INCREASING DESALINATION BY MITIGATING ANOLYTE PH …

The Pennsylvania State University

The Graduate School

College of Engineering

INCREASING DESALINATION BY MITIGATING ANOLYTE PH IMBALANCE

USING CATHOLYTE EFFLUENT ADDITION IN A MULTI-ANODE, BENCH SCALE

MICROBIAL DESALINATION CELL

A Thesis in

Environmental Engineering

by

Robert J. Davis

2013 Robert J. Davis

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

August 2013

The thesis of Robert J. Davis was reviewed and approved* by the following:

Bruce E. Logan

Kappe Professor of Environmental Engineering

Thesis Adviser

John M. Regan

Professor of Environmental Engineering

Fred S. Cannon

Professor of Environmental Engineering

Peggy Johnson

Professor of Civil Engineering

Head of the Department of Civil and Environmental Engineering

*Signatures are on file in the Graduate School

iii

ABSTRACT

A microbial desalination cell (MDC) uses exoelectrogenic bacteria to oxidize organic

matter, producing a current which can be used to desalinate saline water placed between

alternating ion exchange membranes. Protons are produced from the oxidation of organics at the

anode, resulting in acidification of the anolyte which can limit MDC performance. A new method

was used here to mitigate anolyte acidification based on adding non-buffered saline catholyte

effluent from a previous cycle to the anolyte at the beginning of the next desalination cycle. This

method was tested using a larger-scale MDC (267 mL) that contained four anode brushes, an air

cathode, and a three cell pair membrane stack. With an anolyte salt concentration increased by an

equivalent of 75 mM NaCl using the catholyte effluent, salinity was reduced by 26.0 ± 0.5% (35

g/L NaCl initial solution) in a 10 hour cycle, compared to 18.1 ± 2.0% without catholyte addition.

This improvement in performance was primarily due to the increase in buffering capacity of the

anolyte, although raising the conductivity slightly improved performance as well. Substrate was

lost from the anolyte due to diffusion into the desalination membrane stack. This loss of substrate

was decreased from 11% to 2.6% by increasing the anolyte conductivity (7.6 to 14 mS/cm).

These results demonstrated that catholyte effluent can be utilized as a useful product for

mitigating anolyte acidification and improving MDC desalination performance.

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TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................. v

ACKNOWLEDGEMENTS ..................................................................................................... vi

Chapter 1 Introduction ............................................................................................................ 1

1.1 Current global demand for fresh water ...................................................................... 1 1.2 Commercial desalination technologies ...................................................................... 2 1.3 Microbial desalination cells ....................................................................................... 3 1.4 Objectives................................................................................................................... 4 1.5 Literature cited ........................................................................................................... 4

Chapter 2 Literature Review ................................................................................................... 6

2.1 Types of MDCs .......................................................................................................... 6 2.2 Anode acidification in MDCs .................................................................................... 7 2.3 Extent of desalination and power generation in MDCs ............................................. 9 2.4 Multi-electrode bioelectrochemical systems .............................................................. 11 2.5 Anolyte recirculation .................................................................................................. 13 2.6 Literature cited ........................................................................................................... 14

Chapter 3 Increasing desalination by mitigating anolyte pH imbalance using catholyte

effluent addition in a multi-anode, bench scale microbial desalination cell .................... 18

3.1 Introduction ................................................................................................................ 18 3.2 Materials and methods ............................................................................................... 20

3.2.1 MDC construction ........................................................................................... 20 3.2.2 Medium ........................................................................................................... 22 3.2.3 MDC operation and experimental procedures ................................................. 23 3.2.4 Analyses and calculations ............................................................................... 24

3.3 Results ........................................................................................................................ 25 3.3.1 Desalination and current generation ................................................................ 25 3.3.2 Anion transport and the effect on COD removal............................................. 28 3.3.3 Effect of anolyte recirculation on anode performance .................................... 31

3.4 Discussion .................................................................................................................. 32 3.5 Acknowledgments ...................................................................................................... 36 3.6 Literature cited ........................................................................................................... 36

Chapter 4 Conclusions ............................................................................................................ 39

Chapter 5 Future Work ........................................................................................................... 40

Appendix Supplementary Information.................................................................................... 42

v

LIST OF FIGURES

Figure 1. (A) Schematic of bench scale MDC with parallel continuously recycled flow

through the 3 cell pair electrodialysis stack. AEM, anion exchange membrane. CEM,

cation exchange membrane. (B) Photograph of the reactor in operation. ........................ 22

Figure 2. (A) Extent of desalination and total desalination rate with increasing anolyte

salt concentration increase from catholyte effluent addition. (B) Current density

profile during one 10 hour cycle for different amounts of catholyte effluent addition. ... 26

Figure 3. Maximum power density with various amounts of catholyte effluent addition (0

to 75 mM) after ~30 days of operation at 10 Ω external resistance. The internal

resistance is also showed by the slope of the voltage with respect to current. The

closed diamonds “P” represent power density at a given concentration of catholyte

addition, while the open signs “V” are for voltage density points. .................................. 27

Figure 4. Anolyte pH at the beginning and end of one cycle at various concentrations of

anolyte salt increase due to catholyte effluent addition. Increasing the amount of

catholyte effluent addition results in an increase of initial anolyte pH. ........................... 28

Figure 5. Diffusion of acetate and phosphate, across the anion exchange

membrane from the anolyte chamber into the diluate solution. Results are displayed

at various increments of anolyte salt concentration increase due to either NaCl

addition or catholyte effluent addition. ............................................................................ 29

Figure 6. COD removal at various increments of anolyte salt concentration increase due

to NaCl addition with no effect on anolyte pH (open signs) and due to addition of

catholyte effluent with an increase in anolyte pH (closed signs). .................................... 30

Figure 7. Coulombic efficiencies and COD measurements from the anolyte at the

beginning and end of one cycle for various increments of anolyte salt concentration

increase due to catholyte effluent addition. The subscript “diff” indicates data points

where diffusion of acetate out of the anode chamber was taken into account. ................ 31

Figure 8. (A) Current density contribution from each anode with anolyte recycle and (B)

in batch-fed mode. The electrodes are ordered from the bottom (1) to the top anode

(4). .................................................................................................................................... 32

Figure 9. Comparison of diluate solution to anolyte volume ratio and total desalination

rate of various studies. The (50) and (100) values for “this study” indicate PBS

concentration (mM) of the anolyte. An ideal MDC would be in the upper right hand

corner: high diluate to anolyte ratio and high total desalination rate. .............................. 35

vi

ACKNOWLEDGEMENTS

First and foremost, I would like to thank my adviser, Dr. Bruce E. Logan for showing

faith in me and providing me the opportunity to pursue my research interest at the Pennsylvania

State University. His knowledge, patience, and continual guidance have helped me immensely in

my development as a researcher and an engineer. I would also like to thank Dr. Jay Regan and

Dr. Fred Cannon for their kindness, for sharing their expertise with me and for serving as my

committee.

I am extremely appreciative of my lab mates for making my research life colorful and

putting up with me when my anodes died or I experienced other research-related casualties. I

would specifically like to thank Marta Hatzell and Roland Cusick for sharing in the struggles of

stack flow and pressure differences, Dr. Younggy Kim for his patience and constant mentorship

in the early stages of this project when I needed it the most, Dr. Fang Zhang and Dr. Justin

Tokash for their guidance in everything electrochemistry, Dr. Xiuping Zhu for her patience and

help with the HPLC work, Hiroyuki Kashima for his kindness and guidance with the IC, my

classmates Caroline Price and Vanessa Medina, Dr. Michael Siegert for being the second most

interesting man in the lab and especially Hengjing Yan for making every day enjoyable. Some of

those conversations were even work related, though the best were not. It has been my privilege to

work with knowledgeable, kind, and collaborative colleagues, thank you all.

I would also like to thank the King Abdullah University of Science and Technology for

the funding to complete this project.

Robert J. Davis

August 2013

Chapter 1

Introduction

1.1 Current global demand for fresh water

Water is the basis for life on this planet. It is estimated that the Earth contains 332.5

million cubic miles of water, 97% of which are in the oceans, 2% trapped in the icecaps, and less

than 1% available as fresh water in rivers, lakes, and shallow aquifers for human use [1]. This

amount of potable fresh water is becoming even smaller over time due to pollution, over

consumption, and the effects of global warming [2, 3]. To make matters worse, the distribution of

global fresh water resources does not coincide with population distribution. This inherently

makes water supply an international issue as source-water countries can control the volume and

quality of water going to downstream nations. Developing countries are the majority of the water

stressed demographic due to lack of technological advances or water infrastructure for treatment

and distribution. In the developing world, an estimated 1 billion people lack access to safe

affordable drinking water and 2.7 billion lack access to sanitation, with millions dying each year

from preventable waterborne diseases [4]. Water supply is of growing global significance as

nearly two thirds of the world’s population is projected to live in water stressed countries by 2025

[5]. Efforts for conservation and infrastructure improvements will help alleviate this water stress,

but with the global population projected to increase to over 8.9 billion by 2050 and the world’s

water consumption rate estimated to double by 2030, new sources of fresh water are of great

interest and may become necessary to fulfill demand in the near future [6, 7].

2

1.2 Commercial desalination technologies

The increasing global water demand requires new supply streams, which can be

accomplished through water reuse or desalination [8]. While water reuse can be helpful for non-

potable purposes, desalination is the alternative more suited for human consumption. Seawater

desalination is an attractive alternative as nearly half of the world’s major cities are located within

50 kilometers of the coast and the ocean provides an unlimited supply of feed water [9].

Desalination is a process that removes dissolved solids (salts) from water to yield potable water.

In addition to seawater, brackish waters are also commonly used for desalination. Early

desalination was accomplished through thermal processes, in which large amounts of fossil fuels

were used to distill water from saline water that was then condensed and collected. Thermal

processes are still largely used today in fossil fuel rich countries in the Middle East. The energy

consumed by distillation is between 6-12 kWh/m3 (multi-effect distillation) and 10-16 kWh/m

3

(multi-stage flash) based on the water produced [10].

