panel and

the UV sensor status. In order to achieve dose sensing, a two-stage amplifier is used,

with the first stage essentially unchanged from Figure 31. The second stage is an

adjustable-gain non-inverting op-amp amplifier which serves to scale the small voltage

from the first stage into a larger voltage which is easier for the microcontroller to

measure more accurately. This is connected into analog input A1 on the microcontroller.

LM7805

In GND Out

1N4004

10µF + 12V +

-

+ 1000µF 1000µF

+

Freetronics

Leostick

Vcc

GND

A1

A0 4

5

6 7

8

UVTOP270

NSI45020AT1G

STP22NF03L

Power switch

Red LED Green LED

560Ω 560Ω 1kΩ 1kΩ

1kΩ

6.8kΩ

Red LED Green

LED

560Ω 560Ω

M Pump

MCP6273

-

+ AG38S

Vss EN

Vdd

8.2MΩ

10nF

MCP6273

-

+

Vss EN

Vdd

1kΩ 100kΩ

158

Figure 56 - Schematic for prototype System C

System C is a straightforward integration of commercially available modules. The

Victron Energy charge controller contains the necessary logic to manage the charging

and safe discharging of the battery including overload, overcharge and over-discharge

protection. The output is connected to XP Power LDU560S700 current driver modules

which each drive one LED package (containing four dies in series). The dimming input

is left unconnected as it is not required. Likewise, the output is also connected in

parallel with a 24V to 12V converter that powers the camping shower pump which

circulates water through the reactor. Fuses were added to this system due to the higher

current which poses a safety risk, and to meet charge controller manufacturer

requirements.

These three systems were all designed for direct operation on DC, which is unlike some

earlier attempts at PV-powered UV-tube based water disinfection. This confers

additional benefits, as smaller systems below the size of mains-operated units can be

produced. Furthermore, from an electrical efficiency standpoint, the systems can be

more efficient by having only one stage of conversion/loss in the current driver (~50%

efficient for linear models, 90% efficient for switching models) rather than losing on

12V +

-

12V +

-

Victron Energy Charge

Controller & Switch

PV Battery Load

+ - + - + -

12V 18Ah SLA

12V 18Ah SLA

+

- +

-

T4A Fuse

LDU5660S700

+in +out

-in -out

dimming in

LDU5660S700

+in +out

-in -out

dimming in

LDU5660S700

+in +out

-in -out

dimming in

24V to 12V

+in +out

-in -out

T5A Fuse

T4A Fuse

M Pump

+ 470µF

LED Engin

LZ4-00U600

LED Engin

LZ4-00U600

LED Engin

LZ4-00U600

159

conversion to AC (~85% efficient), and then losing again when driving the lamp (~50-

90% efficient) resulting in a net efficiency of approximately 43% to 77%. This

conversion loss problem is exacerbated in small-scale systems where the quiescent (no-

load) power draw of the inverter is around 2.4W, which was a significant proportion of

the power consumed by the systems. The resulting improvements in DC operation

include higher efficiency resulting in smaller PV panels, smaller and lighter systems

which are less complex, generate less heat and have no unsafe high-voltages.

The total bill of materials for each system and their approximate retail cost in US$ at the

time of purchase are detailed in Table 21 for System A, Table 22 for System B and

Table 23 for System C. Where units used are not an integral part of a retail package, the

price has been adjusted to reflect the estimated portion of the package used in

construction. Parts were sourced from Mouser Electronics, element14, Rainbow Power

Company, Jaycar Electronics, Bunnings Warehouse, Woolworths and Anaconda. The

cost of tools used in construction were not included.

Table 21 - Bill of materials and costs for System A

Qty Part

Cost Each

(US$)

Total Cost

(US$)

1 SETi UVTOP270 270nm LED 200 200

1 Freetronics Leostick 35.67 35.67

1 2.5W Solar Panel 16.36 16.36

1 HB6127 115x90x55mm Enclosure 11.12 11.12

1 Vero Board 5.91 5.91

1 Microswitch 2.79 2.79

5 Hook-up wire (m) 0.37 1.85

1 STP22NF03L/IRLZ34 MOSFET 1.78 1.78

50 Silicone Sealant (gm) 0.03 1.5

1 Push button 0.89 0.89

1 LM7805 5v Voltage Regulator 0.68 0.68

1 Red high-intensity 5mm LED 0.63 0.63

1 Green high-intensity 5mm LED 0.63 0.63

160

Qty Part

Cost Each

(US$)

Total Cost

(US$)

10 Solder (grams) 0.06 0.6

2 1000uF 25v capacitor 0.2 0.4

1 1N4004 diode 0.27 0.27

10 Header pins (ea) 0.017 0.17

0.1 Vinyl Hose (m) 1.27 0.13

1 NSI45020AT1G LED Regulator 0.12 0.12


3 1kohm 0.25w 5% resistor 0.007 0.021

2 560ohm 0.25w 5% resistor 0.007 0.014

1 6.8kohm 0.25w 5% resistor 0.007 0.007

TOTAL 281.57

Table 22 - Bill of materials and costs for System B

Qty Part

Cost Each

(US$)

Total Cost

(US$)

8 SETi UVTOP270 270nm LED 200 1600

1 Pump Module 37.175 37.18

1 Freetronics Leostick 35.67 35.67

1 10W Solar Panel 25.6 25.6

6 PV1230 PV rated wire (m) 3.4 20.4

1 AG38S UV Sensor 18.89 18.89


25 Hook-up wire (m) 0.37 9.25

1 12L Bucket 8.93 8.93


4 Vinyl Hose (m) 1.27 5.08

2 Air-hose connectors 1.33 2.66

80 Silicone Sealant (gm) 0.03 2.4

1 STP22NF03L/IRLZ34 MOSFET 1.78 1.78

1 SK0960 Rocker Switch 1.67 1.67

161

Qty Part

Cost Each

(US$)

Total Cost

(US$)

1 Nitto Electrical Tape (reel) 1.64 1.64

2 Red high-intensity 5mm LED 0.63 1.26

2 Green high-intensity 5mm LED 0.63 1.26

2 MCP6273 Rail-to-rail Op-amp 0.59 1.18

1 100kohm trimpot 0.77 0.77

1 LM7805 5v Voltage Regulator 0.68 0.68




1 1N4004 diode 0.27 0.27

1 NSI45020AT1G LED Regulator 0.12 0.12


4 1kohm 0.25w 5% resistor 0.007 0.028

4 560ohm 0.25w 5% resistor 0.007 0.028

1 10nF MKT capacitor 0.02 0.02

1 8.2Mohm 0.25w 5% resistor 0.007 0.007

1 6.8kohm 0.25w 5% resistor 0.007 0.007

1 Coffee Can Reactor 0 0

TOTAL 1799

Table 23 - Bill of materials and costs for System C

Qty Part

Cost Each

(US$)

Total Cost

(US$)

3 LED Engin LZ4-00U600 LEDs 79.04 237.12

2 40W Solar Panel 97.26 194.52

1 Victron Energy Charge Controller 94.47 94.47

2 12v 18Ah Sealed Lead Acid Battery 43.45 86.9

1 Primus Camping Shower 59.5 59.5

3 XP Power LDU5660S700 Drivers 16.07 48.21

1 M3354 24V to 12V converter 44.59 44.59

162

Qty Part

Cost Each

(US$)

Total Cost

(US$)

9 PV1230 PV rated wire (m) 3.4 30.6


4 L-brackets 5.88 23.52

1 Fischer Elektronik SK584/50 SA Heatsink 19.8 19.8

1 2m Water Connector 17.29 17.29

1 12L Bucket 11.16 11.16

1 Plywood base 8.18 8.18

3 Teflon Tape (reels) 2.31 6.93

18 Hook-up wire (m) 0.37 6.66


3 PVC End Caps 1.58 4.74

1.5 PVC 90mm pipe (m) 2.71 4.07

8 Wood Screws 0.49 3.92

3 AG3 Fuse Holder 1.3 3.9

100 Silicone Sealant (gm) 0.03 3

8 Crimp Terminals 0.35 2.8

1 PVC Straight-through Joint 1.79 1.79

2 Male brass fittings 0.86 1.72

2 Female brass fittings 0.86 1.72

1 Thermal Paste (g) 1.65 1.65

1 Nitto Electrical Tape (reel) 1.64 1.64

1 PVC T-shape joint 1.64 1.64

2 T4A Fuses 0.3 0.6


1 PVC Glue (estimated) 0.5 0.5

4 M3 screws 0.1 0.4

1 UV-Transmissive Perspex Round 0.27 0.27



TOTAL 955.70

163

The cost of System A was the least, totalling US$281.57. System B was the most

expensive, totalling US$1799.00. System C, based on UV-A LEDs, cost just over half

as much at US$955.70.


Testing was performed by challenge testing with E. coli K12 suspensions of ~106

CFU/mL in 0.05M NaCl solutions. The single-cup disinfector was tested on a volume of

250mL in a 500 mL glass beaker. The UV-C pumped system was tested with 10L and

the UV-A system tested with 15L supplied from buckets. Samples were taken initially

and at fixed intervals (3 minutes for single-cup, 10 minutes for UV-C pumped system

and 60 minutes for UV-A system), serially diluted and spread plated (0.5mL) on

tryptone soy agar. A 1L control volume of suspension was kept in a foil-covered Schott

bottle next to the reactor under test and was also sampled at the beginning and end of an

experimental run to ensure inactivation was not due to other factors. Sterile water

controls of the NaCl solution prior to inoculation were also analysed. Plates were

incubated for 24 hours at 35°C and number of CFU counted to determine inactivation as

in Chapter 3. This experimental set-up duplicates the techniques used in Chapter 3

allowing for direct comparison of results. Each reactor experiment was repeated to

produce at least duplicate sets of data.

164


The results of challenge testing along with a discussion of the performance and each

reactor design is presented in the following sections.

6.4.1. System A Performance

Figure 57 - System A log10-reduction results (270nm, 1 LED, 250mL volume)

Inactivation in the UV-C single-glass system (Figure 57) surpassed 3-log10 reduction,

but did not achieve complete elimination within the 12 minute test time-frame. On a

per-energy unit basis compared with the bench-scale model system, around 5-log10

reduction within 2-3 minutes was expected.

This poorer performance can be partially explained by the hydrodynamics of the

system. Due to the lack of a pump for circulation, the water is stagnant between each

three-minute disinfection cycle which provides opportunities for bacteria to evade

irradiation. Agitation is only performed after each cycle has been completed providing

limited opportunities for intermixing. Further, the position of the UV-LED within the

water column was fixed and was probably not optimal for exposing the whole volume

of water. The geometry of the cup itself and its material and surface finish are likely to

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 2 4 6 8 10 12

Log

10

Red

uct

ion

Time (minutes)

Run 1

Run 2

Run 3Control: 0.28 log reduction (max)

165

impact upon any potential reflective enhancement, and its tall vertical shape may have

meant that water absorbance played a role in reducing the dose experienced towards the

bottom.

While the same method of preparation was employed as in Chapter 3, cell clumping

could have affected the outcome, with the slope-grown E. coli cells not fully

resuspended and thus, causing shadowing of adjacent cells from UV radiation and

reducing the observed inactivation.

Existing literature on the Steripen™, upon which this design is based, appears to show

that agitation affects the effectiveness of such systems markedly (Timmermann et al.,

2015).

Despite its limitations, the system still showed highly useful levels of disinfection, and

has advantages of being conceptually simple, eschewing the mechanical pump, and only

requiring sunlight - a renewable energy source. Testing showed the device was capable

of operation with solar elevations down to 17° due to the deliberate over-sizing of the

panel relative to the device's energy requirements.

As this system did not contain a UV sensor, it was not possible to provide the user with

feedback to confirm the proper functioning of the UV-LED which could pose a safety

issue. The only feedback available is from visible status LEDs and from an on-board

buzzer which indicates sufficient power input from the PV panel and the status of the

timed disinfection cycle. External checks can be used to confirm the presence of UV,

such as photodiode sensors and fluorochromes.

166

6.4.2. System B Performance

Figure 58 - System B log10-reduction results (270nm, 8 LEDs, 1L reactor, 10L

disinfection volume)

System B, the pumped UV-C system (Figure 58), achieved inactivation of around 2-

log10 reduction. This was less than expected. On a per-energy basis compared with early

model bench-scale tests, 5-log10 reduction in around 12 minutes was expected.

Alternatively, based on the performance of System A, 4-log10 in 60 minutes would be

expected.

It is suggested that the reason for suboptimal performance was similar to the case of

System A. The system utilised a low-rate solar-powered water-fountain pump which

developed a 300mL/min flow rate. This would require about 33 minutes to fully cycle

the 10L volume through the reactor, and due to the slow mixing of the water in the

container, complete disinfection was unlikely to occur. A higher flow-rate may have

been beneficial, although the slow flow rate would have allowed a 3 min residence time

which should have been enough for a single-pass disinfection to occur, thus pumping

from a source container through to a clean container may have been a preferable

configuration compared to recirculation.

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60

Log

10

Red

uct

ion

Time (minutes)

Run 1

Run 2Control: 0.013 log reduction (max)

167

The use of a recycled coffee tin itself appeared to be inappropriate for long-term usage

due to rusting of the tin over time as the tin plating is thin, however, it may have

improved disinfection by allowing for internal reflection and provided an efficient

heatsink for the LEDs. This confirmed in POU settings reactor design and construction

material will be critical to maximising reactor lifetime and LED performance.

A further problem with this design was the interaction between the commercial pump

maximiser circuitry and the microcontroller power supply. This limited the ability for

the system to run during cloudy periods due to the high inrush current consumed by

pump start-up which caused the voltage to dip below the level necessary to run the

LEDs. This, however, could be countered in future by adding a soft-start circuit to

restrict the current flow to the pump during start up and maintaining sufficient voltage.

On the plus side, the sensor proved capable of detecting the UV-C content in the reactor

and hence the functional status of the LEDs. This information was suppliable to the user

through visible red and green status indicator LEDs.

168

6.4.3. System C Performance

Figure 59 - System C log10-reduction results (365nm, 3 LEDs, 10L reactor, 15L

disinfection volume)

By contrast with the UV-C systems, the UV-A system (System C) improved over

expectations (Figure 59), achieving over 4-log10 reduction within the three hour test

period, and within two hours in the case of the second run. On a per-energy basis based

on bench-scale models, 4-log10 reduction in 9.4 hours had been expected. Thus the path-

length generated enhancement appeared to be 3 to 5 fold.

It is suggested that the path-length enhancement was experienced for UV-A due in part

to the lower absorptivity of water which ensured that even with long columns of water,

the dose per unit area declined more slowly. It also suggests that the bench-scale models

were not fully exploiting the potential behind UV-A and visible light disinfection due to

their short path-length.

A secondary reason for the enhancement could be total internal refraction, where the

column of water acts similar to an optical fibre. In line with Snell's Law and the

respective refractive indices of PVC (1.53) and water (1.34), the critical angle is

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 60 120 180

Lo

g1

0R

edu

ctio

n

Time (minutes)

Run 1

Run 2

Control: 0.041 log reduction (max)

169

approximately 60°, indicating any rays of light which hit the PVC at a shallower angle

will be bent back towards the column of water. The increased path length disinfected

15L of water in a similar period of time as the earlier 1L bench-scale model unit. UV-C

is probably less amenable to such enhancement due to the high absorptivity of water in

those wavelengths (Quickenden and Irvin, 1980) and possibly PVC.

A measurement of the PVC reflectivity of a flat end-cap sample was made using a

Perkin-Elmer Lambda 1050 spectrophotometer with 150mm integrating sphere. The

results (Figure 60) indicate that direct reflective enhancement from the PVC's white

surface was not a major contributor to the observed enhancement at 365nm though

reflection of longer wavelengths might be significant. Direct measurement of the optical

characteristics of the system was confounded by the need to have the column filled with

water to maintain conditions representative of the reactor in use. In the future,

computational fluid dynamics (CFD) modelling is one possible route which could allow

for better understanding of the optical characteristics of the disinfector.

Figure 60 - PVC pipe vertical reflectance

0

10

20

30

40

50

60

70

80

90

100

250 350 450 550 650 750

Ref

lect

an

ce (

%)

Wavelength (nm)

170

Some difficulties with System C were encountered during transportation, especially

bulkiness and weight owing to the requirement for battery storage and large energy

requirements. This system configuration also necessitated a higher quality pump to

develop sufficient head to achieve the high head and flow-rate to maintain a fully mixed

system. As a result, the power budget was also not as optimal as initially envisaged, as

the increased power draw of the better pump reduced the power available for charging

the batteries during operation.

Overall, disinfection was achieved with all units, with UV-C systems falling short of

predicted performance due to hydrodynamic issues which could be improved through

the use of CFD modelling in reactor design (Pan and Orava, 2007, Chen et al., 2011b,

Oguma et al., 2016). Optimisation of LED positioning with regards to fluence (Bowker

et al., 2011) and thermal considerations is another possibility. UV-A based systems

showed greater promise than initially expected, due to significant path length

enhancements in the tall reactor design. The majority of the identified issues were the

outcome of the engineering design choices and trialling them in the field. The

experience garnered from this experiment would be useful in refining further prototype

systems. Such systems can have much better flexibility owing to the fact that the size

and number of PV modules can be increased to effectively gather more energy and

concentrate the irradiance within a smaller area of a reactor and more sophisticated

control and monitoring algorithms (aside from time, or UV sensor reading alone) can be

developed and implemented.

The main drawbacks to the prototype devices over the models were their increased

complexity, potential reliability problems with electronics and mechanical pumps, and

increased cost due to the high price of UV-C LEDs. For System A, the LED represented

71% of the system cost, and for System B, the LEDs represented 89% of the system

cost. System C, based on UV-A, only had 25% of the system cost in LEDs, due to the

inclusion of more costly pumps, batteries, charge controller and voltage converter

modules. As technology continues to develop, both UV-C and UV-A LED prices are

likely to continue to reduce with each successive generation implying increased future

significance of non-LED costs. The lifetime of the LEDs could also pose an issue, as

their reliability is not as well proven as with visible light LEDs.

