Multi-electrode bioelectrochemical system for the treatment of high total dissolved solids bearing chemical based wastewater

Accepted Manuscript Multi-Electrode bioelectrochemical system for the treatment of high dissolved solids bearing chemical based wastewater G. Velvizhi, S. Venkata Mohan PII: DOI: Reference:

S0960-8524(17)30687-9 http://dx.doi.org/10.1016/j.biortech.2017.05.048 BITE 18074

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Bioresource Technology

Received Date: Revised Date: Accepted Date:

31 January 2017 6 May 2017 8 May 2017

Please cite this article as: Velvizhi, G., Venkata Mohan, S., Multi-Electrode bioelectrochemical system for the treatment of high dissolved solids bearing chemical based wastewater, Bioresource Technology (2017), doi: http:// dx.doi.org/10.1016/j.biortech.2017.05.048

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ICETB 2016 Abstract No: BR-032 Multi-Electrode bioelectrochemical system for the treatment of high dissolved solids bearing chemical based wastewater G.Velvizhi * and S. Venkata Mohan Bioengineering and Environmental Sciences Lab, EEFF Department CSIR- Indian Institute of Chemical Technology (IICT), Hyderabad - 500 007, India *Corresponding author: E-mail: [email protected] Tel/Fax: 0091-40-27191765 Abstract Multi-electrode bioelectrochemical treatment system (ME-BET; membrane less) consisting of six electrode assemblies (E1 to E6) was developed for the treatment of complex chemical based wastewater with high salt concentrations. The study was also compared with single electrode assembly BET reactor (SE-BET). Enhanced salts and COD removal was observed in ME-BET (32%; 56%) compared to SE-BET (11%; 23%) as a result of in situ bio-potentials generated from multiple electrodes through the oxidation of organic substrate in the wastewater. Inorganic pollutants viz., nitrates (28%; 8%), sulphates (25%; 9%) and phosphates (20%; 7%) removal was higher in ME-BET in comparison with SE-BET and this was also supported with bioelectrogenic activity (584; 160 mW/m3). The study infers that the development of compact reactors with multiple electrodes in a single system enhances the anodic reactions for effective treatment of complex wastewater and simultaneous power production. Key words: Total Dissolved Solids (TDS); Electron losses; Desalination; Microbial fuel cell; Voltage reversal.

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1.0 Introduction Microbial fuel cell (MFC) as bioelectrochemical treatment (BET) unit is gaining much prominence towards treatment of wastewater (Venkata Mohan et al., 2009a; Virdis et al., 2010; Liu et al., 2012). The organic and inorganic pollutant present in the wastewaters are considered to be potential substrates for BET systems due to the dual benefits of treating waste as well as generating bioelectricity (Venkata Mohan et al., 2013). In these systems microorganism act as biocatalyst, advocating the electron flux through the solid electrode from microbial catabolic activities by degrading organic molecules present in the wastewaters (Butti et al., 2016). The variation of redox gradient created between the biocatalyst and the electrodes induces the development of potential difference which acts as a net driving force for bioelectrogenic activity and complex pollutant removal (Venkata Mohan et al., 2014; Kracke et al., 2015; Choi and Sang 2016). BET can generate energy out of waste without the input of external or additional energy, and simultaneously treat real field wastewater like distillery (Mohankrishna et al., 2010), dairy (Venkata Mohan et al., 2010), chemical wastewater (Venkata Mohan et al., 2009a), petroleum refinery wastewater (Venkata Mohan and Chandrasekhar, 2011), steroidal drug industrial effluent (Liu et al., 2012) tetrachloroaurate wastewater (Choi et al., 2013) etc. The pollutants such as phenols (Firman et al., 2012), chromuim (Wang et al., 2008), perchlorate (Thrash et al., 2007), nitrates (Virdis et al., 2010), sulphates (Dutta et al., 2009), etc. (Kumar et al., 2017) increases the salinity of the wastewaters. Generally treatment of these complex wastewater are difficulty in conventional biological processes specially when the salt concentration exceeding 1% (wt/v) (Venkata Mohan et al., 2009a). The physicochemical processes such as reverse osmosis, electro dialysis, ion exchange, sedimentation etc., are expensive and saline wastewater with high concentration of suspended solids increases the buoyancy force and consequently decreases the sedimentation where the settling is difficult in physical unit operations (Jang et al., 2013). Bioelectrochemical treatment is an effective alternative bioprocesses which couples both electrochemical and anaerobic biological processes which triggers the redox reactions for the degradation of complex pollutants specific to salt concentration with simultaneous power generation (Venkata Mohan et al., 2010; Velvizhi and Venkata Mohan, 2011). Based on the complexity of wastewater the requirement of redox reactions vary, however single electrode assembly has the limited electron flux with a theoretically potential of 1.14 V due to the thermodynamic constraints of NADH (−0.32 V) and oxygen (+0.82 V) (Wang and Han, 2009). Studies have been reported that stacked MFCs with multiple electrodes arrangements connecting in series and parallel circuitry attain enhanced voltage and current (Kim et al., 2

