Reclaimed Water Desalination Technologies

Reclaimed Water Desalination Technologies: A Full-Scale Performance and Cost Comparison

Between Electrodialysis Reversal and Microfiltration/Reverse Osmosis

R. Shane Trussell, Ph.D., P.E., BCEE Principal Investigator Gordon J. Williams, Ph.D., P.E. Project Engineer

Webcast August 9, 2012

1

Webcast Overview

•  Background •  Previous Studies & Project Motivation

•  Facility Comparison •  Cost Comparison •  Conclusions

2

Background

3

Why Desalt Recycled Water?

•  Reduce total dissolved solids (TDS) to expand possible uses

•  High salt rejection not necessarily needed – Opportunity to blend membrane product water

•  Membrane desalination – Reverse osmosis (RO) with microfiltration (MF)

pretreatment – Electrodialysis reversal (EDR)

4

Cathode (-)

Anode (+)

Product Water

Concentrate

Product Water

Concentrate

Concentrate

RO EDR Pressure

Saline Fresh

Two Desalting Membrane Types

RO has significantly higher salt removal 2-Stage System: RO 90-98% vs. EDR 50-70% 5

RO Membranes

6

Image from: Baker, R. Membrane Technology and Applications. John Wiley & Sons: West Sussex, England (2004)

Types of RO membranes •  Cellulose acetate •  Thin film composite

RO Vessel Architecture

7

Image from: Baker, R. Membrane Technology and Applications. John Wiley & Sons: West Sussex, England (2004)

EDR Process

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Figure 1.4 Conceptual schematic of how charged ions are removed from the demineralization flow by passing through anion and cation membranes. When the polarity of the electrodes changes (Polarity Mode 1 compared to Polarity Mode 2), the role of the central channels (concentrate or demineralized) switches.

Polarity Mode 1

Polarity Mode 2 (charge reversed from above) 8

EDR Process


Figure 1.4 Conceptual schematic of how charged ions are removed from the demineralization flow by passing through anion and cation membranes. When the polarity of the electrodes changes (Polarity Mode 1 compared to Polarity Mode 2), the role of the central channels (concentrate or demineralized) switches.

Polarity Mode 1

Polarity Mode 2 (charge reversed from above)

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EDR Membranes and Spacers

12 WateReuse Research Foundation

Figure 1.5 Schematic of EDR spacers (aerial view). A spacer is placed in between each membrane to separate the membranes from each other, providing a flow path for water to pass between the membranes. The four holes at the bottom of the spacer (numbered 1 through 4) go through each spacer and membrane. When the membranes and spacers are stacked together, the holes line up and become conduits for (1) concentrate influent, (2) feed water influent, (3) concentrate effluent, and (4) product water effluent (flow through the holes is perpendicular to the membrane). For each spacer, only two of the four holes connect to the flow path (i.e., holes 2 and 4 on the left and holes 1 and 3 on the right; flow path through the spacer marked with dashed line). The spacer orientation is alternated between configurations A and B, such that every other spacer compartment is the flow path for either the concentrate or product flow. When the polarity is switched, the flow designation of holes 1&2 and 3&4 are switched, so what was once the concentrate flow path, now becomes the demineralized flow path, and vice versa.

Spacers

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Two types of membranes: •  Anion Transfer Membrane •  Cation Transfer Membrane

Typical numbers per stack: •  1200 membranes •  1201 spaces

EDR Architecture


Figure 1.6 Side view schematic of an EDR stack (arrows indicate flow direction). When the electrode polarity changes, the following designations are switched (1) “feed water” and “concentrate in,” (2) “product water channel” and “concentrate channel,” and (3) “product water” and “concentrate out” (adapted from Meller, 1984).