171

Further to these constraints, the LED based disinfection systems are also likely sensitive

to the UV absorptive characteristics of the source water such as dissolved natural

organic matter and humic acids, which can serve to limit path length enhancements and

diminish available UV content for disinfection, especially the UV-C systems including

the Steripen™ (Abd-Elmaksoud et al., 2013). Conversely, the presence of these

compounds may serve as photosensitisers for UV-A disinfection and enhance

disinfection. This may be especially true for wastewater applications where high UV

absorbances are experienced (Davies-Colley et al., 2005). A further support for further

UV-A research arises from the bench-scale experiments which showed variations in S90

between E. coli and E. faecalis is much smaller than for UV-C. This finding may hold

true for viruses as well (Davies-Colley et al., 2000, Davies-Colley et al., 1999).

As System B and System C involved enclosed reactors, over time it is possible that

biofilm and fouling of the reactor may occur necessitating maintenance in the form of

cleaning, as the water flow rate, while sufficient to ensure even mixing of pathogens

(Murray and Jackson, 1992, Miller et al., 1977) and oxygenation, is not sufficient to

prevent biofilm accumulation (Roser et al., 2014, McCoy et al., 1981). It was also

observed by Spencer et al. (2014) that chemicals could adsorb onto the quartz window

of the LEDs and hence potentially reduce their output significantly. In their study,

methylene blue adsorption reduced UV-C output by 50%. Large-scale commercial UV

disinfection reactors typically use cleaning in the form of mechanical wipers both with

and without chemicals. One drawback includes the use of mechanical systems which are

prone to wear, require maintenance, and in the case of mercury tubes, can malfunction

and break the quartz glass envelope, releasing mercury into the water

6.5. Conclusion

While bench-scale tests have been conducted, the practical aspects of producing a field-

deployable unit requires addressing numerous engineering aspects. Three prototype PV-

powered, LED-based systems demonstrating various configuration possibilities were

built and tested in field-conditions, with results indicating the reactors were functional

and achieved disinfection of about 4-log10 with 270nm UV-C in 12 minutes for a

250mL glass, 2-log10 in 40 minutes for a 10L container and with 365nm UV-A, greater

172

than 4-log10 in under 3 hours for 15L. System A and System B both fell behind

projected disinfection figures, likely due to hydrodynamic design problems and

limitations in path-length due to absorbance. System C was able to exceed projections

based on per-dose parity with the bench-scale system as the reactor was able to make

use of increased path-length and total internal refraction to better utilise the available

UV-A light.

Compared with SODIS, it appears that PV-powered LED-based systems offer

advantages in controllability and flexibility, with the ability to generate wavelengths not

present in sunlight. Nevertheless, the three LED disinfection systems were still quite

costly and complex, with the two UV-C systems having 71 and 89% of their costs in

LEDs compared with the UV-A system with 25% of the cost in LEDs. However, it was

demonstrated that present technology is sufficient to realise the concept and prices are

anticipated to reduce in the future.

Also importantly, a key benefit of all the systems was confirmed. That is, all systems

appeared to offer remote and regional communities a means of disinfecting water for

periods of years without the need for complex or outside intervention.


Three prototype systems representing a subset of configuration options were

specified, costed, constructed and tested.

Costs of the three systems were US$281 for a single 270nm LED disinfecting

250mL, US$1799 for eight 270nm LEDs disinfecting 10L in 1 hour and US$956

for three quad-die 365nm LEDs disinfecting 15L in 3 hours.

Inactivation achieved was 4-logs, 2-logs and 5-logs respectively, proving the

concept is able to be realised with present-day technology albeit at high cost.

Less than expected inactivation for UV-C 270nm LED based systems were

likely due to hydrodynamic limitations, implying significant benefits if

computational fluid dynamics methods were used in design.

Good results were achieved with UV-A 365nm based systems above

expectations due to longer path length making better utilisation of the energy

and total internal refractive effects.

173

7. Comparing PV-LED Disinfection with SODIS Performance

through Solar Spectrum Based Modelling3

7.1. Introduction

The solar disinfection method (SODIS, 2012) is a conceptually simple point-of-use

technology which relies on direct sunlight and readily available polyethylene

terephthalate (PET) bottles to disinfect water through the processes of photoinactivation

and pasteurisation (Reed, 2004). SODIS appears effective and widely used, but

performance variability has been observed and there has been a desire to enhance its

performance and acceptance (Rainey and Harding, 2005, Graf et al., 2010, Mäusezahl et

al., 2009, Conroy et al., 1996). SODIS performance relies upon the functioning of

photoinactivation, which occurs based upon direct light impacts on pathogens and the

water matrix, and thermal pasteurisation effects which depend on temperature of the

water. Performance variability arises due to a number of factors, including the

geographical location, weather conditions, altitude, time of year, time of day, bottle

orientation, bottle design, bottle material transmissivity, transmissivity of the water

matrix and organic matter within the water matrix.

The process of photoinactivation is complex and wavelength dependent. Short

wavelength UV-B (280-315nm) in sunlight may cause direct damage to DNA, through

the formation of cyclobutane pyrimidine dimers and 6-4 photoproducts. Longer

wavelength UV-A (315-400nm) can produce reactive oxygen species (ROS) through

breakdown of natural organic matter (Tsuyoshi and Akira, 2013). The excitation of

photosensitisers within the medium and cells can also form ROS which further

contribute to damage to pathogens (McGuigan et al., 2012). The relative size of the

ROS contribution to disinfection is an area of active research (Mattle et al., 2015).

The literature review (Chapter 2) identified a need to better understand what was known

about UV-A, UV-B and visible light action spectra and to develop a model to illustrate

the extent to which the disinfective potential of sunlight derived energy could be

3 This chapter is an extension of the work presented at a conference as

Toward a Better Understanding of Solar Disinfection using Action Spectra and Solar Modelling, IWA 3rd

Water Research Conference, 11-14 January 2015, Shenzhen, China.

174

improved upon at the cost of increasing POU complexity and cost using semiconductor

technology.

While the disinfective role of UV-A and UV-B is widely acknowledged, many studies

of SODIS performance and modification (new materials, concentrator systems) tend

only to relate disinfection performance to total sunlight energy (Oates et al., 2003) or

total UV-A energy in addition to total sunlight energy (Dejung et al., 2007, Ubomba-

Jaswa et al., 2010, Polo-Lopez et al., 2011). Other SODIS studies have been based on

observations from the use of various materials with known wavelength cut-offs

(Oppezzo, 2012, Davies-Colley et al., 1997). To my knowledge, only Mbonimpa et al.

(2012) pursued simulated sunlight spectra to estimate disinfection to validate their

reactor. No systematic investigation into the contribution of atmospheric effects from

time of day, location, as well as the contribution of particular wavelengths of sunlight to

the disinfection process appears to have been reported. That said, it is well recognised

that weather and latitude can impact adversely on SODIS effectiveness.

To advance understanding of the disinfective potential of sunlight, an analysis of

SODIS’s photoinactivation potential using a simplified mathematical model was

undertaken similar to, but extending on, Mbonimpa et al. (2012). The current study

relates the following key variables: i) an action spectrum which describes the

inactivation sensitivity of a pathogen to light greater than 280nm, ii) the solar spectrum

which drives photoinactivation to varying extents depending on latitude, season,

elevation and daylight hour, and iii) the transmittance of SODIS containers using PET

plastic bottles as a model. The modelling was necessarily simplified as it did not

account for such factors as ROS, cloud cover and variations in microbial sensitivity.

This modelling did not account for the thermal effects of pasteurisation and did not take

into account the temperature of the water or thermally related inactivation effects due to

difficulties in modelling the process. The focus on photoinactivation parallels that of

LED light induced inactivation and represents rapid inactivation which is effective year-

round and outside of the tropics, and is based on an approach by Mbonimpa et al.

(2012) which was experimentally validated. But it was seen as a first step to a more

systematic understanding and quantification of likely SODIS performance under

175

different environmental conditions and to a better understanding of how using PV to

power LED disinfection compares for POU application.

A better understanding of the relationship of full microbial action spectra to pathogen

disinfection would also be useful for UV light-emitting diode (LED) and medium

pressure based UV disinfection as these might potentially employ a range of longer UV

wavelengths compared to traditional low pressure mercury lamp systems (Beck et al.,

2015).

This research did not seek to validate the SMARTS model for sunlight spectra

modelling, nor using SMARTS outputs and action spectra to model photoinactivation.

This was because the SMARTS model itself has been extensively validated by

Gueymard (2004) against other solar spectra models and instrument readings, and

adopted by ISO/IEC 60904-3 Ed. 2 as the reference for sunlight spectra. The action

spectra modelling was previously performed by Mbonimpa et al. (2012), and was

validated against experimental results.


7.2.1. Solar Spectrum

Solar spectra for different elevation, season, time-of-day and latitude were obtained

using the National Renewable Energy Laboratory's Simple Model for the Atmospheric

Radiative Transfer of Sunshine (SMARTS) software, version 2.9.5 (National

Renewable Energy Laboratory, 2013). Input to the SMARTS model is via ASCII text

files termed ‘input cards’ reflecting the original use of input punch cards when the

model was first developed. These are used to generate the reference solar spectrum in

IEC60904-3 Ed.2 (IEC, 2008) in the range 280-700nm. Modifications were

subsequently made to the input cards to generate zenith angle and altitude variations as

detailed in Appendix A.

In all simulations the tilt angle was set to 0° to simulate a bottle facing the sky (Figure

61), with the solar geometry mode changed from air-mass (AM) to solar zenith angle to

avoid errors due to solar zenith/altitude angle to air-mass conversion. The angle of the

176

collecting area was changed to match the zenith angle (Appendix A). Global solar

irradiation was selected as the output parameter, which includes direct and diffuse

sunlight components. Output intensities were generated in the form of point estimates in

1nm wavelength steps.

Figure 61 - Diagrammatic representation of simulated tilt and zenith angle

conditions

It was noted that the orientation and type of container was likely to have an impact on

SODIS performance. Cylindrical bottles, when oriented in the north-south axis (e.g.

Fisher et al. (2012)), will have the same cross-sectional area collecting solar energy

throughout the day. If oriented along the east-west axis, the projected area is likely to be

less as a function of the zenith angle of the sun, reducing solar collection. Some authors

(e.g. Arvidsson (2013)) suggest flat plastic bags, which can be approximated by a flat

plane collector, which will have reduced collection as the angle of the sun varies as the

cross-sectional area exposed to the sun varies over the course of the day, however, may

be advantageous as it can maximise heating and photoinactivation during the hours

where the sun is strongest by increasing surface area and reducing path length. It

appears that orientation considerations are not clearly stipulated in SODIS guidelines

Bottle at 0 ° Tilt

Sun at Zenith Angle 0 °

Sun at Zenith Angle 9 0 ° N-S Axis

177

(Meierhofer et al., 2002) but is likely to have an impact on performance. The simulation

performed assumes the best case, i.e. by rotating the flat collection plane to match the

zenith angle of the sun, thus simulating a constant area collector, as per the bottle being

oriented north-south.

Variations upon the SODIS method which improve upon conventional SODIS have

been adopted by some users, which include using bottles half-coated in black to

increase thermal absorption (McGuigan et al., 2012), as well as various nano-particle

catalyst impregnated bottles (Carey et al., 2011), bladder bags and concentrators

(Ubomba-Jaswa et al., 2010). These variations are not considered in the modelling

presented, which aims to identify how indirect PV and LED based disinfection

compares with SODIS on a photoinactivation only basis, disregarding thermal

pasteurisation.

The model solar spectrum was generated by SMARTS at 1° zenith-angle steps between

0° and 90° and placed into a look-up table. The solar insolation at any given latitude and

time of day was determined by solving the solar zenith equation for the position of the

sun and retrieving the corresponding spectrum from the look-up table.

where θ is Solar Zenith Angle, φ is the latitude, δ is the declination angle and h is hour

angle in local solar time.

The zenith angle of the sun was computed for each latitude angle (1° steps from 0° to

90°) at 15 minute intervals through all 24 hours of a day. This was undertaken for 0°,

23.44° and -23.44° declination, corresponding to equinox, summer solstice and winter

solstice conditions respectively, to understand the impact of seasonality. The zenith

angle was rounded to the nearest integer degree, and used to look-up the relevant pre-

computed solar spectrum result (Figure 62).

178

Figure 62 - Block diagram of zenith-angle-based model

Another set of input cards was used to generate the solar spectra at 0° latitude, 41.8°

altitude angle (Air Mass 1.5) at 0° declination for altitudes of 0km to 5km in 1km steps.

The input solar spectrum was then multiplied by the action spectrum, to simulate the

effect of altitude on the disinfection dose (Figure 63).

SMARTS

Spectrum at

Zenith Angle

0-90°

Zenith

Angle

Input Card

Zenith

Angle

Equation

Hour Angle

Latitude

Declination

Solar

Spectrum

Action

Spectrum

PET

Transmission

+

+ Direct Sun

Disinfection

With PET

Disinfection +

Inputs Processes Outputs

179

Figure 63 - Block diagram of altitude simulation model

7.2.2. Action Spectrum

An action spectrum describes the sensitivity of a microorganism to light at a given

wavelength, as inactivation rate as a function of wavelength (Bolton and Cotton, 2008).

A survey of literature for action spectra covering visible sunlight wavelengths was

undertaken. Action spectra spanning a wide wavelength range (from UV-B (280nm)

through to red (700nm)) were desired to account for the relative contributions made by

the wide spectral content of sunlight. Only a limited number of action spectra for non

UV-C wavelengths were found, and most were less useful due to the lack of data above

violet wavelengths (> 430nm) e.g. (Kelland et al., 1983, Peak et al., 1982). The most

comprehensive action spectrum was that for E. coli WP2s by Webb and Brown (1979).

It was also judged the most valuable because a major benefit of SODIS is the control of

comparable enteric bacterial pathogens especially pathogenic E. coli, Shigella,

Salmonella and Vibrio spp.

As the data were only available as a graph, the action spectrum for E. coli WP2s was

extracted by curve-tracing using Engauge Digitiser, with linear interpolation between

curve points. The data were output at 1nm intervals from 280nm to 700nm. The data

were unit-converted from inactivation constant (m2) for per incident quantum for 63%

survival to mJ.cm-2

for 1-log10 inactivation.

SMARTS

Spectrum

at Altitude

0-5km

Altitude

Input Card

Altitude

Selection

Action

Spectrum

PET

Transmission

+ Direct Sun

Disinfection

With PET

Disinfection +

Solar

Spectrum +

Inputs Processes Outputs

180

Where S90 is the dose for 1-log10 inactivation, h is the Planck constant in J.s, c is the

speed of light in m.s-1

, λ is the wavelength in m, and xλ is the inactivation constant (m2)

per incident quantum for 63% survival at wavelength λ.

Data generated were combined with the solar spectrum data to determine the per-

wavelength contribution to disinfection, as well as the total disinfection over a 24 hour

period. This curve was used in preference to the curves in Table 9 because it was based

on more point measurements and it included the 280-360nm range. Further details are

found in Section 3.3.6.

7.2.3. PET Transmissivity

A section of 3 x 3cm of 0.3mm thickness PET was excised from a clean, thin-walled

‘Mount Franklin’ (Coca Cola Amatil, North Sydney, Australia) PET water bottle. Its

transmissivity was measured with a Perkin Elmer Lambda 1050 spectrophotometer with

150mm Spectralon integrating sphere attachment between 280nm and 700nm using a

1nm slit width with 100% and 0% transmission calibration. The use of the integrating

sphere ensures that even scattered light is collected in the measurement. These data

were used to scale the solar spectrum data to simulate the effect of the PET bottle on the

incoming sunlight.


7.3.1. Action Spectrum, PET Transmission and Solar Spectrum Inputs

The E. coli WP2s action spectrum extracted from Webb and Brown (1979) was traced

and its units transformed to produce the curve shown in Figure 64. E. coli action spectra

from Peak et al. (1982), Kelland et al. (1983) and rate data for 254nm from Hijnen et al.

(2006) and Bolton and Cotton (2008) are plotted for comparison. It can be seen that the

action spectra show the same pattern as WP2 and the estimated S90s for different

wavelengths appear sufficiently comparable to justify the use of the former for the

modelling. To aid future research, a line and 3 power polynomial was fitted to the WP2s

action spectrum.

181

It is important to note the action spectra lie on a logarithmic scale, and show a 4-order

of magnitude change in sensitivity for E. coli in the 280-340nm range, underscoring the

potential impact of any variation in UV intensity in this range on the solar

photoinactivation process. The interaction between UV variance, the spectral content of

sunlight, the material used in the reactor and also probably organics in the water matrix

(Mattle et al., 2015) will determine the final observed inactivation rate.

Figure 64 - Action spectrum for E. coli WP2s (see also Figure 16)

The measured PET bottle transmission is given in Figure 65. The visible light

transmission (400-700nm) of the bottle averaged 88.6 ± 0.7 (S.D.) %. However, the

transmission rapidly decreased below 355nm, reaching <1% at 313nm. Critically, the

change in opacity of PET to UV wavelengths occurred at similar solar UV wavelengths

which were expected to provide the most effective photoinactivation (Figure 64). This

was further compounded by the possible impacts of non-equatorial geographic locations

and low UV content where the elevation angle of the sun is low.