The majority of the desalination plants being designed today use reverse osmosis (RO)

technology, which uses pressure to push solution through a spiral-wound membrane. Water

diffuses through the RO membrane more quickly than salts, allowing fresh water permeate to be

collected. Energy consumption for salt water reverse osmosis has fallen dramatically in the past

40 years and now nearly approaches the thermodynamic limit [11]. Though production from

seawater desalination plants are expected to double between 2008 and 2016 to 38 billion m3 per

year, that projected capacity would still comprise less than 1% of the global water supply [12].

There are a number of environmental barriers that prevent seawater desalination from more wide

spread implementation, but the biggest problem is its high energy cost. State of the art seawater

RO plants consume 3-4 kWh/m3, which is still much greater than treating surface waters or

pumping from an underground aquifer [11]. With the RO process near the theoretical energy

3

limit, energy consumption can only be reduced by improving the pretreatment process, using

renewable energy sources, or by developing new technologies for seawater desalination.

1.3 Microbial desalination cells

A microbial desalination cell (MDC) is a new desalination technology that uses the

energy in wastewater to desalinate saline water while also producing energy. The MDC is a

modified microbial fuel cell (MFC) which consists of an anode and a cathode separated by at

least one pair of anion and cation exchange membranes. The ion exchange membranes enclose

the salt solution to make a desalination chamber. Bacteria on the anode oxidize organic matter in

the anode solution, producing electrons at the electrode and releasing protons into solution. The

electrons pass through an external circuit to the terminal electron acceptor at the cathode, most

commonly oxygen which is reduced to water. Theoretically, as one mole of electrons passes

through the circuit, one mole of sodium chloride molecules dissociates with one mole of chloride

passing through the anion exchange membrane to the anode chamber, and one mole of sodium

passing the cation exchange membrane to the cathode chamber. This leaves a desalinated solution

in the middle chamber. Similar to other desalination processes, water having different salinities

have been tested in an MDC and salt removals greater than 95% have been achieved [13-15].

Since MFCs and MDCs share many commonalities, many of the advances in MFC technologies

such as in electrode materials have direct application to MDCs.

4

1.4 Objectives

The objective of this study was to increase desalination in a bench scale MDC by adding

catholyte effluent to the anolyte in order to mitigate anode acidification. Various amounts of

catholyte effluent were added to the anolyte at the beginning of a cycle and the results were

compared on the basis of: extent of desalination, power generation, and treatment (COD

removal). The effect of anolyte recycle on current generation and electrode potentials was also

explored as well as anion back diffusion out of the anode chamber.

1.5 Literature cited

1. US Geological Survey. The World’s Water, 2005, page accessed 23 April 2013,

http://ga.water.usgs.gov/edu/earthwherewater.html

2. Oki, T., Kanae, S. Global hydrological cycles and world water resources. Science, 2006,

313, 1068-1072

3. Hoekstra, A., Mekonnen, M. The water footprint of humanity. P National Academy of

Science,2012, 109(9), 3232-3237

4. World Health Organization. Progress on drinking water and sanitation: 2012 update.

2012

5. Service, R. Desalination freshens up. Science, 2006, 313, 1088-1090

6. United Nations, World Population Prospects: The 2010 Revision, Vol. 1, Comprehensive

Tables. United Nations, New York, 2011; accessed 10 May 2011,

http://esa.un.org/unpd/wpp/index.htm

7. 2030 Water Resources Group. Charting our water future; economic frameworks to inform

decision-making. 2009, pg 44, accessed 26 April 2013

http://www.2030waterresourcesgroup.com/water_full/Charting_Our_Water_Future_Final

.pdf

8. Shannon, M., Bohn, P., Elimelech, M., Georgiadis, J., Marinas, B., Mayes, A. Science

and technology for water purification in the coming decades. Nature, 2008, 452, 301-310

9. World Resources Institute, Millennium Ecosystem Assessment, Ecosystems and Human

Well Being. 2005, pg 27, Accessed http://www.unep.org/maweb/en/Synthesis.aspx

10. Semiat, R. Energy issues in desalination processes. Environ. Sci. Technol. 2008, 42 (22),

8193-8201

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11. Elimelech, M., Phillip, W. The future of seawater desalination: energy, technology, and

the environment. Science, 2011, 333, 712-717

12. Schiermeier, Q. Water purification with a pinch of salt. Nature, 2008, 452, 260-261

13. Cao, X., Huang, X., Liang, P., Xiao, K., Zhou, Y., Zhang, X., Logan, B. A new method

of desalination using microbial desalination cells. Environ. Sci. Technol. 2009, 43, 7148-

7152

14. Jacobson, K., Drew, D., He, Z. Efficient salt removal in a continuously operated upflow

microbial desalination cell with an air cathode. Bioresource Technol. 2011, 102, 376-380

15. Luo, H., Jenkins, P., Ren, Z. Concurrent desalination and hydrogen generation using

microbial electrolysis and desalination cells. Environ. Sci. Technol. 2011, 45, 340-344

Chapter 2

Literature Review

2.1 Types of MDCs

The first MDC was developed in 2009 by Cao et al., and there have been a variety of

advancements in this technology since then. The first MDC achieved 93% desalination of a 35

g/L salt solution over a 24 hour desalination cycle [1]. This design was successful at proving the

MDC concept, but volume of electrolyte solution used was much greater than that of the diluate

(66:1), and a ferricyanide catholyte was used as the terminal electron acceptor. MDCs have since

then almost universally used air cathodes since passive oxygen is free and continuously available.

Various concentrations of salt solutions, including artificial seawater, have also been tested in the

desalination chamber [2]. The three-chamber MDC first proposed by Cao et al. has been

extensively modified and adapted to improve specific aspects of desalination performance. Chen

and Kim increased the current efficiency and diluate volume relative to a three chamber system

by increasing the number of ion exchange membranes between the two electrode chambers [3, 4].

This created an electrodialysis (ED) stack with interchanging diluate and concentrate chambers.

In another study, a bipolar membrane was placed next to the anode chamber to make a four-

chamber system which produced acid, alkali, and desalinated solutions [5]. Applying a voltage

and sealing the cathode chamber from oxygen has produced hydrogen while simultaneously

desalinating water in the middle chamber [5-7]. Many researchers have also constructed osmotic

MDCs (oMDC) by replacing the anion exchange membrane next to the anode chamber of an

MDC with a forward osmosis membrane [8, 9]. The forward osmosis process accomplishes

desalination by pulling water from a dilute anolyte feed solution into the saline draw solution.

7

This process increases the recovery of diluate, though the lack of ion selectivity in the FO

membrane decreases the efficiency of ionic separation from current generation. Capacitive

deionization has been coupled with an MDC to prevent salt migration into the anode and cathode

chambers [10]. An activated carbon cloth assembly with a current collector and a CEM was

placed on both sides of the desalination chamber, and electrically connected to each electrode.

During operation, a potential was created by the BES with deionization of the salt solution in the

middle chamber, transporting anions towards the anode and cations towards the cathode. As ions

were collected by the activated carbon cloth, charge was accumulated (as in a capacitor) that

could be discharged to regenerate the adsorption sites [11].

2.2 Anode acidification in MDCs

Low pH is known to significantly inhibit exoelectrogenic bacterial activity in

bioelectrochemical systems (BES) [12]. In a study by He et al. an anolyte pH of 5.0 in a single

chamber MFC resulted in only 10% of the highest peak current, and 3% of the highest coulombic

efficiency, compared to that produced by the system at pH 9.0 [13]. Anode acidification occurs as

bacteria on the anode oxidize organic material, transferring electrons to the electrode and

releasing protons into solution. Torres et al. and Franks et al. studied the kinetics of proton

transfer out of the biofilm in a fed batch system and found that a pH gradient developed within

the biofilm with the lowest pH located closest to the anode [14, 15]. With oxygen as the terminal

electron acceptor, oxygen is reduced to water through consumption of protons, producing

hydroxide ions in the cathode chamber. In an MFC, anolyte pH can be balanced with the

hydroxide produced at the cathode by passive ion transport in a single chamber reactor. Closer

electrode placement has helped facilitate ion transport for balancing electrolyte pH, but it can

result in very low CEs (<20%) due to oxygen diffusion to the anode [16, 17].

8

An MDC has multiple ion exchange membranes placed between the electrodes which

prevents pH balance from passive ion transfer between the anode and cathode. Cycling a large

volume of solution into the anode chamber has been commonly done to avoid acidification at the

anode [1, 3, 6]. Though a large anode solution volume prevents pH from becoming an issue, it

also results in lower COD removals [6]. Storing and pumping large volumes of electrolyte to

produce a much smaller volume of diluate would also increase capital and operational costs.

Electrolyte recycling has been used to specifically address pH imbalance in an MDC. Qu et al.

found that recirculating electrolyte at the end of a batch fed MDC cycle further increased

desalination by 19% [18]. Continuous electrolyte recycling resulted in 40% greater COD removal

and similar levels of desalination as a batch-fed MDC. The final anolyte pH with electrolyte

cycling was 6.7 compared to a pH of 4.1 at the end of a batch cycle without electrolyte

recirculation. Though electrolyte pH was successfully balanced using this method, there was

greater oxygen transfer into the anode chamber, resulting in low coulombic efficiency (25%) and

a biofilm developed on the air cathode, reducing current generation over time. Chen et al.

addressed these problems by using a separator next to the cathode and achieved greater stability

of desalination performance over 60 days compared to tests using MDCs without a separator [19].

Another approach used to mitigate low anolyte pH is placing a bipolar membrane

adjacent to the anode chamber and applying voltage to the system. A bipolar membrane consists

of an anion exchange membrane and cation exchange membrane pressed together. Applying a

potential greater than 0.83 V in theory dissociates water between the two membrane surfaces,

transporting hydroxide ions into the anode chamber to balance anolyte pH. Applying 1.0 V to an

MDC with a bipolar membrane resulted in balanced pH (7.0) compared to a pH of 4.5 at the end

of an electrolysis MDC cycle without a bipolar membrane [7].