1.3.2 EDR History and Types of Membranes Although ED has only been implemented in industrial applications for approximately 40 years, the principles of ED were developed more than 90 years ago. In fact, the multi-compartment ED cell with alternating cation and anion membranes that is used in modern applications has its roots in a design published by Meyer and Straus in 1940. At that time, however, the success of ED was limited by a lack of appropriate materials. The development of proper ion-exchange membranes during the 1950s facilitated great improvements in ED feasibility and application (Langelier, 1952). Ionics Inc. developed the first commercial ED process and began selling the process in 1954, with the first installation in Saudi Arabia (Katz, 1977). The company went on to further innovate the ED process by introducing a polarity reversal step, later termed electrodialysis reversal, in order to control scaling (Katz, 1979). The EDR process essentially replaced conventional ED and still retains a significant percentage of the desalination market today. In the 1990s, EDR was successfully applied to demineralizing nonpotable reclaimed water (Reahl, 2005). ED membranes are typically flat sheets of plastic mounted on a fabric backing to provide structural support (AwwaRF et al., 1996). Previous generations of anionic membranes were

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Comparison Between Membranes Consideration RO EDR

Use in U.S. Recycled Water •  Widely applied •  1 Facility (San Diego)

Pretreatment for Reuse •  MF required •  MF not required

Fouling Concerns

•  Inorganic •  Organic •  Biofouling •  Silica

•  Inorganic •  Organic

O&M

•  MF CIP •  MF maintenance cleans •  RO CIP •  Conductivity profiling •  Interconnector repair

•  EDR CIP •  Cartridge filter replacement •  Manual membrane cleans •  Probing for “hot spots” •  Torque adjustment

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Assuming Two-Stage Systems

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RO: Two-stages

EDR: Two stages

Influent

Influent

Effluent

Effluent

RO: Two passes Effluent Influent

Previous Studies and Project Motivation

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Limitations of Previous Studies

•  Studies based on modeling, piloting or greenfield startup –  None examined longer-term operation

•  Costs will differ between brackish water and reclaimed water

•  Most missing some of life cycle costs –  e.g., membrane replacement, longer-term

maintenance activities

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16

Previous Studies: Costs per Mass of Salt Removed

*See WRF 08-17 report for full references

Cost per Mass Salt Removed ($/ton of TDS)

O&M Total

Study Water MF/RO EDR MF/RO EDR Port Hueneme Demo

(Leitz, 2001) Brackish $120 $124 $276 $319

San Diego Startup (MWH, 2002) Recycled $56 $66 -- --

San Jose Pilot (Adham et al., 2004) Recycled $213 $218 $494 $303

San Diego Pasqual (MWH, 2008) Brackish $275 $248 $430 $437

Overview of This Study •  Motivation: Most existing systems are

RO, but EDR may provide cost savings •  Objective: Document actual full-scale

costs of EDR and MF/RO – Equipment and capital, energy, membrane

replacement, labor, and chemicals •  Approach: Compare two similar facilities

– Size, established operations, feedwater, location

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Facility Comparison

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Comparison of Study Facilities

Parameter MF/RO EDR Location (California) Long Beach San Diego Online Date 2003 1998 Membrane Design Capacity (mgd) 3.0 3.3 Upstream secondary process Nitrified activated sludge Upstream tertiary process Granular Media Filtration Feed Turbidity (NTU) 0.5 0.7 Daily Production (mgd) 2.7 1.5

MF/RO: Leo J. Vander Lans AWT Facility (LVL) EDR: North City Water Reclamation Plant (NCWRP)

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LVL MF/RO Process Train

Tertiary influent

MF RO (1st Stage)

Backwash Flow Brine

Antiscalant RO (2nd Stage)

Effluent to UV and

Barrier

20

LVL Microfiltration (MF) •  Pall Corp Microza •  Module: USV-6203 •  Hollow-fiber •  Nominal pore size: 0.1 µm •  200 modules •  4.2 mgd capacity •  92% design recovery

21

LVL Reverse Osmosis (RO)

•  Hydranautics ESPA-2 •  Spiral wound •  Two stage system

•  72:36 pressure vessels •  3.0 mgd capacity •  Overall RO recovery: 85%

22

MF Performance

23 Data represent 3 years of daily grab samples

RO Performance

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Figure 2.10 Typical RO performance at LVL (data from April/June 2009).