For: 250≤x<324

y = 9.6343E-06x3 - 6.7373E-03x2 + 1.5397E+00x - 1.1523E+02

R² = 0.993

For: 324≤x<700

y = 1.10E-02x + 5.52E-01

R² = 0.983

-2

0

2

4

6

8

10

250 350 450 550 650

log

10(S

90)

mJ/c

m2

Wavelength (nm)

E. coli WP2s based on Webb and Brown (1979) E. coli B/r based on Peak et al (1982)

E. coli SR385/K-12 JG139 based on Kelland et al. (1983) E. coli O157 from Hijnen et al. (2006)

E. coli from Hijnen et al. (2006) E. coli ATCC 11229 from Bolton and Cotton (2008)

E. coli O157:H7 from Bolton and Cotton (2008) E. coli wild type from Bolton and Cotton (2008)

280nm sunlight cut-off

182

Figure 65 - Measured PET bottle transmission

The input solar irradiance spectra generated were too numerous to present in full, so I

selected and plotted an illustrative set of conditions in Figure 66. This plot focuses on

the wavelengths between 280nm and 500nm as the key range of interest for SODIS.

These spectra illustrate a further complication in quantifying disinfective power, how

the solar spectrum reflects in part the atmosphere, selectively absorbing and

disproportionately filtering shorter wavelengths, especially those <340nm. This filtering

especially reduces the UV-B and UV-C content of sunlight to low-to-negligible levels.

This benefits complex life forms but limits the energy available for photoinactivation.

0

10

20

30

40

50

60

70

80

90

100

280 380 480 580 680

Tra

nsm

issi

on

(%

)

Wavelength (nm)

183

Figure 66 - Solar spectrum for selected conditions

To aid comparison I explored the impacts of different factors together relative to a

reference condition of noon, at the equinox, at the equator, and at sea level (Figure 66).

Increasing the altitude to 3km had a slight positive impact on disinfection potential by

increasing the total solar energy available due to reduced atmospheric absorption. This

does not take into account temperature influences. It could be foreseen that in practical

circumstances, the increased altitude would serve to reduce the temperature of the water

and reduce any pasteurisation that may occur. However, in prior bench-scale

experiments, it is understood that thermal pasteurisation requires relatively high water

temperatures as a temperature of 36°C was insufficient to cause measurable

inactivation. Time of day on the other hand rapidly reduced the available energy, which

can be seen in comparison between noon and 2 pm at sea level curve. The most limiting

conditions of the winter solstice + a tropical latitude + PET absorption reduced the

available solar energy in the UV-A by ca 50-75% and eliminated UV-B completely. It

is important to note that these incidence energy reductions do not include additional

impacts of haze, cloud or other anthropogenic atmospheric filtering effects, which

would further reduce SODIS efficacy.

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

280 300 320 340 360 380 400 420 440 460 480 500

Po

wer

(m

W/c

m2/n

m)

Wavelength (nm)

Noon/Equinox/Equator/0km Noon/Equinox/Equator/3km

2pm/Equinox/Equator/0km 2pm/WinterSol/Tropic/0km/PET

184

In summary, the variation in the solar energy and, more importantly, solar spectrum as a

function of time-of-day and latitude can clearly be seen to influence potential sunlight

photoinactivation. Among other things this suggests that experiments based on a fixed-

spectra irradiation or fixed-intensity solar simulators may not generate ideal models of

real-world disinfection of pathogens in water or on the land surface e.g. in animal

excreta.

7.3.2. Diurnal Disinfection Profile

Characterising the daily variation in disinfection potential provided a basis for ensuring

the optimal exploitation of available solar energy. The disinfection profile for the

equinox, at the equator, is plotted in Figure 67 for the 12 hours of the day when the sun

is above the horizon. The plot shows the disinfection power for both no-PET and PET

scenarios in instantaneous and cumulative formats.

The disinfection peaks at the solar noon, as expected, with the six-hour period

straddling noon representing 90% of no-PET, and 80% of with-PET, whole-day

disinfection power. This confirms that the existing recommendation of six-hours of

strong sunlight for SODIS (Meierhofer et al., 2002) is appropriate, as only 10-20 %

more inactivation is available by extending exposure to 12 hours. The effect of the sun

zenith angle is illustrated in Figure 68, with the disinfection power falling off as the

sun's zenith angle approaches the horizon (90°). N.B. Figure 67 is normalised to 100%

of the maximum or total disinfection power. If the no-PET and PET data were plotted

on the same absolute scale the PET peaks would be only ca 5% of their no-PET

equivalents. This difference is illustrated in Figure 68.

The with-PET condition was at a significant disadvantage for disinfection power at

most zenith angles due to the loss of UV-B content. However, as the sun sets, the loss is

relatively less due to atmospheric filtration of UV-B. Thus there is less UV-B content to

lose via PET filtering in this situation but this benefit is likely insignificant as the sun

provides only limited solar energy nearing the horizon, and hence disinfection in any

case.

185

Figure 67 - Daily disinfection profile for equator at equinox

Figure 68 - Predicted disinfection power by sun zenith angle

0

20

40

60

80

100

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Per

cen

tag

e D

isin

fect

ive

Po

wer

(no

rma

lize

d t

o p

eak

)

Hour Angle (15 /hour, solar noon at 0 )

Direct Sun Instantaneous Direct Sun Cumulative

With PET Instantaneous With PET Cumulative

0.00001

0.0001

0.001

0.01

0.1

1

0 20 40 60 80

Ab

solu

te D

isin

fect

ion

Po

wer

(lo

g1

0/m

inu

te)

Sun Zenith Angle (degrees)

Direct Sun With PET

186

7.3.3. Contribution by Wavelength

The total and relative contributions of each wavelength of sunlight to disinfection were

modelled and plotted as a cumulative graph in Figure 69. The graph represents the total

cumulative disinfection power for light of a given wavelength or longer. The use of the

container greatly increased the relative importance of UV-A.

Figure 69 - Cumulative contribution to disinfection of E. coli WP2s by wavelength

The model indicated that light of 520nm and longer contributed relatively little useful

disinfection despite being abundant in sunlight, due to the increasingly limited

sensitivity of E. coli to those wavelengths (Figure 64). Conversely, 90% of disinfection

was due to wavelengths <315nm (UV-B) in the direct sun reference case despite its

small contribution energy wise due to the orders-of-magnitude increased sensitivity of

E. coli. Conversely due to the wavelength filtering effects of PET plastic, in the latter

case disinfection contribution shifted strongly towards the UV-A and visible violet/blue

wavelengths. The inclusion of the PET plastic cut down the total disinfection power to

just 5% of the no-PET case predicted by the model. The size of this reduction was a

surprise.

0.015

0.15

1.5

15

150

0.01

0.1

1

10

100

290310330350370390410430450470490510

Pre

dic

ted

24

h l

og

10

dis

infe

ctio

n

Cu

mu

lati

ve

Dis

infe

ctio

n P

ow

er

(%,

no

rma

lize

d t

o t

ota

l d

irec

t su

n)

Wavelength (nm)

Direct Sun With PET Bottle

187

This modelling of how each wavelength of light contributes to the photoinactivation

part of SODIS is important as it should inform material choices in bottles and reactors,

and may explain some of the performance variability reported (Conroy et al., 1996,

Mäusezahl et al., 2009, Rainey and Harding, 2005, Graf et al., 2010). Conversely as

PET bottles still appear to achieve a satisfactory level of disinfection it may be that in

fact the full disinfection potential of the sun is being missed by using rigid plastic

covering compared to using no cover at all or a more UV transparent plastic. Figure 68

suggests that, in extremis, a mere 10 minutes uncovered exposure to full sunlight near

solar noon in an equatorial area is sufficient to reduce E. coli by 4 orders of magnitude,

a rate much higher that reported in experimental systems (compare T90s in Reed

(2004)). The impacts of temperature were not modelled, and its effects on

photoinactivation are unknown.

7.3.4. Latitude and Seasonality

The results of modelling selected latitude and seasonality effects is presented in Figure

70 as a plot of whole-day non-thermal disinfection power, normalised to equinox at the

equator. Separate curves showing the effect of PET are also included as dashed lines.

Note again that no PET and with PET curves are relative to their respective maxima and

in fact with-PET absolute intensity is much lower.

The effect of seasonality predicted by the model is apparent even at tropic latitudes. The

disinfection power at the tropics for no-PET is seen to swing from 28% in the winter

solstice to 108% in the summer solstice compared to the equator at equinox. This effect

increases for higher latitudes.

When the use of PET bottles is simulated, the effect is less pronounced, noting that the

absolute disinfection powers for the PET bottle scenarios are only ca 5% of that of no-

PET scenarios (see Section 7.3.2). This is due to the with-PET scenario being less

sensitive to the loss of UV-B content in the sunlight, as it has been removed by the PET

plastic. As a result, at the tropics, the disinfection power swings from 54% of the

equator equinox in the case of the winter solstice, to 108% in the case of summer

solstice.

188

Figure 70 - Latitude and seasonality effects

The with-PET curve for the summer solstice also generated an unexpected finding. The

disinfection performance was relatively consistent (>87% of equator at equinox) at all

latitudes indicating that microbial disinfection driven by UV-A and visible light was

comparable during summer at most latitudes. Separately, these plots highlighted how

even if SODIS performs well in summer trials, this is no guarantee that it will perform

sufficiently well during winter.

Overall, latitude and season derived variation in radiation intensity and spectral balance

appeared likely to have a substantial impact on disinfection performance. The modelling

demonstrated clearly how in higher latitudes, reliable year-round SODIS is more

difficult to achieve due to the lack of UV and total sunlight energy in winter. At such

times and localities, alternative technologies, which are not as spectrally sensitive may

be more suitable.

0

30

60

90

120

150

180

0

20

40

60

80

100

120

0 10 20 30 40 50 60 70 80 90

Pre

dic

ted

24

h lo

g10d

isin

fect

ion

Da

ily

Dis

infe

ctiv

e P

ow

er (%

, no

rma

lize

d to

max

. at

0

lati

tud

e a

t eq

uin

ox

)

Latitude (degrees)

Equinox Summer Solstice Winter Solstice

Equinox w/PET Summer Solstice w/PET Winter Solstice w/PET

189

7.3.5. Altitude Effects

Finally, I investigated the effect of altitude between 0km and 5km on predicted

disinfection rates. Results are summarised in Figure 71. The predicted direct sun

disinfection power increased to 141% of sea level at 5km, whereas the PET bottle

disinfection power only rose by 117%. Both figures are higher than the increase in total

solar energy, which only increased to 111% of sea level at 5km. This effect is due to the

disproportionate impact of air-mass on UV-A and UV-B transmission (Gates, 1966).

Overall it appears that for the majority of habitable altitudes, the improvement in

disinfection potential is only modest and is less significant than the variation due to

seasonality and latitude. It is likely that in practical circumstances, the increased altitude

will correlated with reduced ambient temperature and a reduction in thermal

pasteurisation performance.

Figure 71 - Altitude effect on predicted disinfection rate at 0° latitude for 0°

declination at air-mass 1.5

7.3.6. Strengths and Caveats

This modelling of microbial inactivation using solar radiation action spectra was useful

for several reasons. It provided a clearer quantitative picture of the variance in non-

100

110

120

130

140

150

0 1 2 3 4 5

Per

cen

tage

(%, n

orm

ali

zed

to s

ea l

evel

)

Height above Sea Level (km)

Direct Sun With PET Bottle Solar Energy

190

thermal disinfection by SODIS, the potential for environmental disinfection of natural

surface waters and how critical UV-B is to the latter process. It highlighted how

powerful the disinfective power of the midday sun is and the need to

develop/experiment with reactors with minimal UV-B filtering. It illustrated the

potential value of quantifying all solar disinfection processes for optimally deploying

this disinfection technology. Finally it provided a rational basis for comparing the

disinfective capacity of natural sunlight with disinfection potential which could be

realised by effectively transforming both short and long wavelength light into shorter

wavelengths using PV+UV LED systems.

That said, the modelling outcomes have caveats which need also to be borne in mind

when considering using them. SODIS exploits both photoinactivation and thermal

inactivation but the model did not account for any thermal effects, nor synergisms with

it (McGuigan et al., 1998). To my knowledge, existing literature showing thermal

effects of SODIS are primarily field trials which do not distinguish between

photoinactivation and thermal pasteurisation. The methodology varies from study to

study, and relevant parameters such as the spectral content of sunlight are often not

recorded. Thus, reverse-engineering their results to separate the influences of each

mechanism is impractical.

Inclusion of thermal effects would require knowledge of the infrared emissivity of the

surface on which the bottle is placed, the angle-dependent reflectivity of the surface on

which the bottle is placed, the material of which the bottle is made out of, the thickness

of the bottle, the thermal conductivity of the material, the absorbance of the water inside

to various wavelengths, the convective motion of the water, the tilt of the bottle, the

ambient temperature and the speed of the airflow around the bottle(Siple and Passel,

1945) to determine the temperature of the water. From existing literature, with exposure

times of a day, thermal pasteurisation effects on enteric bacteria start at 45°C , requiring

63°C to be safe against most pathogens (Burch and Thomas, 1998). This could be

difficult to achieve in practice, especially at higher latitudes.

This would have made for a highly complicated model. Instead, this first attempt at a

comparison between indirect and direct forms of harnessing sunlight for inactivation

191

focuses solely on photoinactivation, noting that the use of PV and LEDs does not

preclude the use of direct solar heating of the water in parallel. Extending the model to

cover thermal pasteurisation is an area for further research.

The model of photoinactivation was also simplified and does not fully allow predictions

of real world absolute photoinactivation figures. Reasons include the presence of UV

absorbing compounds in natural waters, the level of dissolved oxygen in the water,

differences in ROS and photosensitiser levels in the medium, synergisms between

different wavelengths of light and the non-linearity of the disinfection processes at

extremes (Jagger, 1981, Fisher and Nelson, 2014).

While the model used an E. coli WP2s strain, other pathogens or container materials

need to researched. Recently Beck et al. (2015) provided new action spectra within the

UV-C/B range for important waterborne pathogens. It is suggested that similar action

spectra for a broader range of wavelengths are now also required to provide SODIS

disinfection with a stronger theoretical basis.

The atmospheric model used in this experiment was simplified and did not include

variations due to local weather and atmospheric pollution, which filter UV wavelengths

in particular. Complementing this, a more detailed analysis of altitude impacts might

capture the benefit of elevation by taking into account the cleanliness of high altitude

atmospheres.

In respect to this thesis topic, the most important implication of this modelling was that

most of the total energy content of the sunlight received is of little use for disinfection

compared to the UV-B and shorter UV-A wavelengths. This suggests great benefits

might be achieved by transforming incoming solar radiation into more effective shorter

wavelengths using, for example, photovoltaic-powered UV LED devices while still

employing sustainable, renewable energy sources.

7.4. Comparison with PV-Powered LED Disinfection

While the performance of photovoltaic panels does vary as a function of the input

spectra, they are much less sensitive to the spectral content of sunlight, with silicon

based cells able to utilise all incoming photons below about 1100nm (King et al., 1997).

192

Commercial cells currently achieve around 20% efficiency in conversion from solar

radiation to electrical energy under air-mass 1.5 conditions, with UV-LEDs ranging in

efficiency from 1% to 20% efficiency, resulting in a net conversion efficiency of

between 0.2% (to UV-C) to 4% (to UV-A).

Compared with sunlight, this is advantageous because sunlight provides virtually no

UV-C which are the most effective wavelengths for disinfection, and the UV-A and

UV-B content of sunlight (400nm and shorter) at midday at the equator at equinox only

represents 6.37% of the incoming energy. It would seem that on an area parity basis,

pure sunlight does provide more net UV-A, however this overlooks the fact that at other

latitudes, seasons, time-of-day and atmosphere/cloud-cover influences, the UV-A and

UV-B content varies considerably and SODIS containers may further restrict the UV-A

and UV-B dosage.

Additionally, a photovoltaic-powered system offers the advantage of more control and

the ability to collect and concentrate a larger area of sunlight into a reactor to further

enhance disinfection rates and the potential to disinfect water when the sun is at a low

elevation by over-sizing the power supply and with appropriate panel orientation. This

is especially important for winter disinfection. Furthermore, night-time disinfection is a

possibility where a battery is employed.

193

7.5. Conclusion

A simple model, relating an E. coli WP2s action spectrum, solar spectra and PET

transmission, was used to gain insight into the contributions of solar wavelengths to the

photoinactivation process relevant to SODIS.

Significant spectral sensitivity to the UV-A and UV-B portion of sunlight was evident,

with the addition of PET plastic causing significant loss to UV-B and to predicted

imparted disinfection performance. With the inclusion of PET plastic, disinfection relies

more heavily on longer wavelengths of light.

Significant seasonal and geographical variations in SODIS performance were illustrated

by the model, due to atmospheric absorption of UV light and reduction in total solar

energy in winter when the sun peaks at a lower zenith. The data confirmed how areas

outside the tropics should experience poor SODIS performance for at least part of the

year and that the higher latitude tropics may also experience significantly lower

performance. Increasing altitude was predicted to provide modest improvements in

disinfection due to the increase in UV content, combined with increased solar energy

but not sufficient to offset the impact of solar angle and container materials.

Overall, the modelling suggested that less spectrally sensitive POU technologies, such

as photovoltaic-powered UV-LED disinfection, may be preferable outside of the

equatorial latitudes and even within it, winter months pose a disinfection challenge. PV

powered disinfection also offered the potential to compensate for low solar intensity

through over-provisioning of PV modules, in essence, collecting solar energy over a

larger area to be concentrated by LED emission into a smaller reactor. In addition,

higher UV-transmission alternatives to PET bottles should be explored.


SODIS performance was shown to be sensitive to the spectral content of the sun,

with a majority of inactivation occurring due to UV-B content which is most

affected by time-of-day, altitude and seasonality effects. Even at tropic latitudes,

194

disinfection performance varied significantly and satisfactory year-round

performance of SODIS could not be ensured.

Existing recommendations around SODIS in regards to exposure time are

supported by the results, suggesting the six-hours straddling noon to be most

important to inactivation.

Polyethylene terephthalate (PET) bottles were found to filter a majority of UV-B

radiation, likely impacting on SODIS disinfection rates and supporting focus on

better container materials.

