Sustainable desalination using a microbial capacitive desalination cell

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Cite this: Energy Environ. Sci., 2012, 5, 7161

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Sustainable desalination using a microbial capacitive desalination cell†

Casey Forrestal,a Pei Xub and Zhiyong Ren*a

Received 15th January 2012, Accepted 13th March 2012

DOI: 10.1039/c2ee21121a

Microbial desalination cells (MDCs) use the electrical current generated by microbes to simultaneously

treat wastewater, desalinate water, and produce bioenergy. However, current MDC systems transfer

salts to the treated wastewater and affect wastewater’s beneficial use. A microbial capacitive

desalination cell (MCDC) was developed to address the salt migration and pH fluctuation problems

facing current MDCs and improve the efficiency of capacitive deionization. The anode and cathode

chambers of the MCDC were separated from the middle desalination chamber by two specially

designed membrane assemblies, which consisted of cation exchange membranes and layers of activated

carbon cloth (ACC). Taking advantage of the potential generated across the microbial anode and the

air-cathode, the MCDC was capable of removing 72.7 mg total dissolved solids (TDS) per gram of

ACC without using any external energy. The MCDC desalination efficiency was 7 to 25 times higher

than traditional capacitive deionization processes. Compared to MDC systems, where the volume of

concentrate can be substantial, all of the removed ions in the MCDC were adsorbed in the ACC

assembly double layer capacitors without migrating to the anolyte or catholyte, and the electrically

adsorbed ions could be recovered during assembly regeneration. The two cation exchange membrane

based assemblies allowed the free transfer of protons across the system and thus prevented significant

pH changes observed in traditional MDCs.

Introduction

The increasing awareness of the water-energy nexus is compelling

the development of technologies that reduce energy requirements

during water treatment as well as water demands for energy

production.1,2 Microbial desalination cells (MDCs) recently

aDepartment of Civil Engineering, University of Colorado Denver, Denver,CO 80004, USA. E-mail: [email protected]; Tel: +1 (303)556-5287bCivil and Environmental Engineering, Colorado School of Mines, Golden,CO 80401, USA

† Electronic supplementary information (ESI) available: Two additionalfigures are included. See DOI: 10.1039/c2ee21121a

Broader context

Traditional desalination technologies are energy intensive and gen

desalination cell (MCDC) developed in this study presents an adv

removal and management without using any external energy. The M

with microbial desalination cells (MDCs) including salt migration a

assemblies for capacitive adsorption of ions, the MCDC increases d

(CDI). It is also the first reactor to remove ions from the anode, c

capability in simultaneous salt management, wastewater treatment,

MCDC regeneration also makes salt production possible. The p

directions to the development of sustainable water and energy syst

This journal is ª The Royal Society of Chemistry 2012

emerged as a promising technology to simultaneously treat

wastewater, desalinate saline water, and produce renewable

energy such as electricity or hydrogen gas.3–10 MDCs share the

same principle of bioelectrochemical reactions with microbial

fuel cells (MFCs): electrochemically active bacteria in the anode

chamber oxidize biodegradable substrates and generate electron

flow (i.e. current) to reduce the electron acceptors in the cathode

chamber. The additional desalination function can be achieved in

an MDC by adding a middle chamber containing saline water

and utilizing the anode–cathode potential difference to drive the

migration of anions (e.g., Cl�) to the anode chamber and cations

(e.g., Na+) to the cathode chamber for charge neutrality.3 The

MDC process carries great potential in desalination systems,

erate large amounts of concentrate. The microbial capacitive

anced bioelectrochemical approach to achieve sustainable salt

CDC systematically addressed challenges currently associated

nd pH fluctuation problems. Using high surface area electrode

esalination efficiency by 7–25 times over capacitive deionization

athode, and desalination chamber, which enhances the reactor

and energy production. The nearly full recovery of salt during

roof-of-concept MCDC provides valuable insights and new

ems to address the water-energy nexus.