2011; Winfield et al., 2011). Enhanced active sites of electrode interface might increase the redox reactions for enhanced power production (Venkata Mohan et al., 2009b; Liu et al., 2008; Zhuang, et al., 2012). Currently research following suit with the incorporation of multiple electrodes for power production in MFC could be shifted towards complex pollutant treatment as BET. Keeping this in context, the present study implies a strategy to treat complex chemical based wastewater specific to high salt concentrations by introducing multiple electrodes in a single system. Wastewater originated from chemical based industries with high total dissolved solids (HTDS- IW) was used as feed in ME-BET and SE-BET reactors. The treatment performance was evaluated in terms of organic and inorganic pollutant degradation viz., COD, TDS, nutrients, sulphates, turbidity and colour removal. The electrogenic, electrochemical and electrokinetic studies was performed for all the individual electrodes and for parallel and series circuit connections. 2. Experimental Methodology 2.1 Reactor configuration and Electrode Assembly ME-BET and SE-BET reactors were fabricated using perspex material with a dimension of 27 × 18.6 × 12.5 cm (L/B ratio~1.5), with a total designed volume of 6 l and working volume of 2.5 l. The reactor was layered with gravel bed of varying size as coarse, medium and fine gravel from bottom to top as the details are provided in the Velvizhi et al., (2014) Velvizhi et al. (2014). Non-catalyzed graphite electrodes (with size of 7×3.5 cm and 0.6 cm thickness) were used as anode and cathode. ME-BET was designed with six electrode assembly, the electrodes were arranged such that they were hydraulically connected without partition in two parallel rows (E1 to E3 and E6 to E4) (Fig 1a). Individual electrodes (E1 to E6) and in combination with all the six setups in series and parallel circuitries were evaluated. SE-BET was designed with a single set of anode and cathode assembly with similar operating conditions of ME-BET with only variation of electrode assembly (Fig 1b). Fig 1 2.2 Wastewater Composition and Biocatalyst Complex chemical based industrial wastewater consisting of high salt concentrations was collected from a local bulk drug manufacturing unit with a characteristics of COD, 40±0.1 g/l; TDS, 144 ± 0.5 g/l; pH, 12.8 ± 0.5; colour, 7000 ± 50 Hazen units; turbidity, 2789 ± 50 NTU; BOD, 13.5 ± 0.1g/l; phosphates (as PO43-), 3.2 ± 0.05 g/l; nitrates (as NO32-), 3.2 ± 0.05 g/l). Anaerobic consortia acquired from a full scale anaerobic reactor treating composite chemical

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wastewater were used as biocatalyst (10% v/v). Prior to inoculation, the parent biomass was pretreated as mentioned in Venkata Mohan et al., (2008) (Venkata Mohan et al., 2008). 2.3 Operation The real field HTDS-IW was diluted with tap water and fed to ME-BET and SE-BET reactors with a TDS of 10 g/l (TDS1) with a organic loading (OL1) of 1.25 kg COD/m3. ME-BET was operated with three different TDS concentration viz., TDS2-20 g/l; TDS3-30 g/l and TDS4-40 g/l with a OL of 2.5, 3.75 and 5 kg COD/m3 (OL2-OL4). The feed was adjusted to pH 7.0 using (1N) orthophosphoric acid before feeding the reactors. The reactor was operated at different hydraulic retention time (HRT) of 10, 13, 17 and 23 days corresponding to increase in OL. 2.4Analysis TDS, COD, nitrates, sulphates, turbidity and colour analysis were performed as per standard methods (APHA, 1998). Power generation of the system (voltage and current) was measured using a digital multi-meter with a 100 Ω resistance. Anode and cathode potentials were measured against varying external resistances (30 to 0.05 kilo Ohm (kΩ) against Ag/AgCl as reference electrode. Polarization was measured using varying external resistor (30 to 0.05 kΩ). A potentiostat-glavanostat system (Autolab- PGSTAT12, Ecochemie) was used to record cyclic voltammetry (CV) by applying a potential ramp (0.5 to -0.5 V). Tafel plots were derived from the voltammetric profiles obtained at maximum performance with respect to each TDS loading using GPES software. 3.0 Results and Discussions 3.1 Multi-pollutant Removal The study compares the treatment performance of organic and inorganic pollutant of TDS1 (10 g/l) in ME-BET and SE-BET reactors. Based on the comparative performance in terms of treatment and electrogenic activity the study was further extended with increasing TDS concentration (TDS2-TDS4) in ME-BET reactors. The multiple pollutants such as nitrates, phosphates, sulphates etc., present in the high saline wastewater have both oxidizing and reducing properties. 3.1.1 TDS removal The complex chemical based industrial wastewater specific to high salt concentrations (144 g/l of TDS) was diluted to 10 g/l (TDS1) with tap water and used as feed in the ME-BET and SEBET reactors. During initial days of operation, the mixed biocatalyst consisting of hydrolytic and fermentative micro-organisms has the potential to oxidize the organic waste present in the wastewater through anaerobic oxidation. After few cycles of optimization, the presence of 4

