Desalination 308 (2013) 115–121
Contents lists available at SciVerse ScienceDirect
Desalination
journal homepage: www.elsevier.com/locate/desal
Simultaneous removal of organic matter and salt ions from saline wastewater in
bioelectrochemical systems
Younggy Kim 1, Bruce E. Logan ⁎
Department of Civil and Environmental Engineering, 212 Sackett Building, The Pennsylvania State University, University Park, PA, 16802, USA
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
► Microbial salinity-reduction electrolysis
cells (MSCs) can treat saline wastewater.
► In MSCs, organic matter and salt ions
were removed simultaneously.
► Ion-exchange membranes were not
damaged for three months in MSCs.
► Exoelectrogens can generate high currents with salinities up to 36 g/L TDS.
a r t i c l e
i n f o
Article history:
Received 14 June 2012
Received in revised form 22 July 2012
Accepted 24 July 2012
Available online 17 August 2012
Keywords:
Saline wastewater
Ionic separation
Microbial electrolysis cells
Ion-exchange membranes
Bioelectrochemical systems
a b s t r a c t
A new bioelectrochemical system is proposed for simultaneous removal of salinity and organic matter. In this
process, exoelectrogenic microorganisms oxidize organic matter and transfer electrons to the anode, hydrogen is
evolved at the cathode by supplying additional voltage, and salt is removed from the wastewater due to the electric
potential generated and the use of two ion-exchange membranes. Salinity removal (initial conductivity ~40 mS/cm)
increased from 21 to 84% by increasing the substrate (sodium acetate) from 2 to 8 g/L. A total of 72–94% of the chemical oxygen demand was degraded in the anode and cathode chambers, with 1–4% left in the anode chamber and
the balance lost through the anion-exchange membrane into the concentrate waste chamber. The maximum hydrogen production rate was 3.6 m3-H2/m3-electrolyte per day at an applied potential of 1.2 V. The Coulombic efficiency was ~100%, while the cathode recovery varied from 57 to 100%, depending on the extent of
methanogenesis. Exoelectrogenic microbes generated high current densities (7.8 mA/cm2) at ≤36 g/L of total
dissolved solids, but >41 g/L eliminated current. These results provide a new method for achieving simultaneous
removal of salinity and organic matter from a saline wastewater with H2 production.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author. Tel.: +1 814 863 7908.
E-mail address: [email protected] (B.E. Logan).
1
Present address: Department of Civil Engineering, McMaster University, Hamilton,
ON, Canada.
0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.desal.2012.07.031
Saline wastewaters are generated by many industries, including food
processing plants, leather or tannery industries, and petroleum refineries
[1]. Saline wastewater contains salt ion concentrations that are usually
greater than 20 g/L and sometimes up to 150 g/L. The organic content
of these wastewaters can also be very high, with up to 70 g/L of chemical
116
Y. Kim, B.E. Logan / Desalination 308 (2013) 115–121
oxygen demand (COD) [1]. The treatment of saline wastewaters usually
occurs in two separate processes, with organic contaminants removed in
a biological process [1,2], and salinities reduced in physical and chemical
separation processes such as ion-exchange, electrodialysis, or reverse
osmosis [1]. Both processes are energy-demanding processes, and thus
are not sustainable.
Bioelectrochemcial systems, such as microbial fuel cells (MFCs) or
microbial electrolysis cells (MECs), can be used to remove organic contaminants in wastewater and produce valuable energy such as electrical
power or hydrogen gas. At the anode in MFCs or MECs, exoelectrogenic
bacteria oxidize organic matter and transfer electrons to the anode. At
the cathode, different reduction reactions can occur, such as oxygen reduction in MFCs or hydrogen evolution in MECs. These coupled redox
reactions create an electric field between the electrodes that can drive
ionic transport. Using ion-exchange membranes (IEMs), the driven
ionic transport can be manipulated to separate salt ions in the reactor.
Several new bioelectrochemical systems have been proposed to separate ions using IEMs for desalination of salty water [3–11], efficient recovery of salinity-gradient energy [12–15], and chemical production
[16,17]. In a microbial desalination cell (MDC), for example, water in a
middle chamber between the anode and cathode chambers can be partly or fully desalinated, while the wastewater in the anode chamber
gains salt ions.
