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Microbial mediated desalination for ground water softening with simultaneous power generation
Accepted Manuscript Microbial mediated desalination for ground water softening with simultaneous power generation Manupati Hemalatha, Sai Kishore Butti, G. Velvizhi, S. Venkata Mohan PII: DOI: Reference:
S0960-8524(17)30659-4 http://dx.doi.org/10.1016/j.biortech.2017.05.020 BITE 18046
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
31 January 2017 2 May 2017 3 May 2017
Please cite this article as: Hemalatha, M., Butti, S.K., Velvizhi, G., Venkata Mohan, S., Microbial mediated desalination for ground water softening with simultaneous power generation, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.05.020
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1
Microbial mediated desalination for ground water softening with simultaneous power
2
generation
3
Manupati Hemalatha, Sai Kishore Butti, G.Velvizhi, S.Venkata Mohan*
4 5 6 7 8
Bioengineering and Environmental Sciences Lab, EEFF Department, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India *E-mail: [email protected]; Tel: 0091-40-27161765
9
Abstract
10 11
A novel three-chambered microbial desalination cell (MDC) designed for evaluating desalination
12
of synthetic ground water with simultaneous energy generation and resource recovery. The
13
specific design enabled efficient interelectrode communication by reducing the distance of
14
separation and also maintained an appropriate surface area to volume ratio. MDC were evaluated
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in different circuitry modes (open and closed) for desalination efficiency, bioelectricity
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generation, resource recovery, substrate utilization and bioelectrokinetics. The closed circuit
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operation has showed efficient desalination efficiency (51.5%) and substrate utilization (70%).
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Owing to the effective electron transfer kinetics with closed circuit mode of operation showed
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effective desalination of the synthetic ground water with the simultaneous power production
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(0.35 W/m2). Circuitry specific biocatalyst activity was observed with higher peak currents (10.1
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mA; -5.98 mA) in closed circuit mode. MDC are emerging as sustainable alternative solutions
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for ground and surface water softening with efficient power productivity and resource recovery.
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Keywords: Desalination, Renewable Energy, Bioelectrochemical System, Resource Recovery,
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Water hardness.
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1. Introduction
35 36
Water is an essential and critical commodity to sustain life on Earth. Ground water reserve is
37
one of the major sources apart from the surface water. Current rate of urbanization and
38
population explosion is causing exceptionally high water demand. Excessive withdrawal of sub-
39
surface water, contamination, urban runoffs, domestic activities, etc. are precariously effecting
40
the ground and surface water quality. This vulnerable water resource increases the salinity when
41
the natural sources like rainfall are limited (Khaska et al., 2013). Sustainable water sources is an
42
essential prerequisite to overcome the dearth of usable water (Brastad and Zhen, 2013; Khaska et
43
al., 2013). The water purification and water desalting technologies are highly sought after in
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geographical locations where natural fresh water sources are limited. The conventional water
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desalination or softening technologies employing high pressures, temperature, membranes, etc.
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incur high costs and maintenance, which will cost water approximately 0.5-3.0 $ per liter of
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water (Mathioulakis et al., 2007; Elimelech et al., 2011; Marzooqi et al., 2014,
48
www.lentech.com). Furthermore, the common limitation observed in most of the water
49
purification or desalination technologies is the generation of saline reject in considerable
50
volumes.
51 52
Microbial desalination cell (MDC) is one of the extended application of microbial fuel cell
53
(MFC) (Saeed et al., 2015; Venkata Mohan et al., 2014; Butti et al., 2016). Contrary to MFC,
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MDC involves the inclusion of a desalination chamber in between the anode and the cathode
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chamber of MFC, separated by anion exchange (anode) and cation exchange membrane
56
(Cathode). The operational requirements of MDC include placing the contaminated water or
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saline water in the middle chamber, biocatalyst in the anodic chamber and oxygenated water in
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cathodic chamber (Kim and Logan., 2013). Unlike, the migration of salts based on the
59
concentration gradient through diffusion across the membrane, in MDC the electrochemical
60
gradient created by substrate oxidation in the anodic chamber drives the desalination process.
61
The generated protons and electrons from substrate oxidation enable the transport of anions and
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cations towards the oxidative anodic chamber and reductive cathodic chamber respectively. The
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migration of ions to their respective chambers also enables renewable energy production in the
64
form of bioelectricity and resource recovery in the form of acids and bases (Gude, 2016; Sophia
65
et al., 2016; Nikhil et al., 2016; Saeed et al., 2015; Forrestal et al., 2012). Acidic products (HCl,
66
H2SO4, etc) and bases/salt (NaOH, Ca(OH)2, etc) that are formed can also be recovered from
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MDC.
68 69
While MDC is a novice technology in comparison with the existing water purification
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technologies, it garnered attention due to the inherent advantages viz., low economic burden and
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reducing reject. However, MDC have still significant scope for improvement in terms of the rate
72
and efficiency of desalination through optimization. In this study, MDC are specifically designed
73
to remove TDS and hardness from design synthetic groundwater. The performance of MDC was
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evaluated under closed and open circuit mode of operation to enumerate desalination efficiencies
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and rates, resource recovery potential, power production and bioelectrochemical kinetics. The
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study also focused to understand the regulatory factors of microbial mediated desalination
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process in terms of operational feasibility for real field applications.
78 79
2. Materials and Methods
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2.1 MDC Configuration
81 82
A three chambered specifically designed MDC bioreactor for desalination was fabricated using
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teflon based materials. The reactor consists of three identical chambers with dimensions (7.5 cm
84
× 8 cm × 2.5 cm; 60 ml) with considerations of having the smallest possible distance between the
85
electrodes (2.54 cm) to enhance the inter-electrode communication. The dimensions are designed
86
considering the electrode surface area to have an appropriate volume to surface area ratio of 0.3
87
which enables the efficient biocatalyst activity along with desalination (. The desalination
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chamber (mDC) was sandwiched in between the biotic anode (BA) and abiotic cathode (AC)
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separated by anion exchange membrane (AEM, AMI-Membranes International Inc., USA) and
90
cation exchange membrane (CEM, CMI-7000, Membranes International Inc., USA) on either
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side. The chambers are clamped air tight together with gaskets (silica sheet) and O-rings using
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stainless steel bolts. The either ends of the reactors are closed with perspex sheets, each chamber
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is individually provided with four different ports on the four side, which were used for sampling,
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recirculating and for draining out the contents. The reactors were designed with the possibility to
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operate in both batch and continuous mode with efficient recirculation. Non-catalyzed carbon
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cloth (Ballard AvCarb Co. Ltd.) with geometrical surface area of 28.2cm2 was used as anode and
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cathode. Prior to use, the carbon cloth was treated using NH4Cl solution to increase the
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conductivity (Kondaveeti and Min, 2013, Moon et al., 2014). All the components of the
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bioreactor after being clamped together were sealed using a rubber sealant to prevent leaks and
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sparged with nitrogen to maintain anaerobic conditions in the BA chamber (Fig 1). Fig 1
101 102 103
2.2 Biocatalyst and substrate composition
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The biotic anode chamber (BA) was inoculated with pretreated microbial consortia obtained
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from a already operating microbial fuel cell with 3g/l glucose as the carbon source. The essential
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nutrients are provided as designed synthetic wastewater (DSW g/l: NH4Cl-0.5, KH2PO4-0.25,
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K2HPO4-0.25, MgCl2-0.3, CoCl2-0.025, FeCl3-0.025, ZnCl2-0.0115, NiSO4-0.050, CuCl2-
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0.0105, CaCl2-0.005 and MnCl2-0.015) (Nikhil et al., 2016). The inoculum was administered
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into the reactor with 10%v/v using a long needle syringe and the contents were adjusted to pH 6.
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The cathode chamber (AC) was fed with deionized distilled water at pH 7.4 to avoid the
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development of ionic gradient between mDC and AC. Synthetic ground water (SGW mg/l:
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CaCO3-200, CaSO4-272, 4 MgCO3.Mg (OH2).5H2O -194, NaHCO3-252, KCl -75) (Stewart et
114
al., 2006) was prepared in distilled water. The SGW after adjusting pH to 7 was filtered prior to
115
feeding the mDC. All the redox adjustments were made using 0.1 N HCl and 0.1 N NaOH.