electrodes in BET system creates a bioelectrogenic microenvironment that facilities the formation of electrochemically active biofilm on anode. These electricigens develops in situ biopotentials by the degrading the complex substrate, which imposes direct electric field developing the electrochemical gradient thereby separating the charged ions towards oppositely polarised electrode which resulted in the removal of salts in BET system (Venkata Mohan et al., 2009a; Li et al., 2010). Maximum TDS removal was observed in ME-BET (ξTDS, 32%) in comparison to SE-BET (ξTDS, 11%) (Fig 2a; Table1). Significant difference was observed between both the reactors due to the increased surface area to volume ratio in MEBET (0.168 cm2/m3) which was able to accommodate effective ionic distribution than SE-BET (0.028 cm2/m3). Multiple anodic active site increases the anodic electron transfer reactions through direct anodic oxidation (DAO) and indirect anodic oxidation (IAO) mechanisms which adsorbs the pollutant on the multiple anode surface resulting in the breakdown of salts influencing enhanced reduction of the salt in ME-BET (Mohana Krishna et al., 2010; Velvizhi and Venkata Mohan, 2015). Based on effective performance of ME-BET, the reactor was further operated with increasing TDS concentration (20-40 g/l) (TDS2–TDS4). TDS2 reported, higher removal efficiency (ξTDS 28%) followed by TDS3 (ξTDS 22%) and TDS4 (ξTDS 18%) (Fig.2b). Electrodeposition of white patches of salt was observed on all the cathodes surfaces of BET system which might be due to the salts diffusion. With the increase in concentration the disposition of salts was observed to be higher hence periodic salt recovery was performed by scrapping the electrode surface to enable undisturbed cathodic reduction reactions. Fig 2 and Table 1 3.1.2

Nutrients removal

Bioelectrochemical treatment systems are effective to reduce inorganic pollutants through microbial catalyzed reduction reactions. The nitrates and phosphates present in the high saline wastewater are reduced by using the electrons and protons produced from the organic oxidation through electrochemical active bacteria, denitrifying bacteria and other bacteria which are present in mixed bacterial consortia of BET system. Maximum nitrate and phosphate reduction was observed in ME-BET (ξNO32- 28 %; ξPO43- 20 %)) in comparison to SE-MET (ξNO32-8 %; ξPO43-7 %) for TDS1. Significant difference was observed between single and multiple electrode assemblies indicating that availability of multiple numbers of sites for microbial colonization on solid electrodes which stimulates the adhesion and growth of biomass resulting in higher biocatalyst density in ME-BET. The cumulative potential driven by the multiple electrodes migrate the nitrate and phosphate ions to get reduced with a sequence of reduction 5

reaction from nitrate to nitrogen gas. Since the nitrate has stronger mobility the reduction is more in nitrates than phosphates. With increase in concentration TDS2 (ξNO32- 22 % and ξPO4315%), reported higher followed by TDS3 (ξNO32-18% and ξPO43- 12%) and TDS4 (ξ NO32- 12 % and ξPO43-10 %) (Fig. 2c). Enhanced bacterial biofilm was formed on the multiple electrodes indicates effective utilization of nutrients for bacterial assimilation. 3.1.3 Sulphate removal Sulphate degradation encompasses both oxidation and reduction processes in the bioelectrocatalytic system which could partially recover the energy comprised in the sulfide through its re-oxidation at the anode compared to anaerobic systems (Velvizhi and Venkata Mohan et al., 2011). In anaerobic reactors the hydrogen sulphide remains as end-product, whereas in bioelectrocatalytic operation hydrogen suphide will get further converted to elemental sulphur with the presence of solid electrodes as terminal electron acceptor (Rabaey et al., 2006). ME-BET reported maximum sulphate removal (ξSO 2-4 25%) in comparison to SEBET (ξSO