2.3 Extent of desalination and power generation in MDCs

Current generation in an MDC creates a charge imbalance at the electrodes with protons

being released at the anode and hydroxide generated at the cathode. This charge imbalance

attracts anions (Cl–) towards the anode and cations (Na

+) towards the cathode chamber,

effectively desalting a solution in the middle chamber. Most methods of increasing desalination in

MDCs involve increasing current generation as the two are directly correlated. Decreasing the

external resistance has been shown to increase current. Chen et al. found that the specific

desalination rate increased from 0.25 g/L-h to 1.96 g/L-h when the external resistance decreased

from 500 Ω to 10 Ω. Decreasing the external resistance further to 5 Ω did not improve the

desalination rate [3]. Using a larger anode solution volume can prolong the time until anode

acidification becomes limiting and increase the amount of substrate available to the biofilm for

current generation. Luo et al. reported an 80% improvement in desalination when the anolyte

volume was increased from 25 mL to 425 mL [6]. Though increasing electrolyte volume keeps

electrode activity from becoming the limiting factor in a system, this is not an economically

viable solution.

Increasing the number of cells in the desalination chamber (ED stack) has been found to

increase the total desalination rate and current efficiency. An ED stack with 2 desalination

chambers examined by Chen et al. had a current efficiency of 283% and a 5 cell pair stack by

Kim and Logan increased current efficiency to 430% as compared with 90-100% current

efficiency in a single desalination chambered system [3, 4]. Though a stack can multiply the

efficiency of ionic separation, increasing the distance between the two electrodes and inserting

more membranes increases the ohmic resistance, which decreases the potential that can be used

by the cell to generate current. Ohmic resistance was also found to increase during the

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desalination cycle as the diluate solution became less conductive [3]. The ohmic resistance from

the stack can be decreased by minimizing the distance between each cell [4].

Applying a larger voltage to an MEDC has been shown to slightly improve the rate of

desalination. Increasing the applied voltage from 0.4 V to 0.8 V had a greater effect on hydrogen

production than desalination, increasing hydrogen production by 260% while only improving the

average desalination rate by 35%. The desalination rate and hydrogen production rate showed

parallel variations during a desalination cycle, and followed current production. The maximum

desalination rate at 0.8 V was 0.42 mS/cm-h [6].

Electrolyte recirculation was shown to increase desalination when low buffered

electrolyte solutions were used. Desalination increased by 48% when recirculating between 25

mM buffered electrolytes, but decreased 13% with 50 mM buffered solutions [18]. Recirculating

the electrolyte at the end of an MDC cycle increased desalination the most (84%) as less substrate

was lost to aerobic processes in the cathode chamber. Placing a separator next to the cathode

while circulating electrolyte improved long-term performance by preventing biofouling at catalyst

sites. After 60 days of operation, desalination in a circulation MDC with separator decreased

25%, whereas performance was reduced 41% in circulation MDCs without a separator [19].

Osmotic forces disproportionally assist in desalination of higher saline solutions. The

osmotic pressure is approximately 14.4 atm when a 35 g/L NaCl solution is placed next to a dilute

anolyte with an equivalent salt concentration of 0.5 g/L NaCl [20]. Osmosis resulted in a 68%

conductivity reduction of a 20 g/L NaCl solution [21]. The largest MDC to date is an upflow

reactor designed by Jacobson et al. with a total volume of 2.75 L. This MDC achieved 94.3 ±

2.7% removal of a 35 g/L NaCl solution with a 4 day HRT [2].

Power generation in a BES is a function of the electrode potentials and internal

resistance. In an MDC operating with acetate as the electron donor and an air cathode, the

maximum open circuit potential is 1.11 V (0.81 – (–0.30) V). In practice, the maximum open

11

circuit potential is around 0.8 V due to typical electrode overpotentials, which can be improved

by better catalytic activity or larger electrode surface area [22]. The determining factor of power

production is the internal resistance due electrode overpotentials and ohmic losses. The reactor

with the lowest internal resistance has the largest potential available for power production. Salt

solution quality determines the electrolyte resistance and it is an important factor for power

production in MDCs. Maximum power production decreased from 28.9 W/m3 to 11.1 W/m

3 when

using an artificial seawater solution in place of a 35 g/L NaCl solution [2]. The chosen external

resistance in an MDC represents a trade-off between power production and desalination.

Decreasing the external resistance from 500 Ω to 10 Ω improved desalination by 684%, but

further reducing the resistance to 5 Ω decreased performance [3]. In another study, decreasing the

external resistance from 6 Ω to 0.1 Ω doubled the current output [2].

2.4 Multi-electrode bioelectrochemical systems

Multi-reactor connections of BESs, as well as multi-electro systems can differ in

performance compared to single electrode reactors and therefore an understanding of their

electrochemical behavior is crucial for scaling up these systems. In an MFC or MDC, electrically

connecting the anodes in series theoretically adds the voltages from each electrode, while a

parallel connection increases current. In practice, connecting individual MFCs in series often

results in voltage reversal, which is when a cell cannot deliver the current that is demanded of it

from the system. As a result the polarity of the cell switches and that cell becomes a voltage drain

on the system. For example, in a batch fed system with 2 MFCs connected in series, the voltage at

the beginning of the cycle for reactor 1 and 2 were 0.27 V and 0.20 V, respectively. After 5 hours

of operation, the voltage of reactor 1 increased to 0.6 V, while the voltage in reactor 2 decreased

to –0.6 V, resulting in no net voltage [23]. Fuel starvation is one of the main causes of voltage

12

reversal and but it can be avoided by using a high volumetric loading rate in MFCs with

continuous flow operation [24, 25]. Connecting the circuit to a high external resistance generates

lower current, which can also help prevent reversal [23]. Kim et al. successfully increased voltage

and avoided voltage reversal when using 4 MFCs by linking the MFCs to an array of capacitors

that were incrementally charged and discharged. Voltage and maximum power generation were

also increased in this series connection and lower cell performance did not reverse polarity [26].

Hydraulically connecting MFCs in series can also induce losses in voltage and power due

to parasitic current between the series connected electrodes [27]. In two tubular MFCs connected

in series the open circuit voltage dropped from 1.31 V to 0.96 V immediately after switching

from hydraulically isolated reactors to a system with a shared solution [28]. Higher electrolyte

resistance will decrease ion cross-conduction, and this can be accomplished by extending the

distance between the electrodes, enhancing the electrolyte resistivity, or decreasing the cross-

sectional flow area. Though these factors will reduce parasitic current, they will also increase the

internal resistance of the cell which is known to diminish performance. Wang et al. demonstrated

that voltage did not drop between two anodes connected in series when they were separated by

baffles (~700 mW/m2), but it dropped by almost half (366 mW/m

2) without baffle separation

[29].

Connecting multiple anodes in parallel increased current almost linearly and maintained

similar maximum power output as a single MFC. Zhuang et al. found a 4.2 fold increase in

current generation of 5 MFCs connected in parallel [24]. The short circuit current of 6 MFCs

connected in parallel increased by 5.6 times [25]. There was minimal energy loss in parallel

connected electrode MFCs when they were hydraulically connected, which indicated that there

was little effect of lateral ion cross-conduction on performance. The internal resistance in a multi-

anode baffled electrode system decreased as more anodes were connected in parallel, ranging

from 17.8 Ω (1 anode), 11.3 Ω (2 anodes), 9.0 Ω (3 anodes), to 5.4 Ω (4 anodes) [29]. Electrically

13

connecting a multi-electrode MDC in parallel will increase current generation and maximize

desalination.

2.5 Anolyte recirculation

A substrate gradient may form along the vertical length of a batch-fed reactor, decreasing

substrate concentrations and electrical performance. Karra et al. operated a plug flow MFC with 6

brush anodes/air cathode pairs located in 3 channels separated by baffles. Peak power densities

decreased along the flow path of the reactor from 282 mW/m2 (Channel I), 249 mW/m

2 (Channel

II), 159 mW/m2 (Channel III). COD removal also decreased in consecutive channels from 79%

(I), 71% (II), to 64% (III). The decreased substrate removals along the reactor flow path,

relatively long HRT (10 hours), and placement of the electrodes towards the top of the reactor,

suggests that biomass settling and decreased substrate concentration may have been factors in

reduced performance [30]. You et al. observed a substrate gradient along the height of a tubular

MFC at low influent substrate concentrations (400 mg COD/L) for a reactor operated in fed-batch

mode. Recirculation of the anolyte increased the volumetric power two-fold and decreased the

ohmic resistance by 42% and diffusion resistance by 25% [31]. Increasing the recirculation rate in

an upflow MFC improved current generation up to 46% [32]. Acclimating bioanodes under a

high shear rate formed a biofilm twice as thick and 5 times as dense as under low shear rates and

resulted in 2 to 3 times higher current generation and power production in an MFC [33]. These

results all show that recycling anolyte could produce more robust and electrically active biofilms,

reduce substrate concentration gradients along a reactor, and produce higher power and current

densities by reducing ohmic and diffusion resistances.

The electrolyte flow pattern has also been shown to have a significant effect on MFC

performance. In a recirculation or continuous flow anolyte, the electrolyte will flow along the

14

path of least hydraulic resistance, potentially creating areas of the anode with depleted substrate.