A new type of IEM-based bioelectrochemical system, called a microbial saline-wastewater electrolysis cell (MSC), is proposed here
for simultaneous removal of organic matter and salt ions from saline
wastewater. These goals are different from that of an MDC, which is to
desalinate a separate stream of water. In an MSC, exoelectrogenic microbes on the anode degrade organic contaminants in saline wastewater, and hydrogen gas is produced at the cathode as done in an
MEC [18]. In addition, the electric field created by the exoelectrogens
is used to remove salt ions from the saline wastewater. This allows for
simultaneous wastewater treatment and salt removal in a process
that can generate net energy. Wastewater desalination is accomplished by placing a cation-exchange membrane (CEM) next to the
anode, and an anion-exchange membrane (AEM) next to the cathode.
For the given reactor design, cations are separated from the anode
chamber into the middle chamber while anions are transferred from
the cathode chamber into the middle chamber, desalinating the solutions in the anode and cathode chambers, and producing more concentrated salt solutions in the middle chamber. Oxidation of organic
matter releases protons into the anode solution while hydroxyl ions
are discharged by hydrogen gas evolution at the cathode, creating
pH imbalances between the anode and cathode chambers. To avoid
these pH imbalances, water was recirculated between the anode
and cathode chambers as previously described [19].
(A)
The main objectives of this study were (1) to demonstrate the process of simultaneous removal of organic contaminants and salt ions
from a saline solution, and (2) to examine the integrity of the IEMs
over an extended period of operation (three months). The long-term effects of bacterial activity in the anode chamber on the IEM integrity,
such as biofilm formation or permselectivity changes (capability of
IEMs to block the cross-over of excluded co-ions), have not been well
addressed in bioelectrochemical systems with membranes. In an MSC,
the anode microbes are directly impacted by the initial high salinity of
wastewater. Therefore, the MSC was operated at relatively high current
densities (2.8 mA/cm2) using applied voltages ≥1.0 V in order to enhance the rate of ionic separation here, compared to those used in previous studies with bioelectrochemical systems (b0.6 mA/cm 2) [20–22].
The specific effects of high salinities were also examined on the performance of the exoelectrogenic microbes at these high current densities.
2. Materials and methods
2.1. Reactor construction
A three-chamber MSC was built with cubic Lexan blocks with an
inner cylindrical chamber (7 cm 2 in cross section). The MSC consisted
of the anode (27 mL), concentrate waste (18 mL), and cathode
(33 mL) chambers. A cation-exchange membrane (7 cm 2, Selemion
CMV, AGC Engineering Co., Japan) was placed between the anode
and middle (concentrate waste) chambers to separate cations from
the anode to the middle chamber (Fig. 1). An anion-exchange membrane (7 cm2, Selemion AMV, AGC Engineering Co., Japan) was located
between the middle and cathode chambers. A glass tube was glued on
the top of the cathode chamber to collect H2 [23]. The cathode was prepared with platinum as the catalyst (0.6 mg/cm2; BASF, Germany), in a
mixture of carbon black (BASF, Germany) and Nafion (Sigma-Aldrich,
MO), which was painted on both sides of a 7-cm2 stainless steel mesh
(#50) [24]. A graphite fiber brush anode (2.7 cm in diameter and
2.3 cm in length; Mill-Rose Lab Inc., OH) was inoculated with effluent
from an existing MFC, which was originally started with primary
effluent from a domestic wastewater treatment plant. The anode was initially enriched in a single chamber MFC before it was used in the saline
wastewater treatment system [25]. During this start-up stage, the anode
microbes were exposed to various NaCl concentrations of up to 20 g/L.
In separate experiments, single-chamber microbial electrolysis cells
(MECs) were built without IEMs to examine the effects of various salinities on current generation by anode bacteria. The anode was prepared
using preacclimated bacteria (effluents from existing MFCs) and
adjusted with various NaCl concentrations of up to 20 g/L. The cathode
(B)
CEM
AEM
Conc.
waste
Cathode
Exoelectrogens
Ag/AgCl
Organic
H2 gas
matter
Conc.