2

-4

9 %). Multiple electrodes in single system increase the overall electric field

strength which enhances the ion migration resulting in reduction of sulphate. The compact placing of electrodes in ME-BET effectively formulated the electron flux between the electrodes and enhanced the electrode interface for effective biofilm formation resulting for effective treatment of high saline based wastewater. With increase in concentration, TDS2 reported removal efficiency of 21% followed by TDS3 (ξSO 2-4 %) and TDS4 (ξSO 2-4 14%) (Fig .2c). The negatively charged chlorides and sulphates ions tend to move towards anode and might get reduced and deposited on anode as chlorides and elemental sulphur due to the potential gradient developed in the BET system. The white patches were observed on the anode surface also supports the same. With increase in TDS concentration the deposition of elemental sulphur was observed to higher. Electron sources and sinks occurred sequentially in anode chamber helps to degrade the pollutants. Along with inorganic pollutant, the organic pollutants removal in terms of COD was observed in BET reactors. 3.1.4 COD removal The wastewater consists of high organic content in terms of simple as well as complex forms. The organic substrate is degraded by anaerobic oxidation and creates the electrogenic microenvironment for harvesting bioelectricity. Hydraulically connected ME-BET reported higher removal efficiency (OL1-50%) in comparison to SE-BET (OL1-23%). The insitu biopotential generated in the system degrades the complex pollutant through direct and indirect anodic oxidation processes. The presence of TDS in the solution enhances the anodic oxidation of chloride ions to dissociated hypochlorite ions which act as the main oxidizing agent for 6

pollutant degradation (Venkata Mohan et al., 2009a).The DAO facilitates formation of primary oxidants on the anodic surface area which further react on the anode yielding secondary oxidants such chlorine dioxide and ozone, which have significant positive effect in reducing the complex pollutant (Velvizhi and Venkata Mohan, 2011). ME-BET reported higher biodegradation due to the more number of active site of electrodes for effective electrolytic discharge followed by physical adsorption of reactive hydrox radical on the anode surface treats complex pollutant. At higher concentration the COD reduction was higher in OL2 (ξCOD, 41%) followed by OL3 (ξCOD, 32 %) and OL4 (ξCOD, 20%) (Fig. 2d). The effective contact between biocatalyst, electrolyte in the multiple electrodes resulted higher biofilm growth on the electrodes attributed to significant increase in COD degradation. In addition to electrode assembly, the biofilm attached on the substratum holds up high biomass and facilitates the microorganism to be exposed with the substrate in a controlled unsteady-state conditions degrades the low biodegradable wastewater effectively. The biofilm might also act as a buffer to reduce the concentration of toxic chemicals present in the high saline complex wastewater (Venkata Mohan et al., 2005). The electrogenic activity of the system was also analyzed in terms of substrate utilization to power generation. The power yield for TDS1 is 1.00 W/kg CODR, followed by TDS2 is 0.54 W/kg CODR, TDS3 is 0.43 W/kg CODR, and TDS4 is 0.233 W/kg CODR..The power yield was low with increase in TDS indicating that the electrons generated in the system might get reduced to other soluble electron acceptors rather than harvesting power. 3.1.5 Colour and Turbidity removal The industrial saline wastewater consists of color producing compounds along with organic and inorganic pollutant. Significant difference in colour removal was observed between MEBET (36%) and SE-BET (10%). The in situ biopotential generated in BET system cleaves effectively the bonds of the colour producing compounds in the wastewater (Sreelatha et al., 2015; Velvizhi and Venkata Mohan, 2015). In ME-BET system, the presence of multiple electrodes triggers the redox reactions to reduce the complex color producing compounds. With increase in concentration, TDS2 reported higher removal efficiency (ξColour 31%) followed by TDS3 (ξColour 28%) and TDS4 (ξColour 20%). The studies are also been reported that the influence of sodium chlorides in wastewaters derives the formation of chlorohydroxyl radical based on the potential developed in the system which also influence on the cleavage of colour producing compounds (Guven et al., 2008). Saline wastewaters are also referred to have high concentration of turbidity, the salt present in the wastewaters increases the buoyancy force and which in turn increase the turbidity (Jang et 7

