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Ammonia Recovery From Wastewater
Using a Microbial Electrolysis Cell (MEC)
School of Environmental Science
Murdoch University
Western Australia
Honours Thesis
2011
Raphael Flavigny
Declaration: This thesis is presented for a degree of Bachelor of Science in Environmental Science with
Honours at Murdoch University, 2011.
I declare that this work is my own account of my research and contains as its main content,
work that has not been previously submitted for a degree at this or any other tertiary
educational institution.
Raphael Flavigny
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Abstract: Wastewater contains ammonium that requires removal to prevent environmental degradation.
The most common way of removing ammonium is by using nitrification and denitrification
(i.e.: activated sludge), which requires energy (457 J/mmolNH4+ removed). Microbial
Electrolysis Cell (MEC) is proposed, as a new process, to recover ammonium in the form of
ammonia gas, from mild concentrated ammonium wastewater stream (50 mM). Such process
has the advantage of removing organics and producing a current that enables the migration of
ammonium against its concentration gradient to the cathode. The MEC also produces a high
pH (>9.5) in the cathode, which favours ammonia gas production. It is demonstrated that the
ammonium can accumulate against a maximum concentration gradient of 1 M ammonium in
the catholyte. The ammonium migration from the anode to the cathode is caused by the
electron flow (i.e.: current) due to bacteria biodegrading organics and donating electrons to the
anode. The presence of current enables ammonium migration against its concentration, in a
ratio of 0.47mmolNH4+/mmole-.The ammonium is accumulated in the cathode, and forms
ammonia due to the high pH and dissociation constant (pKa = 9.2).
The energy requirements for this novel process are similar to traditional activated sludge, and
about five times less than electrodialysis. However these two processes are treating urban and
high ammonium concentrated (up to 500 mM) wastewater, while the MEC proposed in this
project treats wastewater with 50 mM ammonium concentration. The MEC is not proposed as
a replacement for current technology but as an alternative to remove ammonium and organics
from specific industrial wastewaters.
Ammonia gas is used in large quantities to produce fertilisers. The MEC process recovers
ammonia at half the cost of conventional technology. However there are limitations to the
application of the system on a large scale, mainly because of the required membrane surface
area.
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Acknowledgments: I would like to acknowledge both my supervisors Professor Goen Ho and Doctor Ralf Cord-
Ruwisch for helping me through this honours journey. Professor Goen Ho in particular for
supporting my application to pursue Honours, and providing me with a drive to dig the best
out of me. Doctor Ralf Cord-Ruwisch in particular for pushing me to become a fully
independent and critical scientist.
I also would like to thank Liang Cheng and Doctor Ka Yu Cheng for providing insight into
bioelectrochemical systems, and scientific research in general. Both of them have helped me
to fully understand and seek the novelty in my project, while providing reassuring comments
in times of great stress.
I wish to thank Emily Quek for lending me her bench throughout the year without
complaining of the mess that was occurring in the lab. Thank for the numerous coffee talk,
and helping me directing my research.
Also to Doctor Lee Walker and Doctor Wipa Charles for their support and advices throughout
this year. I particularly enjoy being able to talk so openly and on many different topics.
I particularly would like to thank Noemie Legendre for her absolute support throughout the
year regardless of the circumstances. I would like to thank my family, back home, for their
understanding and financial help.
I would like to acknowledge Thomas Bowman for sharing time with me in the lab throughout
the year, and to have made me understand how important footy is.
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Abbreviations:
ANAMMOX: ANaerobic AMMonium Oxidising
AS: Activated Sludge
BES: Bioelectrochemical System
BOD: Biological Oxygen Demand
CEM: Cation Exchange Membrane
ED: Electrodialysis
GC: Gas Chromatography
MAP: Magnesium Ammonium Phosphate
MEC: Microbial Electrolysis Cell
MFC: Microbial Fuel Cell
WWT: Wastewater Treatment
WWTP: Wastewater Treatment Plant
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Table of Content:
Chapter Pages
Declaration: .............................................................................................................................. ii
Abstract: .................................................................................................................................. iii
Acknowledgments:.................................................................................................................. iv
Abbreviations: .......................................................................................................................... v
List of Figures .......................................................................................................................... x
List of Tables........................................................................................................................... xi
1. Introduction............................................................................................................................ 1
1.1. Water: a Precious Resource .............................................................................................. 1
1.2. WWT: a Key Process in our Society ................................................................................ 1
1.2.1. Wastewater characteristics ...................................................................................... 1
1.2.2. Environmental degradation prevention ................................................................... 2
1.2.3. Ammonium.............................................................................................................. 2
1.3. The Place of BES in Current WWTP Technology ........................................................... 3
1.3.1. Activated sludge ...................................................................................................... 3
1.3.2. Electrodialysis ......................................................................................................... 4
1.3.3. Bioelectrochemical system (BES) ........................................................................... 4
1.3.3.1. Design ................................................................................................................. 4
1.3.3.2. Microbial Fuel Cell (MFC) vs Microbial Electrolysis Cell (MEC).................... 5
1.3.3.3. Anodic half reaction............................................................................................ 5
1.3.3.4. Cathodic half reaction ......................................................................................... 6
1.3.3.5. Ion exchange membrane ..................................................................................... 7
1.3.3.6. Single or two chambers....................................................................................... 8
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1.3.3.7. Microbiology....................................................................................................... 8
1.3.3.8. Mediator bacteria ................................................................................................ 8
1.3.3.9. Mediatorless bacteria .......................................................................................... 9
1.4. Products and Achievements of BES ................................................................................. 9
1.5. Limitations of BES.......................................................................................................... 10
1.6. Aims of the Thesis: ......................................................................................................... 13
2. Materials and Methods......................................................................................................... 14
2.1. MEC and Data Acquisition ............................................................................................. 14
2.1.1. MEC....................................................................................................................... 14
2.1.2. Data acquisition ..................................................................................................... 15
2.2. Experimental Procedures ................................................................................................ 16
2.2.1. MEC set up ............................................................................................................ 16
2.2.2. Synthetic wastewater ............................................................................................. 16
2.2.3. Ammonium analysis .............................................................................................. 16
2.2.4. Gas Chromatography (GC) analysis...................................................................... 17
2.3. Calculations..................................................................................................................... 17
2.3.1. Electron recovery................................................................................................... 17
2.3.2. Migration efficiency calculations .......................................................................... 18
2.3.3. Energy requirements.............................................................................................. 19
3. Results .................................................................................................................................. 20
3.1. Acetate Degradation for Current Production .................................................................. 20
3.2. Acetate Degradation Rate in Relation to Current Production......................................... 21
3.3. The Effect of pH on Current Production......................................................................... 22
3.4. Maximum Ammonium Removal .................................................................................... 24
3.5. Ammonium Transfer to the Cathode .............................................................................. 25
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3.6. Ammonium Accumulation in the Cathode with 80 mM Ammonium Concentration..... 26
3.7. Ammonium Accumulation in the Cathode with 1 M Ammonium Concentration.......... 27
3.8. Ammonium Accumulation in the Cathode with 2 M Ammonium Concentration .......... 28
3.9. The Effect of Ammonium Concentration in the Cathode on the Ammonium Migration
Rate .................................................................................................................................. 29
3.10. Energy Requirements for Ammonium Migration against its Concentration Gradient. 30
3.11. Sodium Addition from pH Control............................................................................... 31
3.12. Effect of Sodium in the Cathode on the Ammonium Migration .................................. 31
3.13. Ammonia Transfer through the Membrane .................................................................. 33
4. Discussion............................................................................................................................ 35
4.1. Overall Outcomes and Significance................................................................................ 35
4.2. Concentration Effect Interpretation ................................................................................ 35
4.3. Ammonium Accumulation Comparison ......................................................................... 37
4.3.1. Ammonium removal rate comparison ................................................................... 37
4.3.2. AS ammonium removal rate.................................................................................. 37
4.3.3. ED ammonium removal rate.................................................................................. 38
4.3.4. Reactor volume for WWTPs ................................................................................. 38
4.4. Energy Requirements ...................................................................................................... 39
4.4.1. Energy concerns .................................................................................................... 39
4.4.2. Energy required for MEC ...................................................................................... 39
4.4.3. Energy required for ED ......................................................................................... 39
4.4.4. Energy required for AS.......................................................................................... 41
4.4.5. Energy required for ammonia gas production ....................................................... 42
4.5. Limitations ...................................................................................................................... 43
5. Conclusions and Recommendations .................................................................................... 45
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5.1. Conclusions ..................................................................................................................... 45
5.2. Recommendations ........................................................................................................... 45
5.2.1. Wastewater ............................................................................................................ 45
5.2.2. Magnesium Ammonium Phosphate (MAP) .......................................................... 46
References ................................................................................................................................ 47
Appendices ............................................................................................................................... 52
Appendix A ............................................................................................................................ 52
Appendix B ............................................................................................................................ 53
Appendix C ............................................................................................................................ 54
Appendix D ............................................................................................................................ 55
Appendix E............................................................................................................................. 56
Appendix F............................................................................................................................. 56
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List of Figures
Figure 1.1: Activated sludge (AS) technology. .......................................................................... 3
Figure 1.2: Basic BES design. .................................................................................................... 5
Figure 1.3: Potential losses during electron transfer in MFC..................................................... 7
Figure 1.4: The cell voltage of the system affects electricity production................................. 11
Figure 2.1: Microbial Electrolysis Cell with the anode and cathode separated by a cation
exchange membrane . ............................................................................................................... 15
Figure 3.1: The effect of 1.5mmol of acetate addition on current production. . ...................... 20
Figure 3.2: Time course of the current flow over four hours after acetate addition................. 21
Figure 3.3: The effect of acidic pH on the current production in MEC.. ................................. 23
Figure 3.4: The effect of pH > 6.6 on the current production in MEC. ................................... 23
Figure 3.5: Ammonium migration from the anode. ................................................................. 24
Figure 3.6: Time course of ammonium appearance and disappearance in the anolyte and
catholyte respectively. .............................................................................................................. 25
Figure 3.7: The effect of 80 mM ammonium in the catholyte on the ammonium migration rate
to the cathode............................................................................................................................ 26
Figure 3.8: The effect of 1 M ammonium in the catholyte on the ammonium migration rate to
the cathode................................................................................................................................ 27
Figure 3.9: The effect of 2 M ammonium in the catholyte on the ammonium migration rate to
the cathode................................................................................................................................ 28
Figure 3.10: Effect of ammonium concentration on the electron flow and ammonium
migration................................................................................................................................... 29
Figure 3.11: Effect of sodium concentration on the electron flow and ammonium migration 32
Figure 3.12: Effect of current on ammonia back diffusion ...................................................... 33
Figure 4.1: Theoretical voltage requirements for MEC and ED .............................................. 40
Figure 4.2: Schematic diagram comparing energy requirements for ammonia production and
ammonium degradation, and MEC ammonia recovery............................................................ 42
Figure 4.3: MEC with small surface area reduces efficiency .................................................. 43
Figure 4.4: MEC with large surface area enhances ammonium transfer.................................. 44
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List of Tables
Table 2.1: Example of acetate degradation estimated from current integral............................ 18
Table 3.1: Electron transferred ................................................................................................. 22
Table 4.1: Ammonia stripping summary.................................................................................. 36
Table 4.2: Comparison of ammonium removal rates (mM.h-1) of different wastewater
treatment processes................................................................................................................... 38
Table 4.3: Summary of energy requirements per mmol of ammonium migrated .................... 39
Table 4.4: Comparison of energy requirements (J/mmolNH4+) for MEC, ED and AS. ............ 41
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1. Introduction 1.1. Water: a Precious Resource
Agriculture, industries and individuals use water for different processes and in various
quantities (Rutherfurd and Finlayson, 2011). Water is among the most essential resources
required for economic development. However, climate change will alter the rainfall patterns,
and water resources will not be available in the same manner as in past decades (Chiew et al.,
2009). In addition, water resources are becoming polluted and therefore require strong
management plans (Pahl-Wostl, 2006, Zabihollah, 1999). Wastewater is an area of great
prospect for management, because pollutants are diluted and therefore most of the water can
be recovered (Toze, 1997). For example in certain cities, wastewater is recycled for drinking
purposes. One documented significant example is Singapore’s NEWater system (Tortajada,
2006)
Traditionally our society disposed of the wastewater in their surrounding waterways, for
example oceans or wetlands (Lofrano and Brown, 2010). Nowadays society emphasises on
sustainability by reducing, recycling and treating our wastes (Toze, 1997). Unfortunately,
Wastewater Treatment Plants (WWTP) require large amounts of energy to remove pollutants
from wastewater (de Bruin et al., 2004). One challenge is that wastewater has varying
properties according to the source of discharge and its dilution factor (Warith et al., 1998).