Energy Environ. Sci., 2012, 5, 7161–7167 | 7161

http://dx.doi.org/10.1039/c2ee21121a






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Fig. 1 Diagram of MCDC reactor configuration and operation. Two

CEM-ACC assemblies were used to separate the three chambers and

capture the removed salts, as well as allow the free transfer of protons.

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because it can either be used as a stand-alone process or serve as

a pretreatment for conventional desalination processes such as

reverse osmosis (RO) to reduce the salt concentration of RO feed

and minimize energy consumption and the membrane fouling

potential. Current desalination technologies, such as RO and

electrodialysis (ED), are energy and capital intensive. Even the

most advanced large scale seawater RO units require 3–7 kW h

m�3 for water desalination, while conventional multi-stage flash

evaporation requires 68 kW h m�3.11 In contrast, the MDC

system is considered to be an energy gaining process because it

converts the biochemical energy stored in wastewater to elec-

tricity or hydrogen gas. Lab scale MDC studies showed that 180–

231%more energy can be recovered as H2 than the reactor energy

input when desalinating 5–20 g L�1 NaCl solutions,4,6 and

a recent study calculated that a litre-scale MDC can produce up

to 58% of the electrical energy required by downstream RO

systems.8

Current MDC systems use an anion exchange membrane

(AEM) to separate the anode and middle chamber, and a cation

exchange membrane (CEM) to separate the cathode and middle

chamber. Similar to electrodialysis, desalination in MDC is

achieved by direct transport of salts from the middle chamber to

the anode and cathode chamber. This system faces two main

problems. While salts get removed from the middle chamber,

they become concentrated in the anode and cathode chambers,

resulting in an increase of the volume of saline water. This

concern becomes more imperative when wastewater is treated as

the anolyte. Although the addition of ions (or total dissolved

solids, TDS) increases wastewater conductivity and benefits

electricity generation, the increased salinity may affect effluent

water quality and prevent subsequent beneficial use of treated

wastewater.12,13 The high salinity may also affect wastewater

treatment efficiency in MDCs because studies showed that high

chloride concentration is inhibitory to biological treatment,

especially for nutrient removal.14 In addition, the AEM between

the anode and middle chamber inhibits the free transfer of H+

accumulated in the anolyte to other chambers, which causes

a significant pH drop in the anode chamber and pH increase in

the cathode chamber.15,16 A previous study showed that the pH

of the wastewater anolyte dropped to 4.2 in one batch cycle if no

buffer was added to the anolyte.6 Additionally, the catholyte pH

could increase to 11–13 due to the loss of H+.6,16 This pH fluc-

tuation significantly inhibits bioelectrochemical reaction effi-

ciency and reduces system performance.

In order to modulate the movement of salts to the anode and

cathode chambers, the concept of capacitive deionization

(CDI)17 was incorporated in this study to develop a sustainable

desalination system called a microbial capacitive desalination cell

(MCDC). In the proof-of-concept MCDC, salt water can be

deionized through electrochemical ion adsorption driven by the

electrical field generated by microorganisms. Two activated

carbon cloth (ACC) membrane assemblies were designed to

connect with the anode and cathode and adsorb ions from water.

During desalination, the ions are stored in the electrical double

layer capacitors between the solution and the ACC assembly

interfaces, thus preventing the salinity increase in treated

wastewater. After the ACC is saturated with adsorbed ions, the

assembly can be regenerated by removing the electrical potential

and the retained salts can be fully recovered in situ for disposal or

7162 | Energy Environ. Sci., 2012, 5, 7161–7167

further salt recovery. Another innovative aspect of the MCDC,

as compared to conventional MDC, is the use of a second CEM

in lieu of AEM between the anode and desalination chamber

(Fig. 1). This configuration allows cations and protons to move

freely from the anode chamber throughout the reactor and

therefore maintains electrochemical neutrality and prevents pH

fluctuation. In this study, the proof-of-concept MCDC devel-

opment and operation are demonstrated, and its advantages over

current systems and application potentials are discussed.