al., 2013). In anodic oxidation of BET system, the reactive species like primary and secondary oxidants generated at multiple anodic electrode surfaces helps to break the complex chemical structures present in wastewater which also reduces the turbidly. ME-BET reported higher turbidity removal (ξTurbidity43%) to SE-BET (ξTurbidity12%). Enhanced active sites of electrode interface might increase the redox reactions for enhanced colour reduction. The turbidity removal pattern varied with increase in concentration, TDS2 (ξTurbidity 35%), TDS3 (ξTurbidity28%) and TDS4 (ξTurbidity18%) (Table 1). Presence of multiple electrode arrangement in single chamber contributes the bio-electrochemical catalytic mechanism resulted in destabilization of particles, resulting in turbidity removal in HTDS-IW. BET takes the advantage of solid electrodes to treat wastewaters by direct (DAO) and indirect anodic oxidation (IAO) mechanism (Mohan Krishna et al., 2010; Velvizhi and Venkata Mohan et al., 2015). In addition to treatment this compact design also reported effective electrogenic activity. 3.2 Bioelectrogenic activity Bioelectrogenic performance of SE-BET and ME-BET (E1 to E6) was evaluated at TDS1 concentration. Voltage and current was measured for single electrode setup in SE-BET and six electrodes (E1 to E6) were measured individual and series/parallel connections in ME-BET. SE-BET reported OCV of 110 mV and power of 168 mW/m3 with a load of 100 Ω. In MEBET, parallel circuit reported OCV of 468 mV, current 8.64 mA and power 584 mW/m3 and in series connection the OCV was 400 mV, current 4.38 mA and power 496 mW/m3 (Fig 3a and 3b).Significant difference in electrogenic performance was observed in ME-BET in comparison to SE-BET. Increase in surface area of electrodes in ME-BET enhances the electrical signaling between the neighboring electrodes through bacterial ion channels interspecies communication of the biofilms developed on multiple electrodes. In SE-BET reactor, the ion transport distance increases between the electrodes was high and this enhances the internal resistance resulting to low electrogenic activity. Hence the study infers that placing single electrode in a larger reactor leads to volumetric loss and this can be overcome by compactly placing multiple electrodes in a single system. The cells were hydraulically connected without partition hence cumulative voltage and current of six electrodes were not observed. Among the individual electrodes, E1 and E6 depicted higher electrogenic activity (380 mV; 1.25 mW; and 373 mV; 1.18mW), followed by E2 (345 mV and 1.12 mW), E5 (338 mV and 0.9 mW), E3 (287mV and 0.8 mW) and E4 (274 mV and 0.76 mW) (Fig 4a). In case of individual electrodes, the electrogenic performance was observed to vary based on the biofilm developed on the respective electrodes though all the electrode are of similar size and operated 8

in similar conditions. Since the recirculation ports were placed near E1 and E6, the biofilm on the electrodes were observed to be higher which eventually resulted to higher power production.The biofilm which are densely packed and distributed on multi-electrode might have a complex electron network involving various electron transfer components which could be presumed for current generation. For TDS2 concentration, parallel circuit (OCV 456 mV and power 1.3 mW) and series (389 mV and 1.18 mW) followed by TDS3 (Parallel (420 mV and 1.071 mW) series (375 mV and 0.95 mW) and TDS4 (parallel (256 mV and 0.564 mW), series (225 mV and 0.47 mW) (Fig 4a). In series connection, energy losses were observed primarily due to the effect of lateral ion cross-conduction between the electrodes (Ren et al., 2014) (Fig 4a). Based on the effective performance of ME-BET than SE-BET, the further electrogenic studies were carried out for ME-BET at varying TDS concentration. Fig 3 3.2.1 Anode potential The electron flow from the biocatalyst to the anode and the surrounding medium is measured with reference to Ag/AgCl (S) for TDS1. Parallel circuit reported higher anode potential (-481 mV) followed by series (-420 mV), E1 (-385 mV), E6 (-362 mV), E2 (-345 mV), E5 (-312 mV), E3 (-275 mV) and E4 (-264 mV). The anode potential is positively associated with microbial biomass and activity on the anodic electrode, the biofilm was observed to be higher at E1 and E6 due to effective biomass growth on the electrodes compared to E3 and E4. The anodic performance is inextricably dependent on the rate of the bacterial metabolism, and the rate of electron transfer from the microbial cell to the anode (Schroder, 2007). The anode potential measured with varying resistance (30-0.05 kΩ) showed stable performance until 5kΩ of external resistance and later a drop was observed with further decrease in the external resistance. With increase in TDS concentration, the anode potential was observed to be less since the nature of the microbial species was not able to increase the metabolic processes due to high salt concentration (TDS2-TDS4; -300 to -180 mV). The metabolic flux were blocked due to the deactivation of the bacterial activity hence transfer of electrons from the biofilm to anode will be restricted thus resulting low power generation at higher TDS (Wagner et al., 2010). 3.2.2 Polarization behavior Electrode polarization analysis is one of the prerequisite for systematic optimization of fuel cell which helps to determine the electrochemical losses that affect the electrogenic activity of fuel cell. The change in electrode potential and current flow with varying resistance (30-0.05 kΩ) was measured as a function of current density against potential and power density for TDS1 9