Wastewater is usually considered as non-valuable and a mind shift could show that such water
can be turned into an economic product (Cheng, 2008).
1.2. WWT: a Key Process in our Society
1.2.1. Wastewater characteristics
Wastewater is characterised by its Biological Oxygen Demand (BOD) which is the oxygen
required by bacteria to oxidise pollutants, such as nitrogen (N) and organics (carbon source).
In industrialised countries, urban domestic wastewater contains a BOD of 150-250 mg/L
(Warith et al., 1998) and total nitrogen is 50-60 mg/L (Water Corporation, 2011).
Wastewater is also an ideal source for microorganisms development, for example pathogen
and bacteria (Toze, 1997). Untreated wastewater discharged to waterways has negative effects
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on human health and the environment (Camargo and Alonso, 2006). This has led to the
development of WWTP. For example, Woodman Point WWTP (Perth, Western Australia)
treats 120x106 L of urban wastewater per day (Water Corporation, 2009).
1.2.2. Environmental degradation prevention
WWTP prevents eutrophication which is caused by excessive nutrients (nitrogen or
phosphorus) released to the environment (Boulton and Brock, 1999). The excess of nutrients
leads to the development of algae, which is detrimental to the fauna. Fishes and dolphins have
died as a result of eutrophication (Duce et al., 2008). The main nutrient presents in wastewater
is nitrogen in the form of urea (CO(NH2)2), which naturally degrades to ammonium (NH4+)
(Hammes et al., 2003, Maurer et al., 2006). Ammonium needs to be removed to prevent this
environmental damage and will be the focus of this project.
1.2.3. Ammonium
The focus of the project is on ammonium removal, as the main nitrogen constituent of
wastewater. The presence of other nitrogen compounds, namely NOx and nitrous oxides, are
limited by low oxygen dissolved in wastewater (<0.5 mg.L-1). Ammonium is characterised by
its pKa, which represents the pH at which the same amount of ammonium-nitrogen is present
in both ionised ammonium (NH4+) and non-ionised ammonia (NH3) forms. The pKa of
ammonium is alkaline 9.23 (Lide, 2003), therefore at pH > 9.23 the ammonia gas present in
solution increases. Ammonia is a gas which is very soluble in water at room temperature
(ATSDR, 2004).
Ammonia is essential in fertiliser manufacturing. Ammonia production requires 30 GJ.t-1
(EFMA, 2000). This energy is for the nitrogen and hydrogen gases to be compressed and
reacted. If ammonia could be recovered from wastewater, then this energy used by ammonia
fertiliser factory could be saved. It was attempted previously by using lime which increases
pH and leads to ammonia gas being stripped off the wastewater (O'Farrell et al., 1972).
However the wastewater being alkaline cannot be disposed in the environment. Separating the
ammonium from the bulk of wastewater represents a more direct process and therefore a
cheaper alternative. Ammonium recovery from wastewater is of interest not only to protect the
environment, but also because it has an economical value from its energy savings.
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1.3. The Place of BES in Current WWTP Technology
1.3.1. Activated sludge
Activated Sludge (AS) is an effective wastewater treatment used throughout the world
(Kermani et al., 2009). It requires large amounts of aeration for bacteria to degrade organics
and oxidise ammonium to a non pollutant nitrogen gas (N2) (Duce et al., 2008). In AS,
wastewater is continuously flowed to a reactor which is alternately aerated and non-aerated
(Figure 1.1). During the aerated section, ammonium is oxidised to nitrite (NO2-) and nitrate
(NO3-) (Patrick and Reddy, 1976). As the flow continues, the wastewater enters anoxic
conditions, where the denitrification process consumes organics and produces nitrogen gas
(Duce et al., 2008).
This ammonium removal technology uses large quantities of energy (Houillon, 2005) and
does not produce any useful by-products. For example Woodman Point WWTP, in Western
Australia spends about $2.5 millions for aeration per year (Cord-Ruwisch, personal
communication). The energy efficiency has to be addressed even though new technologies
have been used to diminish costs (e.g.: Sequencing Batch Reactor). These technologies are not
reviewed here, because they are detailed in the literature (Mace and Mata-Alvarez, 2002, Yoo
et al., 1999).
Figure 1.1: Activated sludge (AS) technology. The wastewater inflow is primarily treated and is flowed to the bioreactor. Aeration (A) occurs in different parts of the reactor which enables ammonium oxidation to nitrate (NO3
-). In the anaerobic (An) part of the process, the organics are removed and nitrogen (N2) gas is formed. In the clarifier, the excess sludge is settled and disposed of, but some is recycled to maintain a good bacterial activity. The treated wastewater enters the tertiary treatment or is disposed in the environment.
Primary treated wastewater Clarifier Bioreactor
Biomass recycling Biomass disposal
Treated Wastewater
A A An An
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1.3.2. Electrodialysis
Another process used for cations removal is called Electrodialysis (ED). This technology is
mainly used for the production of sodium hydroxide (NaOH) (Zhang et al., 2008), removal of
phosphorus (Yeoman et al., 1988) and heavy metals such as lead (Pb) (Mohammadi et al.,
2004). ED has been used to remove some nitrate and other metal ions from industrial
wastewater (Gain et al., 2002). Alternatively, ED can recover freshwater from saline water
(Mohammadi and Kaviani, 2003)
This process is also important for removing ammonium. In fact, this technology has been used
in combination with reverse osmosis to recover ammonia from swine manure (Mondor et al.,
2008, Ippersiel et al., 2011). ED process has been used for highly concentrated wastewater,
and produces acidic effluent that requires management before disposal (Zhang et al., 2008).
This technology has been refined to enhance efficiency, however it consumes large amounts
of energy and does not treat the organics present in wastewater.
1.3.3. Bioelectrochemical system (BES)
1.3.3.1. Design
BES aims at recovering energy from wastewater and is seen as a technology that would
diversify the energy resources of the world (Franks and Nevin, 2010). BES technology is
widely researched because it is perceived as an energy producer rather than an energy
consumer (Rabaey and Verstraete, 2005). Research areas include, but are not limited to,
electrochemistry, material science and biochemistry (Logan et al., 2006). The most important
principle of a BES is the potential difference generated between its two chambers (Du et al.,
2007), allowing electricity production. Further principles on which BES is based, and its
limitations are described below.
BES are composed of an anodic chamber and cathodic chamber defined respectively by an
oxidation half reaction and a reduction half reaction. The chambers are separated by an ion
selective membrane (Du et al., 2007). Electrons from the organics oxidation are transferred
from the anode to the cathode via an external resistance (Figure 1.2). The transfer of electrons
through the wires and across a resistance represents a current. At the cathode, an electron
acceptor, usually oxygen (O2), is reduced to hydroxyl (He and Angenent, 2006). The circuit is
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completed once positive charges have migrated from the anolyte (i.e.: liquid in anode
chamber) to the catholyte to maintain the charge balance (Figure 1.2).
Figure 1.2: Basic BES design. At the anode an oxidation half reaction occurs, which is a loss of electrons. At the cathode a half reduction reaction occurs, corresponding to a gain of electrons. Both electrodes are separated by a membrane and external resistance enabling the transfer of electrons (Du et al., 2007).
1.3.3.2. Microbial Fuel Cell (MFC) vs Microbial Electrolysis Cell (MEC)
BES is a generic term that covers two types of cells (Rozendal et al., 2008). MFC aims at
producing electricity from the chemical reduction catalysed by bacteria at the anode. In WWT,
MFC attempts to harvest the electrons available from organics (Allen and Bennetto, 1993).
MEC is a technology that uses external power to provide sufficient potential to the cathode to
produce a by-product such as hydrogen or methane (Rozendal et al., 2008). MEC varies from
ED in the sense that the anodic reaction is catalysed by bacteria, and the electron transfer is
enhanced to enable the by-product formation.
1.3.3.3. Anodic half reaction
The anode is the electrode at which an oxidation half reaction occurs, defined by a loss of
electrons. In BES, this half reaction is catalysed by bacteria (Du et al., 2007). The last electron
acceptor from the bacterial respiratory chain defines the potential of the anode (Rabaey and
Verstraete, 2005). This is important because it enables to determine the highest output
possible from BES. The anodic chamber is usually anaerobic, which is characterised by a
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negative redox potential. Anaerobic bacteria are selected, otherwise the electrons are accepted
by oxygen (O2), which prevents the negative anodic potential formation in the system, and
therefore reduces system efficiency (Logan et al., 2006).
1.3.3.4. Cathodic half reaction
At the cathode, electrons are transferred onto chemical species (i.e.: electron acceptor)
(Rozendal et al., 2006b, Boulton and Brock, 1999) . The cathodic chamber is usually abiotic
and a spontaneous chemical reaction occurs (He and Angenent, 2006). The chemical species
are consumed according to their redox potentials from the most positive to the most negative.
The cathode potential is determined by the electron acceptor in the chamber. In a natural
ecosystem, oxygen accepts electrons (potential = +400 mV vs Ag/AgCl) (Atkins and De
Paula, 2010). In BES, oxygen has been used at the cathode because it is readily available and
has a positive potential, which means greater system efficiency. Once a given electron
acceptor is depleted, the cathode’s potential will decrease until the potential of the next
electron acceptor is reached (Boulton and Brock, 1999).
Overpotential is a problem that is described in the literature extensively (Figure 1.3, Rabaey
and Verstraete, 2005, Logan et al., 2006, Rozendal et al., 2008). Overpotential refers to the
difference between the measured potential and the theoretical one, determined from the
thermodynamic reaction taking place at the anode and the cathode. The potential difference
between the two electrodes is increased and therefore the efficiency decreases. Poor conductor
material is a major contributor to overpotential. Numerous materials have been used for the
anode: platinum (Schröder et al., 2003), gold (Malvankar et al., 2011), graphite (Cheng et al.,
2008); but some of the proposed materials are not realistic to be used on the large scale, and
better solutions should be proposed.