Materials and methods

MCDC reactor design

The MCDC reactors consisted of three polycarbonate cube-

shaped blocks with 3 cm diameter holes forming an internal

anode, cathode, and desalination chamber volume of 23 mL,

27 mL, and 10 mL respectively. The anode and cathode cham-

bers had a length of 4 cm, while the desalination chamber had

a length of 1.5 cm. The anode electrode was a graphite brush

(Golden Brush, CA) and was pretreated by washing in acetone

and heating to 350 �C for 30 minutes.18 Traditional air-cathodes

were made by applying 10% Pt/C (0.5 mg cm�2) and four PTFE

diffusion layers on 30% wet-proofed carbon cloth as previously

described.19 The desalination chamber was separated from the

anode and cathode chamber by two assemblies. Each assembly

was constructed by placing a cation exchange membrane (CMX-

SB, Astom Corporation, Japan), a Ni/Cu mesh current collector

(McMaster Carr, IL), and 3 layers of Zorflex� activated carbon

cloth (ACC, Chemviron Carbon, UK) together. Additionally,

the CEM faced the anode/cathode chamber to prevent microbial

growth on the assembly. The total weight of the ACC was 1 gram

with the specific surface area of 1019.8 m2 g�1, determined by the

Brunauer–Emmet–Teller (BET) method (ASAP 2020, Micro-

meritics, Norcross, GA).20 The ACC assemblies were connected

to the anode/cathode by titanium wires (Fig. 1).

MCDC operating conditions

Two reactors were inoculated with anaerobic sludge from the

Englewood-Littleton Wastewater Treatment Plant (Englewood,

CO) and operated in fed-batch MFC mode. When a repeatable



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voltage profile was obtained for consecutive batch cycles, the

reactors were shifted to fed-batch MCDC mode by inserting

a pair of assemblies and adding one middle chamber as described

previously. The anolyte growth media contained per litre: 1.6 g

NaCH3COO, 0.62 g NH4Cl, 4.9 g NaH2PO4$H2O, 9.2 g

Na2HPO4, 0.26 g KCl, and 10 mL trace metals and 10 mL

vitamin solution.21 The catholyte contained per litre: 10 g KCl,

0.68 g KH2PO4, 0.87 g K2HPO4. Potassium chloride was used in

the cathode chamber to differentiate with sodium transport and

monitor the movement of cations from the cathode to the

desalination chamber. The salt solution in the desalination

chamber contained per litre: 10 g NaCl, 0.49 g NaH2PO4$H2O,

0.92 g Na2HPO4. A small amount of buffer was added to the salt

solution to some extent mimic the 300 to 700 mmol kg�1 natural

buffering capacity of seawater and prevent potential scaling at

high pH values.22

Two experimental procedures and two controls were per-

formed to investigate the desalination performance of the

MCDC system. The first experiment investigated simultaneous

physical and electrical adsorption capacity by directly adding salt

solution into the desalination chamber equipped with ACC

assemblies free of adsorbed ion. When the anode and cathode

electrodes were connected to the ACC assemblies, physical and

electrical adsorption on the ACC assemblies could occur

concurrently. The second experimental procedure investigated

only electrical adsorption capacity. Electrical adsorption

capacity of the ACC assemblies was determined by first adding

salt solution to the desalination chamber to allow complete

physical adsorption. Electrical adsorption was then characterized

by replacing the desalted solution with fresh solution, and con-

necting the two assemblies to the anode and cathode, respec-

tively. Any residual water from previous experimental washing

would have been removed when the salt solution was replaced.

Abiotic control experiments were performed by using new

brush anodes without bacterial acclimation. The first control

experiment measured the physical adsorption capacity by short

circuiting the assemblies to ensure no charge was formed across

the electrodes. The adsorption capacity of the assemblies was

defined as the change in initial and final salt concentration. The

second control investigated the electrical adsorption capacity by

first allowing complete physical adsorption to occur then by

connecting the assemblies to an external power supply at

a voltage of 0.53 V to simulate the voltage generated by

a microbial fuel cell. TheMDC control experiment used an anion

exchange membrane next to the anode chamber (Astom

Corporation, Japan) and a CEM next to the cathode chamber

without ACC assemblies in the desalination chamber. An

external resistor of 1000 Ohms was used between the anode and

cathode electrodes, and all other experimental procedures were

identical to the MCDC experiments.