As only photoinactivation is considered, the study is necessarily simplified and

incomplete, neglecting considerations for thermal synergies, atmospheric

weather and producing absolute inactivation figures. Further research is required

to address these areas.

PV-powered LED-based disinfection is mostly immune to spectral effects due to

the broad-band sensitivity of solar cells, which are most efficient in the longer

wavelengths due to their fixed band-gap. They also offer potential for sunlight

concentration by using a larger area of panels to compensate for reduced

sunlight.

195

8. Technical Developments, Market Forces and Disinfector

Cost Modelling

8.1. Introduction

Since the literature review (Chapter 2) was performed at the commencement of this

study, and the purchase of electronic components for bench-scale and prototype

reactors, several years have passed and it was expected that progress in both LED and

PV technology, prices, efficiency and power output per unit would have eventuated.

Separately, the cost of the prototype deployment reactors (Table 21, Table 22 and Table

23) reflected retail prices for small quantities. So realistically, where PV+UV-LED

based POU disinfector units are mass produced, much lower prices should be

achievable.

In this chapter, I re-examined the products available in the market to capture the rate of

progress in LED devices and refine understanding of LED and wavelength costs as they

relate to the present (2016) market. This chapter also examines the market dynamics for

bulk volume discounts from various vendors across several categories of related

products to determine the potential for cost reduction in mass production. Finally, it

relates all the pricing variables to disinfection performance related variables using

Bayes net and spreadsheet models which integrate these data to project unit costs for

different PV-powered LED-based POU devices. A scenario analysis of the costing

models is presented in order to relate likely design configurations to cost drivers which

affect the feasibility of the PV+LED concept.

8.2. UV-LED Device and Pricing Evolution

Progress within the LED market has tended to occur at an exponential rate, following

Haitz's Law (Graydon, 2007), and with strong predictions of LED market growth

occurring in the UV spectral region (Markets and Markets, 2016, Semiconductor Today,

2015), rapid developments in the performance and pricing of UV LEDs are expected in

the near future.

196

8.2.1. UV-C/UV-B LED Market Developments

Pricing of UV-C LEDs is difficult to obtain as few manufacturers sell directly to the

public, and rarely are prices published openly. However, this was not always the case

for SETi's products, where an online shop allowed viewing pricing for individual units

of UVTOP TO-18/TO-39 style hermetically sealed LEDs. It was found that, compared

to a commercial price-list obtained in 2011 (see Appendix D), the prices have generally

only reduced slightly - on average by just 9%. The specifications for the LEDs remained

identical, with no advances in typical output powers or reduction in power consumption.

Nevertheless, while the devices available from SETi, at retail, have not substantially

changed in price or specifications since the literature review, the market apparently has

not been stagnant.

Sensor Electronic Technology Inc (2016) have introduced new product lines with

enticing specifications targeting high powered UV-C applications in surface-mount

device (SMD) and larger TO-3 can packages with multiple wavelength options. These

products are sold under the UVCLEAN® branding and only have limited specifications

available at this stage. However, the product descriptions indicate power ranges from 1-

50mW output at wavelengths between 255-350nm, with optional thermoelectric cooler

for TO-3 packages, are now available.

Crystal IS's merger with Asahi Kasei has seen them moving away from unpackaged

experimental UV-C LEDs into mass produced sealed can and surface mount devices

(SMD) in the 250-280nm range, dubbed Optan and Klaran respectively (Crystal IS,

2016b, Crystal IS, 2016a). Engineering samples are now available for purchase though

detailed specifications are available only on enquiry.

Other vendors include Seoul Optodevice's subsidiary, Seoul Viosys (2014) which has

standardised on a range of 275, 310 and 340nm devices. Nikkiso America (2016) also

offers UV-C LEDs at wavelengths of 265, 285 and 300nm in TO-46, SMD and multi-

chip modules. DOWA Electronics Material Co. Ltd. (2016) now offers a range of

canned, SMD and module LEDs at 265, 280, 310, 325 and 340nm with indications that

at 310nm and longer wavelengths reportedly have >85% of their initial output at 5 000h.

197

International Light Technologies Inc. (2016) offers SMD LEDs with silicon lenses at

260, 273, 305 and 361nm with claimed lifetimes of 1 000h. LG Innotek, also has begun

to offer 275, 278 and 305nm LED modules to the market (Laser Components, 2016,

LEDs Magazine, 2014). Finally, Hexatech Inc. (2011) offers TO-39 and 3.5mm x

3.5mm ceramic packaged LEDs between 250-280nm with a claimed lifetime of 50 000

hours although, again, the product details are not available.

All of the new devices with publically accessible datasheets have been collated in Table

24. Only packaged examples have been collated, as those with flat window lens optics

appear preferable.

While these devices are presently listed online, their availability and pricing are not

available and so obtaining these products from manufacturers directly or through their

limited distributor channels remains a barrier to research, with some vendors insisting

on minimum order quantities. Continuing difficulties with low efficiency are reflected

in SETi's inclusion of an optional in-package thermo-electric cooler, which itself

consumes electricity and further reduces the net electrical efficiency, for their high

powered UV-C arrays to control heating. It is also reflected in the efficiency figures for

UV-C LEDs which still remain near the 1% range. Thus, improvements in efficiency

are still needed, as thermal control is a major hurdle in ensuring that UV-C LEDs

produce their full rated output and achieve their maximum lifetime.

The choice of package for mass production now seems to favour SMD devices, which

should improve automated manufacturing ability and thermal dissipation, although this

requires more careful specialised metal-core printed circuit boards to effectively

conduct heat away from the package. Mass production also seems to have standardised

on the use of 275-278nm for UV-C sterilisation applications, which is supported by

action spectra and power output considerations. It is likely that larger arrays, while

existent and capable of raising the per-package power output, are quite costly due to

their price-on-application nature of their sales. That said, it is encouraging to note that a

single SMD 5050 unit can produce more UV than the 5-LED array used in the bench

scale experiments three years ago.

198

Table 24 - New UV-C LED products

Vendor Part Number

Wavelength

(nm)

Optical

Power (mW)

Input

Power (W)

Efficiency

(%) Package

ILT E265SL 265 0.35 0.132 0.27 SMD, Silicon Lens

ILT E273SL 278 1 0.12 0.83 SMD, Silicon Lens

ILT S305SL 310 1 0.11 0.91 SMD, Silicon Lens

LG Innotek LEUVA66G00KF00 305 8 0.7 1.14 SMD, 6060

LG Innotek LEUVA33B00HF00 278 1 0.0975 1.03 SMD, 3535

LG Innotek LEUVA66B00HF00 278 1.6 0.13 1.23 SMD

LG Innotek LEUVA66G00HF00 265-285 8 0.85 0.94 SMD, 6060

Seoul Viosys CUD8AF1A 275 1.6 0.11 1.45 SMD

Dowa L-1F131 265 1 0.17 0.59 TO-5 Can

Dowa L-1F131 280 1.7 0.13 1.31 TO-5 Can

Dowa L-1F131 310 0.8 0.13 0.62 TO-5 Can

Dowa L-1F131 325 1.2 0.09 1.33 TO-5 Can

Dowa L-1F131 340 1.3 0.08 1.63 TO-5 Can

Dowa L-1F111 265 0.7 0.17 0.41 TO-18 Can

Dowa L-1F111 280 1.3 0.13 1.00 TO-18 Can

199

Vendor Part Number

Wavelength

(nm)

Optical

Power (mW)

Input

Power (W)

Efficiency

(%) Package

Dowa L-1F111 310 0.7 0.13 0.54 TO-18 Can

Dowa L-1F111 325 1.1 0.09 1.22 TO-18 Can

Dowa L-1F111 340 1.1 0.08 1.38 TO-18 Can

Dowa K-2F001 265 0.9 0.17 0.53 SMD, 3020

Dowa K-2F001 280 1.3 0.13 1.00 SMD, 3020

Dowa K-2F001 310 0.83 0.13 0.64 SMD, 3020

Dowa K-2F001 325 1.2 0.09 1.33 SMD, 3020

Dowa K-2F001 340 1.6 0.08 2.00 SMD, 3020

Dowa F-1X009 265 9.5 1.56 0.61 Chip on Board

Dowa F-1X009 280 12 1.2 1.00 Chip on Board

Dowa F-1X009 310 6.2 1.14 0.54 Chip on Board



Dowa DF7VK-2F001 265 0.9 0.17 0.53 SMD, 3020

Dowa DF8VK-2F001 280 1.3 0.13 1.00 SMD, 3020

Dowa UF1VK-2F001 310 0.83 0.13 0.64 SMD, 3020

200

Vendor Part Number

Wavelength

(nm)

Optical

Power (mW)

Input

Power (W)

Efficiency

(%) Package

Dowa UF3VK-2F001 325 1.2 0.09 1.33 SMD, 3020

Dowa UF4VK-2F001 340 1.6 0.08 2.00 SMD, 3020

Dowa DF7VF-1X009 265 5.7 1.71 0.33 3x3 Module

Dowa DF8VF-1X009 280 10.8 1.23 0.88 3x3 Module

Dowa UF1VF-1X009 310 6.2 1.11 0.56 3x3 Module



Dowa DF8VN-2D001 280 2.3 0.13 1.77 SMD, 3838

Dowa UF1VN-2D001 310 2.7 0.118 2.29 SMD, 3838

Dowa UF3VN-2D001 325 3.7 0.09 4.11 SMD, 3838

Dowa UF4VN-2D001 340 3.5 0.08 4.38 SMD, 3838

Dowa DF8VG-2D004 280 8.7 0.48 1.81 SMD, 5050

Dowa DF1VG-2D004 310 10.5 0.48 2.19 SMD, 5050

Dowa DF3VG-2D004 325 14.9 0.32 4.66 SMD, 5050

Dowa DF4VG-2D004 340 16.4 0.316 5.19 SMD, 5050

SETi UVTOP270SMD35EW 273 1.3 0.11 1.18 SMD, 3535

201

8.2.2. UV-A and Visible LED Market Development

When surveying preferred suppliers of UV-A and visible LEDs, it was seen that some

of the chosen products have been discontinued in favour of new generation products

introduced in March 2015. The newer generation 365nm LED offers 61% more output

power at 44% less price and improves on electrical efficiency by 11.5% for a net gain of

187% more output for the same price. Similarly, visible wavelength LEDs have seen an

approximate halving in price. Royal Blue (460nm) options from LED Engin are now

available, and CREE XT-E Royal Blue (455nm) LEDs show slightly increased power.

8.2.3. Market Candidate LED Products for Disinfection

From the revised market survey, the list of current candidate LED products, based on

their availability and ease of construction, was tabulated in Table 25. Their values were

normalized by weighting using the E. coli K12 and E. faecalis ATCC 19433 action

spectra developed in this study as described in Chapter 3 (Table 9). The results are

given in Figure 72 with a logarithmic scale on the vertical axis.

Considering only the disinfection potential and the cost of the LEDs themselves, the

results of optimisation show a local maxima at 270nm in the UV-C and 365 and 390nm

in the UV-A, likely due to the improved efficiency of the new generation UV-A LEDs.

UV-C LEDs, despite cost, still offer more log-reduction per cm2 per second per dollar

due to the high sensitivity of E. coli to UV-C. However, this difference was less for E.

faecalis where 365nm achieves similar performance for LED price as 270nm. It must be

also remembered that this calculation is based upon the bench-scale action spectrum,

which may not take into account the full disinfective potential of UV-A which was only

achieved for the reactor with long path length. With that in mind, the prospects of UV-A

may be still higher. However, further consideration of the increased balance of system

costs in providing more power to run larger UV-A arrays and in thermal management

will also negate the lower LED costs.

202

Table 25 - Present candidate LEDs for POU disinfectors a

Nominal

Wavelength

(nm) Model Vendor

Unit

Cost

(US$)

Output

Power

(mW)

Input

Power

(W)

Electrical

Efficiency

(%)

Price per

mW Output

($/mW)

240 UVTOP240-TO38FW SETi 339.90 0.07 0.20 0.04 4855.714

255 UVTOP255-TO38FW SETi 295.90 0.3 0.13 0.23 986.333

260 UVTOP260-TO38FW SETi 273.90 0.3 0.13 0.23 913.000

270 UVTOP270-TO38FW SETi 218.90 0.8 0.12 0.65 273.625

280 UVTOP280-TO38FW SETi 185.90 0.8 0.12 0.69 232.375

285 UVTOP285-TO38FW SETi 174.90 0.8 0.12 0.69 218.625

295 UVTOP295-TO38FW SETi 163.90 0.5 0.11 0.45 327.800

310 UVTOP310-TO38FW SETi 141.90 0.6 0.11 0.55 236.500

335 UVTOP335-TO38FW SETi 108.90 0.4 0.10 0.40 272.250

355 UVTOP355-TO38FW SETi 64.90 0.8 0.09 0.89 81.125

365 LZ4-44UV0-0000 LEDEngin 66.41 2900 10.64 27.26 0.023

385 LZ4-40UA00-00U4 LEDEngin 57.75 2900 11.00 26.36 0.020

390 LZ4-40UA00-00U5 LEDEngin 46.83 3500 11.00 31.82 0.013

395 LZ4-40UA00-00U6 LEDEngin 57.75 3500 11.00 31.82 0.017

400 LZ4-40UA00-00U7 LEDEngin 57.75 3800 11.00 34.55 0.015

405 LZ4-40UA00-00U8 LEDEngin 57.75 3800 11.00 34.55 0.015

203

Nominal

Wavelength

(nm) Model Vendor

Unit

Cost

(US$)

Output

Power

(mW)

Input

Power

(W)

Electrical

Efficiency

(%)

Price per

mW Output

($/mW)

455 XTEARY-02-0000-000000N04 Cree 3.94 935 2.14 43.69 0.004

460 LZ4-40B208-0000 LEDEngin 15.75 3892 8.96 43.44 0.004

525 LZ4-40G108-0000 LEDEngin 18.00 1119 10.08 11.10 0.016

590 LZ4-40A108-0000 LEDEngin 20.05 775 7.00 11.07 0.026

623 LZ4-40R108-0000 LEDEngin 15.77 1692 7.00 24.18 0.009

660 LZ4-40R200-0000 LEDEngin 16.78 2600 7.35 35.37 0.006

740 LZ4-40R300-0000 LEDEngin 18.35 2100 6.30 33.33 0.009

a: Absolute log-reduction potential may be estimated by cross-referencing with action spectra (Figure 16 and Table 8).

204

Figure 72 - LED cost and wavelength optimisation by action spectra

8.2.4. PV Power Developments

Silicon PV module spot prices continue to reduce, reaching an average of US$0.54/Wp

(PVinsights, 2016), which represents close to halving of the 2013 wholesale price due to

continued demand and increasing supply. In Europe, prices range from EUR 0.37 to

EUR 0.50/Wp (PV Magazine, 2016). Many tier-1 Chinese manufacturers are already

achieving below US$0.50/Wp with targets of US$0.40/Wp by the end of 2016

(Parkinson, 2015). Commercial module efficiencies have continued to increase, with

average commercial silicon wafer-based modules increasing from 12 to 16% over the

past decade (Fraunhofer ISE, 2016), with the leading products reaching 22.1% (Shahan,

2015). Non-silicon technologies, such as cadmium telluride (CdTe) have reached

average module efficiencies of 13%. Multi-junction tandem cells have reached higher

efficiencies of up to 38.9% with better utilization of the solar spectrum, and have been

used in space and solar concentrator applications where utmost efficiency is required.

Their main drawback is high cost and current/voltage matching of the two tandem

junctions.

270

365460

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

240 340 440 540 640 740

Log

10

Red

uct

ion

/ c

m2

/ s

/ $

Wavelength (nm)

E. coli K12


E. coli K12


205

An emerging photovoltaic technology utilizes perovskites, a class of inorganic mineral

materials with crystalline structures similar to calcium titanium oxide (CaTiO3). These

have been receiving attention due to their apparent potential in realising even higher

efficiency solar cells (Niu et al., 2015, Snaith, 2013, Zhou et al., 2014, Green et al.,

2014). The technology is at infancy, although the rate of development in their efficiency

has been staggering, they are not yet ready for commercial manufacturing with observed

stability and degradation issues and require further research (Green and Bein, 2015).

8.3. Bulk Volume Discount

As all parts for the reactor prototypes were obtained at small volume retail prices, they

do not reflect the price which could be obtained if the products were purchased in bulk.

In order to address this, data were collected from suppliers to attempt to determine the

wholesale discount applicable for UV-C LEDs, and mass-produced non-UV-C LEDs

and other components.

8.3.1. UV-C LEDs

The difficulties encountered in obtaining bulk volume pricing for UV-C LEDs are even

more severe than for obtaining retail pricing. It was determined that bulk pricing was no

longer published for SETi LEDs, however, as the prices have only reduced slightly, the

bulk pricing data from 2011 price list (Appendix D) was used to quantify how volume

discount affects the pricing of UV-C LEDs (Table 26).

These data collected strongly suggest that significant reductions in the cost of UV-C

LEDs are possible if purchasing occurs in large volumes. In the absence of alternatives,

this discount factor was applied to retail price to determine the cost of mass production,

however, it may only be applicable to SETi product pricing.

206

Table 26 - UV-C LED volume discount factor

Up to number of units Average Discount (% off)

10 0

20 5.178

50 10.21

100 15.05

500 23.78

1000 33.21

2500 58.99

5000 75.40

10000 85.74

50000 89.55

100000 92.34

500000 95.06

1000000 95.95

8.3.2. Non UV-C LEDs and Other Components

In order to determine the volume cost reduction for more mature and already mass-

produced non-UV-C LEDs and other components, suppliers used to purchase

components for the systems were surveyed for their bulk pricing on a variety of items.