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Figure 1.3: Potential losses during the electron transfer in a MFC. 1. Losses from bacterial electron transfer. 2. Losses owing to electrolyte resistance. 3. Losses at the anode. 4. Losses at the MFC resistance and membrane resistance losses. 5. Losses at the cathode. 6. Losses from electron acceptor reduction (Rabaey and Verstraete, 2005)
1.3.3.5. Ion exchange membrane
In the initial development of BES, the design was an H cell (Logan et al., 2006). However in
such design, the surface area of the membrane to the volume of the chambers was small and
led to a relatively inefficient system (Logan et al., 2006, Rozendal et al., 2008). The ion
exchange membrane separating both chambers can either be anionic or cationic. Considering
that ammonium is the focus of this work, a Cation Exchange Membrane (CEM) is used for the
BES. The driving force for the exchange of positive charges through the CEM is the
concentration difference between the two chambers (Kuntke et al., 2011). The presence of the
CEM has led to a problem known as the pH split (Rozendal et al., 2006b). In the anode the
organic degradation leads to an acidification of the anolyte, while in the cathode the oxygen
reduction produces hydroxyl (OH-). This difference in pH leads to a greater overpotential and
reduces the efficiency of the system. The pH reduction in the anode may eventually lead to the
death of essential bacteria for organics degradation.
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1.3.3.6. Single or two chambers
The initial development of BES was based on a two chambers system. This design is similar
to ED but bacterial activity catalyses the anodic reaction (Rozendal et al., 2008). In MFC, the
catholyte is circulated at a fast rate and aerated to enable the maximum oxygen transfer
(Logan et al., 2006). The low surface area of the membrane compared to the volume of both
solutions was found to be a limiting factor. Logan et al. (2006) proposed a large surface area
to solution ratio, which increased the efficiency of the system. Another issue is oxygen
transfer because it is quite inefficient to pump air into the catholyte. The energy cost is very
high and defeats the purpose for which BES has been proposed as an energy producer.
A single chamber design has been proposed to increase the efficiency of the system (Logan et
al., 2006, Schröder et al., 2003). Oxygen transfer is improved because the cathode is open to
air, which prevents air pumping and saves energy. Yet, it still requires the anolyte pH to be
maintained against the production of protons from the oxidation of organics.
1.3.3.7. Microbiology
Bacteria populations have been analysed by various researchers (Franks and Nevin, 2010,
Rabaey and Verstraete, 2005). This brief review will present the general characteristics of
bacteria that are of importance to run a BES, and not the different bacteria species.
The most important bacteria characteristic is the respiratory electron chain. Organics are
degraded and liberate electrons that are used by bacteria to run the citric acid cycle, the
electron transport chain, both producing adenosine triphosphate (ATP) (Campbell et al.,
2008). Rabaey and Verstraete (2005) showed that the anodic potential corresponds to the
potential of the last electron acceptor of bacteria’s electron transport chain. Electrons transfer
onto electrodes surface occurs by two different mechanisms explained below (Lovley, 2006).
1.3.3.8. Mediator bacteria
The anodic oxidation reaction is catalysed by bacteria. Some bacteria transfer electrons to the
electrode surface using external mediators or electron shuttle (Lovley, 2006). Some research
has shown that bacteria can use mediators present in the wastewater as an electron acceptor,
for example humic substances (Lovley et al., 1996). Some bacteria produce their own electron
mediators, for example Geothrix ferementans. However, soluble mediators are not a
9
sustainable manner to treat wastewater because chemical addition, or mediator formation, is
required at all times, therefore mediators are washed away as the wastewater is replaced.
1.3.3.9. Mediatorless bacteria
Lovley (2006) described electrons transfer that occurred directly to the electrode surface. This
direct transfer was suggested to be done through nanowires (Reguera et al., 2005). Malvankar
et al. (2011) demonstrated that nanowires could transfer electrons across 50 µm. This is
important because understanding the mechanisms of electron transfer would improve the
system efficiency.
1.4. Products and Achievements of BES Potter (1911) demonstrated a potential difference when bacteria degraded organics. MFC
produces electricity based on this principle (Section 1.3.3.2). It was only in the 1970s that the
research focussed on WWTP (Roller et al., 1984). Nowadays, wastewater as a source of
energy seems promising. The most recent achievement for MFC is the production of energy
for a meteorological buoy that was sustained for several years (Franks and Nevin, 2010).
Alternatively to electricity production, Liu et al. (2005) added an external power source, so the
system could be used to form by-products (i.e.: MEC). The energy produced from the organics
degradation is still used but a potentiostat (i.e.: the external power source) enhances the
electron transfer to the cathode. This leads to the cathodic potential reaching the redox
potential of a specific chemical reaction, such as hydrogen production (Liu et al., 2005).
Biohydrogen production using an external power supply was first proposed by Liu et al.
(2005). This process requires energy from fossil fuels. The natural potential difference
between fresh and saline water could be used instead (La Mantia et al., 2011). However such a
technique seems unrealistic because it increases the salinity of fresh water, which is a precious
resource. Cheng (2008) showed that hydrogen production incurred a cost because of
potentiostat energy requirements. Nevertheless it was also demonstrated that biohydrogen
production was less expensive than conventional techniques (Rozendal et al., 2006a).
10
Methane production followed shortly after hydrogen production at the biocathode (Cheng et
al., 2009), because methanogens activity is related to hydrogen concentration. According to
Cheng et al. (2007), hydrogen has less value than methane because of its chemical properties,
for example it is less dense than methane, which makes its transport difficult. Therefore,
focussing on methane production seems a good direction for future applications of MEC.
Biosensors represent an alternative use of BES, aiming to monitor the presence of organics in
effluent water. MFC potential is linearly correlated with the organics content of wastewater
(Du et al., 2007). Kim et al. (2003) claimed that an MFC biosensor could be operated for five
years without important maintenance. Du et al. (2007) have described that MFC can monitor
pollutants by measuring the decrease in potential. For example, heavy metals would reduce
the bacterial activity and hence the potential, which represents a measure of pollution. BES as
a biosensor on industrial scale is yet to be demonstrated.
1.5. Limitations of BES One of the major limitations is that BES has not been demonstrated to function on a large
scale application. The closest application is the meteorological buoy which was deployed in
the ocean, but it required large area (0.03m3) and the authors recommended to have numerous
units to ensure consistent electricity production (Tender et al., 2008). There is a need to
optimise the system to reduce the costs and environmental impacts.
The production of protons (H+) from organic degradation acidifies the anolyte. In theory,
protons can travel through the CEM to balance charges. However because it is in small
concentration (pH 7= [H+] = 10-7 M) other cations tend to move to the cathode (Rozendal et
al., 2006b), as their concentrations are at least 100 times greater (10-3 M). If the anolyte is too
acidic then the bacterial activity stops (Keenan et al., 1984). Therefore regular addition of
hydroxyl is required to maintain the anodic pH, and sodium hydroxide (NaOH) is usually used
(Kuntke et al., 2011). Alternatively a buffer could be used, but it is costly and requires extra
chemicals addition to the wastewater.
11
In the cathode, there is a problem of alkalinity, because protons are consumed with oxygen
reduction. In MEC, water is split into protons that are reduced to produce hydrogen gas
(Equation 1.1, Equation 1.2).
2O2 + 2H+ =2H2O Equation 1.1
2H+ + 2e- = H2 Equation 1.2
Therefore the cathode potential is decreased (Rozendal et al., 2006b) and so is the cell
potential. In MFC, a poor cell potential leads to reduced electricity production (Figure 1.4).
Figure 1.4: The cell voltage (blue lines) affects electricity production significantly. Assuming that the anodic potential is maintained, then at higher cathodic potential (blue circles) the energy output is increased by 45%. (adapted from Cheng 2008).
In MEC, a negative cathodic potential can be beneficial, depending on the chemical redox
potential, but high alkalinity counter acts this reaction. For example, hydrogen production has
a theoretical potential of -1.21 V (Liu et al., 2005). However, due to the constant proton
consumption, a high pH (10) develops and increases the hydrogen production redox potential
by 0.177 V (Wrana et al., 2010). This requires extra electron flow (i.e.: current) to decrease
12
the cathodic potential, which reduces the system efficiency. Therefore it is essential to acidify
the cathode.
BES has been proposed because it reduces the energy requirements from WWTP. However,
some studies use ferrycyanide for electron acceptor in the cathode, which is highly
detrimental to the environment (Oh et al., 2004, Schröder et al., 2003). Others propose using
oxygen in the catholyte to enable a relatively high energy output (Logan et al., 2006, Oh et al.,
2004). However, it is contradictory because BES was proposed as an energy saver by
preventing aeration.
Cord-Ruwisch et al. (2011) suggested using ammonia for proton transfer from the anode to the
cathode. Such idea sustainably overcomes anode acidification in BES. However, recirculating
ammonia from the cathode to the anode would mean that the wastewater is contaminated
again, and cannot be used for continuous treatment system.
Overall, it was shown that ammonium requires to be removed to prevent environmental
degradation. Traditional WWTP removes ammonium by oxidising and reducing it to a useless
by-product: nitrogen gas. ED is an alternative that recovers ammonium, however the organics
in the wastewater remains. BES is a new system that was initially used for electricity
production and then for bio-hydrogen. Ammonium removal using BES has been limitedly
researched. Ammonium has numerous characteristics that make it possible to recover. Using
ammonium for proton shuttling recently opened the opportunity to recover ammonium in the
form of a useful product: ammonia gas. It is currently used for fertiliser production at high
cost, which may be decreased by using MEC. MEC characteristics are: selective migration of
ammonium from anode to cathode, high pH at the cathode assisting with recovery of ammonia
as a gas, and energy production from organics degradation that would reduce the ammonia
production cost.
A research question for this thesis is to test the hypothesis of whether ammonia can be
recovered from wastewater using a MEC, and whether it is energy efficient.
13
1.6. Aims of the Thesis: The aims of the thesis are therefore to investigate:
1. Migration of ammonium against its concentration gradient to the cathode
2. Ability of MEC to concentrate the ammonium in the cathode
3. Ammonia gas recovery from the catholyte
4. Energy comparison of the MEC system to current ammonium removal techniques
(ED and AS).
14
2. Materials and Methods
2.1. MEC and Data Acquisition
2.1.1. MEC
The MEC cell was made of transparent Perspex (Figure 2.1) and had two chambers separated
by a CEM (168 cm2, UltrexTM CMI7000, Membranes International Inc.). The two chambers
were of equal volume (316 mL (14 cm x 12 cm x 1.88 cm)). Granular graphite (El Carb 100,
Graphite Sales, Inc., USA, granules 2-6 mm diameter, porosity of 45 %) was used to fill the
chamber and reduced the liquid volume to 120 mL. Graphite electrodes (5 mm diameter)
provided a contact between the external circuit and the granular graphite and protruded out of
the chambers. The anodic potential was recorded against an Ag/AgCl reference electrode
(BASi, MF-2079). The reference electrode consisted of a sliver chloride in 3 M NaCl
electrolyte.
A potentiostat (manufactured and quality assured by Murdoch University) was connected to
the cell. The anodic potential was set at -300 mV (vs Ag/AgCl) unless otherwise stated
(Appendix A).
Both the anolyte and the catholyte were of equal volume (0.4 L). The anolyte was recirculated
at 7.7 L.h-1 (Masterflex® L/STM, Cole Farmer) and the catholyte recirculated at 1.0 L.h-1
(DEMA peristaltic pump, Australia), to reduce the oxygen concentration within the solution.