To regenerate the ACC assemblies in situ for all experiments,

the assemblies were either allowed to naturally regenerate or

were regenerated by applying an external voltage to increase the

rate of regeneration. The natural regeneration was performed by

disconnecting the anode and cathode from the assemblies and

creating a short circuit between the assemblies with an external

wire. Alternatively, an external voltage of 1 V in reverse polarity

was applied to the assemblies by a programmable power source.

The external voltage was applied for 5–10 minutes and followed


by short circuiting the ACC assemblies, as mentioned above, for

20–30 minutes. When the potential difference reached �0.5 mV,

the ACCs were assumed to be regenerated, meaning that any

electrically adsorbed ions should have been removed from the

electrodes. After regeneration, all electrolytes were emptied and

washed with deionized (DI) water to remove any residual salt

remaining in the chambers before starting a new batch cycle.

Analysis and calculations

Conductivity and pH were measured for all three chambers using

a conductivity meter and pH meter (HACH Co., CO). The

change in the reactor’s internal resistance was determined

through electrochemical impedance spectroscopy (EIS) tests

using a Potentiostat. EIS measurements were performed by using

the anode as the working electrode, the cathode as the counter

electrode, and a saturated Ag/AgCl reference electrode placed in

the anode chamber. Results were fitted into equivalent circuit

models developed in our previous EIS studies and plotted using

Nyquist plots where the ohmic resistance is defined as the

intercept of the Zreal axis.21 Samples of all three chambers were

collected before and after desalination, and after regeneration.

Ion concentrations were measured using the Optima 3000

Inductive Coupled Plasma (ICP) Spectrometer (Perkin Elmer,

CT) and Dionex DC80 ion chromatography system (IC) (Dio-

nex, CA). Using the data from the IC and ICP, the mass balance

of the major ions was determined by summing the concentrations

of the ions in each chamber initially, after desalination, and after

recovery of the salts. Internal power used was calculated using

the following equations:

P ¼ðV 2

Rdt (1)

R ¼ rL

A(2)

where P is power in terms of watt hours, V is the voltage, R is the

resistance, r is resistivity, L is length of the resistance, and A is

the cross-sectional area. Comparisons between the MCDC and

CDI were made based off either presented data or estimations

from figures in published papers. Comparison to membrane

capacitive deionization (MCDI) was not conducted due to the

incompatibility in methodology to the MCDC.

Results and discussion

Reactor desalination performance

During MCDC operation, an electrical potential was generated

across the microbial anode and air-cathode and applied to the

two ACC assemblies to form a double layer capacitor23–30

(Fig. 1). The formation of the double layer capacitor has been

fully modeled using the Gouy Chapman–Stern theory.29 The

potential drives the ions to move from the salt solution and

adsorb on the activated carbon cloths. The ion adsorption can be

observed proportional to the charge formed between the ACC

assemblies (Fig. 2). Fig. 2 shows that, in repeated batch cycles,

when the potential on the assemblies increases from 0 to more

than 530 mV in each cycle, the solution conductivity in the

desalination chamber decreased by 12–18%, from 18 mS cm�1 to



Fig. 2 The correlation between the charge potential across the ACC

assemblies and the conductivity changes in the desalination chamber due

to electrical adsorption. Arrows indicate changes in electrolyte solution in

batch cycles.

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below 16 mS cm�1. The desalination rate was the greatest at the

beginning of each cycle and then decreased gradually, suggesting

the adsorption capacity of the ACC assemblies decreased along

with the increased amount of salt that had been adsorbed in the

assemblies. Salt removal was characterized by both conductivity,

measured using a conductivity meter, and total dissolved solids

(TDS) concentration, measured by IC and ICP (Table 1).