Recently, there have been several investigations on the control the flow path of the electrolyte

through the use of a spiral or helical anode geometry [34, 35]. A helical flow pathway in a porous

anode was shown to improve peak power by 2.2 to 6.8 fold compared to an equivalent system

without a helical flow pattern [36]. Zhang et al. developed a spiral spacer which created a helical

flow around a long brush anode. Using the spiral spacer improved maximum power and current

generation (7.1 W/m3, 62.6 A/m

3) compared to performance without a spiral flow path (4.5 W/m

3,

29.2 A/m3). Increasing the recirculation rate through the spiral spacer from 50 mL/min to 300

mL/min further improved the COD removal 12%, maximum power 49%, and maximum current

45% [37]. COD removal with and without the spiral spacer stayed relatively the same, which

indicated that the improved flow path did not increase substrate distribution to the biofilm. The

improved transport of ions or chemical mediators from the spiral flowpath through the anode

biofilm was likely the reason for the improved electrical performance


1. Cao, X., Huang, X., Liang, P., Xiao, K., Zhou, Y., Zhang, X., Logan, B. A new method

of desalination using microbial desalination cells. Environ. Sci. Technol. 2009, 43, 7148-

7152

2. Jacobson, K., Drew, D., He, Z. Use of a liter-scale microbial desalination cell as a

platform to study bioelectrochemical desalination with salt solution or artificial seawater.

Environ. Sci. Technol. 2011, 45, 4652-4657

3. Chen, X., Xia, X., Liang, P., Cao, X., Sun, H., Huang, X. Stacked microbial desalination

cells to enhance water desalination efficiency. Environ. Sci. Technol. 2011, 45, 2465-

2470

4. Kim, Y., Logan, B. Series assembly of microbial desalination cells containing stacked

electrodialysis cells for partial or complete seawater desalination. Environ. Sci. Technol.

2011, 45, 5840-5845

5. Mehanna, M., Kiely, P., Call, D., Logan, B. Microbial electrodialysis cell for

simultaneous water desalination and hydrogen gas production. Environ. Sci. Technol.

2010, 44, 9578-9583

15

6. Luo, H., Jenkins, P., Ren, Z. Concurrent desalination and hydrogen generation using

microbial electrolysis and desalination cells. Environ. Sci. Technol. 2011, 45, 340-344

7. Chen, S., Liu, G., Zhang, R., Qin, B., Luo, Y. Development of the microbial electrolysis

desalination and chemical-production cell for desalination as well as acid and alkali

productions. Environ. Sci. Technol. 2012, 46, 2467-2472

8. Zhang, B., He, Z. Integrated salinity reduction and water recovery in an osmotic

microbial desalination cell. RSC Adv. 2012, 2, 3265-3269

9. Werner, C., Logan, B., Saikaly, P., Amy, G. Wastewater treatment, energy recovery and

desalination using a forward osmosis membrane in an air-cathode microbial osmotic fuel

cell. J Membr. Sci. 2013, 428, 116-122

10. Forrestal, C., Xu, P., Ren, Z. Sustainable desalination using a microbial capacitive

desalination cell. Energy Environ. Sci. 2012, 5, 7161-7167

11. Welgemoed, T., Schutte, C. Capacitive deionization technology: an alternative

desalination solution. Desalination. 2005, 183, 327-340

12. Patil, S., Harnisch, F., Koch, C., Hubschmann, T., Fetzer, I., Carmona-Martinez, A.,

Muller, S., Schroder, U. Electroactive mixed culture derived biofilms in microbial

bioelectrochemical systems: the role of pH on biofilm formation, performance and

composition. Bioresource Technol.2011, 9683-9690

13. He, Z., Huang, Y., Manohar, A., Mansfeld, F. Effect of electrolyte pH on the rate of the

anodic and cathodic reactions in an air-cathode microbial fuel cell. Bioelectrochemistry,

2008, 78-82

14. Torres, C., Marcus, A., Rittmann, B. Proton transport inside the biofilm limits electrical

current generation by anode-respiring bacteria. Biotechnol. Bioeng., 2008, 100(5), 872-

881

15. Franks, A., Nevin, K., Jia, H., Izallalen, M., Woodard, T., Lovely, D. Novel strategy for

three-dimensional real-time imaging of microbial fuel cell communities: monitoring the

inhibitory effects of proton accumulation within the anode biofilm. Energy Environ. Sci.

2009, 2, 113-119

16. Liu, H., Cheng, S., Logan, B. Power generation in fed-batch microbial fuel cells as a

function of ionic strength, temperature, and reactor configuration. Environ. Sci. Technol.

2005, 39, 14, 5488-5493

17. Cheng, S., Liu, H., Logan, B. Increased power generation in a continuous flow MFC with

advective flow through the porous anode and reduced electrode spacing. Environ. Sci.

Technol. 2006, 40, 7, 2426-2432

18. Qu, Y., Feng, Y., Wang, X., Liu, J., Lv, J., He, W., Logan, B. Simultaneous water

desalination and electricity generation in a microbial desalination cell with electrolyte

recirculation for pH control. Bioresource Technol. 2012, 106, 89-94

19. Chen, X., Liang, P., Wei, Z., Zhang, X., Huang, X. Sustainable water desalination and

electricity generation in a separator coupled stacked microbial desalination cell with

buffer free electrolyte circulation. Bioresource Technol. 2012, 119, 88-93

20. Kim, Y., Logan, B. Microbial desalination cells for energy production and desalination.

Desalination, 2013, 308, 122-130

16

21. Mehanna, M., Saito, T., Yan, J., Hickner, M., Cao, X., Huang, X., Logan, B. Using

microbial desalination cells to reduce water salinity prior to reverse osmosis. Energy

Environ. Sci. 2010, 3, 1114-1120

22. Rabaey, K., Verstraete, W. Microbial fuel cells: novel biotechnology for energy

generation. Trends Biotechnol. 2005, 23(6), 291-298

23. Oh, S., Logan, B.Voltage reversal during microbial fuel cell stack operation. J Power

Sources. 2007, 167, 11-17

24. Aelterman, P., Rabaey, K., Pham, H., Boon, N., Verstraete, W. Continuous electricity

generation at high voltages and currents using stacked microbial fuel cells. Environ. Sci.

Technol. 2006, 40, 3388-3394

25. Zhuang, L., Zheng, Y., Zhou, S., Yuan, Y., Yuan, H., Chen, Y. Scalable microbial fuel

cell (MFC) stack for continuous real wastewater treatment. Bioresource Technol. 2012,

106, 82-88

26. Kim, Y., Hatzell, M., Hutchinson, A., Logan, B. Capturing power at higher voltages from

arrays of microbial fuel cells without voltage reversal. Energy Environ. Sci. 2011, 4,

4662-4667

27. Kim, D., An, J., Kim, B., Jang, J., Kim, B., Chang, I. Scaling-up microbial fuel cells:

configuration and potential drop phenomenon at series connection of unit cells in shared

anolyte. Chem Sus Chem, 2012, 5, 1086-1091

28. Zhuang, L., Zhou, S. Substrate cross-conduction effect on the performance of serially

connected microbial fuel cell stack. Electrochemistry Commun. 2009, 11, 937-940

29. Wang, B., Han, J. A single chamber stackable microbial fuel cell with an air cathode.

Biotechnol Lett, 2009, 31, 387-393

30. Karra, U., Troop, E., Curtis, M., Scheible, K., Tenagler, C., Patel, N., Li, B. Performance

of plug flow microbial fuel cell and complete mixing microbial fuel cell for wastewater

treatment and power generation. Int J Hydrogen Energ, 2013, 38, 13, 5383-5388

31. You, S., Zhao, Q., Zhang, J., Jiang, J., Wan, C., Du, M., Zhao, S. A graphite-granule

membrane-less tubular air-cathode microbial fuel cell for power generation under

continuously operational conditions. J Power Sources, 2007, 173, 172-177

32. Zhang, F., Jacobson, K., Torres, P., He, Z. Effects of anolyte recirculation rates and

catholytes on electricity generation in a litre-scale upflow microbial fuel cell. Energy

Environ. Sci. 2010, 3, 1347-1352

33. Pham, H., Boon, N., Aelterman, P., Clauwaert, P., De Schamphelaire, L., Van Oostveldt,

P., Verbeken, K., Rabaey, K., Verstraete, W. High shear enrichment improves the

performance of the anodophilic microbial consortium in a microbial fuel cell. Microbial

Biotechnology, 2008, 1, 6, 487-496

34. Mardanpour, M., Esfahany, M., Behzad, T., Sedaqatvand, R. Single chamber microbial

fuel cell with spiral anode for dairy wastewater treatment. Biosens Bioelectron, 2012, 38,

264-269

35. Jia, B., Hu, D., Xie, B., Dong, K., Liu, H. Increased power density from a spiral wound

microbial fuel cell.” Biosens Bioelectron, 2013, 41, 894-897

36. Kim, J., Boghani, H., Amini, N., Aguey-Zinsou, K., Michie, I., Dinsdale, R., Guwy, A.,

Guo, Z., Premier, G. Porous anodes with helical flow pathways in bioelectrochemical

17

systems: the effects of fluid dynamics and operating regimes. J Power Sources, 2012,

213, 382-390

37. Zhang, F., Ge, Z., Grimaud, J., Hurst, J., He, Z. Improving electricity production in

tubular microbial fuel cells through optimizing the anolyte flow with spiral spacers.

Bioresource Technol, 2013, 134, 251-256

Chapter 3

Increasing desalination by mitigating anolyte pH imbalance using catholyte

effluent addition in a multi-anode, bench scale microbial desalination cell

3.1 Introduction

It is estimated that more than 4 billion people currently live in high water stressed regions in

the world, and that 2.5 billion lack safe sanitary practices [1, 2]. Conservation and infrastructure

improvement can help alleviate some of the water stress, but with the global water shortage

projected to grow through 2050 due to the effects of climate change, industrialization, and

population growth, new sources of water will be needed [3]. The number of seawater desalination

facilities is predicted to increase exponentially in the next 10 years, but current commercial

desalination techniques such as electrodialysis, thermal desalination and reverse osmosis have

many environmental concerns and high energy costs [4, 5]. Even with state of the art advances in

reverse osmosis technology bringing its energy consumption close to the practical theoretical

minimum, the energy cost for seawater desalination is still too high for widespread

implementation, especially in poorer regions, which are the majority of the water stressed

demographic [6].