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2.3 Operation
118 119
Post the start-up the MDCs were operated in batch mode in two phases viz., initial stabilization
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phase and desalination phase. During the stabilization phase, the MDC was allowed to
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acclimatize to enable biocatalyst- electrode interactions (6-8 days). During the stabilization
122
period the anolyte and catholyte were replaced for every 48 h. The mDC was operated at lower
123
salt concentration (1800 mg/l). The end of the stabilization phase was determined by the stable
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cell voltage and substrate removal recorded. Later, the MDC were operated with SGW (2500
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mg/l) and the hydraulic retention time (HRT) of 48 h at ambient room temperature (24±3 °C).
126
The system performance was analyzed based on desalination efficiency and desalination
127
efficiency.
128 129
2.4 Analysis
130 131
The total dissolved solids (TDS, mg/l), pH and conductivity (EC) were monitored using compact
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multi-parameter analyzer (HANNA-5522-02). The changes in the TDS concentrations in mDC,
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BA and AC were used to calculate the desalination efficiency (DE) using (Eq 1), where Ci and C f
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represent the initial and final TDS concentrations of the middle desalination chamber
135
respectively (Nikhil et al., 2016; Zuo et. al., 2014). The groundwater hardness was quantified
136
titrimetrically using EDTA and was calculated in terms CaCO3 equivalent (mg/L) (Eq 2).
137
(%) =
----------- (1)
138 139
Total Hardness =
×× ×
----------- (2)
140 141
Where, N is normality of EDTA (0.02 N), 50 is equivalent weight of CaCO3, volume of sample
142
taken was 20 ml and volume of EDTA is the burette reading. The chemical oxygen demand
143
(COD) of the anolyte was determined using the closed reflux titrimetric method (APHA 1998).
144
The metabolic intermediates, total volatile fatty acids (VFA) generated during the operation were
145
analyzed using standard methods (APHA 1998) and the composition of VFA was determined by
146
HPLC (Shimadzu LC20A). The samples (0.020 ml) were injected into the Rezex column (300
147
×7.80 mm; Phenomenex) maintained at a temperature of 80°C. The samples were pumped to the
148
refractive index detector (RID 20A) at a flow rate of 0.5 ml/min.
149 150
The open circuit voltages were recorded using source measure unit (Keithiley, 2400) with a 5
151
minutes interval. Post the stabilization phase polarization analysis was performed using a
152
variable resistance box (external resistance from 30 kΩ to 0.05 kΩ) while recoding voltage and
153
current using a multimeter. The bioelectrochemical behavior of the biocatalyst in BA was
154
monitored using a potentiostat (Bio-Logic-VMP3). Cyclic voltammetry (CV) and linear sweep
155
voltammetry (LSV) were carried out with a three-electrode setup using anode as working
156
electrode (Ewe) and cathode as counter electrode (Ece) against Ag/AgCl (3.5 M KCl) reference
157
electrode (Re) for both open and closed modes of operation. CV were recorded at different scan
158
rates (100- 0.5 mV/s) with the voltage range of -1.0 V to +1.0 V. LSV was recorded with 1m/s
159
scan rate with a voltage range (-1.0 V to +1.0 V).
160 161
3. Results and Discussion
162
3.1 Desalination
163 164
The desalination performance of MDC was quantified by monitoring the changes in TDS (mg/l)
165
and conductivity (mS/s) levels in the BA, mDC and CA chambers. The desalination efficiency of
166
the MDC was higher in the CC (52%) mode of operation compared to OC (34%). At the startup
167
(0 h) post the stabilization period, the mDC was fed with an initial TDS concentration of 2500
168
mg/l. A decrement in the TDS was recorded from 0th h to 48th h (OC/CC, 1600/1300 mg/l) (Fig.
169
2a). The TDS removal observed in the mDC depicted a correlative increase in the TDS of BA
170
and the AC chambers. The initial TDS concentration (0 h) in BA and AC chambers was 280 mg/l
171
and 20 mg/l respectively. With the function of varied circuitry, BA operation recorded an
172
increase in TDS concentration (CCOC, 550/440 mg/l) at 48 h. Similarly, the AC showed an
173
increment in TDS concentration (CC/OC, 340/140 mg/l). The initial hardness during feeding to
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mDC was 270 mg/l (equivalent of CaCO3). Significant reduction in hardness reduction was
175
observed after 48 h of operation (CC/OC, 80.7/77%). The relatively higher desalination
176
efficiency observed with the CC operation can be attributed to the additional electrochemical
177
gradient created by the flow of electrons towards AC from BA. This enabled the improved
178
diffusion of ions across the membrane towards the opposite charges (Venkata Mohan et al., 2008
179
and 2009). In the case of OC operation, the desalination is primarily observed as a result of the
180
ionic diffusion without the gradient driving force (Nikhil et al., 2015).
181 182
The rate of desalination (mg/l/h) was monitored to determine the changes in TDS levels with the
183
function of time and to determine the desalination rate. The mDC and AC showed maximum
184
change in TDS during the initial phase of operation up to 24 h due to the presence of higher ionic
185
gradient along with the electrochemical gradient (mDC, CC/OC- 55/33 mg/l/h; AC, CC/OC-
186
14/6 mg/l; 12 h). The rate of desalination decreased as the operation progressed (mDC, CC/OC-
187
17/14 mg/l/h; AC, CC/OC- 4/2 mg/l/h; 48 h). The drop in rate of desalination with the function
188
of time can be attributed to the decrease in ionic gradient due to the mobility of ions (Zuo et. al.,
189
2014; Sophia et al., 2016) and apparent electrochemical gradient that driving the desalination
190
post 24 h of operation. However, in the case of BA the presence of salts in the feed (DSW) have
191
limited the rate of desalination during the initial phase of operation recording 5 mg/l/h (CC) and
192
3 mg/l/h (OC) at 12 h of operation. The maximum rate of desalination was observed at 36 h
193
(CC.OC-9/7 mg/l/h) operation (Fig 2b). The biocatalyst activity and the substrate dynamics
194
played a regulatory role in BA. The operational circuitry significantly influenced the desalination
195
efficiency and rate of desalination owing to the electrochemical gradient induced ionic mobility.
196
Fig 2
197 198
3.2 Power production through desalination
199 200
The salt concentration and the ionic mobility across the membrane provide the advantage of
201
higher cell electromotive force (EMF; bioelectricity) during the MDC operation. The maximum
202
cell voltage recorded in the OC operation (628 mV; 30 h) was significantly higher compared to
203
CC (170 mv; 48 h) (Fig 3a). The desalination independent power production which was
204
primarily based on the biocatalyst electrogenesis was observed in the case of CC operation,
205
where, the cell voltage showed a gradual increase though there was cumulative desalination and
206
low conductivities at 48 h. In the case of OC operation, desalination dependent power production
207
was observed to increase from 0 to 30 h with increase in TDS in BA chamber. This distinct
208
variation in the cell voltage stabilization in both the operations was observed because of the
209
varying ion mobility, where in CC operation the migration of reducing equivalents generated in
210
the BA towards AC through the circuit increased the current output which lowered the cell
211
voltage (Oh and Logan, 2005). On the contrary, in OC operation generated electrons and protons
212
were retained in the BA which increased the potential difference between AC and BA leading to
213
a high cell EMF. The increase in cell voltage can be attributed to the improved biocatalyst
214
efficiency as a result of the CC operation which can also correlates well with the substrate
215
utilization and bioelectrokinetics discussed in the following subsections.