Anode
waste
–
+
HCO3–
H2O
Periodic recirculation
Fig. 1. (A) Schematic design of three-chamber MSC for simultaneous removal of organic matter and salt ions, and (B) photo of the constructed MSC.
Y. Kim, B.E. Logan / Desalination 308 (2013) 115–121
was made with platinum catalysts (0.5 mg/cm2; BASF, Germany) as described above, but using a 7-cm2 piece of carbon cloth [26].
2.2. Solution preparation
For the tests with the three-chamber MSC, a saline medium was prepared with 20 g/L NaCl, 50 mM phosphate buffer (4.58 g/L Na2HPO4;
2.45 g/L NaH2PO4-H2O; 0.31 g/L NH4Cl; 0.13 g/L KCl), and sodium acetate (2, 4, 6, or 8 g/L) with trace amounts of minerals and vitamins [27].
For a given substrate concentration, the three-chamber MSC was operated for at least two fed-batch cycles, or until the experimental results
from each cycle became consistent with the previous one. To control
methanogenic activities in the reactor, 2-bromoethanesulfonate
(10 mM) was occasionally added into the medium [28]. The addition
of 2-bromoethanesulfonate did not appreciably change the solution
conductivity. The saline medium (27 + 33= 60 mL) was recirculated
between the anode and cathode chambers for 2.9 min at 3.5 mL/min
every 7.9 min (i.e., 2.9 min of pumping at 5 min intervals) using a programmable peristaltic pump (Masterflex L/S, Cole-Parmer, IL) (Fig. 1A),
in order to avoid adverse effects of large pH changes on the electrode reactions [19]. The middle chamber was initially filled with the same
salty medium, but it lacked sodium acetate.
For the experiments with the single-chamber MECs without membranes, the medium was a mixture of NaCl, 50 mM phosphate buffer
(7.2 g/L as total dissolved solid, TDS), 4 g/L sodium acetate, and 10 mM
2-bromoethanesulfonate with trace minerals and vitamins [27]. The
NaCl concentration was increased by 5 g/L from 10 to 35 g/L (i.e., from
21.2 to 46.2 g/L as TDS). At a given TDS, the single-chamber MECs were
operated over three fed-batch cycles for each salinity condition.
For the experiments with the three-chamber MSC, gas was collected
in a gas bag (1-L capacity; Cali-5-Bond, Calibrated Instruments Inc., NY)
and analyzed for H2, O2, N2, CH4, and CO2 using a gas chromatograph
(SRI-310 C, SRI Instruments, CA) as previously described [23]. The volume of the collected gas was determined by the gas bag method [29].
Effluent electrolyte and concentrate waste solution were analyzed for
conductivity and pH (SevenMulti, Mettler-Toledo International Inc.,
OH). The chemical oxygen demand (COD) was determined according
to standard methods (Hach Co., CO) [30]. The COD was measured only
for the fed-batch cycles performed without 2-bromoethanesulfonate,
and solution samples were diluted at a relatively high ratio of 1:40 to
meet the maximum chloride concentration [31].
The applied voltage (Eap) from a power supply was 1.0 V unless
otherwise noted (BK Precision, CA). This relatively high voltage was
used to produce high current densities to achieve rapid ionic separation. High applied voltages (≥1.0 V) have previously been used in
other bioelectrochemical studies to achieve high rates of chemical
production [16,32]. Electric current in the reactor was determined
by measuring the voltage drop across an external 10 Ω resistor
using a multi-meter (Keithley Instruments, OH). Current densities
were normalized using the IEM area (7 cm 2, equal to the cathode
projected area). Anode potentials were also recorded using Ag/AgCl
reference electrodes (RE-5B, BASi, IN). All experiments were
performed in a constant temperature room at 30 °C.
2.4. Efficiencies and recoveries
The Coulombic efficiency (CE) is the fraction of electrons transferred to the anode among the total electron released by substrate
oxidation and calculated [33] as
CE ¼
8∫idt
FV el ΔCOD
where i is the electric current, F Faraday's constant, Vel the electrolyte
volume (60 mL), and ΔCOD the removed COD.