These suppliers included Mouser Electronics, Digikey Corporation, element14, Jaycar

Electronics and RS Components. The price breaks for these products were recorded and

divided by the unit cost to determine the average discount multiplier for a given product

volume. As not all types of products are available with the same price breaks,

occasional "jumps" in the curves appeared where discount factors for different types of

products differ. The combined data were plotted in Figure 73. The regression indicated

a logarithmic line of best fit with an R2

of 0.975 indicating generally good agreement

with discount factors. The discount factor was extrapolated through to 1 million units

which showed a 59% discount. This is less than what is achieved with UV-C LEDs

likely due to the maturity of mass-produced products which are more cost-competitive

and have less margin for cost reduction.

207

Figure 73 - Discount factors for non-UV-C LED products and other components

8.4. Bayesian Network Modelling

Bayesian network modelling is a modelling tool consisting of a graph of nodes and

acyclic links that conceptualise a system based on conditional probabilities (Kragt,

2009). Bayesian network (BN) modelling can facilitate learning about causal

relationships between variables, by easing the combination of incomplete knowledge

through the creation of belief networks which can learn from data input (Korb and

Nicholson, 2010). It excels at combining data sources containing uncertainties,

incomplete knowledge, observations, simulations and expert knowledge. Using BNs, it

is possible to perform both diagnostic and predictive reasoning. Its main limitations

include not allowing for modelling feedback loops (Smid et al., 2010).

The construction of Bayes nets can be complex, with different approaches resulting in

different networks with different levels of complexity. It is generally recommended that

nets are built with causal factors first, then moving onto the factors they affect, until the

final outcomes are reached (Korb and Nicholson, 2010). As the network consists of

computed conditional probability tables which take into account every input

combination, nodes should be limited in the number of inputs (preferably three or

y = -4.069E-02ln(x) + 9.779E-01

R² = 9.754E-01

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 10 100 1000 10000 100000 1000000

Mu

ltip

lier

Number of Units

Mouser

Digikey

Element14

Jaycar

RS Components

Trendline Fit

208

fewer) and states (preferably five or fewer) (Marcot et al., 2006) to avoid combinatorial

explosion. Iterative processes of first defining the model, conceptualising the model,

parameterising the model, evaluating the model and analysing the model were used to

refine the network (Kragt, 2009). However, more states may be allowable where

binning reflects reality and probability density functions (Korb and Nicholson, 2010).

The BN model was first conceived as a cost model to assess the financial feasibility of

various configurations of PV-powered LED-based disinfectors based on the knowledge

obtained in this investigation, including the action spectra, market prices of LEDs, PV,

charge controllers, system controllers and ancillaries, LED specifications, engineering

considerations and bulk price discounts. This network was seen as providing insight not

only into costing, but also disinfector performance and key electrical requirements and

factor interrelationships.

The first iteration of the network (Figure 74) was conceptualised using the Netica

Application (Norsys Software Corp., 2016). However, this preliminary version proved

too difficult to parameterize due to the number of nodes and the excessive number of

connections required. Based on an analysis of the bills of materials listing, it was

decided that a second model revision did not need to be as sophisticated, as many of the

smaller balance-of-systems parts make small contributions to the final pricing,

compared to the major components. Further to this, some connections enforced a strict

relationship which were determined to be detrimental to the flexibility of the model, as

while some possibilities were not realistic, the model should not prevent them from

being simulated. Finally, it was determined that the net was also too focused on

secondary objectives in documenting the power consumption, while failing to

conceptualise the financial feasibility of bulk volume pricing.

A revised network was subsequently created, focusing on key variables and ease of

implementation with a focus on minimising the number of non-influential nodes,

reducing node connections at each stage and limiting the number of node states/ranges.

The resulting network is shown in its collapsed form (Figure 75). The network was

parameterised with data and equations detailed in Table 27. Where possible, actual

market data for figures such as pricing, efficiency and discounts were used rather than

209

fitted curves. The latter were found to severely over/underestimate the prospects of

certain wavelengths due to poor fit. Underlined text indicates nodes used in other

calculations. Some calculations were necessarily simplified to avoid overcomplicating

the model with ranges based on sensible values based on the three prototypes. Other

user inputs were unrestricted to facilitate scenario analysis, though some were

implausible (e.g. UV-C LED based device with 1.5m path length). The network thus

represents the larger space of possibilities, encompassing more than the practically

useful configurations.

210

Figure 74 - Initial Bayes net concept

211

212

Figure 75 – Final Bayes net design overview

The full network is displayed with belief bars for the discretised state in Figure 76.

Despite keeping the number of states limited, calculation of the conditional probability

tables still took three minutes on a relatively modern desktop computer (AMD Phenom

II x6 1090T). The network complexity probably pushed this software to its limits.

However, its predictions were comparable to those obtained subsequently using the

spreadsheet model. The network was instrumental in allowing visual identification of

the causes and influences of each of the factors affecting feasibility, and understand the

distribution of outcomes based on the belief bars. This information was used to identify

plausible optimised system specifications.

213

Table 27 - Bayes net model - nodes and algorithms

Node Type Range, Algorithm or Data Source

Wavelength User Input 240-460 nm

Optical Power Proposed User Input 0.0002-15 W

Microorganism User Input E. coli K12 OR E. faecalis ATCC 19433

Safety Factor User Input 1-10 (factor)

Exposure Time User Input 1- 360 minutes

Safe expT Calculated Exposure Time * Safety Factor (minutes)

Cross Sectional Area User Input 50-2000 cm2

Path Length User Input 5-150 cm

S90 Calculated S90 at Wavelength and Microorganism (Action Spectra from Table 9) * Safety

Factor (J/cm2/log)

Volume Calculated Cross Sectional Area * Path Length (L)

LED Optical Power per Unit User Input 0.0005-5 W

log10 Reductions Calculated Optical Power Proposed * Exposure Time * 60 / Cross Sectional Area / S90

(logs)

LED Lifetime User Input 50-50000 hours

LED Efficiency Data Input From Table 25 (fractional)

No. LEDs per reactor Calculated Ceiling (Proposed Optical Power / LED Optical Power per Unit) (count)

LED Cost per Output W Data Input From Table 25 ($/Watt)

214


System Lifetime Treated Volume Calculated LED Lifetime * 60 / Safe expT * Volume (L)

LED Electrical Power Calculated Optical Power Proposed / LED Efficiency (W)

Window User Input $0 OR $0.27 OR $39

Pump Cost User Input $0 OR $10 OR $20 OR $70

Vessel and Other Costs User Input $0-$20

Pipe and Fittings User Input $0-$20

Heatsink Calculated If (LED Electrical Power) > 1W then $20, else $0

Current Driver Calculated If (LED Electrical Power / No. of LEDs per reactor < 1), then No. of LEDs per

reactor * $0.50, else No. of LEDs per reactor * $25

UV Sensor User Input $0 or $22

Enclosure User Input $0-$25

System Controller User Input $0 or $40

Wiring User Input $0-$40

Electrical Power Required Calculated LED Electrical Power + (If (Pump Cost == $70) then 36, else if (Pump Cost ==

$20 || Pump Cost == $10) then 2.5, else 0) (W)

PV Cost per W User Input $1.00-2.50

PV Oversupply Factor User Input 1-10 (factor)

PV Cost Calculated Electrical Power Required * PV Oversupply Factor * PV Cost per W ($)

215


Battery/Controller Oversupply

Multiplier

User Input 0-5 (disinfection cycles)

Battery and Charge Controller

Cost

Calculated If (Battery/Controller Oversupply Multiplier != 0) then 95 + Electrical Power

Required * Safe expT / 60 * Battery/Controller Oversupply Multiplier * 0.20,

else 0 ($)

Cost of Power Supply Calculated PV Cost + Battery and Charge Controller Cost ($)

LED Array Cost Calculated LED Cost per Output Watt * Optical Power Proposed

Reactor Costs Calculated Pipe and Fittings Cost + Vessel and Other Costs + Pump Cost + Window ($)

Balance of System Costs Calculated Heatsink + Current Driver + Wiring + UV Sensor + System Controller +

Enclosure ($)

Total Non LED Costs Calculated Reactor Costs + Cost of Power Supply + Balance of System Costs ($)

Number of Units User Input 1-1000000 (units)

LED Discount Factor Calculated If (Wavelength < 365) then as per Table 26, else 1-4.753188*10-2

*ln(x)

(multiplier)

Non-LED Discount Factor Calculated 1-4.753188*10-2

*ln(x) (multiplier)

Bulk Cost per Unit Calculated LED Array Cost * LED Discount Factor + Total Non-LED Costs * Non-LED

Discount Factor ($)

Project Treatment Cost Calculated (Bulk Cost per Unit * 100) / (System Lifetime Treated Volume /1000 * log10

Reductions (c/logs.kL)

216

Figure 76 – Revised Bayes network belief bars view

217

In order to assure the accuracy of the network, a corresponding spreadsheet containing

the same logic and variables was developed in parallel and used to cross-check the

computed figures. Its limitation was that it could model only point values. It was found

that the Bayes net was in agreement with the spreadsheet.

The program files for the Bayes net and spreadsheet are provided in Appendix D. Due

to the complexity of the net, only fully licensed versions of Netica are able to edit the

Bayes net document. The spreadsheet offers the equivalent calculation functionality and

increased precision for use with Microsoft Excel or compatible spreadsheet software.

8.5. Scenario Analysis

With the refined model, the influence of key parameters could be explored. Key

questions included:

How do the costs of the disinfectors reduce as volume increases?

What is the minimum possible cost configuration for a UV-C or UV-A steriliser

at 20L household volume? At 250mL single glass volume? Are there any

significant disadvantages to these configurations?

What is the impact of incorporating safety margins, or instead, targeting E.

faecalis to drive disinfector cost?

What is the prospect of using the least expensive and most common 455nm

LEDs for household volumes?

What is the lifetime treatment cost for these units and how do they compare?

What would it take to build an effective UV-B disinfector using present-day

LED devices?

What is the final bulk cost per unit most sensitive to?

What is the predictive power of the net?

8.5.1. Modelling Various System Configurations

System configurations which were of interest were modelled using the spreadsheet

model. The inputs to the model are detailed in Appendix C. The result of the modelling

is shown in Table 28 with unique system characteristics highlighted in bold.

218

Table 28 - Cost modelling of different configurations of disinfector units

Node Scenario

A a B

b C

c D1 D2 E1 E2 F1 F2 G H

Wavelength 270 270 365 270 365 270 365 270 365 455 310

Optical Power 0.001 0.008 5.4 0.0005 2.7 0.00003 0.165 0.305 346 22.8 0.0165

Microorganism 0 0 0 0 0 0 0 1 1 0 0

Safety Factor 1 1 1 1 1 1 1 10 10 1 1

Safe expT 15 60 180 180 180 180 180 180 180 180 180

Volume 0.25 10 15 25 25 1.5 1.5 20 20 25 1.5

log10 Reductions 104 83 20 6 6 6 6 61 60 6 6

System Lifetime

Treated Volume

4998 50000 250000 41667 416675 2500 25000 333335 3333500 41668 2500

Electrical Power

Required

0.2 3.7 55.8 2.6 12.4 2.5 3.1 49.8 1305.5 88.2 5.5

Battery/Controller

Oversupply

Multiplier

0 0 2 0 0 0 0 2 2 2 0

LED Array Cost 274 2189 124 137 62 8 4 83456 7923 96 3902

Total Non LED

Costs

59 253 693 61 245 61 67 1734 13815 1237 192

219

Node Scenario

A a B

b C

c D1 D2 E1 E2 F1 F2 G H

Cost per Unit

(1 Unit)

333 2442 817 198 306 69 71 85190 21738 1333 4094

Project Treatment

Cost (c/log.kL)

64 59 16 76 12 440 46 420 11 53 26917

Bulk Cost per Unit

(100 Units)

255 1866 638 152 239 54 55 64966 16980 1041 3124

Project Treatment

Cost (c/log.kL)

49 45 13 58 10 343 36 320 8 41 20541

Bulk Cost per Unit

(1 000 Units)

152 1067 548 97 206 44 47 35390 14601 895 1729

Project Treatment

Cost (c/log.kL)

29 26 11 37 8 282 31 175 7 35 11369

Bulk Cost per Unit

(10 000 Units)

62 371 459 49 172 35 40 9694 12221 749 516

Project Treatment

Cost (c/log.kL)d

12 9 9 19 7 224 26 48 6 30 3390

Values in table rounded to nearest $/c (full values, see Appendix D), bolded values are best in category, a: Original costing in Table 21, b:

Original costing in Table 22, c: Original costing in Table 23, d: Compare with Sydney Water residential usage charges of 227.6c/kL.

220

A total of twelve scenarios were simulated:

Scenarios A, B and C which reflect prototype systems in Table 19 were

simulated by using approximate figures. This was used as a reality check to

discern the predictive power of the model.

Scenarios D1 and D2 look at the costs of a hypothetical UV-C and UV-A

disinfector suited to household 25L volumes, providing 6-log reduction of E.

coli K12 in 3 hours.

Scenarios E1 and E2 focus on a hypothetical cost-minimised UV-C and UV-A

disinfector targeting SODIS-level 1.5L volumes with 6-log reduction of E. coli

K12 in 3 hours.

Scenarios F1 and F2 simulate a hypothetical "deluxe" model steriliser using UV-

C and UV-A to achieve a 10-fold safety factor over 6-logs in 3 hours using E.

faecalis action spectra data with the provision of a battery oversupply multiplier

of 2 hours of operation as a safety against overcast days.

Scenario G models a 455nm based system providing 6-log reduction of E. coli

K12 in 3-hours for a household volume of 25L with a battery oversupply

multiplier of 2 hours of operation.

Scenario H looks at the hypothetical configuration necessary using present-day

310nm LEDs to disinfect 1.5L volumes of water.

For all scenarios, UV-C LED lifetime was taken to be 5,000 hours conservatively

reflecting the lifetime demonstrated with the single UV-C LED in Section 5.3.3. UV-A

LED lifetime was taken to be 50,000 hours. It is acknowledged that while the model

may predict certain levels of log inactivation, this is only possible under optimal

hydrodynamics, for where the deployment system achieves the same sensitivity as the

bench-scale experiments whose action spectra the inactivation prediction is based on. In

reality, it can be possible that this method overestimates the inactivation for UV-C and

potentially underestimates the inactivation for UV-A. When modelling flow-through

reactors, the model's cross-sectional area and path length does not account for dead

volume outside the reactor.

221

While the model contains recommended ranges of values, some of the values for the

prototype designs fall outside these ranges. The model will still function outside these

ranges, which may be necessary when simulating battery based systems utilising an

integrated design rather than relying on an expensive third party controller. Such

configurations were not explored in this analysis. Further to this, it is possible to

simulate conditions (e.g. power levels for LEDs) which are not commercially produced.

8.5.2. Scenario A, B and C

Testing the predictive power of the model using selections which approximately reflect

the deployment models resulted in reasonable levels of agreement. Scenario A's

predicted price was 18% above that of the actual costing. The majority of the difference

was due to the fact that the LED's power is based on the test report data which was

above the typical power level, thus resulting in a higher cost than actual. The same

effect was reflected in Scenario B, which had a predicted price 36% higher than the

actual costing, with the majority of the difference in the LED cost. Finally, Scenario C

was predicted to have a cost 15% below the actual costing due to the use of the most

recent UV-A 365nm LED prices which had cut LED costs in more than half. The

resulting prices from the model are well within the realm of plausibility.

The model breakdown of costs show that for both UV-C systems, the LED costs

dominate the total cost of the unit. The opposite is true for the UV-A system as the

LEDs are much less expensive, and the balance of system costs in battery and power

supply dominate. When volume production is introduced, more significant cost savings

are realised for the UV-C based systems. For a total of 10 000 units, the expected

pricing for a single-glass disinfector (Scenario A) falls to just US$61.73, with the

pumped UV-C disinfector (Scenario B) predicted to cost US$370.73 and the pumped

UV-A disinfector (Scenario C) is ultimately more costly at US$459.10 which is a

reversal of the single-unit retail price where Scenario B was nearly twice as costly as

Scenario C. This clearly indicates the power of bulk volume discount on pricing.

At this volume, both Scenario B and Scenario C represent similar value for money with

Scenario A slightly behind. This, however, is based on the predicted log-reduction,

rather than the achieved log-reduction.

222

8.5.3. Scenario D1 and D2

For the hypothetical cost-minimised 25L disinfectors represented by Scenario D1 and

Scenario D2, again, the cost is dominated by LED costs for the UV-C, and by balance

of system for UV-A. The UV-C based Scenario D1 achieves a lower price per unit at a

scale of 1 through to 10 000, however, due to the shorter simulated 5 000h lifetime

compared to 50 000h lifetime of UV-A LEDs, the UV-A system is able to achieve

almost three times as much treatment for the same price. The cost-minimised

configuration is encouraging, as it suggests that UV-C systems in 10 000 unit quantities

could be had as low as US$48.74, making it potentially affordable. However, as this

system does not include any safety factors for natural water, LED degradation,

operating conditions, and only achieves the 6-log reduction for equivalent

hydrodynamics to the bench-scale apparatus with E. coli, it is not likely to achieve

anywhere near the expected performance in the field. It also requires the manufacturing

of lower-powered UV-C diodes to reduce the unit cost.

8.5.4. Scenario E1 and E2

The cost-optimised SODIS-emulating 1.5L disinfectors showed a different result, with

costs entirely dominated by balance of system costs. Both UV-C and UV-A disinfectors

were configured for 6-logs in 3 hours and both achieved fairly similar per unit bulk

prices in 10 000 quantity of US$34.90 and US$39.69 respectively. These both suffer

similar caveats to Scenarios D1 and D2, again due to lifetime. The UV-A system offers

close to 9 times as much disinfection per dollar.