A 250 mL Schott bottle was part of the recirculation loop. The anolyte was maintained at 28 oC using a water bath (PolyScience, temperature controller). Solutions were introduced to the
chambers at the bottom of the cell and removed from the top. This ensured that all of the air
between the graphite granules was removed. The compartments, the tubing and the
recirculation bottle were covered with aluminium foil to prevent light penetration and algal
development.
The anolyte pH was automatically dosed (set point at 7.5) using 4 M NaOH solution with a
pump flow rate of 2.13 mL.min-1 (Masterflex®C/L®).
15
Samples were taken from both the anolyte and the catholyte, through the recirculation bottles,
using tubing and syringes, to prevent oxygen entering the solutions. The Schott bottles were
covered with a customised rubber bung to fit the tubing for recirculation. 2 mL of samples
were taken, and immediately centrifuged at 3000 rpm for 3 minutes (Hermle Z233M), and the
supernatant used for ammonium analysis (Section 2.2.3) before being stored in a fridge (4 °C).
Figure 2.1: Microbial Electrolysis Cell (MEC) with the anode and cathode separated by a cation exchange membrane (CEM).
2.1.2. Data acquisition
The MEC was controlled and monitored by a purpose-built LabVIEWTM 7.1 virtual
instrument program with a LabJackTM USB data acquisition interface (U12). Anodic potential,
cell potential, and anolyte pH (TPS, 120 x 12 mm) were automatically recorded into an
ExcelTM spreadsheet.
A high precision digital multimeter (resolution 10 µV) (Professional Digital Multimeter
UT70D) was used to confirm the accuracies of all voltage signals detected by the U12 card.
Anode
Anode Cathode
CEM
16
2.2. Experimental Procedures
2.2.1. MEC set up
Activated sludge from a local wastewater treatment was used (Woodman Point wastewater
treatment, Western Australia) to develop a biofilm. The activated sludge and synthetic
wastewater were mixed together (10 % v/v) and recirculated in the anode for 20 hours, at 7.5
mL.min-1 (Masterflex® L/STM, Cole Farmer).
The system was initially run as a MFC (Section 1.3.3.2) to enhance biofilm development. The
catholyte was aerated, and recirculated at a rate of 226 mL.min-1 maximising oxygen transfer.
The circuit was open until the anodic potential reached -450 mV (vs Ag/AgCl). Once this
condition was reached, the anode and cathode were connected by a variable resistor (1 MΩ
maximum). The resistance was lowered in a step-wise manner until a 1 Ω resistance was used.
The pH of the anolyte was maintained manually between 7 and 8 by adding aliquots of 1 M
NaOH solution. After the biofilm was established, the MFC system was connected to a
potentiostat (Appendix A) and the system was run as a MEC (Section 2.1.1).
2.2.2. Synthetic wastewater
The synthetic wastewater composition was (mg.L-1): NaHCO3 1664, KH2PO4 88,
MgSO4.7H2O 50, and 1.87 mL.L-1 of trace element solution, which contained (g.L-1):
ethylene-diamine tetraacetic acid (EDTA) 15, ZnSO4.7H2O 0.43, CoCl2.6H2O 0.24,
MnCl2.4H2O 0.99, CuSO4.5H2O 0.25, NaMoO4.2H2O 0.22, NiCl2.6H2O 0.19,
NaSeO4.10H2O 0.21, H3BO4 0.014 and NaWO4.2H2O 0.050. Sterile concentrated yeast
extract (BBLTM) solution of 1 M was periodically added to the anolyte to support healthy
microbial growth. The ammonium was added in the form of NH4Cl and the carbon source
used was 10 mM NaCH3COOH, unless otherwise stated. Different concentrations of NH4Cl
were added according to the experimental question to be answered.
2.2.3. Ammonium analysis
The ammonium analyses were conducted by using Nesslers’ method. Ammonium
concentrations were measured colorimetrically using a spectrophotometer (Shidmazu,
UVmini1240; Pharmacia Biotech, Novaspec II), at wavelength of 425 nm. This method
measures the total ammonium and ammonia (NH4+ and NH3). The spectrophotometer was
17
blanked with DI water. In a 4 mL cuvette, 200 µL of sample was added to 25 µL of mineral
stabilizer (HACH, USA), 25 µL of polyvinyl alcohol dispersing reagent and 1800 µL of DI
water. Thereafter 100 µL of Nessler reagent was added and the cuvette was inverted 3 times.
After exactly 1 minute the absorbance reading was taken. Samples were diluted such that
absorbance readings were less than 1 (Greenberg et al. 1992). The ammonium analyses were
performed in duplicate (Appendix B)
2.2.4. Gas Chromatography (GC) analysis
GC analysis was used for quantifying the acetate concentration. It was conducted using a GC
(Varian Star 3400) equipped with an auto sampler (Varian 8100) and a flame ionization
detector (FID). Samples were centrifuged (3,000 rpm for 3 minutes) before the supernatant
(900 µL) was acidified (pH< 2) by adding of 10 % (v/v) formic acid (100 µL). 1 µL of the
sample was injected into the capillary column (Alltech ECONOCAPTM, 15 m x 0.53 mm
(internal diameter)) using N2 (flow rate of 5 mL.min-1) as a carrier gas. The oven temperature
program for each injection was as follows: initial temperature 80 oC, ramped to 140 oC at a
rate of 50 oC min-1 where it remained for 1 minute, ramped to 230 oC at a rate of 50 oC.min-1
and held for 2 minutes. Injector and detector temperatures were 200 oC and 250 oC
respectively. The peak area from the FID output signal was integrated using the STAR
Chromatography Software© (1987-1995) and sample concentrations were determined by
interpolation from an acetate standard curve generated by the analysis of 0, 1, 2, 5, 10 mM
acetate samples (Charles et al., 2009).
2.3. Calculations
2.3.1. Electron recovery
The current was monitored (in mA) and corresponded to an amount of coulomb (C) produced
over a second (1 mA = 1 mC.s-1). The Faraday’s constant is the amount of charges (C) per
mole of electron; it is equal to 96,485.33 C.mol-1. Integrating the electron flow rate over time
(i.e.: current) quantifies the total number of electrons produced over a period of time. The
electron recovery calculation is done extensively in the literature (Cheng et al., 2008, Logan et
al., 2006).
18
In addition, it is possible to compare the electrons recovered with the electrons available from
the acetate degradation. The acetate degradation is known to produce 8 electrons (Equation
2.1). So 8 electron mol are produced per mol of acetate. Therefore, it is possible to calculate
the coulomb available per mole of acetate (Equation 2.2). An example of acetate degradation
deduced from the current measurement is shown in Table 2.1. The percentage of the electron
flow to the electron available (by acetate degradation) was calculated using equation 2.2.
1 NaCH3COOH 2CO2 + 1Na+ +7H+ + 8e- Equation 2.1
8mol (electron) / 96,485.33mol (electron)/C = 8 x 10-5C Equation 2.2
Table 2.1: example of acetate degradation estimated from current integral Acetate
initial Time elapse Current Charge Electron Acetate
degraded
Units mmol Sec mA =mC.s-1 mC mmol mmol
Value 10 3600 100 360,000 360,000/ 96,485.33 = 3.7 3.7/8 = 0.46
2.3.2. Migration efficiency calculations
The amount of electron flow can be deduced from the measured current (Section 2.3.1). In
Section 2.2.3, the ammonium migration can be measured over the time of an experiment. To
measure the efficiency of ammonium removal per electron, the amount of ammonium
migrated (mmolNH4+) is divided by the amount of electron flow deduced from the current
(mmole-). This ratio (mmolNH4+/mmole-) is compared in different experiments. The greater the
ratio the most efficient the system is. The expected maximum ratio is explained below.
Organic degradation rate produces current (Section 1.3.3.3), and the oxidation reaction
releases protons (Equation 2.1). Considering that acetate is used, protons and electrons are
released in the anolyte (Equation 2.1). The electrons produced will transfer to the cathode via
the external circuit. Therefore the anolyte will be positively charged. However, the
electroneutrality of the solution must be maintained.
19
To maintain the electroneutrality in solution the same amount of positive charges has to
migrate to the cathode (due to the presence of CEM). Two assumptions were made: firstly,
protons migration is negligible because they are in small concentration (10-7 M at pH 7).
Secondly, only ammonium and sodium migrate because they are the most abundant cations in
the anolyte. Therefore, in the absence of ammonium in the cathode, the maximum ratio of
ammonium removal per electron is one (mmolNH4+/mmole-).
2.3.3. Energy requirements
Using data acquisition from the MEC cell (Section 2.1.2), it is possible to calculate the energy
requirements for ammonium migration. The cell voltage and the current were measured and
recorded for each experiment. Using both parameters, the power (W) can be calculated
(Equation 2.3).
Power (mW) = Cell Voltage (V) x Current (mA) Equation 2.3
Considering that the time of the experiment and the amount of ammonium removed from the
anolyte are known, then the power per mWh can be calculated as follow:
Power requirements (mWh/mmolNH4+) = Power (mW) x time (h) / NH4+ removed (mmol)
To convert this power in Joules, then the following conversion factor would be used:
1 Wh = 3.6 kJ
Therefore:
Energy requirements (J/mmolNH4+) = Power requirement (mWh/mmolNH4+ ) x 3.6 (J/mWh)
20
3. Results
3.1. Acetate Degradation for Current Production
Acetate degradation by bacterial activity produces electrons (Equation 2.2). In MFC, it is
accepted that current is produced by electron flow from acetate degradation in the anode.
In contrast to MFC, MEC systems use additional external power. In the MEC system, a power
supply, referred to as potentiostat, is used to maintain the anodic potential to a determined
redox potential. This enables to maintain the anodic potential to -300 mV (vs Ag/AgCl),
which favours the oxidation of acetate.
The aim of this experiment was to test whether the current was produced by acetate
degradation, or from the potentiostat (Figure 3.1).
Figure 3.1: The effect of 1.5mmol of acetate addition (arrow) on the current production. The potentiostat was connected to the cell during the experiment.
A MEC was set up as described under Section 2.1. After eight weeks of operation as a MFC
with acetate, an increasing current production was observed. The potentiostat was then
connected and the system operated as a MEC (Section 1.3.3.2). Prior to testing the effect of
acetate addition, the MEC was run without acetate feed for 10 hours, in order to set a baseline
in the absence of acetate.
0 20 40 60 80
100 120 140 160
0 10 20 30 40 50
Cur
rent
(mA
)
Time (min)
21
Once acetate was added (Figure 3.1 arrow) the current increased from 20mA to 140mA
(Figure 3.1). This confirmed that current produced was due to acetate degradation by bacterial
activity and not due to other processes.
Acetate degradation releases electrons and can be related to current, which is an electron flow
per second (Section 2.3.1). The focus was then on comparing the acetate degradation rate to
the current produced.
3.2. Acetate Degradation Rate in Relation to Current Production In the previous experiment the electron donor for current flow in the MEC was confirmed to
be acetate (Section 3.1). If acetate was the sole electron donor of the reaction, then the amount
of current should be related to the amount of acetate degraded (Section 2.3.1). By converting
the current flow during a time interval into electron equivalents (Section 2.3.1), an electron
balance can be established. The objective is to quantify how many electrons that are delivered
by acetate can be recovered as electron flow (current) over a time course of 145 minutes
(Figure 3.2 and Table 3.1).
Figure 3.2: Time course of the current flow over four hours after acetate addition. Before the current started the potentiostat was not connected therefore no current was measured. Legend: (▲) Electrons available (mM) from acetate degradation measured by GC, (■) Electrons transferred (mM) from current calculation, (♦) Current (mA).