The cathodic H2 recovery (rcat) represents the contribution of the
mole-H2 evolution (nH2) to the total cathodic charge transfer [34] as
ð1Þ
2nH2 F
r cat ¼
∫idt
ð2Þ
The overall H2 recovery (rH2) was determined by rH2 = rCErcat, indicating the ratio of the produced H2 to the removed organic matter on
the basis of electrons. The volumetric H2 production rate (QH2, m 3-H2/
m 3-Vel/day) was calculated based on the observed maximum current
(imax) and cathode recovery (rcat) at P = 1 atm and T = 298 K as [34]
Q H2 ¼
r cat i max RT
2V el PF
ð3Þ
The H2 yield (YH2, mole-H2/mole-COD) shows the molar ratio of H2
production to COD removal as [34]
Y H2 ¼
32nH2
V el ΔCOD
ð4Þ
The energy recovery from supplied electricity (rE) is the combustion
energy (ΔH) of the produced H2 normalized by the energy provided
from the power supplier as shown in Eq. (5). The overall energy recovery includes the energy provided as substrate (subscript s) as shown
Eq. (6) [34].
rE ¼
2.3. Experimental measurements
117
ΔH H2 nH2
Eap ∫idt
r EþS ¼
ΔH H2 nH2
Eap ∫idt þ ΔH s ns
ð5Þ
ð6Þ
2.5. Ion-exchange membrane permselectivity
Fresh IEMs and used IEMs in the MSC for three months were examined for their permselectivity. With ideal cation-exchange membranes,
for instance, the permselectivity equals unity and decreases as the membrane allows anionic transport. The permselectivity (Ψ) was determined
by measuring the electric potential difference (ΔΦ) between 1.0 and
0.5 M KCl solutions that are separated by an IEM [35] using
Ψ≅
ΔΦ
15:8 mV
ð7Þ
3. Results and discussion
3.1. Ionic separation and current generation in three-chamber reactor
Ionic separation from the saline medium, based on changes in conductivities (initially ~ 40 mS/cm), was successfully achieved with salt
removals of up to 84% (Fig. 2). The extent of salt removal was improved by 21 to 79% by increasing the initial concentration of sodium
acetate from 2 to 8 g/L (Eap = 1.0 V). This trend with the substrate
concentration clearly shows that the ionic separation from the
anode and cathode chambers was driven by microbial activity at the
anode. For a higher applied voltage of Eap = 1.2 V, the conductivity
decreased from 41 to 6.8 mS/cm, which is larger than the maximum
decrease to 8.6 mS/cm using an applied voltage of Eap = 1.0 V (Fig. 2).
The concentrate waste in the middle chamber of the reactor
showed a substantial increase in conductivity from 37 mS/cm to
80 mS/cm (Fig. 2). The final conductivity of the concentrate waste
118
Y. Kim, B.E. Logan / Desalination 308 (2013) 115–121
90
Initial
Electrolyte
was stable at − 0.43 ± 0.01 V (vs. Ag/AgCl), after the current reached
its peak at ~ 1.2 days (Fig. 3B). The 4.5-mA increase in the current
increased the Ohmic losses in the solution by ~ 0.06 V (conductivity = 41.5 mS/cm; average distance between the electrodes =
3.6 cm). Thus, the rest of the 0.14 V must have been lost as cathode
overpotential. This implies that the cathode overpotential can be
the rate-limiting factor for higher current operation of the system.
Higher current densities mean faster ionic separation, as well as
more rapid degradation of organic contaminants at the anode. Therefore, improvements in the cathode catalyst or increases in the cathode area will enhance system performance.
The relatively high current densities (1.4–2.1 mA/cm 2 for Eap =
1.0 V and 2.8 mA/cm 2 Eap = 1.2 V) indicate rapid ionic separation in
the MSC. Current densities reported in previous MDC studies have
not been greater than 0.9 mA/cm 2 [36]. The high current generation
in the MSC is partially due to the very high ionic conductivity
(~ 40 mS/cm) of the anode and cathode solution, which minimizes resistive losses in the solution. Thus, saline wastewater can be better
treated in bioelectrochemical systems than other wastewaters that
have much lower ionic conductivities.