8.5.5. Scenario F1 and F2

Scenario F1 and F2 reflects a more conservative analysis which is likely to more

reliably meet the demands of water quality standards by having an over-irradiation

safety factor of 10 for a volume of 20L, modelled for 6-log reduction of E. faecalis in 3

hours. Furthermore, a battery with two disinfection cycles capability is used to buffer

the system against overcast weather. As a result of their conservative design, the UV-C

system power consumption approaches that of the UV-A prototype system, utilising 39

LEDs. The UV-A system power levels are an unreasonably high 1.3kW utilising 165

223

LEDs. Both systems currently represent an unrealistic cost burden, with individual unit

costs of US$85,189.56 and US$21,738.00 respectively. While the UV-C system is

likely possible to realise given sufficient engineering input, the UV-A system requires a

PV system of approximately twice the average system on a household roof in Sydney.

Improvement in LED technologies may help with this. Bulk costs of the UV-C scenario

reach US$9693.66 in 10 000 unit quantities, which could be affordable for those in

developed world applications.

8.5.6. Scenario G

As royal blue 455nm LEDs underlie the revolution in white-LED lighting for visible

applications and have one of the best LED efficiencies achieved to date, despite their

marginal performance at bench scale, it was considered worthwhile to investigate what

the expected costs might be if it were adapted to 25L household scale applications with

battery back-up. As bacteria are less sensitive to 455nm, the power consumption of such

a system was higher than that of System C by 58%, making it difficult to realise. Its

costs were dominated by balance of system costs, as expected, although its costs were

not clearly superior to the UV-A based systems as the LED cost differential has

narrowed over time and the sensitivity benefits with potential for photocatalysis are

more valued than lower initial LED cost.

8.5.7. Scenario H

Finally, I considered the possibility of a 310nm based disinfection device (Scenario H)

using present technology at bench-scale. I was unable to demonstrate significant

disinfection with an array of 3 x 310nm LEDs, and it turns out, in order to achieve 6-log

disinfection in 3-hours for 1.5L volumes requires 28 LEDs with a system cost of

US$4094.31. Due to its short lifetime, it was clearly the lowest value-for-money option

of the systems considered.

8.5.8. Sensitivity to Findings Analysis

A sensitivity to findings analysis was undertaken using the Bayes net model (Marcot,

2012). For the Bulk Cost per Unit, the "Sensitivity to Findings" is shown in Table 29

with key input parameter nodes bolded.

224

As expected, the sensitivity to findings showed that the immediate neighbouring nodes

were most influential. These included, in order of influence, total non-LED costs, cost

of power supply and PV, electrical power required, LED electrical power and then LED

array cost. This is likely due to the large variance in power supply costs where batteries

are required and high over-provisioning factors are required, throwing the balance of the

system costs towards that of non-LED costs.

A look at only the input nodes within this subset shows that wavelength, optical power

and number of units to be the three most influential inputs. This is very likely as certain

wavelengths can represent order of magnitude changes in pricing (at the same power

level), and that the price scales directly as a function of optical power (which itself, is

modelled across five orders of magnitude). The number of units likely reflects the

significant discount factors, especially for UV-C LEDs.

Other nodes had no influence at all, and were included in the net to provide indicators as

to disinfection performance, and treatment costs for the lifetime of the unit. Some of

these nodes will have an influence if the total treatment cost was analysed instead.

225

Table 29 - Bayes net model sensitivity to findings analysis for 'Bulk Disinfector Cost per Unit'

Node Variance Reduction Percent Mutual Info Percent Variance of Beliefs

Bulk Disinfector Cost per Unit ($) 3.92E+07 100 1.58801 100 0.382153

Total Non LED Cost ($) 1.15E+07 29.3 0.56535 35.6 0.079765

Cost of Power Supply ($) 1.09E+07 27.8 0.47813 30.1 0.072868

PV cost ($) 1.03E+07 26.3 0.43011 27.1 0.066898

Electrical power required (W) 7.98E+06 20.4 0.33843 21.3 0.04383

LED Electrical power (W) 5.87E+06 15 0.24949 15.7 0.041615

LED Array cost ($) 5.77E+06 14.7 0.22402 14.1 0.037063

Current Driver ($) 4.41E+06 11.2 0.18101 11.4 0.02713

Heatsink ($) 4.20E+06 10.7 0.1657 10.4 0.024803

Balance of System Costs ($) 3.38E+06 8.63 0.15453 9.73 0.022712

log10 Reductions (logs) 3.33E+06 8.49 0.12879 8.11 0.025107

LED discount (fraction) 3.07E+06 7.84 0.13612 8.57 0.017191

Wavelength (nm) 3.05E+06 7.78 0.10861 6.84 0.021483

LED efficiency (fraction) 3.05E+06 7.78 0.10846 6.83 0.021464

LED cost per Output Watt ($/W) 3.05E+06 7.78 0.10844 6.83 0.021441

Battery and charge controller cost ($) 2.95E+06 7.52 0.14213 8.95 0.025478

Optical Power (W) 2.90E+06 7.39 0.12684 7.99 0.021122

Number of Units (count) 2.61E+06 6.65 0.10546 6.64 0.012515

226


Non LED discount (fraction) 2.60E+06 6.63 0.10492 6.61 0.012453

Project Treatment Cost (c/log.kL) 2.54E+06 6.48 0.09175 5.78 0.017207

S90 (J/cm2/log) 2.53E+06 6.45 0.08946 5.63 0.017752

No. of LEDs per Reactor 1.77E+06 4.52 0.07243 4.56 0.013063

Pump cost ($) 1.74E+06 4.44 0.07298 4.6 0.011847

Reactor Costs ($) 7.57E+05 1.93 0.03915 2.47 0.005205

PV oversupply multiplier 7.17E+05 1.83 0.02371 1.49 0.005087

PV cost per W ($) 1.05E+05 0.267 0.00344 0.217 0.000748

Battery/controller oversupply multiplier 3.66E+04 0.0933 0.02185 1.38 0.000416

Safe expT (min) 2.78E+04 0.0711 0.00096 0.0605 0.000205

Exposure (min) 2.02E+04 0.0514 0.00069 0.0438 0.000148

LED Optical Power per Unit (W) 1.39E+04 0.0354 0.00067 0.0422 9.63E-05

System Lifetime Treated Volume (L) 5243 0.0134 0.00018 0.0114 0.000038

Safety factor 3202 0.00817 0.00011 0.00692 2.33E-05

Window ($) 1430 0.00365 0.00207 0.13 2.85E-05

System Controller ($) 1181 0.00301 0.00376 0.237 4.81E-05

Wiring ($) 567.4 0.00145 0.00161 0.101 2.01E-05

UV Sensor ($) 320 0.000817 0.00091 0.0574 1.15E-05

Vessel & other costs ($) 273.1 0.000697 0.00041 0.026 5.6E-06

227


Pipe and fittings cost ($) 273.1 0.000697 0.00041 0.026 5.6E-06

Enclosure ($) 203.2 0.000519 0.00061 0.0383 7.6E-06

Microorganism 0 0 0 0 0

LED Lifetime (h) 0 0 0 0 0

Volume (L) 0 0 0 0 0

Path length (cm) 0 0 0 0 0

X-section area (cm2) 0 0 0 0 0

228

The sensitivity to findings analysis was repeated for the Project Treatment Cost node to

better understand which factors have the most influence in the final value-for-money

indicator (Table 30). This analysis reaffirmed the importance of LED lifetime, as it is

one of the most influential factors in the total treatment cost. This is intuitive, as an

LED which lasts 10-times longer will disinfect 10-times more volume of water,

ignoring the reduction in output as the LED ages. The next factor is safe exposure time,

which ties in with exposure time in general. This interacts with LED lifetime, as a

longer exposure time means that the LED's treatment life is shorter as it takes less

cycles to expire the total lifetime of the LED. The third-most influential input is

volume, with larger volumes meaning better throughput and lower treatment costs.

Wavelength is also seen as significant, as it has impacts on the volume achievable,

lifetime and exposure time requirements through S90s.

This analysis does have a significant caveat, as it takes into account all of the possible

input combinations (the space of all solutions). Due to the design intent to allow

flexibility in the net, unreasonable combinations are often not excluded - for example, a

15W optical power disinfector made from approximately 15 000 x 270nm LEDs at a

cost of US$3m is considered a possibility. As a result, the sensitivity analysis may have

some bias towards these extreme cases. This could possibly be improved by

constraining the combinatorial possibilities between the inputs to exclude unreasonable

inputs. Due to the fast pace of LED development and the possibility for some

configurations to become reasonable given enough engineering effort, it was not

considered viable to attempt to distinguish reasonable configurations from unreasonable

at this point.

229

Table 30 - Bayes net model sensitivity to findings analysis for 'Project Treatment Cost'


Project Treatment Cost (c/log.kL) 3.77E+11 100 2.51697 100 0.659452

System Lifetime Treated Volume (L) 9.85E+10 26.2 0.60125 23.9 0.031503

LED Lifetime (h) 5.60E+10 14.9 0.2811 11.2 0.008801

Bulk Disinfector Cost per Unit ($) 2.28E+10 6.05 0.09175 3.65 0.001837

Safe expT (min) 1.36E+10 3.62 0.08737 3.47 0.002084

Electrical power required (W) 1.29E+10 3.42 0.03534 1.4 0.000825

Volume (L) 1.28E+10 3.39 0.06942 2.76 0.001942

Exposure (min) 1.19E+10 3.16 0.07583 3.01 0.001788

Wavelength (nm) 1.16E+10 3.07 0.03936 1.56 0.000661

S90 (J/cm2/log) 9.99E+09 2.65 0.03511 1.4 0.000536

log10 Reductions (logs) 9.82E+09 2.61 0.02862 1.14 0.000401

Total Non LED Cost ($) 9.54E+09 2.53 0.03716 1.48 0.000785

Cost of Power Supply ($) 9.29E+09 2.47 0.03542 1.41 0.000788

Path length (cm) 8.28E+09 2.2 0.03625 1.44 0.001009

Battery and charge controller cost ($) 7.75E+09 2.06 0.03271 1.3 0.000957

Pump cost ($) 7.41E+09 1.97 0.01944 0.772 0.000407

PV cost ($) 6.07E+09 1.61 0.02392 0.951 0.000536

LED discount (fraction) 5.79E+09 1.54 0.02853 1.13 0.000542

230


Number of Units (count) 5.54E+09 1.47 0.02856 1.13 0.000552

Non LED discount (fraction) 5.52E+09 1.47 0.02845 1.13 0.000551

X-section area (cm2) 4.49E+09 1.19 0.02933 1.17 0.000796

Reactor Costs ($) 3.34E+09 0.887 0.00902 0.358 0.000192

LED Electrical power (W) 3.30E+09 0.875 0.0122 0.485 0.000223

Optical Power (W) 2.94E+09 0.78 0.00994 0.395 0.000225

PV oversupply multiplier 2.27E+09 0.602 0.00648 0.257 0.000135

LED cost per Output Watt ($/W) 2.08E+09 0.553 0.00757 0.301 0.000106

Current Driver ($) 1.83E+09 0.485 0.0071 0.282 0.000168

LED Array cost ($) 1.59E+09 0.423 0.00623 0.248 0.000134

Balance of System Costs ($) 1.42E+09 0.377 0.0062 0.246 0.000148

Safety factor 1.38E+09 0.365 0.0086 0.342 0.000203

No. of LEDs per Reactor 1.30E+09 0.344 0.00511 0.203 0.000123

Heatsink ($) 1.18E+09 0.314 0.00425 0.169 0.000112

LED efficiency (fraction) 1.13E+09 0.299 0.00403 0.16 6.12E-05

PV cost per W ($) 3.25E+08 0.0862 0.00094 0.0374 0.00002

Battery/controller oversupply multiplier 2.72E+08 0.0723 0.00124 0.0491 2.46E-05

Microorganism 1.32E+08 0.0351 0.00045 0.0177 8.2E-06

LED Optical Power per Unit (W) 3.69E+07 0.00978 0.00015 0.00613 3.7E-06

231


System Controller ($) 1.57E+07 0.00418 0.00011 0.0044 1.9E-06

Window ($) 1.24E+07 0.00328 0.00008 0.00303 1.4E-06

Wiring ($) 6.95E+06 0.00184 0.00005 0.0019 8E-07

UV Sensor ($) 3.93E+06 0.00104 0.00003 0.00111 5E-07

Enclosure ($) 2.59E+06 0.000687 0.00002 0.000724 3E-07

Pipe and fittings cost ($) 2.42E+06 0.000642 0.00002 0.000612 3E-07

Vessel & other costs ($) 2.42E+06 0.000642 0.00002 0.000612 3E-07

232

8.6. Comparison with Technical and Economic Analyses in Literature

Attempts have been made to predict the technical progress and economic feasibility of

UV-C LEDs for POU applications in the literature. In this section, their projections are

compared with actual technical and economic progress.

A presentation by Schowalter (2008), CEO of Crystal IS, focused on the technological

challenges of pseudomorphic UV-C LED manufacturing and progress, while providing

projections of future technical progress. They used the historical development progress

of red and blue LED development to justify projections that by 2014, wall-plug

efficiencies of 280nm LEDs would reach 40%. Price was also projected to fall to below

$0.1/mW by 2014. As evident from our analysis, clearly these benchmarks have not

been realised yet.

Chatterley and Linden (2010) offered economic projections based on information

supplied by SETi and Crystal IS. Their projections focused on UV-C LEDs in 2013-

2014 timeframe, with optimism that they should be a viable and economic option

around 2013. Their projections suggested that 100mW/lamp output with 10 000h

lifetime would be similarly available with US$0.1/mW pricing by 2014.

Ibrahim et al. (2013) projected UV-LED performance into the future using projections

obtained from S. Schujman from Crystal IS. Their LED projections are tabulated in

Table 31, with prices converted to US$/mW (1GBP = US$1.60, June 2013) to facilitate

easy comparison. Again, these projections have proved overly optimistic.

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Table 31 - Projected UV-C LED parameters derived from Table 3, Ibrahim et al.

(2013)

Year Efficiency (%) Lifetime (h) Output Power (mW) Cost per Output Power

(US$/mW)

2007 1 200 0.18

2010 2 1 500 0.36 75.55556

2012 8 10 000 7 1.028571

2015 16 30 000 64 0.1125

2020 75 100 000 675 0.000166

Time has proven the projections made by these publications to be mostly very

optimistic, as today, the efficiency of commercially-available UV-C LEDs remain in the

1% range, with world record efficiencies still at 11%. Likewise, the largest unit-sized

LEDs are 50mW, although these are comprised of arrays of emitters rather than single-

emitters which remain at the <10mW range. Costs remain high at US$232/mW for

280nm LEDs at single-unit prices, and US$11.60/mW assuming maximum bulk

discount for orders of 1 000 000 units. However, it was found in this study that lifetime

is at least projected to be 39 600h so the goal of resilient long-lived LEDs seems closer

to reality. As a result, UV-C LED costs still not have reached parity with LP mercury

tube technology, and so projections as to their feasibility and future applications should

still be made with some caution.

As several water disinfection standards require a fluence level of 40mJ/cm2 UV-C

254nm for 3-log inactivation credit for Cryptosporidium and Giardia, this requirement

is also likely to increase costs at least for disinfection application in advanced

economies. This requirement is based on the conservative need to ensure sufficient

reduction in the case of wastewater with reduced transmissivity. That said, on pure

theoretical calculations, prototype System A and System B both provided above this

level (50mJ/cm2 and 43mJ/cm

2 respectively) of fluence within their reactors and

operating time envelope assuming optimal hydrodynamics. But Scenario D1 and E1

both provided around only 3mJ/cm2, under a tenth of the required level. Despite these,

low-cost systems may still be useful in POU settings as, on a risk basis, enteric bacteria

represent the greatest risk (Enger et al., 2012).

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8.7. Conclusion

An assessment of the market showed that progress was being made in reducing the costs

of both LEDs and PV. While the available UV-C products and pricing remained

stagnant, the increasing number of manufacturers moving into mass production with

standardised, higher-power output, surface-mount devices provided encouragement that

progress is being made. In the UV-A and visible region, LEDs halved their prices and in

some cases, also increased their output power. PV panels showed a similar halving in

wholesale price with slow ramp-up in efficiencies.

Wavelength optimisation based on LED costs alone showed that 270nm was a near

optimum choice for E. coli and that 365nm LEDs were the optimal choice out of the

UV-A and visible LEDs. When optimising for E. faecalis, the 270nm and 365nm LEDs

represented surprisingly similar value for money due to the difference in bacterial

sensitivity of E. faecalis being larger for UV-C than for UV-A. Such optimisation did

not consider the impact of balance of system costs and reactor geometry.

Bayes net and spreadsheet models were developed that allowed costing of various

possible configurations to be undertaken. The models had good cost predictive ability,

with cost variations from the prototype model costings (Table 21, Table 22 and Table

23) being due to changes in market pricing and variations in LED outputs per real unit

with calculations based on typical figures. The models confirmed that UV-C based

disinfectors are mostly dominated by LED-related costs, whereas UV-A based systems

are more likely to have higher balance-of-system costs. The greater discount factor for

UV-C based systems sometimes permitted UV-C systems to be cheaper than UV-A

systems in bulk, when the opposite is the case in single unit quantities. Estimations of

disinfection power based on action spectra, however, require improved hydrodynamics

and system design to be achieved in practice.

While it was possible to simulate SODIS-scale bare-minimum configurations starting at

US$35 to US$40 and household scale bare-minimum configurations ranging from

US$48 to US$173, it was found that such configurations offered no safety factor against

water absorbances, LED output deviations, reactor sub-optimalities, etc. When a safety

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factor of 10 was combined with battery storage, the cost of reactors becomes

unaffordable and in the case of UV-A, impractical to realise due to high power

consumption and heat generation. Further improvements in LED efficiency and pricing

are needed to realise systems which deliver performance above that required of

developed-world water standards.


A market survey revealed that obtainable UV-C LEDs remained stagnant in

terms of specifications and pricing, although new entrants to the UV-C market

and new products are being listed for engineering sampling. Visible and UV-A

LEDs have generally halved in their prices since 2013, with some products also

benefiting from increased output and efficiency figures.