2.2.4 RO System Salt Removal The average salt removal from the RO system from March 2003 through July 2009 was 87%, which is uncharacteristically low for an RO system (see Figure 2.11). The low average TDS rejection was a result of a prolonged period of operation with underperforming second stage membranes (January 2007 through early April 2009), where the average salt rejection during this period was 80%. The second stage was underperforming because the membranes were subject to frequent CIPs because of aluminum carryover from the LBWRP and improperly positioned valves that caused backpressure on the second stage elements. Unlike an MF CIP, an RO CIP is completely manual and requires two operators to properly position valves while cleaning each stage. The manual CIP nature of RO systems make these cleans more operator intensive than MF CIPs, and there is also greater potential for human error. The entire second stage was replaced in mid-April 2009, and the salt removal has since ranged from 94–96% of the dissolved solids. Excluding the period of uncharacteristic salt rejection (i.e., averaging data from March 2003 through 2006 and mid-April 2009 through July 2009) the average RO membrane salt rejection was 94%, which is the salt removal value used for comparison with EDR.

The RO membranes in the first stage (504 elements) have been in service since the plant commissioning was completed in 2003 and recent conductivity profiles (March 2008) show the increased salt passage that has occurred over the 5 years (see Figure 2.12A). The first stage product water conductivity has increased from an average of 15 to 40 PS/cm, which is a reasonable increase in salt passage, but is still reflective of the frequent CIPs experienced early on at LVL.

LVL RO performance from April to June 2009

RO Performance

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MF/RO Recovery


Figure 2.12 Conductivity profile probability plot from (A) first RO stage and (B) second RO stage.

2.2.5 Overall MF/RO System Feed Water Recovery

Because MF pretreatment is a requirement for a successful application of RO membranes for wastewater applications, the cumulative losses through both systems need to be considered. The variability in overall feed water recovery at LVL is presented in Figure 2.13. The feed water recovery is typically 80%, which is a combination of an operational recovery of 94% through the MF process and 85% through the RO system.

Figure 2.13 Variability in overall LVL water recovery (combined recovery of MF and RO processes).

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NCWRP EDR Process Train

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NCWRP EDR Membranes

•  GE/Ionics EDR 2020 •  Two stage system •  15 EDR lines

•  30 EDR stacks •  1,200 membranes per

stack •  36,000 membranes

total •  3.3 mgd capacity

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Pretreatment Performance: GMF

29 Turbidity data from 2005 through April 2009

EDR Performance

30


Although operating at a target of 65% salt removal, the actual salt removed was 60% on average and was below the absolute minimum of 55% about 4% of the time (Figure 3.6). The salt removal decreases as the membranes foul, but also varies with changes in the power applied to the stacks. On a time scale of minutes, a 3% decline in salt removal is typically seen as a result of fouling during the 15 min before the charge is reversed (Figure 3.7; salt removal drops from 61% to 58%), but the salt removal consistently rebounds after the polarity switches. On a time scale of weeks, the peak level of salt removal seen after charge reversal will gradually decline. In order to maintain adequate salt removal, operators occasionally increase the power applied to an EDR unit by increasing the voltage. After necessary voltage exceeds 400V, the EDR unit will be shut down and cleaned by salt CIP to restore performance. After cleaning, the voltage requirements are much lower, but eventually, the restorative power of the CIP will diminish and a manual clean is necessary to restore the EDR unit’s performance.

Figure 3.7 EDR current, power, and salt removal over a 6-h period (4/24/09). Total power and current represent all five lines and both stages of a single EDR unit. The electrode charges are reversed every 15 min to prevent scaling on the membrane. Average feed water conductivity was 1770 PS/cm and last CIP was 49 days prior.

Day-to-day, the electrical potential applied to the EDR stacks remains fairly constant. As the stack begins to foul over a matter of weeks, the plant operator will make discrete increases in the voltage to maintain salt removal. During February through March 2009, the electrical potential in EDR Unit 1 was initially 360V, but after 3 weeks of operation, the voltage was increased to 390V. Then after another 2 weeks, voltage was increased again to 415V, requiring a CIP with salt and acid only 3 days later (Figures 3.8 and 3.9). After the CIP, the required voltage to achieve the target salt removal was restored to 360V. These incremental

EDR operation on April 24, 2009

EDR Performance

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Figure 3.8 EDR performance data after manual clean (performed January 30, 2009) through the first CIP event (Unit 1).