Conc. waste
80
60
50
40
30
20
10
0
2g/L
1.0V
4g/L
1.0V
6g/L
1.0V
8g/L
1.0V
8g/L
1.2V
Sodium acetate concenctration; Eap
Fig. 2. Ionic separation from synthetic saline wastewater in the three-chamber MEC.
in the middle chamber increased with initial substrate concentrations
up to 6 g/L. This lack of further increase in conductivity of the waste
chamber solution was due to osmotic water transport from the adjoining anode and cathode chambers through the IEMs, as a result
of the substantial ionic concentration differences across the IEMs.
The electrical currents in the three-chamber MSC ranged between 10 and 15 mA (1.4–2.1 mA/cm 2) for Eap = 1.0 V (Fig. 3A). Increasing the applied voltage from 1.0 to 1.2 V increased the
maximum current to 19.5 mA (2.8 mA/cm 2), which was an increase
of 4.5 mA (30%). Even with the increased Eap, the anode potential
Current (mA)
20
(A)
15
10
4g/L (1.0V)
6g/L (1.0V)
5
3.2. Water transport through ion-exchange membranes
Water volume in the middle chamber (concentrate waste) almost
doubled over a fed-batch cycle, increasing from 18 to 33.4 mL (8-g/L
sodium acetate and Eap = 1.2 V), with the amount of water transport
proportional to the initial amount of the substrate (Fig. 4). Water
transport is inevitable in this system as ionic separation creates a concentration difference between the concentrate waste in the middle
chamber and the electrolyte in the anode and cathode chambers.
This concentration difference drives osmotic water transport into
the middle chamber. In addition, the movement of ions in the nanoscale channels in IEMs drags water molecules in the same direction
as the ion movement, transferring water into the middle chamber
(electroosmosis). Thus, both osmosis and electroosmosis contributed
to increasing the water volume in the middle chamber.
Water transport into the middle chamber reduces treated water recovery in the anode and cathode chambers, and increases the volume of
concentrate waste in the middle chamber. Thus, the extent of water
transport needs to be minimized during saline wastewater treatment.
In order to try to minimize the osmotic water transport, the applied
voltage was increased from Eap = 1.0 to 1.2 V. This higher voltage reduced the fed-batch cycle time from 2.6 to 2.3 days. However, water
transport was increased from 14.6 to 15.4 mL (Fig. 4). This result
8g/L (1.0V)
20
8g/L (1.2V)
100
Water transport
0
Water recovery
16
(B)
0.1
Water transport (mL)
Anode potential (V vs. Ag/AgCl)
0.2
0.0
-0.1
-0.2
-0.3
80
12
60
8
40
4
20
Water recovery (%)
Conductivity (mS/cm)
70
-0.4
0
0
-0.5
0 .0
0 .5
1 .0
1 .5
2 .0
2 .5
Time (days)
2g/L
1.0V
4g/L
1.0V
6g/L
1.0V
8g/L
1.0V
8g/L
1.2V
Sodium acetate concentration; Eap
Fig. 3. (A) Current production and (B) anode potential in the three-chamber MEC
treating synthetic saline wastewater.
Fig. 4. Water transport through ion-exchange membranes and resulting water recovery.
Y. Kim, B.E. Logan / Desalination 308 (2013) 115–121
implies that the electroosmotic water transport was the dominant contributor to water transport, even though its contribution was not separately examined in this study.
3.3. Hydrogen production
Hydrogen production increased from 61 to 338 mL per cycle, in proportion to the increase in sodium acetate concentration from 2 to 8 g/L
(Fig. 5A). Carbon dioxide or oxygen was not detected in the gas, indicating that there were no leaks into the reactor. As 2-bromoethanesulfonate
was occasionally added in the medium for sodium acetate concentrations 6 g/L and higher, methane contributed less than 4% of the total
gas generated in the reactor for these conditions, resulting in very high
cathode recoveries (rcat of 95–100%, Fig. 5B). The Coulombic efficiency
was also high (~100%) and thus the overall H2 recovery (rH2) was identical to the cathode recovery (rcat). However, for cycles where 2- or 4g/L sodium acetate was added without 2-bromoethanesulfonate, there
was substantial methanogenic activity which reduced the cathodic
recovery (rcat of 60–70%, Fig. 5B). Even with substantial methanogensis
(2- and 4-g/L sodium acetate), Coulombic efficiencies were very high
(~100%), implying that the methanogenic activities were mainly
hydrogenotrophic and not acetoclastic. If acetoclastic methanogenesis
had been dominant, the Coulombic efficiency would have been lower.