and the maximum performance was observed for ME-BET. Parallel circuit reported a cell design point (CDP) of 300 Ω with a power of 0.47 mW and series circuit reported a CDP of 200 Ω with a power of 0.36 mW (Fig 4b). Ren et al., (2014) reported that voltage reversal occurs when fuel cells with different emfs are connected and the same was also observed in the present study in series connection (Ren et al., 2014). The potential was observed to be maximum at high resistance and gradually dropped with decrease in resistance and correspondingly there was increase in current. Optimized resistance for each electrode assembly varied based on the electrogenic activity E1 and E6 (CDP of 300 Ω and power of 120 mW/m3) and (200 Ω /112 mW/m3) reported higher followed by E2 (200 Ω / 102 mW/m3), E5 (100 Ω /92 mW/m3), E3 (100 Ω/ 76 mW/m3) and E4 (100 Ω/ 60 mW/m3) (Fig 4b). Voltage drop curves reveals the electron losses during their transfer of electrons from the biocatalyst to the anode and then to the cathode. The losses are due to the anodic overpotentials viz., activation, ohmic and concentration polarization (CP) losses which are usually encountered during bioelectrochemical cell operation. Initially at higher resistance, the lower ranges of current densities are observed where the activation losses occur (Velvizhi and Venkata Mohan, 2012). In the present system, the activation losses were observed to be high since non catalyzed graphite electrodes were used. With increasing current the polarization curve show linear drop representing the ohmic resistance, however in the present study the linearity was less since the ionic conductivity was higher for the anolyte indicating less ohmic resistance and also due to the placement of electrodes in the compact design. With further increase in current a sharp drop in cell voltage was observed which is generally attributed to mass transfer losses due limited turnover of bacterial reaction Fig 4 3.2.3 Electrochemical and Kinetic Behavior Electron transfer dynamics of redox species between the electrode and biofilm was analyzed in cyclic voltammtery by applying external potential (0.5 to -0.5 V) on all the electrodes individually and by connecting in parallel and series connection for ME-BET. The maximum redox current was observed in TDS1 followed by TDS2, TDS3 and TDS4 as depicted in the figure 5a (Fig 5a). For TDS1, the parallel circuit documented higher oxidative and reductive current (90 mA,-75 mA) followed by series (65 mA, -45 mA), E1 (24 mA, -15 mA), E6 (21 mA, -13 mA), E2 (17 mA, -10 mA), E5 (15 mA,-7 mA), E4 (4 mA, -3 mA) and E3 (2.5 mA, 2.5 mA). The study depicted enhanced redox current in the parallel circuit depicting the cumulative effect of all individual electrodes in comparison to series connection. The lateral ionic cross conductance which is common for the fuel cell arrays sharing the same electrolyte 10

and it also depends on the number of units connected with each other (Zhuang et al., 2012) whereas the parallel circuit maintains stability to avoid voltage reversal and depicted higher current (Hatzell et al., 2013). Conductive nature of the anolyte depicted equal distribution of oxidative and reductive reaction. With increase in TDS concentration, the redox reactions was favoring more on oxidation rather than reduction. This might be attributed to the utilization of electrons on the working electrode and relatively less reduction of electrons on the counter electrode due to the existing inherent internal resistance of system and less bacterial activity. The electrodeposition of white patches of salt was observed on all the cathodes which might also influence on the decrement in current production with increase in concentration, disposition of salts was observed to be high for TDS3 and TDS4 resulting less electrochemical activity. Electrode kinetics through Tafel plots provides an understanding to interpret the reaction rate of biocatalytic activity based on the derived kinetic parameters in terms of losses through oxidative and reductive slope (Velvizhi and Venkata Mohan, 2012). The electrons (and protons) need to overcome different barriers to get transferred from the biocatalyst to the terminal electron acceptor through anode and cathode prior to their reduction. These losses are accounted under activation losses especially at low current densities. The oxidative and reductive slope was observed for parallel circuit for all the TDS concentration. Gradual decrease in the redox slope was observed for TDS4 to TDS1 from 0.562 to 0.124 V/dec (oxidative solpe) and 0.342 to 0.115 V/dec (reductive slope) (Fig 5b). The low redox slope infers efficient transfer of electrons by overcoming the losses. The salt concentration increases the conductivity of ions and enhances the electron mobility favouring electrogenic activity HTDS –IW. Fig 5 4.0 Conclusion The study infers that the strategy of introducing multiple electrodes in BET systems increases the electrode interface for the biofilm development resulting effective treatment of high saline wastewater than single electrode BET system. The vital advantage of this compact design is to treat specific complex inorganic pollutants due to the increased surface area in ME-BET system. The spacing of electrodes is a critical factor for this compact design hence studies could be carried on optimizing the electrode distance. This low cost design can easily be scaled up for practical applications with less operational problems.