A new desalination technology has recently been developed, called a microbial desalination

cell (MDC), that uses exoelectrogenic microorganisms to degrade organic matter in a wastewater

and generate electricity [7]. This process is coupled with a stack of ion exchange membranes to

desalinate water and produce energy. There have been a number of advancements and

modifications of this technology since its development in 2009, that include: use of an

electrodialysis stack rather than a single desalination chamber to increase the current efficiency

19

and ionic separation [8, 9]; applying a voltage to produce hydrogen gas while desalinating water

rather than producing net electrical power [10, 11]; using a non-buffered solution with limited

impacts on desalination rates [9]; and examining the use of actual wastewaters as the anolyte [12].

One of the main factors limiting MDC performance has been anolyte acidification. As organic

matter in wastewater is oxidized by exoelectrogenic bacteria on the anode and electrons are

transferred to the electrode, protons are released into solution, lowering the anolyte pH. The rate

of proton production at the anode is greater than the rate of buffer diffusion into the anode

biofilm, which results in a pH gradient between the solution and the biofilm. This creates an

acidic environment for the anodic microbial community than can occur even before it is detected

in bulk solution [13]. A decrease in the pH below neutral inhibits the respiration of anodic

bacteria [14, 15]. This effect on pH is heightened in MDCs as transport of protons from the anode

chamber is limited due to the anion exchange membrane in the membrane stack located adjacent

to the anode chamber. Similarly, at the cathode the pH increases when a cation exchange

membrane is placed next to the cathode chamber.

Several different approaches have been used to avoid decreases in anode pH such as using

larger volumes of electrolyte solutions [7, 8], applying electrolyte recirculation between the

cathode and anode chambers [16, 17], or inserting a bipolar membrane next to the anode chamber

[18]. Larger electrolyte solution volumes increase cycle time by providing more solution to

balance pH, but they require higher operational and capital costs associated with pumping and

storing larger volumes of water, and they do not solve the inherent pH problem. Electrolyte

recirculation extends cycle time by balancing protons accumulated in the anode chamber with

hydroxide ions which are formed in the cathode chamber, but the introduction of organic matter

into the cathode chamber can result in extensive biofouling of the cathode. A bipolar membrane

can be used instead of the anion exchange membrane next to the anode chamber to dissociate

20

water and balance pH. However, a large voltage (1.0 V) must be used due to the high resistance

of the bipolar membrane, which makes the process energy intensive.

A different approach was used here to mitigate anolyte pH imbalance based on using the

cathode solution. Instead of recycling electrolyte solutions between the electrode chambers, the

non-buffered and saline catholyte effluent was mixed once with fresh anolyte to increase the

anolyte alkalinity and ionic conductivity. Oxygen reduction reaction at the cathode consumes

protons, and increases the pH to approximately 12.8. The catholyte has a high conductivity due to

the salinity of the water being desalinated (~70 mS/cm with 35 g/L NaCl). Therefore, adding the

catholyte to the anolyte increases the conductivity of the anolyte solution and reduces internal

resistance. The effectiveness of amending the anolyte with catholyte was examined here using a

larger-scale, multi- anode electrode MDC in terms of current production, desalination, power

generation, and COD removal. The transport of anions out of the anode chamber during a cycle

by back-diffusion was also measured to determine substrate losses from the anode chamber.

Additional experiments were conducted by adding NaCl directly to the anolyte, allowing

observation of the effect of conductivity separately from that produced by catholyte pH.

3.2 Materials and methods

3.2.1 MDC construction

The anode and cathode chambers were constructed using high density polyethylene (HDPE)

material with a cross sectional area of 52.5 cm2 (17.5 cm × 3 cm) (Fig. 1). The anode chamber

had a volume of 160 mL and the cathode chamber had a volume of 53 mL. Four bars of HDPE

(0.5 cm height) were evenly spaced and placed on the inner face of both chambers in order to

21

prevent membrane deformation. Four heat treated graphite fiber brushes 2.7 cm in diameter and

2.3 cm long were used as the anodes (Mill-Rose Lab Inc., USA). The air cathodes consisted of

wet proofed carbon cloth (30%), with four layers of polytetrafluoroethylene diffusion layers, a

Nafion binder and 0.5 mg Pt/cm2 [19]. The anodes were each connected to individual 10 Ω

external resistors and connected in parallel to the cathode through a single titanium wire current

collector along the length of the cathode.

The electrodialysis stack consisted of 3 cell pairs made of interchanging anion and cation

exchange membranes (Selemion CMV and AMV, Asahi glass, Japan) pretreated in a 0.6 M NaCl

solution for 24 hours, and then rinsed with deionized water. The silicone gaskets used to make a

water tight seal in the stack had a thickness of 1.3 mm, and polyethylene mesh spacers (2.5 × 16

cm2) were used to maintain cell thickness. Each cell held approximately 9 mL. The gaskets and

membranes were cut to allow parallel flow through the stack, entering and leaving through the

anode side of the reactor. Ag/AgCl reference electrodes (RE-5B; BASi, West Lafayette, IN) were

placed between the anodes in the anode chamber, and directly across from the anodes in the

cathode chamber. The reactor was held in place by two anodized aluminum end plates.

22

Figure 1. (A) Schematic of bench scale MDC with parallel continuously recycled flow through

the 3 cell pair electrodialysis stack. AEM, anion exchange membrane. CEM, cation exchange

membrane. (B) Photograph of the reactor in operation.

3.2.2 Medium

The anodes were inoculated (50% v/v) with a pre-acclimated mixed culture community

of microorganisms from a functioning acetate-fed MFC, and acclimated individually in 4 cm cube

reactors with a 10 Ω external resistor for over one month. NaCl concentrations were gradually

increased from 0 to 200 mM NaCl in order to preacclimate bacteria to higher Cl– conditions

typically produced in the MDC [9]. The anode chamber of the MDC was fed a solution of sodium

acetate (1 g/L) in a 50 mM phosphate buffer solution containing (per liter of deionized water):

0.31g NH4Cl, 2.45g NaH2PO4·H2O, 4.58g Na2HPO4, 0.13g KCl, 5 mL vitamins, and 6.25 mL

trace minerals [20]. The catholyte, diluate, and concentrate solutions were all synthetic seawater

consisting of 35 g/L NaCl prepared in deionized water.

23

3.2.3 MDC operation and experimental procedures

The cathode chamber was operated in fed-batch mode. The solution in the anode chamber

was continuously recycled at 1.0 mL/min (from the bottom to the top of the chamber) to avoid

localized differences in substrate concentrations that could affect reactor performance [21]. The

reactor was left in open circuit for an hour previous to operation, with 35 g/L NaCl solution

flushed through the stack at 5 mL/min to remove solution from the previous cycle. During

operation, the diluate solution was continuously recycled using a 100 mL salt water reservoir. The

concentrate stream was continuously recycled to a larger solution reservoir (~450 mL). Both

streams had a flowrate of 1 mL/min. The cycle time was set to 10 hours, to minimize osmotic

losses at the end of a cycle when current decreased to lower levels.

Catholyte effluent (~70 mS/cm, pH~12.8) collected from a previous cycle was added to

the anolyte influent, rather than using the catholyte from the same cycle, to simplify MDC

operation. The volume of catholyte used was evaluated in terms of the equivalent change in salt

concentration, producing increments of 25 mM, 50 mM, and 75 mM higher anolyte salt

concentrations. The substrate concentration was maintained at 1 g/L acetate to avoid the effects of

the different initial substrate concentrations on performance. The amount of PBS added to the

anolyte was also constant for each experimental condition (Fig A1). At 100 mM PBS current was

produced with a maximum salt concentration addition from catholyte of 150 mM. Therefore, with

50 mM PBS as the anolyte, the highest salt concentration from catholyte addition used was 75

mM. As a control, a fed batch cycle was run without addition of catholyte effluent to the anolyte

solution. Addition of catholyte effluent increased both conductivity and pH of the anolyte. In

order to observe the effect of conductivity separately from pH, in separate experiments sodium

chloride was added to the anolyte at 25 mM, 50 mM, and 75 mM NaCl increments, avoiding a pH

change.

24

3.2.4 Analyses and calculations

The voltage (U) for each anode across a 10 Ω external resistor [8] was measured at 10

minute intervals using a multimeter (Keithley Instruments, USA) connected to a personal

computer. Current was calculated as i=U/R, and current density was normalized by the cathode

surface area (52.5 cm2). Influent and effluent solutions for diluate, concentrate, and anolyte and

catholyte solutions were analyzed using conductivity and pH probes (SevenMulti, Mettler-Toledo

International Inc., USA). The total desalination rate (g/L-d) was calculated as the change in

salinity based on total dissolved solids. The salinity was estimated from conductivity

measurements using an in situ conductivity conversion as previously outlined by Bennett [22],

and assuming the conductivity measured was due only to NaCl. Current efficiency (η) was

determined as ratio of ionic separation of NaCl to the total number of electrons passed through

the circuit, as

(1)

where F is Faraday’s constant, c the molar concentration of NaCl in the diluate, v the volume of

the diluate, Ncp the number of cell pairs in the electrodialysis stack, and i the current generated in

the reactor. The subscript “in” indicates conditions at the beginning of the cycle, “out” the end of

the cycle, and the superscript “D” indicates diluate [9].

The chemical oxygen demand (COD) was measured for influent and effluent anolyte

solutions using standard methods (Hach Co., USA). The COD sample was diluted at a 1:10 ratio

in order to minimize the effect of chloride ions on measurements. The coulombic efficiency (CE)

was calculated based on the total COD removed and the number of coulombs collected during the

cycle as previously described [23]. The coulombic efficiency calculated here was modified to

account for acetate losses due to diffusion out of the anode chamber, as

25

(2)

where MO2 is the molecular weight of O2 (32 g/mol), bes the number of electrons exchanged per

mole of oxygen (4 mol e–/mol O2), CODin the measured substrate concentration at the beginning

of the cycle, CODeff the measured concentration at the end of the cycle, and CODdiff the substrate

concentration measured in the adjacent diluate solution due to diffusion through the membrane.