PV panels continued their price decline, falling to approximately half the 2013

cost. Further demand, increased manufacturing volume and new technologies

with improved efficiencies are expected to continue driving the price down.

Volume discounting was found to be significant with UV-C LEDs which

reduced in price by 95% at 1 million units. A lower level of volume discount

applied to mass-produced products, achieving a 58% discount for 1 million

units.

A Bayes net costing model was developed concurrently with a spreadsheet

model which was used to model 11 different configurations. It showed good

agreement with prototype model costings, and showed that in most UV-C

systems, LED costs are the dominant factor. However, due to strong discounting

of UV-C LEDs in large volumes, high per-unit costs of UV-C disinfectors could

reach parity or be even cheaper than UV-A systems if produced in high

numbers.

Systems with adequate over-irradiation are hard to achieve in UV-C, are

extremely costly, and impractical with UV-A. Improvements in LED technology

might help with this, although improvements in LED electrical efficiency are

required.

In many cases UV-A systems provided more disinfection per dollar, mainly due

to the assumption of 50 000 hours lifetime in the model, as compared to 5 000

hours for UV-C LEDs.

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Royal blue 455nm based systems were not clearly cheaper than UV-A based

systems mainly due to the high balance of system costs which dominate, and the

lower bacterial sensitivity which increases power demand.

UV-B systems are impractical to realise with present-day technology.

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9. Conclusions, Future Potential and Recommendations

This interdisciplinary investigation considered the application of PV and LED

technologies together for POU disinfection. It identified opportunities in emerging LED

and existing mature LED technologies and sought to bridge several interdisciplinary

knowledge gaps in how the key technologies might be optimally combined to realise

robust, simple and reliable POU devices that are a better match to regional and remote

user needs. A summary of the knowledge gaps identified from the literature review and

progress made by this investigation are summarised in Table 32.

Bench-scale experiments contributed critical knowledge to understanding how different

wavelengths of LED irradiation ranging from 270-740nm can directly contribute to

disinfection, modelled using E. coli K12 and E. faecalis ATCC 19433. Successful

disinfection (~5-logs) was achieved for wavelengths of 270, 365, 385 and 405nm, and

marginal disinfection (~2-4-logs) was achieved with 430, 455nm. Wavelengths ≥525nm

were found to be ineffective. Disinfection using selected wavelengths was trialled in the

presence of TiO2 photocatalyst (no enhancement), and chlorophyllin photosensitisers

(variable enhancement up to 5-logs). Consideration of other enhancement routes was

undertaken based on the literature. These were found to have limited applicability to

POU. These experiments and literature analyses also identified a need to better

characterise commercial LEDs as manufacturers’ power output specification ranges are

broad, often ranging by a factor of over three. Likewise, wavelength specifications were

also found to range over 10nm. Precisely measuring LED emission flux was found to be

difficult to achieve in the field and prone to error.

Important engineering considerations were identified during the bench-scale reactor

construction. These included a need to experimentally confirm LED lifetime, measure

quartz solarisation and develop UV-selective sensor systems. UV-C LED lifetime

greatly surpassed literature-based expectations, achieving 5 000h operation at 65% of

initial output. This testing also showed no detectable solarisation at 270nm or sensor

degradation over the 5 000h trial. Self-heating related output power losses were

estimated via thermal resistance modelling predictions and validated experimentally,

confirming the need for careful thermal management of LEDs. Very low-cost UV

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sensors for diagnostic purposes were successfully trialled, based on modified royal blue

LEDs and household fluorochromes. Eye safety was assessed and accidental exposure

appeared unlikely to cause immediate harm with current LEDs. However, this is likely

to require reassessment in the future.

Based on model reactor performance and the engineering assessment, full-scale

deployment prototype construction was initiated. This necessitated development of

circuitry and microcontroller programs as well as physical hardware construction which

aided system pricing. Three prototype systems, representing different design concepts,

were successfully trialled under simulated field conditions to disinfect water containing

E. coli K12 at single-glass (250mL) (1 x 1mW 270nm LED[US$281]=> 4-logs

reduction in 15 min) and household (10L and 15 L) (8 x 1mW 270nm LEDs [US$1799]

=> 2-logs in 1 h; 3 x 365nm 1.8W ‘quad’ LED arrays [US$956] => 5-logs reduction in

3h respectively).

An analysis of photoinactivation by conventional SODIS was undertaken for direct

comparison. Simulated solar spectra clearly showed a number of limitations of SODIS

which could be largely avoided with PV-powered LED-based disinfectors.

While it was clearly demonstrated that the PV+LED concept is realisable with present-

day technology, costs remain high with LED costs dominating system costs, especially

in UV-C based systems. UV-A based systems saw a larger proportion of costs from

balance of system items due to the higher power demand. Costs for charge controllers,

batteries and pumps were not insignificant, where required in the system's design.

Reassessment of the market provided hope that the mass production of devices may be

possible in the next 5 years even though earlier projections proved overly optimistic.

This was because prices for UV-A/visible LEDs and PV panels have halved in the

period between 2013 and 2016, and new entrants to the UV-C market with announced

high-power UV-C LED products suited to disinfection roles signal the imminent

reduction in price once they become available at retail. Bulk purchases were also found

to attract significant discounts.

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Table 32 - Progress on identified issues

Theme Knowledge Gap Result

UV-C

LEDs

Efficiency

Output Power

Lifetime

Unit Cost

New entrants to the market gearing up for mass production are bringing products with approximately

double the efficiency and over five-fold emission power. While not yet available at retail, it is expected

that cost reductions are imminent as mass manufacturing has begun and competition has increased. UV-

C lifetime was demonstrated to far exceed initial expectations, exceeding 5 000h with 65% of initial

output. Best results were from 270nm LEDs.

Pulsing Literature report claims it is energy efficient at low duty cycles of 10% and 25%, but is less time-

efficient than continuous operation.

Other Disinfection

Technologies

Miniature LP tubes, e.g. Steripen™, are effective but limited in operational life (up to 8 000 cycles of 90

seconds) and require batteries. Newer units incorporate hand crank mechanisms which are mechanical

and potentially a reliability issue. Small PV panels are well suited to SODIS scale devices.

UV-A,

violet,

blue, UV-

B LEDs

Disinfection

Mechanisms

Observed lag-phase disinfection behaviour with UV-A and violet-blue LEDs, with slight

photoreactivation, suggestive of different disinfection mechanism. Observed disinfection dose

requirements are less variable than for UV-C when comparing E. coli and E. faecalis. Other research

suggests mechanisms targeting repair mechanisms, proteins, and cell wall with different bond breakages

due to lower photon energy. Best results were from 365nm LEDs.

Advanced

Oxidation

Processes

Positive literature reports on TiO2 photocatalysis were not able to be replicated using ceramic Raschig

rings. The enhancement reported elsewhere appeared to be significant, with further investigation

necessary. Photosensitisers showed promise in preliminary trials.

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UV-B Of interest as it may penetrate water better than UV-C. However, LEDs remain inefficient and unsuitable

due to expensive technology being very similar to UV-C and facing similar challenges but having far

less disinfection potential.

Oxygenation and

mixing

Oxygenation not assessed, however, believed to be critical for UV-A performance. Mixing was

determined to be important from literature and likely accounted for poor deployment prototype

performance in UV-C and good in UV-A. Optimizing hydrodynamics using pumps should promote

oxygenation.

Wavelength

combination

Benefits reported were limited, and were generally not considered great as combinations eroded the cost-

advantage of UV-A LEDs to require the use of UV-C LEDs, and UV-C LEDs performed well alone.

Additionally, it increases complexity of the POU device.

PV Power

Supply

Current and

voltage balancing

and matching

Systems were developed using direct DC power. LED array power demands were kept within reasonable

ranges to ensure success. Pumping power demands found to be significant and interaction between

pumping power demands and controller power demands were experienced. A strategy to manage this

was proposed.

Battery cost vs.

benefit

PV costs found to be insignificant compared to LEDs, and replacement cost and difficulty of batteries

made over-provisioning of PV panels an option for avoiding battery storage. Batteries do allow for

higher power systems to be realised with less over-provisioning and permit for year-round anytime

disinfection unlike SODIS outside the tropics or during overcast conditions.

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PV-LED

Integration

Scale-up/large

units

Array reliability requires design approaches which increase cost by using independent channels. Large

LED arrays are prone to thermal problems which can reduce output and lifetime.

Cost drivers Market costs appear to be driven by demand and technological advance. A halving of costs for UV-A

and visible LEDs and PV has occurred, with some increases in output and efficiency. However,

obtainable UV-C options remained stagnant in pricing, although it seems likely to change in the next 1-3

years given the reported emergence of much higher powered UV-C modules.

POU

Suitability

Safety UV-selective sensors were constructed at low prices (1/10th of commercial retail cost) and found to

perform reliably, and could be used to provide input for dose-monitoring and control hardware. Lower

cost sensors utilising modified blue LEDs and household fluorochromes were also successful for

qualitative checks of system operation. Microswitches were also suitable forinterlocking safety in

ensuring placement or positioning of LEDs. Safe UV exposure times were calculated for the bench-scale

model arrays and exceeded 3 minutes at 30cm distance.

Maintenance Designs were aimed at minimising failure likelihood. Decline in UV-C LED output needs to be

considered despite greater than expected lifetime. Biofilms within reactors and adsorption of

contaminants onto windows were identified as key maintenance requirements. Large reactors may need

pump maintenance and battery replacement.

Training/Local

Construction

Designs for models and prototypes developed and provides basis for training.

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Particles and

Organics

Experiments with chlorophyllin suggest that organic matter may allow long-wave visible wavelengths to

contribute to disinfection if photosensitisation occurs. Wastewater disinfection using UV-A is worth

considering.

Reactor and

Module Design

Simple reactors appeared to provide suboptimal results. Application of computational fluid dynamics

techniques is highly recommended to optimise exposure and ensure maximum microbial inactivation per

millijoule. Material selection for long lifetimes in service is critical, with reflective enhancement

possible for some materials. UV-A utilization of total internal reflection (long path lengths) found to be

likely highly beneficial.

Cost-effective

production and use

Bulk discounts were scoped and found to be significant for UV-C due to its non-mass-produced nature.

Smaller discounts generally applied to mass-produced products. Costs were reduced during design

phase, with cost models allowing identification of promising options.

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9.1. Potential Improvements to Existing Technology

The performance and applicability of the prototype designs could likely be improved

with potential modifications and permutations including:

Altered reactor geometry including the use of long horizontal reactors as

opposed to vertical reactors, and reactors which are wider than they are tall.

Application of computational fluid dynamics (CFD) to reactor design to

significantly improve hydrodynamic performance.

Incorporation of a small rechargeable batteries (e.g. "9V" Ni-MH) or

supercapacitors in UV-C designs to allow for any-time disinfection.

Adaptation of existing microcontroller to perform charge control functions to

eliminate the need for expensive third-party controllers.

Utilising newer generation of LEDs with higher output and efficiency.

Reusing waste heat by capturing it in a heatpipe and dissipating it in water for

thermal pasteurisation while minimising junction heating.

Larger LED arrays distributed as groups, or utilising heatsinks with fans,

thermo-electric or water cooling.

Modification of control systems for dose integration with larger UV sensors or

arrays of UV sensors and implement feedback based power control and

detection of water turbidity and absorption.

Use of switching converter LED drive with boost to reduce operating voltage

requirements enabling smaller low-voltage batteries and PV modules to be used,

improving efficiency and reduce system volume.

Modification of design to make UV-LED systems compatible with mains

electricity and other sources of DC power (e.g. existing PV systems, truck, car

and bike batteries, mobile phone power banks, etc.). This could greatly improve

the practicality of UV-A disinfectors.

Use polished stainless steel, or other suitable mirrored surfaces (perhaps

nanomaterial based), for reactor material to enhance disinfection by reflection of

light.

Use of other types of pump with more sustainable flow rates and potentially

lower costs.

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Use of pulsed UV-C for increased energy efficiency, if time trade-offs prove

acceptable.

More careful approach to integrating photocatalysts and photosensitisers.

Utilisation of agitators instead of pumps for in-container disinfection and

control of biofilms especially with container batch reactors comparable to the

model systems.

9.2. Further Research

Important areas for further research include:

UV-C compatible LED packaging thermal optimisation.

Developing a further understanding of how different materials are compatible

with UV wavelengths.

Developing a further understanding of how different pathogens, especially

viruses and protozoa, are inactivated at UV-A and developing full UV/Visible

action-spectra.

UV-C LED lifetime testing of larger batches of LEDs to provide a clearer

picture on degradation and improve cost projection accuracy.

Exploring the potential for high-powered UV-B LEDs to be developed in the

future to provide a balance between UV-A penetrating capability and UV-C

disinfection capability.

Understanding how to service and clean biofilms from reactors to prevent

spread of contamination and improve service life.

Re-examination of safety issues with higher powered LEDs in mind.

A better understanding of pulsed irradiation disinfection dynamics by

developing a predictive mathematical model relating duty cycle, frequency and

sensitivity for a variety of pathogens.

Characterisation of the performance of the systems using real world natural

water and water with different absorbances.

Continued focus on UV-A as a possible alternative for real waters with high

absorbance in UV-C.

A better understanding of the conditions where TiO2 photocatalysis can reliably

enhance disinfection and how it performs in regards to other pathogens,

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Investigating use of such systems for wastewater treatment.

Investigate the use of electronics to automate large-batch SODIS operations

rather than use LED irradiation.

Further develop the model for SODIS inactivation to encompass thermal

pasteurisation impacts.

Of these areas for further research, the areas I would consider priorities include

understanding how different pathogens are inactivated by different wavelengths and

developing further extended action spectra. This would promote a better understanding

of wavelength-based photoinactivation which can extend into solar inactivation, and a

more complete understanding of wavelength-based inactivation mechanisms.

Validation of devices will provide further information as to whether such systems

would be of value in the field.

A better understanding of how real world natural waters affect the disinfection system

would also be necessary to understand the interaction between the water matrix and

UV/light absorbing compounds and disinfection performance.

Further to this, a more refined SODIS model would promote better comparison between

competing sunlight-based POU water disinfection technologies.

9.3. Key Recommendations

Key recommendations for prospective designers of PV-powered LED-based POU

disinfection devices include:

Assessing the market for the most suitable (high power, high efficiency, low-

cost) LEDs available with as stringent output specifications as possible.

Ensuring adequate thermal management to maintain high output and achieve full

LED lifetime.

Avoid moving mechanical parts where possible and ensure adequate margin in

specifications to provide robustness.

Consider a battery-free approach, as batteries represent consumable items which

need periodic replacement.

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Consider characterising LEDs prior to use to ensure they meet specifications.

Use CFD based models where possible to design reactors and validate them

through challenge testing.

While action spectra provides a predictive tool, its results are unlikely to provide

exact figures.

Due to LED batch variability (up to 3-fold), losses of output as LEDs age (up to

50%), losses due to high temperature (up to 30%) and potential for further losses

due to fouling (up to 50%), over-irradiation by a significant factor above the

design goal is the only way to ensure the safety of the output water.

Consider incorporating sensors and microcontrollers for more sophisticated

control regimes and improved safety.

Consider enhancement via reflection, increased path length and TiO2 as being

the most promising avenues.

9.4. Is the PV+LED Concept Viable?

The concept of PV-powered LED-based POU disinfection has many merits with a

myriad of configuration options and flexibility to adapt to various user scenarios. Its

further adoption is expected as LED costs fall in the near future, and stands to offer a

real chance at providing high-tech, sustainable water disinfection, providing those who

rely on decentralised sources of water a better quality of life through reducing the

possibility of contracting waterborne illness.

However, while the concept itself is viable, it does face numerous challenges and

opportunities along its path to commercialisation and wider adoption. A brief

consideration of these factors is presented below.

9.4.1. Technology and Market Forces

While the concept is realisable using present-day technology, its result is not optimal.

To achieve the levels of over-irradiation called for by water standards requires

significant infrastructure and expense. However, improvements in the technology offer

the opportunity to push the concept to much wider acceptance by overcoming present-

day limitations.

247

Existing UV-C LEDs are still relatively low power per unit and low efficiency.

Developments in UV-C semiconductor technology to push up the efficiency can address

the problems of self-heating, which limit the power per unit, and reduce the size of PV

panels and batteries required. This advance will pave the way for more compact and

lighter disinfection units requiring less heatsinks and should also permit affordable

faster disinfection (or alternatively, safer disinfection through over-irradiation).

The issue of lifetime with present-day UV-C LEDs were not found to be a problem with

the tested SETi unit. However, longer lifetimes with lower degradation rates would be

welcome as it will simplify design and enable longer service life. Furthermore,

addressing the thermal co-efficient of power through new materials, substrates and

manufacturing techniques may also enable the production of more temperature-robust

LEDs which maintain better output at high temperatures, reducing the need for cooling

and enabling more reliable operation in hot climates.

The increasing number of manufacturers entering the UV-C market has the potential to

further the concept by providing a wider variety of LEDs for design flexibility, with

more suitable packaging to reduce self-heating losses. Additionally, the competition

provided by a multiple-source market should drive prices down dramatically, while

ensuring a stable supply of products to the market. The design expertise of new entrants

will need to be assessed, as their products may be inferior to those from established

manufacturers and, thus, their value for money proposition may not be as obvious.

Collaboration with existing LED manufacturers may provide a fruitful synergy, where

tighter specifications for LEDs are made available from the manufacturer, minimising

time in testing and characterisation of the incoming inventory by the manufacturer of

the disinfection units. With UV-A LEDs, which are more established, adoption of UV-

A disinfection for wastewater allows for the development of a new market for an

otherwise mature technology.