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Acknowledgements The authors wish to thank the Director, CSIR-IICT for support and encouragement. GV acknowledges CSIR for providing research support under the scientist pool scheme. Department

of

Science

and

Technology

(DST)

in

the

form

of

project

(DST/TM/WTI/2K15/35(G)) supported the research. References 1. APHA,Water Environment Federation and American Water Works Association, In Standard Methods for the Examination of Water and Wastewater, 1998. 20th Ed.Washington DC. 2. Butti, S.K., Velvizhi,G., Mira L.K., Johanna S.M., Haavisto, Koroglu,E.O., Cetinkaya,A.Y., Singh, S., Arya,D., Modestra, J.A., Vamsi K., Verma,A., Ozkaya, B., Lakaniemi,A., Puhakka,J.A.,Venkata Mohan,S 2016. Microbial Electrochemical Technologies with the perspective of Harnessing Bioenergy: Maneuvering towards Upscaling.Rene. Sust. Energy Reviews, 53, 462-476. 3. Choi,O., Sang, B., 2016. Extracellular electron transfer from cathode to microbes: application for biofuel production.Biotechnol. Biofuels. 9, 11-25. 4. Dutta, P.K., Keller, J., Yuan, Z., Rozendal, R.A., Rabaey, K. 2009. Role of sulfur during acetate oxidation in biological anodes. Environ Sci Technol. 43, 3839-45. 5.

Friman, H., Schechte,A., Nitzan, Y., Cahan, R., 2013. Phenol degradation in bioelectrochemical cells. Int. Biodeter. Biodeg 84, 155–160.

6. Hatzell, M.V., Kim, Y., Logan, B.E., 2013. Powering microbial electrolysis cells by capacitor circuits charged using microbial fuel cell J. Power. Sources, 229, 198-202. 7. Jang, D., Hwang, Y., Shin, H., Lee, W. 2013. Effects of salinity on the characteristics of biomass and membrane fouling in membrane bioreactors. Bioresour Technol. 141, 50–6. 8. Kim, JR.,Rodríguez, J.,Hawkes, FR.,Dinsdale, RM., Guwy,AJ., Premier GC., 2011. Increasing power recovery and organic removal efficiency using extended longitudinal tubular microbial fuel cell (MFC) reactors. Energy Environ. Sci. 4, 459–465. 9. Kracke, F., Vassilev, I., and Krömer, J. O. 2015. Microbial electron transport and energy conservation – the foundation for optimizing bioelectrochemical systems. Front. Micro, 6, 575-623

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Table 1 Comparative performance of single and multi-electrode assembly at varying TDS concentrations Parameters

SETDS1 110 0.43 15

ME-TDS1

ME-TDS2

ME-TDS3

ME-TDS4

389 1.23 32

372 1.12 28

265 0.9 22

200 0.41 18

TDS removal (mg/l) COD (%)

1500

3200

5600

6600

7200

23

56

41

32

26

COD removal (mg/l) Nitrates (%) Sulphates (%) Phosphates (%) Colour (%) Turbidity (%)

690

1550

2460

2880

3120

8 9 7 10 12

28 25 20 36 43

22 21 15 31 35

18 16 12 28 26

12 13 10 20 20

OCV (mV) Power (mW) TDS (%)

16

Figure Captions Fig 1: Schematic representation of (a) Multi-electrode assembly (ME-BET) and (b) Single electrode assembly (SE-BET) systems Fig 2: a) TDS concentration with respect to time (TDS1 to TDS2), (b) Comparative profiles of SE-BET and ME-BET system for TDS1 concentration with respect to time (c) Multipollutant removal of Nitrates, Sulphates and Phosphates for ME-BET with varying loading TDS1 –TDS4 and SE-BET (TDS1) (d) COD removal efficiency with increasing OL (OL1 to OL4). Fig 3: (a) Power profiles with respect to time for increasing loading TDS1-TDS4 for six individual fuel cell (E1 to E6) and series and parallel circuitry connection (The arrow mark indicates the increase in loading) (b) Comparative profiles of OCV and power for SE-BET and ME-BET system for TDS1 concentration with respect to time. Fig 4: (a) OCV profiles with respect to time for increasing loading TDS1-TDS4 for six individual fuel cells (E1 to E6) and series and parallel circuitry connection. (b) Polarization profiles for TDS1 for six individual fuel cell (E1 to E6). Fig 5: (a) Cyclic voltametry profiles for six individual fuel cell (E1 to E6) (b) Tafel plots for series and parallel circuitry connection for varying loading TDS1–TDS4. .