216
217
The electrochemical behavior of the MDC was analyzed to estimate the internal resistance
218
(electrochemical losses) and the maximum power density by plotting current densities against
219
power density with a function of varying external resistances. The cell design point (CDP) was
220
calculated, which represents the highest external resistance at which point the power density is
221
maximum. The specifically designed MDC enabled higher power output with low
222
electrochemical losses with the maximum power output of 348 mW/m2 and the cell design point
223
of 1 kΩ (Fig 3b). With the decrease in the applied external resistance (30 to 0.05 kΩ) the voltage
224
dropped (602 to 489 mV) initially ascribing to lower activation losses owing to the high
225
conductivity of the anolyte (Mohanakrishna et al., 2017; Nikhil et al., 2015). Further, with the
226
decrease in external resistance the current density increased (30 kΩ to 1 kΩ) resulting in
227
maximum power production. At 1 kΩ an effective electron discharge was observed and hence 1
228
kΩ (CDP) was applied for all the CC operations where the internal resistance is close to the
229
external resistance enabling stabilized bioelectrogenic performance. The MDC operates with
230
zero net energy consumption as the process is self-sustained and operates based on the
231
electrochemical and ionic gradients derived through the bacterial metabolism (Wilson and Kim.,
232
2016). On the contrary, the conventional desalination process requires external energy input (1
233
KWH per m3 of water produced) (http://www.lenntech.com). MDC system also facilitates to
234
recover salts in a way reducing the reject, which is primarily a major limitation with the
235
convention desalination process. Fig 3
236 237 238
3.3 Substrate utilization
239 240
The biocatalytic activity, substrate utilization, redox change and metabolite formation play a
241
crucial role in developing the electrochemical gradient which also determines the desalination in
242
mDC. Substrate removal/utilization was monitored by measuring COD in relation to volatile
243
fatty acid (VFA) formation along with the changes in the pH. The initial COD concentration of
244
3229 mg/l was fed to BA for both OC and CC operations. At 48 h of operation, maximum COD
245
removal efficiency was observed in CC mode (70%) which was significantly higher compared to
246
OC (52%) operation (Fig 4a). The COD removal profile correlates with the improved
247
bioelectrochemical activity in CC and the desalination efficiency.
248
3.4 VFA synthesis
249 250
The anaerobic exo-electrogens in BA metabolize the organic substrate to produce VFA (acetic
251
acid, butyric acid and propionic acid). The concentration of VFA increased with time showing a
252
maximum VFA concentration of 800 mg/l in CC mode of operation compared to OC (580 mg/l)
253
(Fig 4b). The VFA profile was evaluated to understand the variations in the acidogenic
254
metabolism. CC operation depicted a highest concentration of acetic acid (95.3%) followed by
255
butyric acid (3.1%) and propionic acid (1.5%) (Fig 4c). OC operation also showed more or less
256
similar trend in acid metabolite production in the case of acetic acid (83.3%) and butyric acid
257
(16.6%). Both the operations showed a typical acidogenic metabolites profile, however, the
258
production of higher acetic acid fraction in CC operation can be attributed to the increase in
259
electrogenesis as the production of longer chain fatty acids consume more electrons (Sarkar et
260
al., 2016).
261 262
3.5 Redox Changes
263 264
With the production/consumption of the metabolites and developing ionic flux across the AEM
265
and CEM in the three chambers results in the alteration of the system redox conditions (pH) with
266
function of time. Initial pH of the anolyte, catholyte and SGW was set to 6, 7.5 and 7
267
respectively (Fig. 4d). The pH showed a decremental trend in BA (OC: 5.2, CC: 4.8) due to the
268
formation of VFA and the migration of anions across the AEM to form acidic intermediates in
269
the presence of protons which was relatively more amenable in CC compared to OC. The AC
270
showed an increase in the pH as the result of migrating cations across the CEM to form alkalis in
271
the catholyte. The change in catholyte pH correlates with the trend observed in BA, under OC
272
operation, wherein the increase was marginal (7.5 to 7.6) compared to CC (7.5 to 7.85). The pH
273
of mDC remained in the range of 7-7.2 showing minimal change with time. The drop and
274
increase in the anolyte and catholyte pH respectively not only depicts the progress of
275
desalination but also documents the resource recovery potential from MDC. The effluents from
276
BA and AC can be used for producing acids and bases.
277 278
Fig 4
279
3.6 Bioelectrokinetics: Cyclic Voltammetry and Tafel Analysis
280 281
The electro-metabolism of anodic biocatalyst was evaluated employing CV in both closed and
282
open circuit operation at varying scan rates (0.5 to 100 mV/s) with a potential range of −1.0 V to
283
+1.0 V. The voltammograms assisted to quantitatively compare the electrochemical interaction
284
between the biocatalyst and electrode based on electron transfer kinetics and peak currents
285
(Velvizhi and Venkata Mohan, 2012). CC operation showed significant increase in oxidative
286
(15.2 mA) and reductive current (-20 mA) than OC (4.5 mA and -2.5 mA) at 100 mV/s (Fig
287
5a,b). The increase in catalytic currents infers the continuous flow of electrons resulting from
288
substrate oxidation and stable the biofilm formation, which correlates with the higher power
289
generation in the CC operation compared to OC conditions (Srikanth et al., 2011). The increase
290
in reductive currents under CC operation indicates the effective reduction of cations at the
291
cathode in the presence of oxygen which relates with the increase in pH and desalination rate
292
(Mehanna et al., 2010). Linear increase in current was observed with increase in scan rate in both
293
CC and OC operations, however, the capacitive current was observed to be higher in OC
294
conditions which may be because of a double ion layer formation. The double ion layer might
295
have formed as a result of the high charge deposition around the anode owing to the lower
296
desalination rate in OC.
297 298
Linear sweep Voltammetry was recorded at a scan rate of 1 mV/s (+1 to -1 V) to understand the
299
biocatalyst-electrode interaction in terms of oxidation and reduction currents with a function of
300
potential. The maximum peak currents of 4.74 mA (oxidative); -1.98 mA (reductive) in OC
301
model and 10.1 mA (oxidative); -5.98 mA (reductive) in CC (Fig 5c). The LSV profiles of the
302
CC showed higher catalytic currents as a result of the enhanced biocatalyst activity with an
303
efficient biocatalyst and electrode interactions. Whereas, in the LSV profile under OC operation
304
showed a linear trend which resembles the limited electrode- biocatalyst ratio owing to the
305
double ion layer formed due to deposited ions in the BA correlating to the higher capacitance
306
observed in cyclic voltammetry.
307 308
Tafel slopes were plotted in derive oxidation (βa) and reduction (βc) slopes (Raghavulu et al.,
309
2012) which helps to delineate the electrons transfer from biocatalyst to anode dependent on the
310
cathode activity inclusive of the electrochemical losses.
The CC operation has depicted a
311
oxidative slope (βa: 0.225 V/dec) and reductive slope (βa: 0.225V/dec) which are comparatively
312
higher than the OC (βa: 0.368V/dec) and reductive slope (βc: 1.392V/dec) (Fig 5d). The higher
313
electrochemical losses in the open circuit operation has resulted in relatively lower current
314
versus the potential compared to the lower slopes showing proportional increase in the current
315
with the increase of current in CC operation. The lower βa than βc in CC operation infers the
316
product formation at the AC with the utilization of electrons generated from the substrate in the
317
BA. The bioelectrochemical analysis inferred higher desalination efficiency owing to the
318
lowered electrochemical losses and enhanced biocatalyst performance correlating well with the
319
substrate removal and desalination rate. Fig 5
320 321 322
4. Conclusion
323 324
The study establishes MDC as a viable solution for ground as well as surface water softening
325
apart from production of bioelectricity and biobased products. The variations in the circuitry
326
mode operations with the specific design showed good desalination efficiency of ground water
327
without reject and with power production. The low-cost design can be used in water plants as
328
pretreatment option prior to RO purification. The prospects of MDC are diverse and the scaling-
329
up impediments of MDC can be overcome by process optimizations, stacking and effective
330
usage of low cost materials.
331 332
Acknowledgement
333
The authors thank The Director, CSIR-IICT for encouragement. Department of Science and
334
Technology (DST) in the form of project (DST/TM/WTI/2K15/35(G)) supported the research.
335
SKB acknowledges University Grants Commission (UGC) for providing research fellowship.
336 337 338 339 340
341
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Energy Environ. Sci. 3 (8), 1114–1120.
374
13. Mohanakrishna, G., Butti, S.K., Kannaiah Goud, R., Venkata Mohan, S., 2017.
375
Spatiometabolic stratification of anoxic biofilm in prototype bioelectrogenic system.
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Bioelectrochemistry 115, 11–18.
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14. Moon, J.M., Sanath, K., Booki. M., 2014. Evaluation of low-cost separators for increased
378
power generation in single chamber microbial fuel cells with membrane electrode
379
assembly. Fuel Cells. 15, 230-238.