0 2 4 6 8 10 12 14 16 18 20
0 20 40 60 80
100 120 140 160 180
0.00 50.00 100.00 150.00 200.00
Ele
ctro
n co
ncen
tratio
n (m
M)
Cur
rent
(mA
)
Time (min)
22
Table 3.1: Electrons transferred (mmol) compared to electrons available (mmol) from acetate degradation measured by GC and the equivalent electron recovery (%) (Section 2.3.1).
Average current
Time elapse Charge Electrons
transferred
Actual
Acetate
degraded
Electrons available
Electron Recovery
Units mA sec mC mmol mmol mmol %
Value 115.42 8,700 1,004,181 10.4 1.58 12.6 82
The electrons recovered represented 82% of the acetate degraded (Figure 3.2). This level of
recovery is in line with the literature (Rabaey and Verstraete, 2005, Franks and Nevin, 2010).
The electron recovery cannot achieve 100 %, because some energy is required for bacterial
growth (Campbell et al., 2008). Some energy is used within the cell for cell wall and proteins
production.
Having established that the MEC operated as designed, the next step was to observe acetate
degradation over an extended period of time and ammonium transfer from the anode to the
cathode.
3.3. The Effect of pH on Current Production
The addition of 0.5 mM of acetate typically resulted in a predictable current peak (Figure 3.3
arrow 1), followed by a current drop caused by acetate depletion. However, repeated acetate
additions resulted in less (Figure 3.3 arrow 2) or no (Figure 3.3 arrow 3) current, while at the
same time, pH decreased to 4.9. The pH decrease can be explained by the anodic reaction
(Equation 2.1).
To test whether the pH decrease reduced the current production, the pH was adjusted (Figure
3.3 arrow 4). The resulting current recovery showed that the low pH had inhibited the
bacterial current production.
23
Maintaining the pH enabled a constant current peak after acetate addition (Figure 3.4). The
focus of the investigation was then on ammonium migration, with pH control in the anodic
chamber.
Figure 3.3: The effect of acidic pH on the current production in MEC. 1.5 mmol acetate was added (arrows 1, 2 and 3) when the current decreased to 20 mA. At arrow 4, the pH was adjusted using 1 M sodium hydroxide solution. Legend: Current (mA) (♦) pH (■).
Figure 3.4: The effect of pH> 6.6 on the current production in MEC. When the current fell to 20 mA, a constant amount of acetate (1.5 mmol) was added (each arrow). Legend: Current (mA) (♦) pH (■).
0
2
4
6
8
10
0
50
100
150
200
250
0 100 200 300 400
pH
Cur
rent
(mA
)
Time (min)
0
2
4
6
8
10
0
50
100
150
200
250
0 100 200 300 400
pH
Cur
rent
(mA
)
Time (min)
1
2 3 4
24
3.4. Maximum Ammonium Removal The focus of this thesis is to evaluate the effectiveness of ammonium removal from
wastewater, using a MEC. Previously, acetate degradation created a current (Section 3.1) and
its rate could be quantified by the electron flow (Section 3.2). The aim of this experiment is to
quantify the ammonium migration rate during acetate degradation, using an equal ratio of
ammonium and acetate in the anolyte (Figure 3.5).
Figure 3.5: Ammonium migration from the anode in mM. The ammonium and acetate concentrations were equal in the feed. The cathode chamber was flushed with 1 L of DI water. Legend: (■) ammonium concentration in the anode (mM).
Most of the ammonium (16.5 mM) was removed, initially at a rate of 2.7 mM.h-1. However
the rate decreased to 0.19 mM.h-1 after eight hours. Assuming that the ammonium has
migrated to the cathode, then after eight hours the ammonium migrated against its
concentration gradient. This showed that the ammonium removal from wastewater is feasible
using an MEC. However, it was important to confirm that the ammonium migrated to the
cathode, and was not removed from the anolyte by other processes (e.g.: nitrification).
0 2 4 6 8
10 12 14 16 18 20
0 5 10 15 20 25
Con
cent
ratio
n (m
M)
Time (h)
25
3.5. Ammonium Transfer to the Cathode To test whether the ammonium disappearance in the anolyte was equal to the ammonium
accumulation in the catholyte, the system was run with a new feed containing 42 mM
ammonium in the anode chamber and DI water in the cathode chamber (Figure 3.6).
Figure 3.6: Time course of ammonium disappearance and appearance in the anolyte and catholyte respectively. The anolyte initially contained 42 mM ammonium and the catholyte contained no ammonium. Legend: (■) Ammonium concentration in the anolyte (mM), (▲) Ammonium concentration in the catholyte (mM).
The ammonium disappearance in the anode corresponded to the ammonium appearance in the
cathode (Figure 3.6). Until 200 minutes, the movement from the anode to the cathode could
have been achieved by diffusion caused by concentration gradient. However, after that, the
ammonium moved against its gradient. This can only be explained by an electron flow from
the anode to the cathode via the external circuit and the need for corresponding ammonium
movement from the anolyte to the catholyte. The next step in the investigation was to start
with a high ammonium concentration the cathodic chamber.
0 5
10 15 20 25 30 35 40 45
0 100 200 300 400 500 600
Con
cent
ratio
n (m
M)
Time (min)
26
3.6. Ammonium Accumulation in the Cathode with 80 mM Ammonium Concentration
Ammonium migration, from the anode to the cathode, was shown to occur (Section 3.4). If the
system was run repeatedly, for example for another two cycles, then the ammonium
concentration in the cathode would reach 80 mM. The aim of the experiment was to test
whether the ammonium could migrate against such a strong gradient by placing 80 mM
ammonium in the cathode (Figure 3.7).
Figure 3.7: The effect of 80 mM ammonium in the catholyte on the ammonium migration. Legend: (♦) ammonium concentration in the anolyte (mM), (▲) ammonium concentration in the catholyte (mM).
The ammonium in the anode could migrate (27.7 mM) and accumulate against a concentration
gradient of 40 mM (Figure 3.7). The catholyte concentration increased by the same amount of
ammonium to reach about 110 mM. One aim of the thesis was to concentrate the ammonium
in solution. This experiment demonstrated that using a MEC, concentrating ammonium in the
catholyte is possible. However, in order to recover ammonia gas by air stripping, ammonium
concentration needs to be about 10 times more concentrated (O'Farrell et al., 1972, Cheung et
al., 1995).
0
20
40
60
80
100
120
0 100 200 300 400 500 600
Cco
ncen
tratio
n (m
M)
Time (min)
27
3.7. Ammonium Accumulation in the Cathode with 1 M Ammonium Concentration
To achieve a stronger ammonium concentration in the catholyte, the system should be run
longer than previously. If the system operated for another 40 cycles then the ammonium
would be concentrated to 1 M in the catholyte. In this experiment the ammonium
concentration in the cathode is not shown, because it has been demonstrated in previously
(Sections 3.5 and 3.6) that all ammonium in the anode migrated to the cathode. The
experiment was run for 340 minutes only, because total ammonium removal was not tested
here. The aim of this experiment was to test whether ammonium can be transferred from the
anode against 1 M ammonium concentration in the cathode (Figure 3.8).
Figure 3.8: The effect of 1 M ammonium in the catholyte on the ammonium migration. The anolyte was made of a new batch of acetate. The catholyte ammonium concentration is equivalent to a system run for 40 cycles with synthetic wastewater. The pH in the anode was maintained at 7.5, while the initial pH in the cathode was 9.5 and was left to increase. Legend: (■) ammonium concentration in the anode (mM), (♦) current produced (mA).
The ammonium decreased in the anode (by 9.6 mM) demonstrating that the ammonium could
accumulate against 1 M ammonium in the cathode (Figure 3.8). This also showed that
ammonium can accumulate to 1 M in the catholyte and continue migrating. The ammonium
migration rate decreased with an increase in ammonium concentration in the cathode. In the
previous experiment the ammonium migration rate was 2.7 mM.h-1, while in this experiment it
0 20 40 60 80 100 120 140 160
0 5
10 15 20 25 30 35 40
0 100 200 300 400 C
urre
nt (m
A)
Con
cent
ratio
n (m
M)
Time (min)
28
is 1.7 mM.h-1. However, the comparison needs to be done with respect to the electron flow
(i.e.: current), because it changes according to bacterial activity.
Ammonia stripping would be more efficient in the presence of a concentration greater than 1
M (Bonmati and Flotats, 2003). If the system was run for another 100 cycles then the
ammonium concentration would be 2 M in the cathode.
3.8. Ammonium Accumulation in the Cathode with 2 M Ammonium
Concentration Ammonium accumulation in a cathode with 2 M ammonium concentration would enhance
ammonia gas recovery. The aim of this experiment is to test whether the ammonium
can migrate from the anode against a 2 M concentration in the cathode (Figure
3.9).
Figure 3.9: The effect of 2 M ammonium in the catholyte on the ammonium migration. The pH in the anode was maintained at 7.5, while the initial pH in the cathode was 9.5 and was left to increase. Legend: (■) ammonium concentration in the anode (mM), (♦) current (mA)
Results showed that the ammonium could not migrate from the anode against 2 M
concentration gradient (Figure 3.9), even with a current ranging from 80 to 130 mA from
acetate degradation in the anode. With a high current production, the charges transported are
expected to be maximum. Out of all charges present in the wastewater, it is expected that
0
20
40
60
80
100
120
140
0
10
20
30
40
50
60
0 100 200 300 400
Cur
rent
(mA
)
Con
cent
ratio
n (m
M)
Time (min)
29
some ammonium would migrate to the cathode. However, at first, the ammonium
concentration in the anode increased to 55 mM and then returned to the initial concentration
(50 mM) (Figure 3.9).
In the previous experiments, the rates of ammonium disappearance were shown independently
of the current produced in the MEC. To allow comparison, the ammonium migration needs to
be related to the mol of electron flow in the system (Section 2.3.2).
3.9. The Effect of Ammonium Concentration in the Cathode on the Ammonium Migration Rate
The ratio of ammonium flow to electron flow (mmolNH4+/mmole-) was used to compare
the previous experiments (Section 2.3.2). The current can be converted to electron mmol,
which can be compared with the ammonium disappearance (mmolNH4+) in the anode. Two
experiments were conducted, one without and one with ammonium present in the cathode
(Figure 3.10).
Figure 3.10: Comparison of electron flow (♦) and ammonia migration (■) in the absence (―) and presence (- -) of ammonium (500 mM) in the cathode. The experiments were conducted with new feed, containing 10 mM acetate and 50 mM ammonium in the anode. In the experiment without ammonium, the cathode chamber was flushed with 1 L of DI water. In the experiment with ammonium, 500 mM ammonium chloride was added to the catholyte. In both cases the pH in the cathode was above 9.5.
0 20 40 60 80 100 120 140 160 180 200
10
15
20
25
30
35
40
45
0 100 200 300
Cur
rent
(mA
)
Con
cent
ratio
n (m
M)
Time (min)
30
The migration of ammonium from the anode was about 30 % greater (27.5 %) in the presence
of low ammonium in the cathode (Figure 3.10). With no ammonium in the cathode the ratio
was 0.51 mmolNH4+/mmole- , compared to 0.37 with 500 mM ammonium concentration. This
reflects that in the presence of high ammonium concentration in the catholyte, more electron
flow (i.e.: current or acetate degraded) is required to concentrate the ammonium further.