Higher efficiency PV panels will also appear in the future, mainly driven by the demand

for electricity in grid-connected applications. However, the flow-on effects of

improving technology should mean lower cost and less weight which can make such

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systems easier to handle, easier to install and more attractive. Integration with other PV-

based services, such as water pumping and lighting, may be an avenue for the future as

an "integrated services" model taking advantage of existing infrastructure and reducing

redundancy and duplication.

For the concept to remain attractive, it must remain competitive with the other

disinfection technologies, especially at POU. In the time this concept has been

developed, conventional UV disinfection has further advanced and numerous other

solar-based POU concepts using nano-materials have been mentioned. It is hence

important to continually evaluate the competing technologies for their merits and deploy

the most suited technology available at the time.

9.4.2. Training and Support Required

The unit, as it has been conceived, can be operated with the most basic of skills and

features indicator LED lights and buzzer which do not require any literacy or numeracy

to interpret and use. The unit does not require any consumables, thus eliminating one

potential source of ongoing support requirements.

However, in the long term, basic training of the community in general maintenance (e.g.

cleaning) and basic diagnostics to ensure functionality may be required. Simplified

instructions in the form of a leaflet, using graphics and a local language, may be

provided to accompany the units to inform all users of its operation and provide safety

warnings where necessary.

It is probably also advantageous if at least one member is trained in basic electronics

debugging and repair, so that a local workshop with components and adequate test

equipment can be established for fast turnaround repairs of deployed units where they

are needed. Ongoing parts supply may be required in the rare case of component

problems, however, most of the components used are expected to have a 20+ year

service life (with the exception of pumps and buzzers, everything else is "solid state").

Prior SODIS interventions provide a base upon which the requirements can be modelled

upon.

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9.4.3. Possible Trial Locations

While the concept has the possibility of being deployed wherever it is needed, there are

some site characteristics which may be desirable to better understand whether this

concept meets the needs of end users and the goals of the sponsoring body.

Deployment in areas where SODIS interventions have already taken place may allow

for the comparison of outcomes with direct-sun SODIS disinfection. Areas which are of

particular interest may be those where SODIS has seen low acceptance. One of the

design goals of this concept was to "modernize" SODIS by making it "high tech" in the

hopes that this could improve acceptance and foster an understanding that the people in

developing areas are not being neglected, and it is hoped that this can improve the

acceptance.

Deployment in areas where SODIS typically does not perform well could provide

valuable data as to the strengths of the system and whether any unanticipated design

faults occur. For example, in cooler, high altitude and high latitude climates, thermal

pasteurisation may not work and solar access is limited. In these environments, the

improved performance of UV-C LEDs and PV panels at low temperatures, along with

the concentrating ability of connecting larger arrays of PV panels, may allow for

positive outcomes which wouldn't otherwise be achieved by direct solar disinfection.

Desirable features of the source water are, at least initially, relatively clean and low

organic matter to reduce UV absorbance, until such time that higher powered UV LEDs

become affordable and common-place.

9.4.4. Policy and Governance Issues

Problems with policy and governance may form a barrier to adoption of simple systems

like those developed in this paper, especially in developed countries where drinking

water guidelines and standards and UV disinfection guidelines have been developed.

These guidelines mainly target developed world centralised applications where access to

disinfection chemicals, monitoring and design expertise is unrestricted. However, in

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developing areas and remote off-grid areas, in practice, such guidelines are often not

met and treatment may be simple or non-existent (Sobsey, 2007).

Early adoption has the advantage of early risk reduction, with the proceeds of sales of

relevant technologies going towards funding advances in the technology which would

then deliver the final goal of risk mitigation through achieving guideline-levels of

disinfection. This is one way I see the "chicken and egg" problem of infant UV-C LED

technology being broken.

The SODIS interventions that have occurred to date can provide a model on which

policy and governance issues can be addressed, as the technology and concerns are

likely to be fairly similar, and the success of SODIS through field trials may prove to be

the foundation on which the success of PV and LED based disinfection may be built.

9.4.5. Funding and Cost Issues

Aid operations, non-government organisations and charities generally operate on very

limited funding, and the amount they can afford to expend on an intervention is

normally only a few dollars per device. While the concept is technically feasible, the

financial feasibility, in light of the stringent funding parameters, is problematic.

While advances in technology are anticipated to significantly lower the price for LEDs,

the balance of system components which are mature are expected to see much less price

reductions over time. As a result, the units are still expected to be at least 10-times more

expensive than the funding available.

While the units can be designed robustly to ensure a long service life, and thus the

actual amortized cost over the lifetime is within parameters, high upfront costs are a

barrier to adoption. In light of this, pursuing even more simple "disposable" style

devices made with a short lifetime to minimise costs could make sense if LED costs

were to fall to a point where their costs mirrored those of visible LEDs. This is,

however, environmentally undesirable.

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9.4.6. Patents and Intellectual Property

A significant issue, especially in developed countries, that can restrict the adoption of

this technology is existing patents. From a cursory search of patents, numerous UV

disinfection-related companies have filed speculative patents covering hypothetical

products with unrealistic characteristics which they have never actually demonstrated in

the hope that such patents could be enforced in the future when the technology

eventuates.

Unfortunately, due to the long 25-year lifespan of most patents, the wider adoption of

the technology may be limited in the countries where the patents are enforced due to the

risk of litigation and the likelihood that the companies holding the patents would not

license them under fair and reasonable non-discriminatory terms.

It is, hence, my hope that research in this area continues with information released to the

public, for public benefit, rather than being restricted within the patent system which

results in uncertainty for those who might otherwise wish to manufacture and sell such

products commercially.

9.4.7. Socioeconomic Problems

In many areas where this system may be usefully deployed, the local population subsists

on very little income and societal problems are rampant. Deployers of such systems

may have to consider issues such as equality and equity, as unfair distribution of such

devices could lead to theft, vandalism and sabotage. Ruggedised designs with solid

fixed installation foundations may be necessary to guard against such threats. Further to

this, methods whereby such systems can be authenticated as "genuine" and tested to

prove workability may be necessary to protect against imitation devices which may not

be effective, but appear to end users to be similar or identical and sold as genuine.

Failure to guard against these threats may result in the compromise of end user safety

and a loss of trust and goodwill towards the concept due to the failure of the high

upfront investment to meet its targets.

252

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Appendix A: SMARTS Input Cards

This appendix details the input cards to SMARTS v.2.9.5 used to derive the input solar

reference spectrum for zenith angle, and for altitude.

Zenith Angle Computations

The zenith angle computations were performed on 1° steps from 0° to 90°. This was

done by replacing the first variable of the last line of the input card with the required

zenith angle. The input card was based upon the IEC60904-3 Ed.2 Annex A, with

modifications as below:

Card 2a modified to add new height parameter of 0.0m as required by SMARTS

v.2.9.5.

Card 10c modified with tilt angle of 37° replaced by tilt angle of 0° to simulate

bottle facing sky on a flat surface.

Card 12a modified with wavelength interval changed from 0.5nm to 1nm steps.

Card 13 modified to disable circumsolar calculation mode as the output is global

solar irradiation, and enabling the calculation has no impact on the output.

Card 17 modified to input solar geometry instead of air-mass.

Zenith Angle Input Card Example

'ZENITH-ANGLE-SIMULATION'

1

1013.25 0. 0.

1

'USSA'

1

1

1

370

1

'S&F_RURAL'

0

0.084

38

1

38 0 180

280 4000 1.0 1367.0

2

280 4000 1

286

1

8

0

0

0

0

0

0 180

Altitude Computations

The altitude computations were performed in 1 km steps from 0 km to 5 km at a fixed

condition of equator at equinox with zero tilt angle and 60 degree solar elevation. The

altitude is controlled by the second variable of Card 2a. The input card was based upon

the IEC60904-3 Ed.2 Annex A, with modifications as below:

Card 2 modified to use mode 2 to specify latitude, altitude and height, as modes

0 and 1 are not suitable due to site pressure being used as supplied rather than

calculated based on altitude.

Card 10c modified with tilt angle of 37° replaced by tilt angle of 0° to simulate

bottle facing sky on a flat surface.

Card 12a modified with wavelength interval changed from 0.5nm to 1nm steps.

Card 13 modified to disable circumsolar calculation mode as the output is global

solar irradiation, and enabling the calculation has no impact on the output.

Card 17 modified to input solar geometry instead of air-mass, with the sun

specified at 60 degrees elevation for air-mass 1.5 conditions.

Altitude Input Card Example

'ALTITUDE-SIMULATION'

2

0 0 0.

1

'USSA'

1

1

1

370

1

'S&F_RURAL'

0

0.084

287

38

1

38 0 180

280 4000 1.0 1367.0

2

280 4000 1

1

8

0

0

0

0

0

41.8 180

288

Appendix B: Arduino Microcontroller Code

This appendix provides the source code used with the Arduino for assessment of pulsed

irradiation warm-up characteristics of LEDs and for the resulting disinfector System A

and System B.

Pulsed Irradiation Test

void setup()

Serial.begin(9600);

pinMode(3,OUTPUT);

pinMode(2,OUTPUT);

pinMode(13,OUTPUT);

digitalWrite(3,LOW);



void loop()

int c;

if(Serial.available())

c=Serial.read();

if(c=='0')



Serial.println("OFF");

else if (c=='1')

digitalWrite(13,HIGH);


Serial.println("ON");

else if (c=='2')


analogWrite(3,128);

Serial.println("Pulse 50%");

else if (c=='3')


analogWrite(3,25);

Serial.println("Pulse 10%");

Disinfection System A

// Simple UV-C Timed Disinfector

// By Gough Lui

289

#define STARTBUTTON 3

#define STARTBUTTONGND 2

#define REDLED 4

#define GREENLED 5

#define UVLEDDRIVE 6

#define ALTGREENLED 9

#define ALTBLUELED 10

#define ALTREDLED 13

#define BUZZER 11

#define DISINFECT_PERIOD 180

#define POWERMON A0

#define MINVOLTS 217

void powerFinder();

void startHandler();

void clearLEDs();

void greenLEDs();

void redLEDs();

void bothLEDs();

void runDisinfection();

volatile int startbutton = 0;

void setup()

pinMode(STARTBUTTONGND,OUTPUT);

pinMode(STARTBUTTON,INPUT_PULLUP);

pinMode(REDLED,OUTPUT);

pinMode(GREENLED,OUTPUT);

pinMode(UVLEDDRIVE,OUTPUT);

pinMode(ALTREDLED,OUTPUT);

pinMode(ALTGREENLED,OUTPUT);

pinMode(ALTBLUELED,OUTPUT);

pinMode(BUZZER,OUTPUT);

digitalWrite(STARTBUTTONGND,LOW);

digitalWrite(REDLED,HIGH);

digitalWrite(GREENLED,LOW);

digitalWrite(UVLEDDRIVE,LOW);

digitalWrite(ALTREDLED,HIGH);

digitalWrite(ALTGREENLED,LOW);

digitalWrite(ALTBLUELED,LOW);

digitalWrite(BUZZER,LOW);

pinMode(POWERMON,INPUT);

attachInterrupt(digitalPinToInterrupt(3),startHandler,LOW);

while(!startbutton)

powerFinder();

290

void loop()

if (startbutton)

clearLEDs();

startbutton = 0;

runDisinfection();

startbutton = 0;

void powerFinder()

// Solar Panel Alignment Aid

// 218 = 7V

// 310 = 10V

// 372 = 12V

int reading;

reading=analogRead(POWERMON);

if(reading > 372)

greenLEDs();

tone(BUZZER,4000,100);

else if (reading > 310)

bothLEDs();



redLEDs();


else

clearLEDs();

delay(750);

void startHandler()

startbutton=1;

delay(50);

void clearLEDs()



digitalWrite(REDLED,LOW);

digitalWrite(ALTREDLED,LOW);

void greenLEDs()

digitalWrite(GREENLED,HIGH);

digitalWrite(ALTGREENLED,HIGH);


digitalWrite(ALTREDLED,LOW);

void redLEDs()

291





void bothLEDs()


digitalWrite(ALTGREENLED,HIGH);



void runDisinfection()

unsigned long time;

int voltflag=1;


delay(1000);


delay(1000);


delay(1000);

digitalWrite(UVLEDDRIVE,HIGH);

time = millis();

time = time + (unsigned long) DISINFECT_PERIOD *

(unsigned long) 1000;

while ((voltflag=(analogRead(POWERMON)>MINVOLTS)) &&

time>millis())


greenLEDs();

delay(500);

redLEDs();

delay(500);


clearLEDs();

if(!voltflag)

redLEDs();


delay(1000);


delay(1000);


delay(1000);

else

greenLEDs();


delay(250);


delay(250);

292


delay(250);


delay(250);


delay(250);


delay(250);

Disinfection System B

// Simple UV-C Pumped Disinfector Monitor

// By Gough Lui

#define REDLED 4

#define GREENLED 5

#define UVLEDDRIVE 6

#define REDLEDB 7

#define GREENLEDB 8

#define BUZZER 11

#define POWERMON A0

#define UVDOSE A1

#define SENSORPRESENT A2

void powerFinder();

void uvSensor();

void greenLEDs();

void redLEDs();

void bothLEDs();

void greenbLEDs();

void redbLEDs();

void bothbLEDs();

void startupWarn();

void setup()

pinMode(REDLED,OUTPUT);

pinMode(GREENLED,OUTPUT);

pinMode(REDLEDB,OUTPUT);

pinMode(GREENLEDB,OUTPUT);

pinMode(UVLEDDRIVE,OUTPUT);

pinMode(BUZZER,OUTPUT);



digitalWrite(REDLEDB,HIGH);

digitalWrite(GREENLEDB,LOW);


digitalWrite(BUZZER,LOW);

293

pinMode(POWERMON,INPUT);

pinMode(SENSORPRESENT,INPUT);

pinMode(UVDOSE,INPUT);

startupWarn();

void loop()


powerFinder();

uvSensor();

delay(500);

void powerFinder()

int reading;

reading=analogRead(POWERMON);

if(reading > 372)

greenLEDs();


bothLEDs();

else

redLEDs();

void uvSensor()

if(analogRead(SENSORPRESENT) >= 512)

int reading;

reading=analogRead(UVDOSE);

if(reading > 5)

greenbLEDs();


bothbLEDs();

else

redbLEDs();

void greenLEDs()



void redLEDs()



void bothLEDs()


294


void greenbLEDs()

digitalWrite(GREENLEDB,HIGH);

digitalWrite(REDLEDB,LOW);

void redbLEDs()

digitalWrite(GREENLEDB,LOW);


void bothbLEDs()

digitalWrite(GREENLEDB,HIGH);


void startupWarn()


delay(1000);


delay(1000);


delay(1000);

digitalWrite(UVLEDDRIVE,HIGH);

295

Appendix C: System Configuration Cost Model Inputs

All the input parameters used to generate the findings in Section 8.5 are tabulated below. These worksheets can be found within the spreadsheet

files in Appendix D.

Node System

A

System

B

System

C

System

D1

System

D2

System

E1

System

E2

System

F1

System

F2

System

G

System

H

Wavelength 270 270 365 270 365 270 365 270 365 455 310

Optical Power

Proposed

0.001 0.008 5.4 0.0005 2.7 0.00003 0.165 0.305 346 22.8 0.0165

Microorganism 0 0 0 0 0 0 0 1 1 0 0

Safety Factor 1 1 1 1 1 1 1 10 10 1 1

Exposure Time 15 60 180 180 180 180 180 18 18 180 180

Cross Sectional

Area

16.66 666.67 100 1666.67 166.67 100 10 1333.34 133.34 166.67 100

Path Length 15 15 150 15 150 15 150 15 150 150 15

LED Optical

Power per Unit

0.001 0.001 2.1 0.001 2.1 0.001 2.1 0.008 2.1 3.892 0.0006

LED Lifetime 5000 5000 50000 5000 50000 5000 50000 5000 50000 50000 5000

Window 0 0 0.27 0 0.27 0 0.27 39 0.27 0.27 39

296

Node System

A

System

B

System

C

System

D1

System

D2

System

E1

System

E2

System

F1

System

F2

System

G

System

H

Pump Cost 0 20 70 10 10 10 10 20 70 70 10

Vessel and Other

Costs

0 7 63 5 20 5 5 10 20 20 15

Pipe and Fittings 2.5 20 18 5 20 5 5 10 20 20 5

UV Sensor 0 22 0 0 0 0 0 22 0 0 0

Enclosure 11 14.84 25.25 10 15 10 10 20 25 20 15

System Controller 40 40 0 0 0 0 0 40 0 0 0

Wiring 1.85 30 92 5 10 5 5 25 40 30 5

PV Cost per W 2.5 2.5 2 2.5 2 2.5 2.5 2 1.5 2 2.5

PV Oversupply

Factor

8 8 1.5 4 4 4 4 4 4 4 5

Battery/Controller

Oversupply

Multiplier

0 0 2 0 0 0 0 2 2 2 0

297

Appendix D: Digital Appendix Listing

As the thesis investigations involved extensive use of third party components, software

and sourcing of data which may no longer be easily available, this data has been

provided digitally on optical disc.

The digital appendix is split into six folders:

Code and Miscellaneous Data - this includes the SMARTS input cards detailed

in Appendix A, microcontroller code as detailed in Appendix B and LED market

survey data.

Costing Model - this contains the Netica file containing the Bayes net program

and the corresponding spreadsheet which contains the same equations for the

costing model.

Datasheets - this includes datasheets, manuals and specification sheets for LED

and electronic components and test equipment used within the investigation.

Design Recommendations - this includes whitepapers, application notes and

design recommendation documents from component manufacturers which

provide additional design and engineering information.

Price Lists - these are historical price listing tables for Sensor Electronic

Technology Inc. UV LEDs including volume discount information which is no

longer available.

Software - this contains the software used in the investigation including those

which are used to interface with, program microcontrollers, log data from meters

and oscilloscopes, control power supplies and perform solar simulations.

Test Reports - this contains PDF scans of the manufacturer supplied LED test

reports for the 270nm and 310nm LEDs procured from Sensor Electronic

Technology Inc.

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