Through simultaneous physical and electrical adsorption, the

MCDC removed 26.9% of the conductivity or 25.5% of TDS

from the desalination chamber in one batch cycle. In addition,

a small percentage of salt was removed from the anolyte (4.4%)

and catholyte (10.4%) as well. This is likely due to the ions that

were driven across the membranes by the electrical potential of

the ACC assemblies from the anode and cathode chamber and

then adsorbed onto the ACC. Further experiments showed that

electrical adsorption alone removed 22.3% TDS from the desa-

lination chamber, which contributed up to 88% of the TDS

removal compared to the combined physical and electrical

adsorption experiments.

Table 2 compares the normalized TDS removal between the

MCDC and CDI studies. The results showed that for the same

amount of adsorptive material (ACC), the MCDC improved

TDS adsorption by 7–25 times. Both MCDC and CDI use an

electric field between two electrodes that electrochemically

adsorb ions, but the high adsorption from the MCDC may be

attributed to the unique feature of the MCDC that uses the

internal potential generated by microorganisms. This in situ

Table 1 Salt removal in terms of conductivity and total dissolved solids in t

Physical/electrical adsorption

Desalinationchamber

Anodechamber

% Removal in conductivity 26.9 � 5.1 13.1 � 3.8% Removal in TDS 25.2 � 3.6 4.4 � 3.6Total TDS adsorption(mg TDS per g ACC)


approach avoided the use of an external power supply and circuit

and reduced transportation energy loss, so it demonstrated

higher efficiency than traditional CDI processes. The salt

adsorption rate inMCDC, however, is lower than published CDI

studies, and that is mainly due to the low kinetics of the fed-batch

operation and the limited amount of ACC available for ion

adsorption. In this study, theMCDC configuration was modified

from traditional cubic type MDCs, which only allowed for

a total of 1 g activated carbon cloth being used in the assembly.

This may explain why the amount of salt removed in the desa-

lination chamber was relatively small. It was calculated that the

amount of salt added in the desalination chamber (114 mg TDS)

was drastically beyond the control electrical adsorption capacity

of the ACC (8.5 mg TDS for the 1 g ACC applied). Moreover,

compared to CDI systems that consume 0.21–1.78 Watt hour

external energy to generate the potential to remove 1 g TDS, the

MCDC system does not use any external energy but instead

utilized the in situ potential difference between the ACC assem-

blies generated during microbial activities. It was calculated that

the MCDC reactor saved 2.18 Watt hour for 1 g of TDS

removed. That is why in Table 2 the net energy used for the

MCDC is negative, indicating that 2.18 W h g�1 TDS removed

was not required, while for the CDI systems an external energy of

0.2–1.78 W h is required for removal of 1 g TDS. While the

MCDC reactor directly uses generated current for desalination,

it is possible for electricity to be generated by applying an

external load across the ACC assemblies during regeneration.

Reactor configuration optimization is underway to increase the

ACC loading and further improve desalination efficiency.

Sodium, chloride, potassium, and phosphate accounted for

greater than 85% of the TDS, and their specific concentration

changes in the three chambers are shown in Fig. 3. In addition to

direct capacitive electrical adsorption that caused concentration

decreases in the desalination chamber, a small amount of charged

ions migrated from the anode and cathode chamber to the

desalination chamber due to the electrical potential or concen-

tration gradient. However, the desalination efficiency for the

anode and cathode chambers is low compared to the salt removal

in the desalination chamber due to the lack of electrical double

layer adsorption and the inhibited anion transfer across cation

exchange membranes. Results in Table 1 showed that saline

water can also be used as the catholyte and partially desalinated.

Further desalination can be achieved by feeding the treated

catholyte to the subsequent reactor’s desalination chamber.

The reactor’s internal resistance as measured by EIS at the

beginning of the desalination cycle was on average 8.5 Ohms.