380
15. Nikhil, G., Subhash, G.V., Yeruva, D.K., Venkata Mohan, S., 2015. Closed circuitry
381
operation influence on microbial electrofermentation: proton/electron effluxes on electro-
382
fuels productivity. Bioresour. Technol. 195, 37–45.
383
16. Nikhil, G.N., Dileep Kumar, Y., Venkata Mohan, S., Swamy, Y.V., 2016. Assessing
384
potential cathodes for resource recovery through wastewater treatment and salinity
385
removal using non-buffered microbial electrochemical systems. Bioresour. Technol. 215,
386
247-253.
387
17. Raghavulu, S.V., Babu, P.S., Goud, R.K., Subhash, G.V., Srikanth, S., Venkata Mohan,
388
S., 2012. Bioaugmentation of electrochemically active strain to enhance the electron
389
discharge of mixed culture: process evaluation through electrokinetic analysis. RSC.
390
Adv. 2, 677–688.
391
18. Saeed, H.M., Husseini, G.A., Yousef, S., Saif, J., Al-asheh, S., Fara, A.A., 2015.
392
Microbial desalination cell technology: a review and a case study, Desalination. 359, 1–
393
13.
394
19. Sarkar, O., Kumar, A.N., Dahiya, S., Krishna, K. V, Yeruva, D.K., Venkata Mohan, S.,
395
2016. Regulation of acidogenic metabolism towards enhanced short chain fatty acid
396
biosynthesis from waste: Metagenomic profiling. RSC Adv. 6, 18641–18653.
397
20. Sophia, A.C., Bhalambaala, V.M., Limab, E.C., Thirunavoukkarasua, M., 2016.
398
Microbial desalination cell technology: Contribution to sustainable waste water treatment
399
process, current status and future applications. J Environ. Chem. Eng. 4, 3468–3478.
400
21. Srikanth, S., Pavani, T., Sarma, P.N., Venkata Mohan, S., 2011. Synergistic interaction of
401
biocatalyst with bio-anode as a function of electrode materials. Int J of Hydrogen Energ.
402
36, 2271-2280.
403
22. Stewart, D.I., Csovari, M., Barton, C.S., Morris, K., and Bryant, D.E., 2006. Performance
404
of a functionalized Polymer-coated silica at Treating Uranium Contaminated
405
Groundwater from a Hungarian Mine Site. Engineering Geology. 85, 174-183.
406
23. Velvizhi, G., Venkata Mohan, S., 2012. Electrogenic activity and electron losses under
407
increasing organic load of recalcitrant pharmaceutical wastewater. Int J of Hydrogen
408
Energ. 37, 5969-5978.
409
24. Venkata Mohan, S., Raghavulu, S., Sarma, P.N., 2008. Biochemical evaluation of
410
bioelectricity production process from anaerobic wastewater treatment in a single
411
chambered microbial fuel cell (MFC) employing glass wool membrane. Biosens
412
Bioelectrons. 23, 1326-1332.
413
25. Venkata Mohan, S., Veer Raghavulu, S., Dinakar, P., Sarma, P.N., 2009. Integrated
414
function of microbial fuel cell (MFC) as bio-electrochemical treatment system associated
415
with bioelectricity generation under higher substrate load, Biosens. Bioelectron. 24,
416
2021-2027.
417
26. Venkata Mohan, S., Velvizhi, G., Vamshi Krishna, K., Lenin Babu, M., 2014. Microbial
418
catalyzed electrochemical systems: A bio-factory with multi-facet applications.
419
Bioresource Technol. 165, 355-364.
420 421 422 423
27. Gude, V.G., 2016. Desalination and sustainability – An appraisal and current perspective. Water Res. 89, 87–106. 28. Wilson, E.L., Kim, Y., 2016. The yield and decay coefficients of exoelectrogenic bacteria in bioelectrochemical systems. Water Res. 94, 233–239.
424
29. Zuo, K., Cai, J., Liang, S., Wu, S., Zhang, C., Liang, P., Huang, X., 2014. A ten liter
425
stacked microbial desalination cell packed with mixed ion-exchange resins for secondary
426
effluent desalination. Environ. Sci. Technol. 48 (16), 9917–9924.
427
Figure captions
428 429 430
Figure 1: Schematic representation of MDC block reactor (Triple Chamber) for efficient
431
desalination and resource recovery with bioelectricity generation.
432
Figure 2: (a): TDS removal in the three chambers (mg/l) with a function of time, (b): Rate of
433
change of TDS (mg/l/h) with the function of time for different chambers of MDC block reactor –
434
Open circuit and Closed circuit (1KΩ).
435
Figure 3: (a) Cell voltage of MDC block reactor in open circuit operation and closed circuit
436
operation (1 KΩ), (b) Polarization profile showing the maximum power point (MPP) of MDC
437
block reactor.
438
Figure 4: (a): COD removal in BA of MDC block reactor under Open circuit and Closed circuit
439
(1KΩ) operations, (b): VFA profile in BA of MDC block reactor under Open circuit and Closed
440
circuit (1KΩ), (c): VFA composition profile (acetic acid, butyric acid and propionic acid)
441
quantified at the maximum VFA production, (d): pH changes in the three chambers of MDC
442
block reactor- Open circuit and Closed circuit (1KΩ).
443
Figure 5: Cyclic voltammetry (CV) profile of MDC block reactor performed with different scan
444
rates (a) Open circuit operation, (b) Closed circuit operation (1kΩ), (c) Linear sweep
445
voltammetry (LSV) profile of MDC block reactor with different operating conditions (d) Tafel
446
analysis of MDC reactor with different operating conditions.
447 448 449 450 451 452 453 454 455 456 457
458 459 460
Figure 1: Schematic representation of MDC block reactor (Triple Chamber) for efficient
461
desalination and resource recovery with bioelectricity generation.
462 463 464 465 466 467 468 469 470 471 472 473
600
2600
BC
350
mDC
2400
500
a
AC
300
300 200 CC OC
100
250
2000
TDS (mg/l)
TDS (mg/l)
TDS (mg/l)
2200 400
1800 1600 1400
12
24
36
48
Time (h)
474
150 100
CC OC
1200
CC OC
50
1000 0
200
0
0
12
24
36
48
0
12
24
36
48
Time (h)
Time (h)
20 BC
mDC
b
AC
TDS Removal Rate (mg/l/h)
10 0 -10 -20 -30 -40
CC OC
-50 -60 0
475
12 24 36 48 60 72 84 96 108 120 132 144 156 168
Time (h)
476
Figure 2: (a): TDS removal in the three chambers (mg/l) with a function of time, (b): Rate
477
of change of TDS (mg/l/h) with the function of time for different chambers of MDC block
478
reactor – Open circuit and Closed circuit (1KΩ).
479 480 481 482 483
800
a
600
Cell Voltage (mV)
400
200 140
120
OC CC
100 0
6
12
18
24
30
36
42
48
Time (h)
484 485
b
600
0.35
0.30 400 0.25 300 0.20
200
100 0.0000
Voltage Power Density 0.0005
0.0010
2) Power Density (W/m
Voltage (mV)
500
0.15 0.0015
2
0.0020
0.0025
486
Current Density (A/m )
487
Figure 3: (a) Cell voltage of MDC block reactor in open circuit operation and closed circuit
488
operation (1 KΩ), (b) Polarization profile showing the maximum power point (MPP) of
489
MDC block reactor.
490
a
3000 OC CC
COD (mg/l)
2500 2000
52%
1500 70%
1000 500 0 0
12
24
36
48
Time (h)
491
VFA (mg/l)
800
b
OC CC
600
400
200
0 12
492 493 494 495 496 497 498 499 500
24
36
Time (h)
48
501 95.36% 95.36% 83.33% 83.33%
OC
CC 16.67% 16.67%
1.55% 1.55% 3.1% 3.1%
Acetic acid Butyric Acid
Acetic Acid Butyric acid Propionic Acid
502 503
7.9
BC
6.0
7.4
5.5
7.3
5.0
7.2
d
AC
mDC
7.8 7.7
4.5
pH
pH
pH
7.6
7.4
7.1 CC OC
4.0
7.5
CC OC
7.0
CC OC
7.3 7.2
0
12
24
Time (h)
504
36
48
0
12
24
36
48
0
12
Time (h)
24
36
Time (h)
505
Figure 4: (a): COD removal in BA of MDC block reactor under Open circuit and Closed
506
circuit (1KΩ) operations, (b): VFA profile in BA of MDC block reactor under Open circuit
507
and Closed circuit (1KΩ), (c): VFA composition profile (acetic acid, butyric acid and
508
propionic acid) quantified at the maximum VFA production, (d): pH changes in the three
509
chambers of MDC block reactor- Open circuit and Closed circuit (1KΩ).