3.10. Energy Requirements for Ammonium Migration against its Concentration Gradient
Concentrating ammonium against its gradient requires energy. The energy needed for
ammonium removal is important to evaluate the prospect of full scale application. The energy
requirements can be calculated from the current (mA) and the cell voltage (V) (Section 2.3.3).
In the previous experiment (Figure 3.10), the amount of energy required for migrating
ammonium, in the absence of ammonium in the cathode, was 97.2 J/mmolNH4+, compared to
482 J/mmolNH4+ for the migration against 0.5 M (Section 2.3.3). It is expected that the energy
requirements would be smaller when the concentration is low, because some ammonium
migrates by simple diffusion due to concentration differences.
In theory, all charges are transferred by ammonium because migration occurs due to diffusion
with the concentration gradient, and that other cations are in smaller concentration. The ratio
of ammonium per electron transferred should be one. In the previous experiment, the ratio was
about half the theoretical ratio in the absence of ammonium, and about a quarter in the
presence of 0.5 M ammonium in the cathode. Low ratios indicate that some electrons are
transferred without moving ammonium. This leads to an increase in energy requirements to
obtain a total ammonium removal from wastewater. Improving ammonium migration is the
next focus, looking in particular at the migration of alternative cations described by Kuntke et
al. (2011) in MFC.
31
3.11. Sodium Addition from pH Control It was shown that the system stopped in the absence of pH control (Section 3.3). Adding
sodium hydroxide to the anolyte to control pH increased sodium cations and their migration to
the catholyte. In the previous experiment, the ratio of ammonium to electron was less than half
the theoretical ratio (Section 2.3.2). It is expected that the electron balance was transferred by
sodium. Considering that the focus of the thesis is ammonium accumulation, it is essential to
understand the transfer of sodium to obtain a better efficiency of ammonium removal. It is
important to quantify the amount of sodium hydroxide required to balance the pH in the
synthetic wastewater used.
The synthetic wastewater made for the experiments contained 5 mmol of sodium acetate and
25 mmol of ammonium in the anolyte. After the full degradation of acetate, 35 mmol of
protons (H+) (Equation 2.1) were produced, hence the same amount of sodium hydroxide was
added to balance pH. The increasing build-up of sodium would compete with the ammonium
to migrate from the anode to the cathode.
3.12. Effect of Sodium in the Cathode on the Ammonium Migration To test whether sodium prevented ammonium migration to the cathode, the catholyte was
concentrated in sodium chloride (1 M) to simulate the effect of numerous cycles of the
system. Such concentration was expected to prevent the migration of sodium from the anode
and favour ammonium migration (Figure 3.11).
32
Figure 3.11: Comparison of electron flow (♦) and ammonium migration (■) in the absence (―) and presence (- -) of sodium (1 M) in the cathode. In the experiment without sodium, the cathode chamber was flushed with 750 mL of DI water to remove all cations. In the high sodium concentration experiment, 1 M sodium chloride was added. In both cases, the catholyte contained 0.5 M ammonium and pH drifted above 9.5.
The presence of sodium in the cathode increased the migration of ammonium to the cathode
from 0.37 to 0.47 mmolNH4+/mmole- (Figure 3.11). It was found that the ammonium movement
was enhanced by 25 % when high sodium (1 M) was present in the cathode. Furthermore, the
addition of sodium to the cathode favours ammonium migration to balance charges. The ratio
of ammonium to electron obtained was similar to the one found in the absence of ammonium
and sodium in the cathode (0.51 mmolNH4+/mmole- Section 3.9).
In the absence of sodium, energy requirements were 482.4 J/mmolNH4+, even with 0.5 M
ammonium in the cathode. In the presence of sodium (1 M) and ammonium (0.5 M) in the
cathode, energy requirements decreased to 238 J/mmolNH4+. The energy cost is reduced when
both sodium and ammonium were present in the catholyte, even though it is still twice the
energy cost without both cations in the catholyte (Section 3.10). However, the use of sodium
is required to maximise ammonium migration against its concentration gradient and therefore
the ratio of ammonium to electron.
0 20 40 60 80 100 120 140 160
0 5
10 15 20 25 30 35 40 45
0 50 100 150 200 250 300
Cur
rent
(mA
)
Con
cent
ratio
n (m
M)
Time (min)
33
3.13. Ammonia Transfer through the Membrane The cathodic pH reached values that were above the pKa (Section 1.2.3). In addition the
catholyte was maintained at about 25 oC, at which ammonia is soluble and therefore only
small amounts could be expected to volatilise.
Ammonia gas (NH3) diffusion back through the membrane is one possible reason for the ratio
of ammonium to electron flux being less than one. To test whether ammonia could transfer
through the membrane, a fresh anolyte was used and the ammonium was monitored until
MEC current production stopped because of acetate depletion (Figure 3.12 arrow 1).
Figure 3.12: Effect of current on ammonia migrating back to the anode. Fresh anolyte was used with 12mM ammonium and acetate. The catholyte contained 0.5 M ammonium at approximatly pH 9.5, to enable about 0.25 M of ammonia to be formed by dissociation. Legend: (■) ammonium concentration in the anode (mM), (♦) current (mA), (- -) sodium hydroxide addition, (○) Cathode pH, (●) Anode pH.
In the presence of current, the ammonium migrated against its concentration (4.3 mM), while
once the current stopped (Figure 3.12 arrow 1) the ammonia migrated back to the anode (1
mM.h-1). In the absence of current, charges cannot migrate because electroneutrality must be
maintained, so it was hypothesised that only ammonia (i.e.: non ionised) in solution
transferred through the membrane. The pH increase in the anode (from 7.5 to 8.2), in the
absence of sodium hydroxide, supports the concept of ammonia migration hypothesis (Figure
3.12). When the current resumed (Figure 3.12 arrow 2), ammonium migration against its
concentration also resumed.
0 50 100 150 200 250 300 350 400 450 500
0 2 4 6 8
10 12 14 16 18 20
0 500 1000 1500 2000
Cur
rent
(mA
) / N
aOH
add
ition
Con
cent
ratio
n (m
M)/
pH
Time (minutes)
1 2
34
The transfer of ammonia from the cathode could explain that the observed ratio of ammonium
to electron was lower than the theoretical value of one. While one ammonium migrated to the
cathode with one electron, simultaneously an ammonia molecule, which had previously
migrated from the anode, could transfer back. Therefore, it is possible to think that one
ammonium could migrate twice from the anode, reducing the efficiency of ammonium
removal from the anolyte. If the hypothesis, is correct then a membrane that does not allow
ammonia to migrate is needed.
35
4. Discussion
4.1. Overall Outcomes and Significance
The results demonstrated that it is possible to accumulate the ammonium from wastewater in a
separate solution, the catholyte (Section 3.6). It was shown that the current was produced from
organics degradation, and that without current ammonium accumulation cannot occur (Section
3.1 and 3.13). The accumulation against concentration gradients was demonstrated up to 1 M
ammonium in the cathode. The cathodic pH increased above 10 because of the proton-
consuming reduction half reaction occurring at the electrode. It was also observed that the
presence of sodium in the catholyte improved efficiency migration and reduced energy
requirements (Section 3.12). The migration of ammonia through the membrane, from cathode
to anode, reduced the ammonium to electron ratio, because one molecule of ammonium
migrated several times (Section 3.13).
MFC was proposed to recover the energy (i.e.: electricity) from organics degradation using
urban wastewater (Section 1.3.3.2, Rozendal et al., 2008). MEC was used to produce
hydrogen gas with less energy input than conventional processes (Section 1.4). Research was
done on ammonium migration in MFC (Kuntke et al., 2011). In addition, ammonium was
proposed as a proton shuttle to overcome the acidification problem in the anolyte (Cord-
Ruwisch et al., 2011). Using this knowledge, ammonium recovery from wastewater was
attempted in this thesis, which had not been studied prior to this work.
Traditionally, ammonium is removed from wastewater by nitrification and denitrification (i.e.:
AS) producing nitrogen gas. Such process treats urban wastewater, which is relatively diluted.
On the other hand, high ammonium containing wastewater (up to 0.5 M) can be treated using
ED. However, ED is energy intensive and does not remove the organics, but attempt at
recovering ammonia. MEC has the advantage of removing organics and recovering ammonia.
4.2. Concentration Effect Interpretation The aim of the thesis is to recover ammonia gas from wastewater using a MEC. Section 3.12
showed that a MEC can produce the conditions required for ammonia gas recovery. These
36
conditions have been established to be an ammonium concentration of 1 M and pH greater
than 9.5. Together with ammonia gas recovery via air stripping, the described ammonia
concentration could represent a veritable ammonia production process.
The acid dissociation constant of ammonium is 9.23 (Lide, 2003). When the cathodic pH is
greater than the pKa of ammonia (NH3) (pH > 9.5), then ammonia is present in greater
quantities than ammonium in solution. An alkaline pH was demonstrated as being the most
effective way of obtaining ammonia gas in solution (Bonmati and Flotats, 2003).
Assuming a concentration of 1 M concentration in the cathode at pH 9.5 (Section 3.7), this
represents 651 mM ammonia gas in solution (Equation 4.1). Cheung et al. (1995)
demonstrated that, with greater ammonia concentration in solution, there is an increase in
ammonia gas recovered. In the proposed MEC, the cathodic pH can be further increased and
increase ammonia recovery (pH 12; Cord-Ruwisch et al., 2011).
[NH3]= [NH3 + NH4+]/(1+10 pKa-pH) Equation 4.1
Ammonia gas recovery was not attempted in this thesis because numerous studies have
described means of effective recovery of ammonia gas from wastewater (Cheung et al., 1995,
Başakçilardan-Kabakci et al., 2007, Zeng et al., 2006, Cord-Ruwisch et al., 2011). Table 4.1
presents the different techniques and percentage recovery. Most studies recover 90 % of
ammonia, at pH ranging from 10 to 12 (Appendix C). Considering that the pH was 9.5 to 10 in
this thesis, then it is expected that about 80 % of the ammonia (800 mM) could be recovered,
using a high air flow rate in the stripping (> 100 L.h-1).
Table 4.1: Summary of the literature where ammonia stripping was attempted.
Reference Stripping apparatus
Air Flow (L.h-1) Wastewater type Recovery
Başakçilardan-Kabakci et al.,
2007 Air pumps 2700 Urine 97 % pH 12 and 16 oC
Zeng et al., 2006 Stripping tower and
heating 660 Anaerobic
digester effluent 93.5 % pH 8-11 and 80oC
Cheung et al., 1995 Air pumps 300 Landfill
leachates 90 % pH > 11 and 22oC
90 % pH > 12 and 55 oC Cord-Ruwisch et al., 2011
Nitrogen pumping 60 Synthetic
wastewater 47 % pH > 12 and 25 oC
37
The MEC of this thesis offers the possibility of recovering ammonia by flushing nitrogen gas
through the catholyte (Cord-Ruwisch et al., 2011). An increase in temperature would enhance
volatilization of ammonia. There would be an interest for further research on thermophilic
MEC for ammonia recovery, to reduce the volume of the process.
4.3. Ammonium Accumulation Comparison
4.3.1. Ammonium removal rate comparison
The ammonium removal rate plays an important role in determining the reactor volume
required for the process. If a process has twice the ammonium removal rate compared to
another, then this process would require half the volume to remove the same amount of
ammonium (Appendix D).
The MEC system would operate most of the time under 0.5 M ammonium and 1 M sodium
concentrations. The ammonium removal rate was determined to be 3.1 mM.h-1 (Section 3.12;
Appendix E). This rate will be compared to other WWTP processes, namely AS and ED. Then
their reactor volume is deduced from the rates and compared.
