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

iii

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.

iv

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.

v

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

vi

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  

vii

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  

ix

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  

x

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  

xi

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

1

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

2

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.

3

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

4

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

5

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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6

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.

7

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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8

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