After desalination, the internal resistance increased to an average

he MCDC

Electrical adsorption

Cathodechamber

Desalinationchamber

Anodechamber

Cathodechamber

5.6 � 4.4 10.0 � 0.2 10.6 � 3.5 �2.0 � 2.710.4 � 3.6 22.3 � 3.6 7.6 � 3.6 2 � 3.672.7 50.7



Table 2 Desalination efficiencies in the MCDC and CDI reactorsa

Method Electrode materialsElectrodedistance (mm)

Net W h/g TDSremoved

mg TDS/g adsorptivematerial Reference #

MCDC Activated carbon cloth 15 �2.18 50.74 This paperCDI Carbon aerogel 2.3 +0.21 7.00 17CDI Activated carbon powder NA +1 1.95 24CDI Activated carbon powder 0.1 +1.78 2.88 25CDI Activated carbon powder 0.1 +1.68 3.11 26CDI Activated carbon powder with

mesoporous carbon black0.22 NA 3.82 27

CDI MnO2/nanoporous carbon composite NA NA 0.10 28CDI Activated carbon cloth NA +0.52 NA 30CDI Activated carbon cloth with titania NA NA 4.38 34

a NA ¼ not available.

Fig. 3 The concentration changes of the four major ions (potassium,

sodium, chloride, phosphate) before and after one typical batch cycle of

MCDC operation. Fig. 4 Mass balance of the four major ions (potassium, sodium, chlo-

ride, phosphate) in the MCDC reactor before and after regeneration.Dow

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of 13 Ohms (ESI†). The change in conductivity in the desalination

chamber correlated closely with the change in internal resistance

for the reactor over the course of desalination. The MCDC

reactor’s ability to transfer electrons was not inhibited as occurs

over the course of desalination in standard MDCs. It is theorized

that this is due to the MCDC’s ability to maintain charge

neutrality better than MDC reactors. In standardMDC reactors,

charge neutrality is reached by ion migrating out of the desali-

nation chamber, while in the MCDC reactor charge neutrality is

performed by ion migrating through the entire reactor.

Assembly regeneration and salt recovery

The ion saturated ACC assemblies were regenerated using two

approaches. The natural regeneration was accomplished by

directly connecting the two assemblies in short circuit. The

electrical potential across the assemblies was dissipated with

the adsorbed salts being released back into solution. When the

potential difference across the ACC assemblies reached

�0.5 mV, it was assumed that the ACC assemblies were regen-

erated with complete electrical salt desorption. The regeneration

rate can be significantly increased by connecting the assemblies

to an external power supply of 1 V with reverse polarity to

facilitate ion desorption (ESI†). Fig. 4 shows that among the four

major ion species, almost all of the electrical adsorbed salts were


recovered during assembly regeneration, shown as a direct

correlation between the initial and recovered salt concentrations.

The capability of in situ regeneration of the ACC assemblies is

another advantage of the MCDC, because the assemblies can be

reused many times without investing significantly in materials.

Almost all of the adsorbed salts can be recovered in concentrates

during regeneration, and the recovered salts can be dewatered or

extracted for beneficial uses. Furthermore, MCDC stacks can be

developed and integrated with reverse electrodialysis (RED) to

capture the energy generated due to the salinity gradient across

the concentrate and freshwater.31,32 The current MCDC is

operated in batch mode, and the desalination and regenerated

processes were conducted sequentially. More efficient operation

can be achieved by connecting multiple reactors in series or in

parallel and operating them in complementary sequential batch

reactor (SBR) modes. While some of the units perform desali-

nation, others conduct assembly regeneration at the same time.

This operation not only provides a continuous flow of produced

freshwater but also allows for the direct usage of the electricity

produced from regeneration units for desalination units.

Reduced pH fluctuation

Fig. 5 shows the change in pH units among the three chambers

over one typical batch cycle for both the MCDC and the control



Fig. 5 The change in pH for the MCDC anode, cathode, and desali-

nation chambers compared to the control MDC in one batch cycle. The

initial pH values in the chambers of MDC orMCDC all ranged 7.0� 0.2.