Diluate samples were analyzed for phosphate using ion chromatography (IC, Dionex

ICS-1100) and acetate using high performance liquid chromatography (HPLC, Shimadzu LC-

20AT). The power density was measured for each experimental condition using an external

resistance ranging from 10 to 10,000 Ω at 20 minute intervals, after the reactor was initially set at

an open circuit for ~1 hour. During polarization tests, salt solution (35 g/L NaCl) was

continuously flowed through the electrodialysis stack at a flow rate of 5 mL/min in order to

minimize losses due to junction potential, and to decrease internal resistance as previously

demonstrated [9]. Power densities (mW/m2) were normalized by the cathode projected surface

area.

3.3 Results

3.3.1 Desalination and current generation

The MDC was run under four different operating conditions, each with varying amounts

of catholyte effluent added to the anolyte. The salt concentration in the diluate solution (35 g/L

NaCl) was reduced by 26.0 ± 0.5% when the anolyte salt concentration was increased by 75 mM

using the catholyte effluent, compared to 18.1 ± 2.0% in the control (no catholyte addition). The

total rate of desalination and extent of desalination increased linearly with catholyte

26

concentration, since the cycle time was fixed (Fig 2a). The peak current density was 7.21 ± 0.08

A/m2, shortly after the circuit was closed, for two lowest catholyte additions (25 and 50 mM) and

the control. For tests with 75 mM catholyte, the current density was initially lower at 6.10 A/m2,

but it increased to a maximum of 6.95 A/m2 after 40-60 minutes (Fig 2b). The number of

coulombs recovered increased with catholyte addition, from 532 C (no catholyte) to 833 C (75

mM catholyte). Diluate recovery averaged 90%, with a anolyte:diluate effluent ratio of 1.4:1.

Fouling was not observed on the cathode.

Figure 2. (A) Extent of desalination and total desalination rate with increasing anolyte salt

concentration increase from catholyte effluent addition. (B) Current density profile during one 10

hour cycle for different amounts of catholyte effluent addition.

The maximum power density was 685 mW/m2 with either 50 mM or 75 mM catholyte

addition (Fig 3). When the anolyte conductivity was nearly doubled from 7.6 ± 0.1 mS/cm to 14.3

± 0.9 mS/cm through addition of the 75 mM equivalent catholyte, the total internal resistance

decreased by only 5 Ω to 73 Ω (based on the slopes of the polarization data). This suggests that

the anolyte solution resistance was not a large fraction of the overall internal resistance. The

average power production over the fed batch cycle was 318 ± 14 mW/m2 with the 75 mM

catholyte addition, and this power density decreased linearly with smaller volumes of catholyte

addition (Fig A2). The external resistance of the MDC was set to maximize current and therefore

27

desalination rate, rather than power production. Increasing the external resistance could yield

higher power densities as discussed by Jacobson et al. [24]

Figure 3. Maximum power density with various amounts of catholyte effluent addition (0 to 75

mM) after ~30 days of operation at 10 Ω external resistance. The internal resistance is also

showed by the slope of the voltage with respect to current. The closed diamonds “P” represent

power density at a given concentration of catholyte addition, while the open signs “V” are for

voltage density points.

The catholyte effluent pH was 12.8 ± 0.3, so amending fresh anolyte solution with

catholyte effluent resulted in a higher initial anolyte pH. The initial anolyte pH was 8.12 ± 0.40

with 75 mM catholyte addition. The initial pH more approached a more neutral pH with smaller

volumes of catholyte addition, with a pH of 7.17 ± 0.03 for the control (no catholyte addition)

(Fig 4). Greater volumes of catholyte effluent addition increased the extent of desalination (Fig

2a), but too high an initial pH limited current generation by the microorganisms. In separate tests

with greater amounts of catholyte addition than those reported here, it was observed that an initial

anolyte pH above 9.0 irreversibly decreased current generation to zero (Fig A3), as also shown by

others [25]. The pH at the end of the cycle was 5.0 ± 0.1 under all experimental conditions,

0

0.1

0.2

0.3

0.4

0.5

0

100

200

300

400

500

600

700

800

0 2 4 6

Vo

ltag

e (V

)

Po

wer

Den

sity

(m

W/m

2)

Current Density (A/m2)

P0P25P50P75V0V25V50V75

28

which is in the low pH range known to inhibit current generation [15].

Figure 4. Anolyte pH at the beginning and end of one cycle at various concentrations of anolyte

salt increase due to catholyte effluent addition. Increasing the amount of catholyte effluent

addition results in an increase of initial anolyte pH.

3.3.2 Anion transport and the effect on COD removal

The current efficiency is the fractional contribution of the total electrons transferred

through the circuit for ionic separation in the dilute solution. The average current efficiency here

was 96 ± 7% (eq. 1), demonstrating that the ion exchange membranes effectively transported

sodium or chloride counter-ions with current generation, with little back-diffusion of these ions.

Other counter-ions could back-diffuse across the ion exchange membranes, although this is not

accounted for in eq. 1. There was back-diffusion of acetate and phosphate anions across the anion

exchange membrane into the adjacent diluate chamber. This diffusion resulted in a loss of 0.03 ±

0.02 g/L of acetate from the anode chamber with 75 mM catholyte addition, and this increased to

as much as 0.11 ± 0.02 g/L with no catholyte addition (control). Phosphate ions were also lost

from the anode chamber, increasing from 2.3 mM PO43–

(75 mM catholyte addition, 14.3 mS/cm)

to 3.2 mM PO43–

(control, 7.6 mS/cm) (Fig 5). This trend of decreasing diffusive anion transport

was also apparent when NaCl (not catholyte) was directly added to the anolyte, demonstrating

4.0

5.0

6.0

7.0

8.0

9.0

0 25 50 75

An

oly

te p

H

Anolyte salt concentration increase

(NaCl equivalent, mM)

pH in pH out

29

that anion back-diffusion out of the anode chamber was a function of anolyte conductivity.

Figure 5. Diffusion of acetate and phosphate, across the anion exchange membrane from

the anolyte chamber into the diluate solution. Results are displayed at various increments of

anolyte salt concentration increase due to either NaCl addition or catholyte effluent addition.

Final acetate concentrations in the anode chamber decreased with higher concentrations

of catholyte added to the anolyte, with acetate removals ranging from 55.2 ± 1.7% (control) to

62.8 ± 0.4% (75 mM catholyte addition). The greater COD removal was due to higher current

densities associated with catholyte addition. When increasing anolyte conductivity using only

NaCl, there was less COD removal, (Fig 6) which indicated that it was the additional buffering

capacity due to the catholyte rather than the increased conductivity that improved the extent of

substrate oxidation. COD removal from biochemical processes was less than that measured in the

anode chamber, due to acetate diffusion through the adjacent ion exchange membrane. The effect

of diffusion on correcting microbial COD removal was greatest at lower conductivity anode

0

1

2

3

4

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0 25 50 75

Ph

osp

ha

te (

mM

)

Ace

tate

(g

/L)

Anolyte salt concentration increase(NaCl equivalent, mM)

Acetate (cat)

Acetate (NaCl)

Phosphate (cat)

Phosphate (NaCl)

30

solutions, decreasing COD removal by 17% compared to the control (7.6 mS/cm) (Fig 7

Figure 6. COD removal at various increments of anolyte salt concentration increase due to NaCl

addition with no effect on anolyte pH (open signs) and due to addition of catholyte effluent with

an increase in anolyte pH (closed signs).

The CE ranged from 59% to 87%, indicating that very little oxygen diffused to the anode

biofilm. CE increased with greater amounts of catholyte effluent addition, from 59 ± 3%

(control), 66 ± 1% (25 mM catholyte), 80 ± 6% (50 mM catholyte), to 84 ± 1% (75 mM

catholyte). The changes in the CEdiff were less variable when acetate losses due to diffusion were

included, ranging from 75 ± 3% (control) to 87 ± 4% (75 mM). The effect of acetate diffusion on

CEdiff was greatest at lower anolyte conductivities (28% increase at 7.6 mS/cm) and lessened with

higher anolyte conductivity (4% increase at 14.3 mS/cm) (Fig 7).

40

45

50

55

60

65

0 25 50 75

CO

D r

emov

al (

%)



catholyte addition

NaCl addition

31

Figure 7. Coulombic efficiencies and COD measurements from the anolyte at the beginning and

end of one cycle for various increments of anolyte salt concentration increase due to catholyte

effluent addition. The subscript “diff” indicates data points where diffusion of acetate out of the

anode chamber was taken into account.

3.3.3 Effect of anolyte recirculation on anode performance

Anolyte recirculation resulted in a more equal distribution of current generation along the

height of the reactor and improved overall current density. When the anode chamber was initially

filled (batch mode), each anode contributed almost equally to performance based on measured

currents. The nearly equal current production by each anode at the beginning of the cycle

indicates that when the anode chamber was initially filled, it functioned as a completely mixed

reactor. As the cycle progressed, however, the current produced by the anode at the top of the

reactor first declined, followed successively by the other anodes down along the length of the

reactor (Fig 8b). The difference in current generation from each anode was most likely caused by

a substrate gradient that developed over time as has been observed by others [26, 27]. Over eight

24 hour batch fed cycles, the contribution of the top anode towards current generation decreased

from 37% to 11% suggesting that there were difference in substrate concentrations developing

within the reactor over time.

40

50

60

70

80

90

100

0

400

800

1200

1600

0 25 50 75

Co

ulo

mb

ic E

ffic

ien

cy (

%)

CO

D (m

g/L

)

Anolyte salt concentration increase(NaCl equivalent, mM)

COD in COD outCOD out_diff CECE_diff

32

To test the hypothesis that substrate gradients within the anode chamber were producing

different current production by the anodes, the anolyte was recycled from the bottom to the top of

the reactor at 1 mL/min. Anolyte recirculation successfully balanced the contribution of current

from each anode (Fig 8a) and increased total current generation by 7.9 ± 2.2% based on total

coulombs recovered in the circuit. The COD removal increased to 86 ± 4% (recycle) from 78±

3% (batch). Recirculation minimized the difference in anode potentials between electrodes and

had a minimal effect on cathode potentials (Fig A4).