4.3.2. AS ammonium removal rate
AS ammonium removal rate varies with different processes. These processes involve energy
consuming nitrification, and subsequent or simultaneous denitrification. The removal rates are
comprised between 0.2 and 2 mM.h-1. If settling and decanting (e.g.: sequencing batch
reactor) are included, then the degradation rates are halved (Holman and Wareham, 2005, Yoo
et al., 1999, Yang et al., 2004).
The most recently described ANAMMOX process involves ANaerobic AMMonium
OXidizing bacteria that were found to remove ammonium faster: up to 12 mM.h-1, (van der
Star et al., 2007). However, this process requires an additional step for nitrite formation via
nitrification, and does not remove organics from the wastewater.
In conventional processes, ammonium is oxidised to nitrogen gas, which is not a useful
product, while MEC produces an ammonium concentrated solution, which has value for
fertilisers manufacturing.
38
4.3.3. ED ammonium removal rate
In contrast to biological removal, electrochemical ammonia removal from concentrated
wastewater streams can be very high. Gain et al. (2002) achieved a 625 mM.h-1 ammonium
removal from swine wastewater, using ED. Such removal rate is 200 times faster than the
MEC proposed. This means that the reactor volume would be 200 times smaller. On first sight
this may question the usefulness of the proposed MEC.
However, ED does not totally remove the ammonium, and also produces a highly acidic
effluent that needs to be further treated before disposal. This technology also requires more
energy to remove the ammonium (Section 4.4). ED has been used for 10 times more
concentrated wastewater (0.5 M) than the one fed to the MEC of this project. MEC has the
additional advantage of removing organics.
4.3.4. Reactor volume for WWTPs
Woodman Point WWTP (Perth, Western Australia) treats 120x106 L.d-1 of wastewater
containing 55 mg.L-1 nitrogen (i.e.: 3.9 mM) (Section 1.2.1). The reactor volume for each
process was calculated according to the ammonium removal rate and the concentration of this
urban wastewater (Section 1.2.1 and Appendix D), and is presented in Table 4.2, along with
some advantages and disadvantages of each process.
Table 4.2: Comparison of ammonium removal rate (mM.h-1) from urban wastewater (Water Corporation, 2011), using different WWTP processes. The reactor volume was calculated according to Appendix D.
Treatment process
Ammonium removal rate
(mM.h-1)
Reactor volume required
(m3)
Advantages Disadvantages Authors
MEC 3.1 13,730 Organic removal Electrical input Present
study
AS 0.8 53,200 Organic removal
Nitrogen production and no beneficial by-products
Yang et al., (2004)
ED 625 68 High removal rate
High electrical input and no
organic removal
Gain et al., (2002)
39
4.4. Energy Requirements
4.4.1. Energy concerns
Society is concerned about the environment and aims at increasing its sustainability (Chiu et
al., 2000), which starts with pollution prevention and energy savings (Gentil et al., 2011). In
this project, the proposed MEC system should be compared to different WWTPs in terms of
energy requirements, to be able to establish its feasibility on industrial scale. The energy
required is compared according to Joules per mmol of ammonium removed (J/mmolNH4+)
(Section 2.3.3)
4.4.2. Energy required for MEC
The conditions under which the MEC is most likely to operate are 1 M ammonium and 500
mM sodium concentrations in the catholyte (Section 3.12). The voltage and current were
measured and recorded (Section 2.1.2). This enabled to calculate power consumption (W = V
x I). The energy requirements in such conditions are 238 J/mmolNH4+ (Table 4.3 and Appendix
F).
Table 4.3: Summary of the energy requirements per mmol of ammonium migrated from the anode in the experimental section (Appendix F). These requirements are purely based on electrical energy used and not pumping, stripping, pH control or energy gained from inadvertent hydrogen production.
Catholyte Conditions Energy requirement
Experiment # NH4+ (mM) Na+ (mM) mWh/mmolNH4
removed J/mmolNH4 removed
3.9 0 0 27 97
3.9 500 0 134 482
3.12 500 1000 66 238
4.4.3. Energy required for ED
The MEC system in the present project is similar to an ED, with the main difference being
that bacterial activity provides electrons for the current, which leads to an anodic potential
(from acetate degradation -280 mV) being close to the cathodic reaction (-827 mV). On the
contrary, ED has highly different electrode potentials, which increases the overall cell voltage.
40
For example Gain et al. (2002) produced oxygen at the anode (+ 400 mV) and hydrogen at the
cathode (- 827 mV).
In theory the MEC cell voltage is 547 mV, while the ED cell requires twice this voltage (1,227
mV Figure 4.1), using Ohm’s Law:
I (A) = V / R (Ω)
∴ I (A) x R (Ω) = V
Assuming that the resistance and the current are the same in both processes, then the voltage is
proportional to the amount of energy used. This suggests that the MEC system enables about
50 % savings compared to ED, while degrading acetate (i.e.: organics) and concentrating
ammonium. In the ED process the voltage is increased further than the theoretical voltage
required, in order to enhance the current. Such increase in electron flow improves the
ammonium migration rate, but also raises the energy requirements. For example, Ippersiel et
al. (2011) used 17.5 V for their ED cell. In the MEC system, the cell average was about 1.04
V. Assuming the same current and resistor apply to both systems, then the energy difference
would be about 17 times.
Figure 4.1: Voltage requirements difference (against standard hydrogen electrode) between MEC and ED processes. The redox potentials were obtained from Lide (2003).
H2 (from H2O) -827 mV
Acetate -280 mV
Oxygen +400 mV
Total ED process voltage 1,227 mV
Total MEC process voltage 547 mV
Redox potential
41
Gain et al. (2002) used an ED with a current of 20 A, but the voltage was not given. To
calculate the power, the most efficient voltage (17.5 V) from Ippersiel et al. (2011) was used.
Gain et al. (2002) removed 1 M ammonium in 100 min. Their working volume was 2 L,
therefore the energy cost for their process was 1000 J/mmolNH4+ (Appendix F). This energy
requirement is about five times the MEC’s (238 J/mmolNH4+). Such energy requirement shows
some limits to the ED process, especially considering that the wastewater is not fully treated.
In ED, the electrode voltage is provided by an external power supply. If this process was
complemented with bacteria in the anolyte, then the energy requirement could be reduced. For
example, in this project, bacterial activity decreased the anode potential to -500 mV. If the
bacteria were added to Gain et al. (2002) ED system, then the cell voltage could be reduced to
17V. Assuming the same current (20 A) would be produced, the energy cost would be
decreased about 3 %.
4.4.4. Energy required for AS
Hu et al. (2000) estimated that an average size (500-5000 m3.d-1) plant required 500 Wh.m-3 to
remove pollutants. Using the wastewater described in Section 1.2.1 (3.9 mM ammonium), the
energy requirements per mmol of ammonium removed would be 457 J/mmolNH4+ (Appendix
F). This represent approximately twice the MEC energy requirements (238 J/mmolNH4+)
(Table 4.4). There are further energy savings from the MEC that have not been taken into
account, the ammonia recovery and the hydrogen production at the cathode.
Table 4.4: Comparison of energy requirements (J/mmolNH4+) for MEC, ED and AS. Wastewater
processes
Energy Requirements
(J/mmolNH4+) References
238 Project
MEC 162
Cord-Ruwisch et al.
(2011)
ED 1000 Ippersiel et al. (2011)
Gain et al. (2002)
AS 457 Hu et al. (2000)
42
4.4.5. Energy required for ammonia gas production
Nitrogen gas is used for ammonia production in the fertiliser industry, which requires high
energy inputs. The European Fertilizer Manufacturers Association (EFMA) states that 30.2
GJ/t of ammonia (EFMA, 2000) is required, being equivalent to 423 J/mmolNH4+-N (Appendix
F). According to this estimate, the described MEC has the advantage of recovering ammonium
at about half the energy cost (238 J/mmolNH4+-N). This savings are related to the AS process,
because AS produces nitrogen gas from ammonium degradation, which is used for ammonia
production. MEC removes ammonium at 50% of AS’ costs. In addition, MEC recovers
ammonia gas which can be directly used rather than manufacturing it for fertiliser. This
represents 75% savings from both systems (Figure 4.2).
Figure 4.2: Schematic diagram of conventional ammonia production and ammonium removal using arbitrary units. MEC ammonia recovery avoids the ammonia production process, while using half the cost of the normal ammonium removal process.
It is important to note that some significant factors have not been taken into account in the
MEC energy requirements, such as mixing and heating of anolyte, NaOH dosing for pH
balance, but also some savings from the hydrogen production.
N2
NH3
Fertilizer manufacturing = 100 units energy
AS = 100 units energy
MEC = 50 units energy
Diluted NH4
+
Diluted NH4+
NH3
43
4.5. Limitations One limit to this project is the high technicality of the process. The understanding of this
technology requires numerous areas of expertise such as biological science, electrochemistry,
material science and engineering, physical and chemical sciences. This limits the applicability
of MEC to industrialised countries.
The MEC proposed in this thesis was studied on a laboratory scale, and the large scale
application has limitations. A large gap separating the electrodes is a known hurdle to MEC
efficiency (Figure 4.3) (Logan et al., 2006). In Section 4.3.4, the needed volume for an
industrial reactor (treating 3.9 mM ammonium wastewater) was determined to be 6,300 m3,
but the surface area would need to be large to maintain a minimum gap between electrodes
(Figure 4.4). The CEM area required would be very large because the ratio of surface area to
volume of wastewater has to be high to maximise ammonium transfer (Section 1.3.3.5). This
means that there is an extra cost associated with using a large amount of CEM.
It is important to note that, as MEC is used for wastewater treatment, there is a need to look at
the catholyte requirements. In the present study, the catholyte was of the same volume as the
anolyte. However to obtain a high concentration more rapidly it is possible to think of a small
catholyte volume. This would require maintenance and further engineering to maintain the pH
and develop the gas stripping device.
Figure 4.3: A MEC with a small surface area has a reduced efficiency because of the distance between the two electrodes. The small surface area of the CEM compared to the reactor volume reduces the possibility of ammonium migration from the anode to the cathode.
44
Figure 4.4: A MEC with a large surface area overcomes the problem of distance between anode and cathode. It also enhances the ion transfer from the anolyte to the catholyte. However it increases the environmental impact of the WWTP if applied on industrial scale.
Ammonia gas is present in large quantities at high pH, which is beneficial for ammonia
stripping. However, the back diffusion of ammonia gas, as detected in Section 3.13, is one of
the main concerns about the application of the project. This can be improved by using a
membrane preventing ammonia movements, but also by increasing the temperature which
would lead to a greater volatilisation and therefore would reduce the amount of ammonia
present in solution.
45
5. Conclusions and Recommendations
5.1. Conclusions
Overall, the MEC system, as a new process for ammonium removal from wastewater, has
prospects because it enables organics removal, ammonium accumulation, and ammonia gas
recovery. Ammonium can be concentrated to 1 M (Section 3.7) and be recovered as ammonia
gas (Section 4.2), which is used for fertilisers manufacturing. This study has indentified that
adding sodium in the catholyte enhanced ammonium accumulation (Section 3.12).
The MEC energy requirements per mmol of ammonium removed is half the AS costs, and is
five times cheaper than ED process. If this MEC system could be developed and operated on
an industrial scale, then it would be an example of saving energy for both nitrogen removal
from wastewater, and nitrogen fertiliser production. As such, it could represent 75% energy
savings.