510 511 512 513 514 515
48
5 50mV/s 5mV/s
15
30mV/s 1mV/s
10
Current (mA)
Current (mA)
3
100mV/s 10mV/s 0.5mV/s
2 1
b
20
a 4
100mV/s 10mV/s 0.5mV/s
50mV/s 5mV/s
30mV/s 1mV/s
5 0 -5
-10
0
-15
-1
-20 -2
-25
-3
-30 -1.0
-0.5
0.0
0.5
1.0
-1.0
E (mV) vs Ag/AgCl (3.5M KCl)
-0.5
0.0
0.5
1.0
E (mV) vs Ag/AgCl (3.5M KCl)
516 517 12
2
c
10
1
8 6
log (|/mA|)
Current (mA)
d
100mV/s
OC CC
4 2 0 -2 -4
0 -1
OC CC
-2 -3
-6 -8 -1.0
-0.5
0.0
0.5
E (mV) vs Ag/AgCl (3.5M KCl)
1.0
-1.0
-0.5
0.0
0.5
1.0
E (mV) vs Ag/AgCl (3.5M KCl)
518
Figure 5: Cyclic voltammetry (CV) profile of MDC block reactor performed with different
519
scan rates (a) Open circuit operation, (b) Closed circuit operation (1kΩ), (c) Linear sweep
520
voltammetry (LSV) profile of MDC block reactor with different operating conditions
521
(d) Tafel analysis of MDC reactor with different operating conditions.
522 523
Highlights
524 525
•
Desalination of ground and surface water is an emerging green technology.
526
•
Triple chambered MDC for desalination, waste remediation and product recovery.
527
•
Exoelectrogenic activity is the drives desalination under varied circuitries.
528
•
Salts and hardness removal using microbial desalination.
529 530 531 532 533 534 535
S0960-8524(17)30659-4 http://dx.doi.org/10.1016/j.biortech.2017.05.020 BITE 18046
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
31 January 2017 2 May 2017 3 May 2017
Please cite this article as: Hemalatha, M., Butti, S.K., Velvizhi, G., Venkata Mohan, S., Microbial mediated desalination for ground water softening with simultaneous power generation, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.05.020
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1
Microbial mediated desalination for ground water softening with simultaneous power
2
generation
3
Manupati Hemalatha, Sai Kishore Butti, G.Velvizhi, S.Venkata Mohan*
4 5 6 7 8
Bioengineering and Environmental Sciences Lab, EEFF Department, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India *E-mail: [email protected]; Tel: 0091-40-27161765
9
Abstract
10 11
A novel three-chambered microbial desalination cell (MDC) designed for evaluating desalination
12
of synthetic ground water with simultaneous energy generation and resource recovery. The
13
specific design enabled efficient interelectrode communication by reducing the distance of
14
separation and also maintained an appropriate surface area to volume ratio. MDC were evaluated
15
in different circuitry modes (open and closed) for desalination efficiency, bioelectricity
16
generation, resource recovery, substrate utilization and bioelectrokinetics. The closed circuit
17
operation has showed efficient desalination efficiency (51.5%) and substrate utilization (70%).
18
Owing to the effective electron transfer kinetics with closed circuit mode of operation showed
19
effective desalination of the synthetic ground water with the simultaneous power production
20
(0.35 W/m2). Circuitry specific biocatalyst activity was observed with higher peak currents (10.1
21
mA; -5.98 mA) in closed circuit mode. MDC are emerging as sustainable alternative solutions
22
for ground and surface water softening with efficient power productivity and resource recovery.
23 24
Keywords: Desalination, Renewable Energy, Bioelectrochemical System, Resource Recovery,
25
Water hardness.
26 27 28 29 30 31 32 33
34
1. Introduction
35 36
Water is an essential and critical commodity to sustain life on Earth. Ground water reserve is
37
one of the major sources apart from the surface water. Current rate of urbanization and
38
population explosion is causing exceptionally high water demand. Excessive withdrawal of sub-
39
surface water, contamination, urban runoffs, domestic activities, etc. are precariously effecting
40
the ground and surface water quality. This vulnerable water resource increases the salinity when
41
the natural sources like rainfall are limited (Khaska et al., 2013). Sustainable water sources is an
42
essential prerequisite to overcome the dearth of usable water (Brastad and Zhen, 2013; Khaska et
43
al., 2013). The water purification and water desalting technologies are highly sought after in
44
geographical locations where natural fresh water sources are limited. The conventional water
45
desalination or softening technologies employing high pressures, temperature, membranes, etc.
46
incur high costs and maintenance, which will cost water approximately 0.5-3.0 $ per liter of
47
water (Mathioulakis et al., 2007; Elimelech et al., 2011; Marzooqi et al., 2014,
48
www.lentech.com). Furthermore, the common limitation observed in most of the water
49
purification or desalination technologies is the generation of saline reject in considerable
50
volumes.
51 52
Microbial desalination cell (MDC) is one of the extended application of microbial fuel cell
53
(MFC) (Saeed et al., 2015; Venkata Mohan et al., 2014; Butti et al., 2016). Contrary to MFC,
54
MDC involves the inclusion of a desalination chamber in between the anode and the cathode
55
chamber of MFC, separated by anion exchange (anode) and cation exchange membrane
56
(Cathode). The operational requirements of MDC include placing the contaminated water or
57
saline water in the middle chamber, biocatalyst in the anodic chamber and oxygenated water in
58
cathodic chamber (Kim and Logan., 2013). Unlike, the migration of salts based on the
59
concentration gradient through diffusion across the membrane, in MDC the electrochemical
60
gradient created by substrate oxidation in the anodic chamber drives the desalination process.
61
The generated protons and electrons from substrate oxidation enable the transport of anions and
62
cations towards the oxidative anodic chamber and reductive cathodic chamber respectively. The
63
migration of ions to their respective chambers also enables renewable energy production in the
64
form of bioelectricity and resource recovery in the form of acids and bases (Gude, 2016; Sophia
65
et al., 2016; Nikhil et al., 2016; Saeed et al., 2015; Forrestal et al., 2012). Acidic products (HCl,
66
H2SO4, etc) and bases/salt (NaOH, Ca(OH)2, etc) that are formed can also be recovered from
67
MDC.
68 69
While MDC is a novice technology in comparison with the existing water purification
70
technologies, it garnered attention due to the inherent advantages viz., low economic burden and
71
reducing reject. However, MDC have still significant scope for improvement in terms of the rate
72
and efficiency of desalination through optimization. In this study, MDC are specifically designed
73
to remove TDS and hardness from design synthetic groundwater. The performance of MDC was
74
evaluated under closed and open circuit mode of operation to enumerate desalination efficiencies
75
and rates, resource recovery potential, power production and bioelectrochemical kinetics. The
76
study also focused to understand the regulatory factors of microbial mediated desalination
77
process in terms of operational feasibility for real field applications.