Figure 8. (A) Current density contribution from each anode with anolyte recycle and (B) in

batch-fed mode. The electrodes are ordered from the bottom (1) to the top anode (4).

3.4 Discussion

Catholyte effluent addition increased desalination and substrate removal in the MDC by

delaying anode acidification, although a near-neutral anolyte pH was not sustained for the full

cycle. Batch fed addition of catholyte to the anolyte at the beginning of the cycle was only partly

successful in improving anode performance, as the initial current density was slightly inhibited

with 75 mM addition (6.10 A/m2) compared to the control (7.21 A/m

2), and adding too much

catholyte detached the biofilm. In future tests, an incremental addition of catholyte could be used

to better mitigate anolyte acidification throughout the whole cycle. This addition of catholyte over

33

the whole cycle would avoid a high initial pH, allowing for a greater total catholyte volume to be

used. For incremental addition to be effective, the catholyte must have a high pH at an early stage

of the cycle. This could be done by using a large cathode surface area to volume ratio.

Performance would not improve indefinitely with greater catholyte addition, as Cl– ions from the

adjacent diluate stream, with the addition of high concentrations of NaCl from the catholyte,

could also inhibit microbial activity.

COD removal and number of recovered coulombs increased with catholyte effluent

addition, but the approach used here did not take into account substrate dilution resulting from

catholyte addition. The initial substrate concentration (1 g/L sodium acetate) was kept constant in

order to minimize the number of variables in the system during experiments. The addition of

catholyte would have decreased the substrate concentration by 6% (25 mM), 10% (50 mM), and

15% (75 mM catholyte addition). Small changes in the initial concentrations of COD (>850 mg

COD/L) would have had little effect on power generation, but at lower COD concentrations

power can decrease substantially with the COD concentration [26, 28, 29]. Substrate

concentration is therefore only a factor at lower COD concentrations. At high COD

concentrations, other factors are more important for maximizing current production such as pH

and conductivity. Substrate dilution could therefore have an impact on performance of the MDC

with either incremental or continuous addition of the catholyte to anolyte, in terms of reduced

cycle times, desalination rates, or extent of desalination.

Substrate losses by anion diffusion out of the anode chamber affected both the CEdiff and

COD removal. The CE is a measure of the ratio of recovered coulombs as current to the total

theoretical amount of coulombs that could be produced by the oxidized substrate. Losses in CE

typically arise from the use of alternate electron acceptors by the bacteria on the anode, such as

oxygen, nitrate, or sulfate, or from the incomplete oxidation of a substrate. The CE typically is

calculated based only on current production, with the assumption that all of the other substrate

34

losses are due to other biological processes [23]. However, as shown in MDC tests here, there

was significant loss (11%) of substrate through physical diffusion through the membrane. When

an anion exchange membrane is placed adjacent to the anode chamber, transport of negatively

charged species, such as acetate anions, should be measured and accounted for in the COD

removal and CE calculations (eq. 2). In a two chamber MFC, substrate losses through the

membrane have also been show to decrease cathode electrode performance [30]. Without

considering substrate diffusion, total COD degradation will be overestimated, and CE under

estimated.

In considering scale up of MDCs, two important factors are the ratio of substrate to

diluate volumes, and the desalination rate. Many MDCs have been shown to achieve over 90%

desalination of saline water, but these extents of desalination have required 13 to 66 times more

anolyte volumes than the volume of desalinated water produced in the process (Fig 9). The use of

such large volumes of anolyte solutions is not practical for scale up due to high capital and

operational costs. A high level of desalination has also previously required over 48 hours [12, 24].

A long fed-batch cycle time results in a small rate of desalted water production [31, 32]. The

desalination rate produced here using the bench scale MDC was 22.5 g/L-d, with an anolyte

volume 1.4 times that of the diluate effluent. The only report of a similar desalination rate

required the use of 66 times more anolyte to desalination effluent [7]. Design aspects used here

that improved performance compared to these previous studies were: the rectangular shape of the

reactor, which minimized the possibility for dead zones in the stack; use of a larger concentrate

solution to decrease osmotic water losses from adjacent cells; use of thin electrodialysis cells to

minimize solution resistances; and choosing a more optimal cycle time, based on ending the cycle

at a time that maximized the extent of desalination, avoiding back diffusion of ions from the

concentrate to diluate chambers (Fig A5). The diluate volume in this operation was also easily

adjustable since diluate was collected in a separate container and recycled through the stack. If a

35

smaller diluate volume had been used, greater desalination would be expected.

Figure 9. Comparison of diluate solution to anolyte volume ratio and total desalination rate of

various studies. The (50) and (100) values for “this study” indicate PBS concentration (mM) of

the anolyte. An ideal MDC would be in the upper right hand corner: high diluate to anolyte ratio

and high total desalination rate.

Recycling the anolyte was also shown here to have a positive effect on MDC

performance. The average current output when the anolyte was recycled was 8% higher than that

obtained in batch mode operation (no recycle). While the exact reason for this change is not

known, current generation without recycle may have been a result in the development of a

substrate gradient in the anolyte chamber over time. It was observed that electrodes near the top

of the reactor produced less current than those at the bottom. Recycling anolyte from the bottom

to the top would have eliminated the development of substrate gradients in the anode chamber,

facilitated better mass transport of substrate to the biofilm, and increased ion transport into and

out of the biofilm. Transport of protons from the biofilm into solution can hinder microbial

activity on the anode due to the development of localized low pH [13]. Pumping fluid through the

anode chamber could have reduced concentration boundary layers around the anode, and

improved ion transport rates and therefore pH gradients in the biofilm. The recycle rate used here

1:35

1:2.2

1:1.4

1:5.9

1:66

1:3

1:1.3

1:30

10

20

30

40

50

60

0.00 0.20 0.40 0.60 0.80 1.00

Tota

l Des

alin

atio

n R

ate

(g/L

-d)

Ratio (Vdiluate : Vanolyte)

[8] [24]

this study (50) [11]

[7] [18]

this study (100) [16]

36

was low (1 mL/min), resulting in replacement of about 0.4 times the volume of the anolyte per

hour. However, increasing the recycle rate to 2 mL/min decreased current densities over 3 cycles,

for reasons which are not known (data not shown). Other studies have reported the use of much

higher recycle rates. For example, an upflow MFC recycled 3.5 and 12.5 times the volume of

anolyte per hour and reported 38% and 46% improvement in current generation, respectively

[33]. The results obtained here suggest that reactor performance can be improved using only

minimal recirculation.

3.5 Acknowledgments

The authors would like to thank Siemens Corp. for kindly donating an electrodialysis

reactor as a design reference for this project, Hiroyuki Kashima for his help with ion

chromatography and Dr. Xiuping Zhu for conducting the HPLC analysis. This research was

supported by Award KUS-11-003-13 from the King Abdullah University of Science and

Technology (KAUST).


1. WHO, Progress on Drinking Water and Sanitation, 2012, World Health Organization

and UNICEF.

2. Vorosmarty, C., McIntyre, P., Gessner, M., Dudgeon, D., Prusevich, A., Green, P.,

Glidden, S., Bunn, S., Sullivan, C., Liermann, C., Davies, P., Global threats to human

water security and river biodiversity. Nature, 2010. 467: p. 555.

3. Oki, T., Kanae, S., Global hydrological cycles and world water resources. Science, 2006.

313: p. 1068.

4. Zhou, Y., Tol, R., Evaluating the costs of desalination and water transport. Water

Resour. Res., 2005. 41(3): p. 1-10.

5. Christen, K., Environmental costs of desalination. Environ. Sci. Technol., 2007. 41: p.

5579.

6. Elimelech, M., Phillip, W., The future of seawater desalination: energy, technology, and

the environment. Science, 2011. 333: p. 712.

37

7. Cao, X., Huang, X., Liang, P., Xiao, K., Zhou, Y., Zhang, X., Logan, B., A new method

for water desalination using microbial desalination cells. Environ. Sci. Technol., 2009.

43: p. 7148-7152.

8. Chen, X., Xia, X., Liang, P., Cao, X., Sun, H., Huang, X., Stacked Microbial

Desalination Cells to Enhance Water Desalination Efficiency. Environ. Sci. Technol.,

2011. 45: p. 2465-2470.

9. Kim, Y., Logan, B., Series assembly of microbial desalination cells containing stacked

electrodialysis cells for partial or complete seawater desalination. Environ. Sci.

Technol., 2011. 45: p. 5840-5845.

10. Mehanna, M., Kiely, P., Call, D., Logan, B., Microbial electrodialysis cell for

simultaneous water desalination and hydrogen gas production. Environ. Sci. Technol.,

2010. 44: p. 9578-9583.

11. Luo, H., Jenkins, P., Ren, Z., Concurrent desalinationa and hydrogen generation using

microbial electrolysis and desalination cells. Environ. Sci. Technol., 2011. 45: p. 340-

344.

12. Luo, H., Xu, P., Roane, T., Jenkins, P., Ren, Z., Microbial desalination cells for

improved performance in wastewater treatment, electricity production, and desalination.

Bioresour. Technol., 2012. 105: p. 60-66.

13. Torres, C., Marcus, A., Rittmann, B., Proton transport inside the biofilm limits electrical

current generation by anode-respiring bacteria. Biotechnol. Bioeng., 2008. 100(5): p.

872-881.

14. Franks, A., Nevin, K., Jia, H., Izallalen, M., Woodard, T., Lovely, D., Novel strategy for

three-dimensional real-time imaging of microbial fuel cell communities: monitoring the

inhibitory effects of proton accumulation within the anode biofilm. Energy Environ. Sci.,

2009. 2: p. 113-119.