17

E1

E2

E6

E5

E3

V

E4

R

RP

A

Anode

Recirculation Port Cathode

R

Gravel

A

Ammeter

V

Voltmeter

Resistor

V

E1

A

Recirculation Port

Cathode

R

Anode Gravel

A

Ammeter

V

Voltmeter

Resistor Fig 1b 18

10500

TDS1

b

SE-BET ME-BET

10000

TDS4

36000

9500

32000 TDS (mg/l)

9000 TDS (mg/l)

40000

8500 8000

TDS3

28000 24000 10500

TDS2

20000

10000 TDS1

9500 9000

16000

7500

TDS (mg/l)

a

TDS1 12000

7000

7000 6500 -1 0 1 2 3 4

6500 2

4

6

8

10

0

5

10 15 20 25 30 35 40 45 50 55 60 65

Time (Days)

Time (Days)

40

60

d

30 25

50

20

COD Removal Efficiency (%)

25

Nitrates Sulphates Phosphates

Removal Efficiency (%)

Removal Efficiency (%)

30

5 6 7 8 9 10 11

Time (Days)

0

35

8000 7500

8000

c

8500

15 10 5 0 ME-TDS1

SE-TDS1

20 15 10

40

30

20

10

5 0

0 ME-TDS1 ME-TDS2 ME-TDS3

METDS4

ME-OL1

SE-TDS1

Fig 2

19

SE-OL1

ME-OL2

ME-OL3

ME-OL4

400

SE-BET OCV ME- BET OCV SE-BET Power ME-BET Power

TDS1

350

1.2 1.0

250

0.8

200

0.6

150

Power (mW)

OCV (mV)

300

0.4 100

0.2

50

0

2

4

6

8

10

Time (Days)

Fig 3a 1.5

1.5

Parallel

1.2

1.2

0.9

0.9

0.6

0.6

0.3

0.3

1.5

1.5

E6

0.9

0.9

0.6

0.6

0.3 1.5 E2

1.2

0.3 1.5 0.9

0.6

0.6

0.3

0.3

0.0 1.5

0.0 1.5

E4

0.9

0.6

0.6

0.3

0.3 20

30

40

50

60

E3

1.2

0.9

10

E5

1.2

0.9

1.2

E1

1.2

Power (mW)

Power (mW)

1.2

0.0 0

Series

70

Time (Days)

0

10

20

30

40

50

Time (Days)

Fig 3b

20

60

70

500

TDS1

TDS2

TDS3

TDS4

450

350 300 250 200 150 100 0

10

20

30

40

50

60

70

Time (Days)

Fig 4a

400

E6 Voltage E5 Voltage

E1 Voltage E2 Voltage E3 Voltage

120

E4 Voltage

100

3

350 300

80

250

60

200

40

150 20 100

E5 Power E1 Power

50 0.0

Power Density (mW/m )

450

Voltage (mV)

Voltage (mV)

400

Parallel Series E1 E6 E2 E5 E3 E4

0.2

0.4

0.6

E4 Power

E3 Power

E6 Power

0.8

1.0

E2 Power

1.2

Current Density (mA)

Fig 4b

21

1.4

1.6

0 1.8

TDS1

TDS2

E1

E6

E2

E2

E1 E6

E5

E5 E4

TDS3

E6

E4 E3

E3

E1

E1

TDS4

E2

E5

E5

E6

E2

E3

E4

E3

E4

Fig 5a

0

TDS1 -2

TDS2

-2

ln(i)

ln (i)

-4

-4

-6

-6

-8

-8 -10 -0.8

-2

Parallel

Parallel Series

Series

-10

-0.6

-0.4

-0.2

0.0 E/V

0.2

0.4

0.6

0.8

-0.6

TDS3

-2

-0.4

-0.2

0.0 E/V

0.2

0.4

0.6

TDS4

-4

ln(i)

ln (i)

-4 -6

-6

-8

-8 -10

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Parallel Series

Parallel Series

-10

0.8

-0.8

E/V

-0.6

-0.4

-0.2

0.0 E/V

Fig 5b

22

0.2

0.4

0.6

0.8

23

Highlights .

   

ME-BET reported higher treatment than SE-BET Multiple electrodes in a single system treats complex pollutant Membrane-less BET is a economical viable processes BET reported higher electrogenic in parallel circuit than series

24