In batch cycles with 6- and 8-g/L sodium acetate, hydrogen yields
were very high (>1.95 mole-H2/mole-COD) as methanogensis was
inhibited by 2-bromoethanesulfonate (not shown). The maximum
hydrogen production rate ranged from QH2 = 2.7 to 3.6 m 3-H2/m 3electrolyte/day for the batch cycles with 6- and 8-g/L sodium acetate
(rcat = 97.6%; imax = 15–20 mA). The energy recovery from the applied
electric energy (rE) was 140–171%, and the overall energy recovery
from the supplied electricity and substrate (rE+S) ranged 52 to 60%
(Fig. 5B). However, these parameters decreased by up to 50% for the
fed-batch cycles with 4-g/L sodium acetate due to methanogenesis in
the absence of 2-bromoethanesulfonate.
400
Gas produced (mL)
(A)
H2
CH4
300
3.4. COD removal
COD removals decreased from 94 to 72% with increasing substrate
concentration (Fig. 5B). Since the influents and effluents were always
neutral with a pH =6.8±0.1, the substrate was present primarily as
anionic acetate (pKa of acetic acid=4.8). The anionic acetate in the
cathode chamber can pass through the AEM into the middle chamber.
This substrate loss through the AEM can explain the decrease in COD removal with increased substrate concentrations. At the end of the fedbatch cycles with the high sodium acetate concentrations (6 and
8 g/L), most of the residual COD (18 to 24% of the initial COD) was observed in the concentrate waste, while only 1–4% of the initial COD
was measured as residual in the anode and cathode chambers.
The periodic recirculation of solutions between the anode and
cathode chambers successfully limited changes in electrolyte pH, as
previously described [19]. The low COD residuals in the anode and
cathode chambers (i.e., 1–4% of the initial COD) indicated that the
MSC performance was not limited by pH imbalances, as incomplete
COD removals were previously reported with low anolyte pH [10].
The successful control of pH in this study implies that the periodic
recirculation will be especially useful for treating wastewaters with
low pH buffering capacities.
3.5. IEM integrity after three-month operation
New IEMs had permselectivities of Ψ = 0.97 ± 0.04, which agrees
well with the permselectivity given by the manufacturer of > 0.96
[37]. The permselectivities of the IEMs after three months of operation were 0.96 ± 0.06. This comparison indicates that three months
of operation did not impair the permselectivities of either CEMs or
AEMs.
During this three-month period of operation, the membranes
were not either physically or chemically cleaned. When the IEMs
were taken out from the reactor after three months, the AEM
appeared clean based on the absence of visible biofilms. Thin biofilms
(b1 mm) were observed on the both sides of the CEM, but they readily detached from the membrane surface when the membrane was
rinsed with a NaCl solution. These findings demonstrate the biological
reactor does not impose an aggressive environment for the IEM integrity even with high organic concentrations and microbial activity by
exoelectrogens and other anaerobic microbes such as methanogens.
3.6. Effects of salinity on anode bacteria
200
100
0
2.0
(B)
Removal, recoveries
119
1.5
COD removal
r_cat
r_E
r_E+S
1.0
0.5
0.0
2g/L
1.0V
4g/L
1.0V
6g/L
1.0V
8g/L
1.0V
8g/L
1.2V
Sodium acetate concentration; Eap
Fig. 5. (A) Gas production from the three-chamber MSC and (B) COD removal, cathode
recovery, and energy recovery.
During a single-chamber MEC experiment, exoelectrogenic microorganisms generated current with saline solutions of up to 36 g/L of
TDS (25 g/L NaCl; 7.2 g/L phosphate buffer medium; 4 g/L sodium acetate) (Fig. 6). This result indicates that the exoelectrogens were capable
of generating current at the maximum capacity possible in this system
at salinities typical of seawater (~35 g/L TDS). However, at 41 g/L TDS,
exoelectrogenic activity was impaired, as there was a gradual decrease
in current, and at 46 g/L TDS, the maximum current decreased to less
than 4.1 mA (Fig. 6). This high salinity condition for 9 days permanently
damaged the exoelectrogenic bacteria at the anode as the current
remained low in subsequent fed-batch cycles with only 21 g/L TDS
(Fig. 6).