78 79
2. Materials and Methods
80
2.1 MDC Configuration
81 82
A three chambered specifically designed MDC bioreactor for desalination was fabricated using
83
teflon based materials. The reactor consists of three identical chambers with dimensions (7.5 cm
84
× 8 cm × 2.5 cm; 60 ml) with considerations of having the smallest possible distance between the
85
electrodes (2.54 cm) to enhance the inter-electrode communication. The dimensions are designed
86
considering the electrode surface area to have an appropriate volume to surface area ratio of 0.3
87
which enables the efficient biocatalyst activity along with desalination (. The desalination
88
chamber (mDC) was sandwiched in between the biotic anode (BA) and abiotic cathode (AC)
89
separated by anion exchange membrane (AEM, AMI-Membranes International Inc., USA) and
90
cation exchange membrane (CEM, CMI-7000, Membranes International Inc., USA) on either
91
side. The chambers are clamped air tight together with gaskets (silica sheet) and O-rings using
92
stainless steel bolts. The either ends of the reactors are closed with perspex sheets, each chamber
93
is individually provided with four different ports on the four side, which were used for sampling,
94
recirculating and for draining out the contents. The reactors were designed with the possibility to
95
operate in both batch and continuous mode with efficient recirculation. Non-catalyzed carbon
96
cloth (Ballard AvCarb Co. Ltd.) with geometrical surface area of 28.2cm2 was used as anode and
97
cathode. Prior to use, the carbon cloth was treated using NH4Cl solution to increase the
98
conductivity (Kondaveeti and Min, 2013, Moon et al., 2014). All the components of the
99
bioreactor after being clamped together were sealed using a rubber sealant to prevent leaks and
100
sparged with nitrogen to maintain anaerobic conditions in the BA chamber (Fig 1). Fig 1
101 102 103
2.2 Biocatalyst and substrate composition
104 105
The biotic anode chamber (BA) was inoculated with pretreated microbial consortia obtained
106
from a already operating microbial fuel cell with 3g/l glucose as the carbon source. The essential
107
nutrients are provided as designed synthetic wastewater (DSW g/l: NH4Cl-0.5, KH2PO4-0.25,
108
K2HPO4-0.25, MgCl2-0.3, CoCl2-0.025, FeCl3-0.025, ZnCl2-0.0115, NiSO4-0.050, CuCl2-
109
0.0105, CaCl2-0.005 and MnCl2-0.015) (Nikhil et al., 2016). The inoculum was administered
110
into the reactor with 10%v/v using a long needle syringe and the contents were adjusted to pH 6.
111
The cathode chamber (AC) was fed with deionized distilled water at pH 7.4 to avoid the
112
development of ionic gradient between mDC and AC. Synthetic ground water (SGW mg/l:
113
CaCO3-200, CaSO4-272, 4 MgCO3.Mg (OH2).5H2O -194, NaHCO3-252, KCl -75) (Stewart et
114
al., 2006) was prepared in distilled water. The SGW after adjusting pH to 7 was filtered prior to
115
feeding the mDC. All the redox adjustments were made using 0.1 N HCl and 0.1 N NaOH.
116 117
2.3 Operation
118 119
Post the start-up the MDCs were operated in batch mode in two phases viz., initial stabilization
120
phase and desalination phase. During the stabilization phase, the MDC was allowed to
121
acclimatize to enable biocatalyst- electrode interactions (6-8 days). During the stabilization
122
period the anolyte and catholyte were replaced for every 48 h. The mDC was operated at lower
123
salt concentration (1800 mg/l). The end of the stabilization phase was determined by the stable
124
cell voltage and substrate removal recorded. Later, the MDC were operated with SGW (2500
125
mg/l) and the hydraulic retention time (HRT) of 48 h at ambient room temperature (24±3 °C).
126
The system performance was analyzed based on desalination efficiency and desalination
127
efficiency.
128 129
2.4 Analysis
130 131
The total dissolved solids (TDS, mg/l), pH and conductivity (EC) were monitored using compact
132
multi-parameter analyzer (HANNA-5522-02). The changes in the TDS concentrations in mDC,
133
BA and AC were used to calculate the desalination efficiency (DE) using (Eq 1), where Ci and C f
134
represent the initial and final TDS concentrations of the middle desalination chamber
135
respectively (Nikhil et al., 2016; Zuo et. al., 2014). The groundwater hardness was quantified
136
titrimetrically using EDTA and was calculated in terms CaCO3 equivalent (mg/L) (Eq 2).
137
(%) =
----------- (1)
138 139
Total Hardness =
×× ×
----------- (2)
140 141
Where, N is normality of EDTA (0.02 N), 50 is equivalent weight of CaCO3, volume of sample
142
taken was 20 ml and volume of EDTA is the burette reading. The chemical oxygen demand
143
(COD) of the anolyte was determined using the closed reflux titrimetric method (APHA 1998).
144
The metabolic intermediates, total volatile fatty acids (VFA) generated during the operation were
145
analyzed using standard methods (APHA 1998) and the composition of VFA was determined by
146
HPLC (Shimadzu LC20A). The samples (0.020 ml) were injected into the Rezex column (300
147
×7.80 mm; Phenomenex) maintained at a temperature of 80°C. The samples were pumped to the
148
refractive index detector (RID 20A) at a flow rate of 0.5 ml/min.
149 150
The open circuit voltages were recorded using source measure unit (Keithiley, 2400) with a 5
151
minutes interval. Post the stabilization phase polarization analysis was performed using a
152
variable resistance box (external resistance from 30 kΩ to 0.05 kΩ) while recoding voltage and
153
current using a multimeter. The bioelectrochemical behavior of the biocatalyst in BA was
154
monitored using a potentiostat (Bio-Logic-VMP3). Cyclic voltammetry (CV) and linear sweep
155
voltammetry (LSV) were carried out with a three-electrode setup using anode as working
156
electrode (Ewe) and cathode as counter electrode (Ece) against Ag/AgCl (3.5 M KCl) reference
157
electrode (Re) for both open and closed modes of operation. CV were recorded at different scan
158
rates (100- 0.5 mV/s) with the voltage range of -1.0 V to +1.0 V. LSV was recorded with 1m/s
159
scan rate with a voltage range (-1.0 V to +1.0 V).
160 161
3. Results and Discussion
162
3.1 Desalination
163 164
The desalination performance of MDC was quantified by monitoring the changes in TDS (mg/l)
165
and conductivity (mS/s) levels in the BA, mDC and CA chambers. The desalination efficiency of
166
the MDC was higher in the CC (52%) mode of operation compared to OC (34%). At the startup
167
(0 h) post the stabilization period, the mDC was fed with an initial TDS concentration of 2500
168
mg/l. A decrement in the TDS was recorded from 0th h to 48th h (OC/CC, 1600/1300 mg/l) (Fig.
169
2a). The TDS removal observed in the mDC depicted a correlative increase in the TDS of BA
170
and the AC chambers. The initial TDS concentration (0 h) in BA and AC chambers was 280 mg/l
171
and 20 mg/l respectively. With the function of varied circuitry, BA operation recorded an
172
increase in TDS concentration (CCOC, 550/440 mg/l) at 48 h. Similarly, the AC showed an
173
increment in TDS concentration (CC/OC, 340/140 mg/l). The initial hardness during feeding to
174
mDC was 270 mg/l (equivalent of CaCO3). Significant reduction in hardness reduction was
175
observed after 48 h of operation (CC/OC, 80.7/77%). The relatively higher desalination
176
efficiency observed with the CC operation can be attributed to the additional electrochemical
177
gradient created by the flow of electrons towards AC from BA. This enabled the improved
178
diffusion of ions across the membrane towards the opposite charges (Venkata Mohan et al., 2008
179
and 2009). In the case of OC operation, the desalination is primarily observed as a result of the
180
ionic diffusion without the gradient driving force (Nikhil et al., 2015).
181 182
The rate of desalination (mg/l/h) was monitored to determine the changes in TDS levels with the
183
function of time and to determine the desalination rate. The mDC and AC showed maximum
184
change in TDS during the initial phase of operation up to 24 h due to the presence of higher ionic
185
gradient along with the electrochemical gradient (mDC, CC/OC- 55/33 mg/l/h; AC, CC/OC-
186
14/6 mg/l; 12 h). The rate of desalination decreased as the operation progressed (mDC, CC/OC-
187
17/14 mg/l/h; AC, CC/OC- 4/2 mg/l/h; 48 h). The drop in rate of desalination with the function
188
of time can be attributed to the decrease in ionic gradient due to the mobility of ions (Zuo et. al.,
189
2014; Sophia et al., 2016) and apparent electrochemical gradient that driving the desalination
190
post 24 h of operation. However, in the case of BA the presence of salts in the feed (DSW) have
191
limited the rate of desalination during the initial phase of operation recording 5 mg/l/h (CC) and
192
3 mg/l/h (OC) at 12 h of operation. The maximum rate of desalination was observed at 36 h
193
(CC.OC-9/7 mg/l/h) operation (Fig 2b). The biocatalyst activity and the substrate dynamics
194
played a regulatory role in BA. The operational circuitry significantly influenced the desalination
195
efficiency and rate of desalination owing to the electrochemical gradient induced ionic mobility.