15. He, Z., Huang, Y., Manohar, A., Mansfeld, F., Effect of electrolyte pH on the rate of the

anodic and cathodic reactions in an air-cathode microbial fuel cell. Bioelectrochemistry,

2008. 74: p. 78-82.

16. Qu, Y., Feng, Y., Wang, X., Liu, J., Lv, J., He, W., Logan, B., Simultaneous water

desalination and electricity generation in a microbial desalination cell with electrolyte

recirculation for pH control. Bioresour. Technol., 2012. 106: p. 89-94.

17. Chen, X., Liang, P., Wei, Z., Zhang, X., Huang, X., Sustainable water desalination and

electricity generation in a separator coupled stacked microbial desalination cell with

buffer free electrolyte circulation. Bioresour. Technol., 2012. 119: p. 88-93.

18. Chen, S., Liu, G., Zhang, R., Qin, B., Luo, Y., Development of the microbial electrolysis

desalination and chemical-production cell for desalination as well as acid and alkali

productions. Environ. Sci. Technol., 2012. 46: p. 2467-2472.

19. Cheng, S., Liu, H., Logan, B.E., Increased performance of single-chamber microbial fuel

cells using an improved cathode structure. Electrochem. Commun., 2006. 8: p. 489-494.

20. Cheng, S., Xing, D., Call, D., Logan, B., Direct biological conversion of electrical

current into methane by electromethanogenesis. Environ. Sci. Technol., 2009. 43(10): p.

3953-3958.

21. Ahn, Y., Logan, B., Domestic wastewater treatment using multi-electrode continuous

flow MFCs with a separator electrode assembly design. Appl. Microbiol. Biotechnol.,

2013. 97: p. 409-416.

22. Bennett, A.S., Conversion of in situ measurements of conductivity to salinity. Deep -Sea

Res., 1976. 23: p. 157-165.

23. Logan, B., Microbial Fuel Cells2008, Hoboken, New Jersey: John Wiley & Sons, Inc.

38

24. Jacobson, K., Drew, D., He, Z., Use of a liter-scale microbial desalination cell as a

platform to study bioelectrochemical desalination with salt solution or artificail

seawater. Environ. Sci. Technol., 2011. 45: p. 4652-4657.

25. Patil, S., Harnisch, F., Koch, C., Hubschmann, T., Fetzer, I., Carmona-Martinez, A.,

Muller, S., Schroder, U., Electroactive mixed culture derived biofilms in microbial

bioelectrochemical systems: The role of pH on biofilm formation, performance and

composition. Bioresour. Technol., 2011. 102: p. 9683-9690.

26. You, S., Zhao, Q., Zhang, J., Jiang, J., Wan, C., Du, M., Zhao, S., A graphite-granule

membrane-less tubular air-cathode microbial fuel cell for power generation under

continuously operational conditions. J. Power Sources, 2007. 173: p. 172-177.

27. Karra, U., Troop, E., Curtis, M., Scheible, K., Tenaglier, C., Patel, N., Li, B.,

Performance of plug flow microbial fuel cell (PF-MFC) and complete mixing microbial

fuel cell (CM-MFC) for wastewater treatment and power generation. Int J Hydrogen

Energ, 2013. 38(13): p. 5383-5388.

28. Jiang, D., Li, B., Granular activated carbon single-chamber microbial fuel cells (GAC-

SCMFCs): a design suitable for large-scale wastewater treatment processes. Biochem.

Eng. J., 2009. 47: p. 31-37.

29. Cheng, S., Logan, B., Increasing power generation for scaling up single-chamber air

cathode microbial fuel cells. Bioresour. Technol., 2011. 102: p. 4468-4473.

30. Harnisch, F., Wirth, S., Schroder, U., Effects of substrate and metabolite crossover on the

cathodic oxygen reduction reaction in microbial fuel cells: Platinum vs. iron(II)

phthalocyanine based electrodes. Electrochem. Commun., 2009. 11: p. 2253-2256.

31. Mehanna, M., Saito, T., Yan, J., Hickner, M., Cao, X., Huang, X., Logan, B., Using

microbial desalination cells to reduce water salinity prior to reverse osmosis. Energy

Environ. Sci., 2010. 3: p. 1114-1120.

32. Kim, Y., Logan, B., Microbial desalination cells for energy production and desalination.

Desalination, 2013. 308: p. 122-130.

33. Zhang, F., Jacobson, K., Torres, P., He, Z., Effects of anolyte recirculation rates and

catholytes on electricity generation in a litre-scale upflow microbial fuel cell. Energy

Environ. Sci., 2010. 3: p. 1347-1352.

Chapter 4

Conclusions

Adding catholyte effluent to the anolyte successfully increased MDC performance in

terms of: extent of desalination and desalination rate (46% improvement); coulombs recovered

(36%); COD removal (24%); and coulombic efficiency, CEdiff (17%). Acetate was lost by

diffusion from the anode chamber through the adjacent ion exchange membrane into the adjacent

diluate chamber (0.11 g/L). This acetate loss should be included in COD and coulombic balance

calculations when an anion exchange membrane is placed next to the anode chamber. Recycling

anolyte balanced the performance of the four anodes connected in parallel, and increased current

generation (8%) and COD removal (10%). Catholyte effluent could be a useful resource for

mitigating anolyte acidification in MDCs.

Chapter 5

Future Work

There are several other aspects of catholyte effluent addition that should to be addressed

to assess the practicality of this approach and improve performance in the system, including:

1) The effect of substrate dilution on desalination performance and power generation.

Instead of keeping the initial substrate concentration constant, there should be a

change in initial COD concentrations depending on the volume of cathode solution

added to the anolyte.

2) Incremental addition of catholyte could also be used instead of batch addition at the

beginning of the cycle. The anode and cathode chamber pH values should also be

continuously monitored to determine the best time intervals for catholyte addition.

An energy balance for the electricity required for pumping compared to the

additional desalination and power generation from the system would be of interest.

3) The anodes could be acclimated under higher flow rate conditions for longer than

tested previously in this system (>3 cycles). The effect of higher recirculation rates

on the potential and current generation of each anode along the reactor could be

studied. Different flow paths could also be created to optimize substrate and ion

transport into the biofilm.

The reactor design could also be modified by adding another AEM next to the anode

chamber, to have 4 concentrate chambers and 3 diluate chambers in the ED stack. Following the

same procedures as outlined in this study, hydroxide being produced at the cathode would pass

into the concentrate stream, raising the pH of the concentrate through the desalination cycle. The

41

AEM between the concentrate and anode chamber would facilitate hydroxide transport into the

anolyte. Hydroxide ions will be more readily transported into the biofilm with a high anolyte

recirculation rate to more effectively balance anolyte pH. The optimal ED stack flowpath would

connect the concentrate cells next to the anode and cathode chambers in order to maximize the

concentration and likelihood of hydroxide ions transporting into the anode solution.

Appendix

Supplementary Information

Figure A1. This shows the method for making the anolyte for “catholyte addition” solutions.

The amount of PBS for each experimental condition was constant, while the amount of DI water

and catholyte effluent used to dilute the solution to 50 mM PBS was adjusted according to the

amount of salt concentration increase desired. The anolyte conductivity for each experimental

condition (with catholyte addition) was equal to the anolyte conductivity of the control solution

plus a given amount of NaCl addition (25 mM, 50 mM, or 75 mM).

0%

25%

50%

75%

100%

0 25 50 75

Ano

lyte

com

posi

tion

(%

)



Catholyte

DI water

200 mM PBS

43

Figure A2. The average power density calculated during operation over one desalination cycle at

10 Ω external resistance, using various amounts of catholyte addition.

149

199

265

318

0

50

100

150

200

250

300

350

0 25 50 75Ave

rag

e P

ow

er

De

nsi

ty (

mW

/m2

)



44

Figure A3. (A) Current density as a function of catholyte addition using 100 mM PBS. Current

went to zero with 200 mM catholyte addition and did not recover in subsequent cycles. (B) Initial

anolyte pH for the various amounts of catholyte effluent addition.

0

1

2

3

4

5

6

7

8

9

0 2 4 6

Cur

rent

Den

sity

(A/m

2)

cycle time (hours)

0mM50mM100mM150mM200mM

6.0

7.0

8.0

9.0

10.0

0mM 50mM 100mM 150mM 200mM

pH

Anolyte salt concentration increase (NaCl equivalent, mM) at 100 PBS

50 PBS

100 PBS

45

Figure A4. (A) Electrode potentials in 1 mL/min anolyte recirculation mode. The cathode

potentials are on the top graph and anode potentials on the bottom. (B) Electrode potentials in

anolyte batch-fed mode. The electrodes are ordered by placement in the reactor from bottom (1)

to the top electrode (4). Note: the top cathode electrode potentials are not shown because of a

malfunction in the reference electrode.

0.0

0.1

0.2

0.3

0 2 4 6 8 10

Cat

ho

de

Po

ten

tial

(V

)

321

0.0

0.1

0.2

0.3

0 2 4 6 8 10

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 2 4 6 8 10

An

od

e P

ote

nti

al (

V)

cycle time (hours)

4321

-0.5

-0.4

-0.3

-0.2

-0.1

0

0 2 4 6 8 10

cycle time (hours)

46

Figure A5. (A) Conductivity of diluate solution as a function of time for various volumes of

catholyte effluent addition. The cycle time was set for when the diluate conductivity was at a

minimum, which was at about 10 hours for all conditions. (B) The diluate concentration increased

when osmotic water loss into the adjacent concentrate chamber was greater than ionic separation

from current generation. Current generation decreased along the cycle and reverse desalination is

shown at the end of this cycle.

40

42.5

45

47.5

50

52.5

55

0 2 4 6 8 10

Co

nd

uct

ivit

y (m

S/cm

)

time (hours)

0mM25mM50mM75mM

35

40

45

50

55

0 5 10 15 20 25

Co

nd

uct

ivit

y (m

S/cm

)

time (hours)

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