The two MECs operated as duplicates showed slightly different responses to the increasing salinity. The current in MEC-1 was always
higher than that in MEC-2 (Fig. 6). At TDS = 36 g/L, the maximum
current in MEC-1 was constant (19.8 to 19.9 mA) over three consecutive cycles. However, MEC-2 showed a gradual decrease in the maximum current from 16.2 to 14.7 mA under the same conditions. This
difference between the MECs was more pronounced in the cycles
that followed at TDS = 41 g/L: the current was 15.4–17.9 mA in
MEC-1, and 8.5–11.4 mA in MEC-2. In addition, when the salinity
was reduced from 46 to 21 g/L TDS, the maximum current in MEC-1
120
Y. Kim, B.E. Logan / Desalination 308 (2013) 115–121
Current (mA)
25
21 g/L
26 g/L
31 g/L
36 g/L
41 g/L
46 g/L
21 g/L
20
MEC-1
MEC-2
15
10
5
0
0
5
10
15
20
25
30
Time (days)
Fig. 6. Effects of total dissolved solids on current generation in single chamber MECs.
was 6.5 mA which was 33% of the maximum current (19.9 mA) before the anode was damaged by the high salinity. However, the recovered maximum current in MEC-2 was 4.4 mA, which was only 25% of
the maximum current (17.3 mA) before the anode was damaged
(Fig. 6). The lack of replication on the effects of salinities shows that
there were slight differences in the performance of the biofilms that
developed in the reactors in response to increases in salinity. This
finding suggests that it might be possible in the future to develop
exoelectrogenic biofilms more resistant to the higher salinities
through extensive pre-acclimation to high salinities.
4. Conclusions
The microbial salinity-reduction electrolysis cell (MSC) developed
here can be used to simultaneously remove organic matter and salt
from saline wastewaters. Exoelectrogenic bacteria at the anode degraded 72–94% of the COD. The conductivity of the anode solution was
decreased by up to 84%, with the middle waste chamber doubling in
conductivity by the end of the fed-batch cycles. The Coulombic
efficiencies were very high (~100%) indicating a lack of substrate losses
to aerobic microbes due to oxygen leaks. Hydrogen gas was produced at
a rate of QH2 = 3.6 m 3-H2/m 3/d (sodium acetate = 8 g/L; Eap = 1.2 V),
although this was reduced in the absence of 2-bromoethanesulfonate
due to methanogensis. High Coulombic efficiencies in the absence of
2-bromoethanesulfonate indicated that hydrotrophic methanogens
were the predominant source of methane.
Water transport through IEMs was found to decrease not only the
rate of ionic separation but also the volume of treated water that was recovered. Water recovery decreased from 71 to 57% when conductivity
removals increased, creating greater water losses by both osmosis and
electroosmosis.
The IEM integrity was investigated under high microbial activities
and organic contents because high IEM costs can be a limiting factor
for practical applications of the suggested system [35]. Over the
three-month operation, there was no noticeable reduction in the
permselectivity of either the AEM or CEM. Also, there was no physical
deterioration on the membrane surface, and biofilms developed on
the CEM surface were easily removed by rinsing the membrane. Even
though further tests are necessary, our results indicate that the microbes, including the exoelectrogens in this system, do not create an environment that adversely affects IEM integrities.
The key to successful treatment and salinity removal in this system is good performance of the microbial communities at different
wastewater salinities. Exoelectrogens were found to be versatile at
salinities as high as those of typical seawater. However, salinities
above 41 g/L TDS reduced current generation, and very high salinities
(46 g/L TDS) permanently damaged the exoelectrogenic activities of
the anodes as reducing the salinities could not restore current generation to the previous levels. However, the different responses of the
replicate reactors to higher salt concentrations suggest that greater
tolerance to high salinities might be possible through extended acclimation of the biofilms.
Acknowledgements
This research was supported by funding through the King Abdullah
University of Science and Technology (KAUST) (Award KUS-I1-003-13).
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