196
Fig 2
197 198
3.2 Power production through desalination
199 200
The salt concentration and the ionic mobility across the membrane provide the advantage of
201
higher cell electromotive force (EMF; bioelectricity) during the MDC operation. The maximum
202
cell voltage recorded in the OC operation (628 mV; 30 h) was significantly higher compared to
203
CC (170 mv; 48 h) (Fig 3a). The desalination independent power production which was
204
primarily based on the biocatalyst electrogenesis was observed in the case of CC operation,
205
where, the cell voltage showed a gradual increase though there was cumulative desalination and
206
low conductivities at 48 h. In the case of OC operation, desalination dependent power production
207
was observed to increase from 0 to 30 h with increase in TDS in BA chamber. This distinct
208
variation in the cell voltage stabilization in both the operations was observed because of the
209
varying ion mobility, where in CC operation the migration of reducing equivalents generated in
210
the BA towards AC through the circuit increased the current output which lowered the cell
211
voltage (Oh and Logan, 2005). On the contrary, in OC operation generated electrons and protons
212
were retained in the BA which increased the potential difference between AC and BA leading to
213
a high cell EMF. The increase in cell voltage can be attributed to the improved biocatalyst
214
efficiency as a result of the CC operation which can also correlates well with the substrate
215
utilization and bioelectrokinetics discussed in the following subsections.
216
217
The electrochemical behavior of the MDC was analyzed to estimate the internal resistance
218
(electrochemical losses) and the maximum power density by plotting current densities against
219
power density with a function of varying external resistances. The cell design point (CDP) was
220
calculated, which represents the highest external resistance at which point the power density is
221
maximum. The specifically designed MDC enabled higher power output with low
222
electrochemical losses with the maximum power output of 348 mW/m2 and the cell design point
223
of 1 kΩ (Fig 3b). With the decrease in the applied external resistance (30 to 0.05 kΩ) the voltage
224
dropped (602 to 489 mV) initially ascribing to lower activation losses owing to the high
225
conductivity of the anolyte (Mohanakrishna et al., 2017; Nikhil et al., 2015). Further, with the
226
decrease in external resistance the current density increased (30 kΩ to 1 kΩ) resulting in
227
maximum power production. At 1 kΩ an effective electron discharge was observed and hence 1
228
kΩ (CDP) was applied for all the CC operations where the internal resistance is close to the
229
external resistance enabling stabilized bioelectrogenic performance. The MDC operates with
230
zero net energy consumption as the process is self-sustained and operates based on the
231
electrochemical and ionic gradients derived through the bacterial metabolism (Wilson and Kim.,
232
2016). On the contrary, the conventional desalination process requires external energy input (1
233
KWH per m3 of water produced) (http://www.lenntech.com). MDC system also facilitates to
234
recover salts in a way reducing the reject, which is primarily a major limitation with the
235
convention desalination process. Fig 3
236 237 238
3.3 Substrate utilization
239 240
The biocatalytic activity, substrate utilization, redox change and metabolite formation play a
241
crucial role in developing the electrochemical gradient which also determines the desalination in
242
mDC. Substrate removal/utilization was monitored by measuring COD in relation to volatile
243
fatty acid (VFA) formation along with the changes in the pH. The initial COD concentration of
244
3229 mg/l was fed to BA for both OC and CC operations. At 48 h of operation, maximum COD
245
removal efficiency was observed in CC mode (70%) which was significantly higher compared to
246
OC (52%) operation (Fig 4a). The COD removal profile correlates with the improved
247
bioelectrochemical activity in CC and the desalination efficiency.
248
3.4 VFA synthesis
249 250
The anaerobic exo-electrogens in BA metabolize the organic substrate to produce VFA (acetic
251
acid, butyric acid and propionic acid). The concentration of VFA increased with time showing a
252
maximum VFA concentration of 800 mg/l in CC mode of operation compared to OC (580 mg/l)
253
(Fig 4b). The VFA profile was evaluated to understand the variations in the acidogenic
254
metabolism. CC operation depicted a highest concentration of acetic acid (95.3%) followed by
255
butyric acid (3.1%) and propionic acid (1.5%) (Fig 4c). OC operation also showed more or less
256
similar trend in acid metabolite production in the case of acetic acid (83.3%) and butyric acid
257
(16.6%). Both the operations showed a typical acidogenic metabolites profile, however, the
258
production of higher acetic acid fraction in CC operation can be attributed to the increase in
259
electrogenesis as the production of longer chain fatty acids consume more electrons (Sarkar et
260
al., 2016).
261 262
3.5 Redox Changes
263 264
With the production/consumption of the metabolites and developing ionic flux across the AEM
265
and CEM in the three chambers results in the alteration of the system redox conditions (pH) with
266
function of time. Initial pH of the anolyte, catholyte and SGW was set to 6, 7.5 and 7
267
respectively (Fig. 4d). The pH showed a decremental trend in BA (OC: 5.2, CC: 4.8) due to the
268
formation of VFA and the migration of anions across the AEM to form acidic intermediates in
269
the presence of protons which was relatively more amenable in CC compared to OC. The AC
270
showed an increase in the pH as the result of migrating cations across the CEM to form alkalis in
271
the catholyte. The change in catholyte pH correlates with the trend observed in BA, under OC
272
operation, wherein the increase was marginal (7.5 to 7.6) compared to CC (7.5 to 7.85). The pH
273
of mDC remained in the range of 7-7.2 showing minimal change with time. The drop and
274
increase in the anolyte and catholyte pH respectively not only depicts the progress of
275
desalination but also documents the resource recovery potential from MDC. The effluents from
276
BA and AC can be used for producing acids and bases.
277 278
Fig 4
279
3.6 Bioelectrokinetics: Cyclic Voltammetry and Tafel Analysis
280 281
The electro-metabolism of anodic biocatalyst was evaluated employing CV in both closed and
282
open circuit operation at varying scan rates (0.5 to 100 mV/s) with a potential range of −1.0 V to
283
+1.0 V. The voltammograms assisted to quantitatively compare the electrochemical interaction
284
between the biocatalyst and electrode based on electron transfer kinetics and peak currents
285
(Velvizhi and Venkata Mohan, 2012). CC operation showed significant increase in oxidative
286
(15.2 mA) and reductive current (-20 mA) than OC (4.5 mA and -2.5 mA) at 100 mV/s (Fig
287
5a,b). The increase in catalytic currents infers the continuous flow of electrons resulting from
288
substrate oxidation and stable the biofilm formation, which correlates with the higher power
289
generation in the CC operation compared to OC conditions (Srikanth et al., 2011). The increase
290
in reductive currents under CC operation indicates the effective reduction of cations at the
291
cathode in the presence of oxygen which relates with the increase in pH and desalination rate
292
(Mehanna et al., 2010). Linear increase in current was observed with increase in scan rate in both
293
CC and OC operations, however, the capacitive current was observed to be higher in OC
294
conditions which may be because of a double ion layer formation. The double ion layer might
295
have formed as a result of the high charge deposition around the anode owing to the lower
296
desalination rate in OC.
297 298
Linear sweep Voltammetry was recorded at a scan rate of 1 mV/s (+1 to -1 V) to understand the
299
biocatalyst-electrode interaction in terms of oxidation and reduction currents with a function of
300
potential. The maximum peak currents of 4.74 mA (oxidative); -1.98 mA (reductive) in OC
301
model and 10.1 mA (oxidative); -5.98 mA (reductive) in CC (Fig 5c). The LSV profiles of the
302
CC showed higher catalytic currents as a result of the enhanced biocatalyst activity with an
303
efficient biocatalyst and electrode interactions. Whereas, in the LSV profile under OC operation
304
showed a linear trend which resembles the limited electrode- biocatalyst ratio owing to the
305
double ion layer formed due to deposited ions in the BA correlating to the higher capacitance
306
observed in cyclic voltammetry.