However, because of the lack of industrial scale application, the MEC described in this study
may not be considered practical or economical enough. The membrane cost is very high, and
the use of catholyte increases the maintenance and the reactor volume. The high technology of
the system limits its applicability to industrialised countries.
5.2. Recommendations
5.2.1. Wastewater
The project used synthetic wastewater, but it would be advisable to use real wastewater. This
would determine the efficiency of the system in an industrial environment. In this project, the
bacteria were selected by only providing acetate. However, real wastewater contains a variety
of organics, and using such wastewater would reflect the potential efficiency under real world
parameters.
In the proposed MEC, the wastewater contained 50 mM ammonium, which is about 13 times
higher than in municipal wastewater, and usually only produced by industrial processes or
landfill leachate. One example of high ammonium containing wastewater is leachate from
anaerobic digester. An anaerobic digester is a thermophilic process of solid waste treatment,
46
which effluent contains 100-150 mM of ammonium (Çelen and Türker, 2001, Zeng et al.,
2006) .
Ammonium toxicity to microorganisms is a known phenomenon in AS processes (Camargo
and Alonso, 2006). In the proposed system, ammonium toxicity has not been tested, but
Kuntke et al (2011) tested that 4 mg.L-1 ammonium did not affect their MFC efficiency. It is
recommended to test ammonium toxicity on MEC, to ensure a maximum bacterial activity and
hence a maximum ammonium transfer.
An interesting observation was made on several experiments (Figure 3.7 and Figure 3.6). The
ammonium disappearance in the anode was not immediately followed by appearance in the
cathode. One could test the possibility of ammonium retention by the membrane.
5.2.2. Magnesium Ammonium Phosphate (MAP)
Çelen and Türker (2001) demonstrated that ammonium could be recovered as MAP from the
effluent of an anaerobic digester, which can be used as fertiliser. In the catholyte, it would be
possible to use a specific ratio of magnesium (Mg) and phosphate (PO43-), as well as a pH
between 8.5 and 9 to favour MAP precipitation. Magnesium is present in seawater in a
concentration of 3.1 M (Dickson and Goyet, 1994). If it was used with phosphate as a
catholyte in MEC, then the pH would become alkaline (Section 1.3.3.4), and MAP could be
precipitated and recovered as a fertiliser.
MAP has been recovered at 85.8 % after purification (i.e.: separation from the organics and
other untreated ions) (Çelen and Türker, 2001). The recovery can probably be increased using
MEC. The ammonium is collected in a separate solution from the wastewater, which reduces
the purification process. Using this technique increases the value of the MEC system, because
it produces a finished by-product: MAP, which can be readily used as slow release fertiliser.
Such process could be worth testing, however some limitations would need to be addressed,
for example the relative shortage of phosphate compared to ammonia in most wastewaters.
47
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AppendicesAppendix A Anodic potential and current calibration
To determine the electropotential output from the potentiostat, the counter and reference
(brown and white ports, Figure A.2) electrode were short circuited, and a known resistor
connected between them and the working electrode (Blue port, Figure A.2). The voltage input
was changed from 0 to 5V, and the multimeter readings were recorded across the resistor. The
correlation between the input and the output provides the equation to calculate the anodic
potential (Figure A.1)
The current can be deduced from the voltage and the resistor, using Ohm’s law (Equation
A.1). The potentiostat can read the current output (black and red ports, Figure A.2), it is
measured in volts, but because there is 1Ω present, 1 volt is equal to 1 ampere (A).
The calculated current is equated with the current measured by the LabJACKTM USB
acquisition card (Labjack U12). From the equation obtained the current measured by the card
can be adjusted.
I (mA) = V (mV) / R (Ω) Equation A.1
Figure A.1: Equation of the current measured by the computer and the current readings from the multimeter.
y = 0.4868x + 2.3005 R² = 0.99997
0
10
20
30
40
50
60
-20 0 20 40 60 80 100 120
Cur
rent
from
am
perm
eter
(m
A)
Current reading from computer (mA)
53
Figure A.2: Three electrodes potentiostat connected to the microbial fuel cell (MFC) created a microbial electrolysis cell (MEC).
Appendix B
The colorimetrically ammonium standard curve (Figure A.3) was determined using solutions
of known concentration (0.0, 0.5, 1.0, 1.5, 2.0 and 3.0 mM) and their respective absorbance
readings (Table A.1).
Table A.1: Standard solution preparation for ammonium standard curve determination.
Standard (mM)
Volume of 1mM NH4Cl (µL)
Volume of DI water (mL)
Average Absorbance reading at 425 nM
0.0 0 2.00 0.035
0.5 100 1.90 0.196
1.0 200 1.80 0.377
1.5 300 1.70 0.501
2.0 400 1.60 0.666
3.0 600 1.40 0.936
54
Figure A.3: One example of ammonium standard curve used in the thesis.
Appendix C Theoretical ammonia gas recovery calculation:
pKa=9.23 (Lide, 2003)
KH: e(-4092/T)+9.7 atm.mol/L (Dasgupta and Dong, 1986)
Henry’s Law: P=KH C
Where P is the partial pressure in atm, KH Henry’s law constant (atm.mol-1.L-1) and C the
concentration of ammonia in solution (M)
At room temperature of 25oC (298.15K)
Assuming the flushing gas flow rate (GF) is 1L.sec-1
At 1M of total ammonium N in the catholyte.
NH3(liquid)= 1 / (1+ 10(pKa-pH)) = 855mM
Therefore:
PNH3 = e(-4092/T)+9.7 x 855 = 1.5 x 10-2 atm
y = 0.3002x + 0.0516 R² = 0.9974
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
0 1 2 3 4
Abs
orba
nce
read
ing
Ammonium concentration (mM)
55
Assuming an interval of 10minutes (i.e.: 600sec) and using the gas constant law:
Gas removed (mol) = P x GF x 600 / R x T
Where P is the partial pressure of ammonia (Atm), GF is the gas flow rate (L.sec-1), R is the
gas constant (0.082 L atm.K-1.mol-1) and T is temperature in Kelvin (K).
Appendix D Reactor volume calculations:
In order to determine the reactor volume (Table A.2), some parameters have to be known or
calculated. The rate at which a pollutant is removed (mM.h-1) can be determined based on the
literature, and the concentration of pollutant (mM) in the wastewater to be treated is known
from WWTP. These two parameters are used to determine the hydraulic retention time (HRT
in h):
Concentration of wastewater (mmol/L-1) / Removal rate (mmol/L.h-1) = HRT (h)
The organics and ammonium contents in wastewater are known (Section 1.2.1). The reactor’s
volume can be calculated using the concentration of the inflow wastewater (mmol.L-1), the
volume of wastewater (L.h-1) and the HRT (h):
HRT (h) x (Volume of wastewater (L.d-1) / 24h.d-1) / 1000L/m3 = reactor volume (m3)
Table A.2: Excel calculations of the reactor volume for different WWTP.
Process WW V WW conc
NH4 rate HRT time volume Reactor v
L/d mM mM/h h h/d L/m3 m3
MEC 2.6E+08 3.93 3.1 1.2673 24 1000 13728.9
2.6E+08 3.93 0.8 4.9107 24 1000 53199.4 AS
2.6E+08 3.93 12 0.3274 24 1000 3546.6
ED 2.6E+08 3.93 625 0.0063 24 1000 68.1
56
Appendix E
Table A.3: Summary of the ammonium removal rate (mM.h-1) under different experimental conditions in the cathode.
Catholyte Conditions Ammonium removal rate Experiment # NH4
+ (mM) Na+ (mM) mM.h-1
3.5 0 0 3.7
3.6 80 0 2.7
3.7 1000 1000 1.7
3.8 2000 1000 -0.2
3.10 500 0 1.3
3.10 500 1000 3.1
Table A.4: Summary of the energy requirements per mmol of ammonium migrated from the anode in the experimental section.
Catholyte Conditions Energy requirement
Experiment # NH4+ (mM) Na+ (mM) mWh/mmolNH4
removed J/mmolNH4 removed
3.9 0 0 27 97
3.9 500 0 134 482
3.10 500 1000 66 238
Appendix F Energy Calculation for the MEC used in this project:
The cell voltage and the current were measured and recorded for each experiment. Using both
parameters the power (W) can be calculated.
Power (mW) = Cell Voltage (V) x Current (mA)
Considering that the time of the experiment is known and the amount of ammonium removed
from the anolyte known, then the power per mWh can be calculated:
Power requirements (mWh/mmolNH4+) = Power (mW) x time (h) / NH4+ removed (mmol)
57
To convert this power in Joules, then the following conversion factor would be used:
1 Wh = 3.6 kJ
Therefore:
Energy requirements (J/mmolNH4+) = Power requirement (mWh/mmolNH4+) x 3.6 (J/mWh)
Energy requirements (Cord-Ruwisch et al 2011):
Ammonium migration rate: 0.18mmol.h-1 over 45 hours.
Total ammonium removal = 8.1mmol.
Cell voltage = 0.9V
Current 9mA
Power required= 9mA x 0.9Vx45h/8.1mmolNH4 = 45mWh/mmolNH4
Energy requirements = 45 mWh.mmolNH4+ x 3.6 J/mWh = 162 J/mmolNH4+
Activated Sludge energy requirements (Hu et al. 2000):
Power required: 500Wh.m-3
Ammonium concentration in wastewater:
55mg/L x1000L/m3 / 1000mg/g = 55g/m3 / 14g/mol =3.9 mol.m-3
Power required per mmol ammonium = 500Wh.m-3 / 3900 mmol.m-3 = 127mWh/mmolNH4+
Energy required = 127mWh/mmolNH4+ x 3.6J /mWh = 457 J/mmolNH4+
Energy calculation for ED:
The energy requirements to migrate one mmol of ammonium using an ED process (Table A.5)
were made using the following assumption:
a) Current was 20 A (Gain et al. 2002)
b) Voltage was 17.5 V (Ippersiel et al. 2011).
c) 1 Wh = 3600 J
58
Table A.5: Example of the energy calculcation using Excel spreadsheet. ED Process
current Cell voltage Power Time Powe NH4 remov
A V W h Wh mmol Wh/mmol J/Wh J/mmol 20 17.5 350 1.6 560 2000 0.28 3600 1008
Energy calculation for conventional Ammonia gas production:
1. Cost to treat ammonium per day in WWTP
Wastewater treated: 120x106 L/d
Cost from MEC: 238 J/mmolN
55mg/L of N, assumed to be only ammonium: 55 mg/L/14 g/mol=3.9 mmolN/L
Ammonium to be treated: 120x106 x 3.9/1000 = 468,000 molN/d
MEC energy cost: 18.5 x 468,000x103 = 8.7 GJ/d
2. Cost to Produce Ammonium from nitrogen gas according to the European Fertilizer
Manufacturers Association (EFMA, 2000):
36.9 GJ.t-1NH3 = 36.9 MJ.kg-1
NH3 = 36.9 kJ.g-1NH3 x 14 g.mol-1= 517 kJ.molNH3 = 517 J.mmolN
-1
3. Cost savings from recovering ammonia from wastewater to form ammonia:
Energy cost to recover ammonia from wastewater with MEC = 8.7 GJ
Energy cost to produce the same amount ammonia with normal process =
423 kJ.mol-1 x 468,000 mol.d-1 =198x106 kJ = 197 GJ