Figure 3.9 EDR performance directly before and after a CIP clean (data subset of Figure 3.8).

EDR Performance

42 WateReuse Research Foundation

Figure 3.5 Variability in EDR feed water and product water conductivities.

3.3.2 Salt Removal, Electrical Potential, and Current Unlike RO membranes, salt removal through the EDR process can be controlled to a certain

degree by varying the electrical current. When the NCWRP first began operating the EDR

facility, the system was operated to achieve a salt removal of 70–72%, but this relatively high

salt removal for a two-stage EDR system led to difficulties maintaining the system, such as

more frequent cleanings, and operating problems, such as hot spots. After a few years of

operating at a high salt removal, NCWRP decided to lower the salt removal target by

lowering the electrical potential. They settled on an operating target of 65% with an absolute

minimum of 55%, below which a CIP would be performed to restore the performance. In the

case of the NCWRP, lowering the salt reduction also meant that they needed to construct

additional EDR capacity to meet their salinity reduction goals.

Figure 3.6 Variability in EDR salt removal and water recovery (Unit 1).

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Membrane Performance Summary

Parameter Units MF/RO EDR Water recovery % 80% 85% Salt Removal % 94% 60% Avg. feedwater TDS mg/L 654 1115 Avg. product water TDS mg/L 39 444

33

Cost Comparison

34

Costs Included in Comparison

•  Capital •  Membrane replacement •  Energy •  Labor (maintenance) •  Chemicals for operation •  Chemicals for cleaning

35 Note: All costs presented are in 2009 dollars

Costs Excluded from Comparison •  Footprint costs

– Land acquisitions, site improvements, structure •  General construction costs

– Management, engineering & design •  Concentrate disposal •  Facility operations

– Labor beyond maintenance/cleaning

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Cost Comparison Assumptions •  Everything but membrane type is the same

–  Influent TDS, production, salt reduction goals •  Operated at design capacity, year round •  20 year facility life span; no salvage value •  Electricity: $0.105 per kWh •  Labor: $65/h •  Present worth calculations

– 3% inflation, 6% discount

37

Costs (normalized by flow)

Cost MF/RO EDR Initial equipment and construction $249 $114 Membrane replacement $20 $26 Energy costs $62 $51 Chemical costs (process) $22 $1.3 Chemical costs (CIP) $3.0 $3.2 Labor cost (maintenance only) $16 $32 Total $371 $228

Dollars per acre-foot of membrane product

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Costs (normalized by flow)

Cost MF/RO EDR Initial equipment and construction $249 $114 Membrane replacement $20 $26 Energy costs $62 $51 Chemical costs (process) $22 $1.3 Chemical costs (CIP) $3.0 $3.2 Labor cost (maintenance only) $16 $32 Total $371 $228

Dollars per acre-foot of membrane product

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Salt removals not equal

Normalize costs based on salt removed

Normalized by Salt Removal: A Blending Example

•  Assume a system is needed to produce 10 mgd recycled water

•  Current TDS is 1000 mg/L, but users need 800 mg/L

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41

20% salt reduction




42

20% salt reduction

What size membrane system is needed?

Membrane System Size Related to Salt Removal Efficiency

43

Membrane capcity (mgd) = overall salt removal from blendmembrane salt removal

× blended flow (mgd)

Parameter Units MF/RO EDR Salt Removal % 94 60


44



MF/RO capcity (mgd) = 20%94%

×10 mgd = 2.1 mgd


45




×10 mgd = 2.1 mgd

EDR capcity (mgd) = 20%60%

×10 mgd = 3.3 mgd


46




×10 mgd = 2.1 mgd

EDR capcity (mgd) = 20%60%

×10 mgd = 3.3 mgd

EDR is ~60% larger than the MF/RO

Costs (normalized for 20% salt reduction) MF/RO ($79/ac-ft*) EDR ($76/ac-ft*)

Capital $53

Capital $38

Energy $17

Energy $13

Maintenance $10.5

*In dollars per acre-foot of blended water

Maintenance $3.3

Replace membranes $8.6

Replace membranes $4.2 Chemicals - $5.3 Chemicals - $1.5

Other $13

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Footprint and Brine Flow from Example •  Membrane system footprint:

•  Waste flow produced:

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Additional Factors to Consider •  Need to Remove Other Constituents •  Presence of Certain Foulants (silica) •  Occupational Hazards •  Usage: Seasonal vs. Year-Round •  Footprint Constraints •  Brine Disposal Method

49

A Need to Remove Other Constituents When removal of other constituents is needed

•  Comparison based on salt removal only

•  RO provides an effective barrier to pathogens and TOC

•  EDR does not provide this type of barrier

•  MF/RO is well-suited for groundwater recharge

Effectively Removed by MF/RO

TOC particles

pathogens uncharged molecules

50

•  Silica fouling a problem for RO

•  Silica does not concentrate in an EDR

•  EDR is well-suited for high silica applications

51

Presence of Certain Foulants: When high levels of silica are present

Image from: Gabelich, C. J..; Chen, W. R.; Yun, T. I.; Coffey, B. M.; Suffet, I. H. (2005). The role of dissolved aluminum in silica chemistry for membrane processes. Desalination 2005, 180, 307-319.

•  Voltage: EDR membranes stack often exposed (e.g., checking for “hot spots”) and improper handling could result in shock –  Measures can be taken to minimize risk

•  Pressure: both systems under high pressure –  RO (~200 psi) and EDR (~50 psi) –  Risk low due to overdesign for safety

52

Occupational Hazards: High Voltages and High Pressures

•  Many reuse applications are seasonal – e.g., agriculture, landscape irrigation

•  Seasonal flow favors EDR, with the lower capital cost and higher O&M – Chemical and energy costs reduced

proportional to flow – Labor and membrane replacements costs

reduced but not directly proportional

53

Usage: Seasonal vs. Year-Round: When the system is only used seasonally

•  Cost of acquiring and developing land not considered in comparison: – Footprints similar on flow normalized basis – MF/RO will be smaller for same salt removal

54

Footprint Constraints: When the size of footprint is important

Assuming 10 mgd blended flow with 20% TDS reduction

Impact of Footprint on Blend Cost Comparison (10 mgd with 20% salt reduction)

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Footprint Constraints: When the size of footprint is important

Footprint Cost ($/ft2)

MF-RO ($/ac-ft blend)

EDR ($/ac-ft blend)

MF-RO:EDR

$0 $78.9 $75.7 1.04 $10 $79.3 $76.2 1.04

$100 $82.6 $81.2 1.02 $1000 $116 $130 0.89

•  Significant consideration for inland utilities •  RO system will have:

–  Less brine flow – More concentrated brine – Anti-scalants in the brine

•  Brine disposal cost will vary based on variety of factors including location, method, quality

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Brine Disposal Method: When disposal of concentrate is difficult

•  Examples of brine disposal methods: – Brine line/ocean outfall – Zero-liquid discharge (e.g., ponds, crystallizer) – Deep-well injection – Trucking

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Impact of Brine Disposal Cost on Blend Cost Comparison (10 mgd with 20% salt reduction) Disposal Cost

($/ac-ft brine) MF-RO

($/ac-ft blend)

EDR ($/ac-ft blend)

MF-RO:EDR

$0 $79 $76 1.04 $100 $83 $82 1.01

$1,000 $116 $134 0.86 $10,000 $446 $659 0.68

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Other Factors •  Size of treatment facilities •  Chemical price volatility •  Tax implications of capital investment •  Deviations from cost analysis assumptions

–  Discount rate of 6% –  Facility lifespan of 20 years –  Electricity costs –  Labor rate (average cost of $65/h)

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Conclusions

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Conclusions •  Study first to provide full-scale cost comparison •  Both technologies feasible for desalting recycled water •  EDR less expensive (~4%) than MF/RO on salt removal

basis •  MF/RO has higher capital costs, but less maintenance

labor required •  Cost comparison close enough that site specific

impacts on cost must be considered –  Seasonal usage will favor EDR (lower capital cost) –  High brine disposal and/or land use cost will favor RO

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Acknowledgements

City of San Diego

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