307 308
Tafel slopes were plotted in derive oxidation (βa) and reduction (βc) slopes (Raghavulu et al.,
309
2012) which helps to delineate the electrons transfer from biocatalyst to anode dependent on the
310
cathode activity inclusive of the electrochemical losses.
The CC operation has depicted a
311
oxidative slope (βa: 0.225 V/dec) and reductive slope (βa: 0.225V/dec) which are comparatively
312
higher than the OC (βa: 0.368V/dec) and reductive slope (βc: 1.392V/dec) (Fig 5d). The higher
313
electrochemical losses in the open circuit operation has resulted in relatively lower current
314
versus the potential compared to the lower slopes showing proportional increase in the current
315
with the increase of current in CC operation. The lower βa than βc in CC operation infers the
316
product formation at the AC with the utilization of electrons generated from the substrate in the
317
BA. The bioelectrochemical analysis inferred higher desalination efficiency owing to the
318
lowered electrochemical losses and enhanced biocatalyst performance correlating well with the
319
substrate removal and desalination rate. Fig 5
320 321 322
4. Conclusion
323 324
The study establishes MDC as a viable solution for ground as well as surface water softening
325
apart from production of bioelectricity and biobased products. The variations in the circuitry
326
mode operations with the specific design showed good desalination efficiency of ground water
327
without reject and with power production. The low-cost design can be used in water plants as
328
pretreatment option prior to RO purification. The prospects of MDC are diverse and the scaling-
329
up impediments of MDC can be overcome by process optimizations, stacking and effective
330
usage of low cost materials.
331 332
Acknowledgement
333
The authors thank The Director, CSIR-IICT for encouragement. Department of Science and
334
Technology (DST) in the form of project (DST/TM/WTI/2K15/35(G)) supported the research.
335
SKB acknowledges University Grants Commission (UGC) for providing research fellowship.
336 337 338 339 340
341
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427
Figure captions
428 429 430
Figure 1: Schematic representation of MDC block reactor (Triple Chamber) for efficient
431
desalination and resource recovery with bioelectricity generation.
432
Figure 2: (a): TDS removal in the three chambers (mg/l) with a function of time, (b): Rate of
433
change of TDS (mg/l/h) with the function of time for different chambers of MDC block reactor –
434
Open circuit and Closed circuit (1KΩ).
435
Figure 3: (a) Cell voltage of MDC block reactor in open circuit operation and closed circuit
436
operation (1 KΩ), (b) Polarization profile showing the maximum power point (MPP) of MDC
437
block reactor.
438
Figure 4: (a): COD removal in BA of MDC block reactor under Open circuit and Closed circuit
439
(1KΩ) operations, (b): VFA profile in BA of MDC block reactor under Open circuit and Closed
440
circuit (1KΩ), (c): VFA composition profile (acetic acid, butyric acid and propionic acid)
441
quantified at the maximum VFA production, (d): pH changes in the three chambers of MDC
442
block reactor- Open circuit and Closed circuit (1KΩ).
443
Figure 5: Cyclic voltammetry (CV) profile of MDC block reactor performed with different scan
444
rates (a) Open circuit operation, (b) Closed circuit operation (1kΩ), (c) Linear sweep
445
voltammetry (LSV) profile of MDC block reactor with different operating conditions (d) Tafel
446
analysis of MDC reactor with different operating conditions.
447 448 449 450 451 452 453 454 455 456 457
458 459 460
Figure 1: Schematic representation of MDC block reactor (Triple Chamber) for efficient
461
desalination and resource recovery with bioelectricity generation.
462 463 464 465 466 467 468 469 470 471 472 473
600
2600
BC
350
mDC
2400
500
a
AC
300
300 200 CC OC
100
250
2000
TDS (mg/l)
TDS (mg/l)
TDS (mg/l)
2200 400
1800 1600 1400
12
24
36
48
Time (h)
474
150 100
CC OC
1200
CC OC
50
1000 0
200
0
0
12
24
36
48
0
12
24
36
48
Time (h)
Time (h)
20 BC
mDC
b
AC
TDS Removal Rate (mg/l/h)
10 0 -10 -20 -30 -40
CC OC
-50 -60 0
475
12 24 36 48 60 72 84 96 108 120 132 144 156 168
Time (h)
476
Figure 2: (a): TDS removal in the three chambers (mg/l) with a function of time, (b): Rate
477
of change of TDS (mg/l/h) with the function of time for different chambers of MDC block
478
reactor – Open circuit and Closed circuit (1KΩ).
479 480 481 482 483
800
a
600
Cell Voltage (mV)
400
200 140
120
OC CC
100 0
6
12
18
24
30
36
42
48
Time (h)
484 485
b
600
0.35
0.30 400 0.25 300 0.20
200
100 0.0000
Voltage Power Density 0.0005
0.0010
2) Power Density (W/m
Voltage (mV)
500
0.15 0.0015
2
0.0020
0.0025
486
Current Density (A/m )
487
Figure 3: (a) Cell voltage of MDC block reactor in open circuit operation and closed circuit
488
operation (1 KΩ), (b) Polarization profile showing the maximum power point (MPP) of
489
MDC block reactor.
490
a
3000 OC CC
COD (mg/l)
2500 2000
52%
1500 70%
1000 500 0 0
12
24
36
48
Time (h)
491
VFA (mg/l)
800
b
OC CC
600
400
200
0 12
492 493 494 495 496 497 498 499 500
24
36
Time (h)
48
501 95.36% 95.36% 83.33% 83.33%
OC
CC 16.67% 16.67%
1.55% 1.55% 3.1% 3.1%
Acetic acid Butyric Acid
Acetic Acid Butyric acid Propionic Acid
502 503
7.9
BC
6.0
7.4
5.5
7.3
5.0
7.2
d
AC
mDC
7.8 7.7
4.5
pH
pH
pH
7.6
7.4
7.1 CC OC
4.0
7.5
CC OC
7.0
CC OC
7.3 7.2
0
12
24
Time (h)
504
36
48
0
12
24
36
48
0
12
Time (h)
24
36
Time (h)
505
Figure 4: (a): COD removal in BA of MDC block reactor under Open circuit and Closed
506
circuit (1KΩ) operations, (b): VFA profile in BA of MDC block reactor under Open circuit
507
and Closed circuit (1KΩ), (c): VFA composition profile (acetic acid, butyric acid and
508
propionic acid) quantified at the maximum VFA production, (d): pH changes in the three
509
chambers of MDC block reactor- Open circuit and Closed circuit (1KΩ).
510 511 512 513 514 515
48
5 50mV/s 5mV/s
15
30mV/s 1mV/s
10
Current (mA)
Current (mA)
3
100mV/s 10mV/s 0.5mV/s
2 1
b
20
a 4
100mV/s 10mV/s 0.5mV/s
50mV/s 5mV/s
30mV/s 1mV/s
5 0 -5
-10
0
-15
-1
-20 -2
-25
-3
-30 -1.0
-0.5
0.0
0.5
1.0
-1.0
E (mV) vs Ag/AgCl (3.5M KCl)
-0.5
0.0
0.5
1.0
E (mV) vs Ag/AgCl (3.5M KCl)
516 517 12
2
c
10
1
8 6
log (|/mA|)
Current (mA)
d
100mV/s
OC CC
4 2 0 -2 -4
0 -1
OC CC
-2 -3
-6 -8 -1.0
-0.5
0.0
0.5
E (mV) vs Ag/AgCl (3.5M KCl)
1.0
-1.0
-0.5
0.0
0.5
1.0
E (mV) vs Ag/AgCl (3.5M KCl)
518
Figure 5: Cyclic voltammetry (CV) profile of MDC block reactor performed with different
519
scan rates (a) Open circuit operation, (b) Closed circuit operation (1kΩ), (c) Linear sweep
520
voltammetry (LSV) profile of MDC block reactor with different operating conditions
521
(d) Tafel analysis of MDC reactor with different operating conditions.
522 523
Highlights
524 525
•
Desalination of ground and surface water is an emerging green technology.
526
•
Triple chambered MDC for desalination, waste remediation and product recovery.
527
•
Exoelectrogenic activity is the drives desalination under varied circuitries.
528
•
Salts and hardness removal using microbial desalination.
529 530 531 532 533 534 535