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MDC. The initial pH values in the chambers were all within 7.0�0.2. The change in pH for the anode chamber in both the MCDC

and the MDC was relatively small, with a drop in pH of between

0.2 and 0.5 pH units, which was presumably attributed to the

high buffering capacity of the anolyte. However, the catholyte

had drastically different results between the MCDC and MDC,

with the MCDC increasing in pH on average 1.5 pH units and

the MDC increasing 4.4 pH units. Interestingly the change in pH

for the desalination chamber for the MCDC is greater than for

the MDC control. Previous capacitive deionization studies

showed that water electrolysis may cause slight pH variation at

low voltages, which may explain the pH increase in the MCDC

desalination chamber.30 It is difficult to compare the MCDC

results directly with CDI studies, because no known CDI

experiments have been conducted at a set potential lower than

0.6 V.23–30 Further investigations should explore the cause of this

phenomenon.

Because the average percent change between the cathode and

desalination chamber were essentially the same, it is assumed

that the proton transfer capability of the reactor was not

inhibited. TheMCDC employs a CEM to separate the anode and

desalination chamber. This is different from the AEM used in

current MDCs and releases the pH fluctuations in the reactor. In

traditional MDCs, anions (Cl�) migrate from the desalination

chamber to the anode chamber to compensate for the accumu-

lation of H+, but because the AEM prevents the transfer of H+

out of the anode chamber, a decrease in pH is observed. By using

a CEM, the accumulated H+ not only can transfer to the desa-

lination chamber but also can transfer further to the cathode

chamber and therefore solves the pH change problem in the

entire MCDC reactor. Previous studies show that other ions such

as Na+ and K+ also play important roles in maintaining charge

balances across different chambers in microbial fuel cells,33 but

the majority of such ions are adsorbed in the ACC assemblies, so

electrolyte charge balance due to ion transfer is not a concern in

the MCDC.

Outlook

The integration of capacitive deionization with microbial desa-

lination provides a sustainable solution that not only addresses


the salt migration and pH fluctuation problems facing current

MDC systems, but also improves salt removal and energy effi-

ciency compared to CDI systems. Traditional MDCs remove

salts from the desalination chamber, but they also add TDS to

the anode and cathode chambers and may increase the volume of

saline water significantly, depending on different operation

configurations.3–10 The MCDC reactor demonstrated that desa-

lination can be accomplished in the middle chamber without

adding salts to the anolyte and catholyte, and therefore released

the concerns on the viability of wastewater treatment and reuse

due to increased TDS concentration. This proof-of-concept

system also demonstrates a microbial desalination reactor to

reduce salinity in all three chambers of the reactor. The MCDC

system offers a sustainable desalination, renewable energy

production, and wastewater treatment. To maximize the benefits

and prevent negative effects of salinity changes on the waste-

water anolyte, salt migration from the desalination chamber

could be modulated by constructing modular plate-shaped ACC-

membrane assemblies. If added salt is desired in wastewater to

improve the anolyte conductivity, regular MDC operation could

be performed. If salt should be prevented frommigrating into the

anode chamber, the modular ACC assembly plate can be inserted

into the reactor to perform salt adsorption. This system inte-

gration and operation will provide microbial desalination

systems with great flexibility in salt migration management as

well as better pH fluctuation control.

Despite the potential benefits offered by the MCDC system,

many challenges remain to be addressed based on the informa-

tion collected from this proof-of-concept study. In addition to

the low-cost material development that is required for all bio-

electrochemical systems, the adsorptive material can be

improved, such as with silica or titanium modification.34,35 The

reactor configuration needs to be optimized to provide more

ACC loading and improve diffusion rate and adsorption capa-

bility. Modular stack reactors and flexible operation strategies

need to be developed to maximize the integration of desalination

and assembly regeneration in multiple units, optimize water

recovery, and enhance salt migration management. Improve-

ments in MCDCs will also benefit from the continued advances

of other bioelectrochemical systems such as microbial fuel cells

and capacitive deionization, with the eventual goal of developing

a full scale sustainable system directed toward the integration of

multiple functions, such as extracting energy from wastewater

and water desalination.

Acknowledgements

This work was supported by the Office of Naval Research (ONR)

under Awards N0001410M0232. We thank Dr Peter Jenkins for

his suggestions and reviewers for their helpful comments.

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Sustainable desalination using a microbial capacitive desalination cell

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