Bioresource Technology xxx (2015) xxx–xxx

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Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater Dileep Kumar Yeruva, Srinivas Jukuri, G. Velvizhi, A. Naresh Kumar, Y.V. Swamy, S. Venkata Mohan ⇑ Bioengineering and Environmental Sciences (BEES), CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Integration strategy of SBR–BET for

the remediation of petrochemical wastewater.  GC–MS/FTIR confirmed the increment of biodegradability in PCW.  SBR is an effective pre-treatment processes.  Enhanced bioelectrochemical behavior was observed in BET system.

a r t i c l e

i n f o

Article history: Received 30 November 2014 Received in revised form 5 February 2015 Accepted 6 February 2015 Available online xxxx Keywords: Periodic discontinuous batch process Bioelectrochemical system (BES) Anoxic microenvironment Bioelectrogenesis Wastewater

a b s t r a c t The present study evaluates the sequential integration of two advanced biological treatment methods viz., sequencing batch reactor (SBR) and bioelectrochemical treatment systems (BET) for the treatment of real-field petrochemical wastewater (PCW). Initially two SBR reactors were operated in aerobic (SBRAe) and anoxic (SBRAx) microenvironments with an organic loading rate (OLR) of 9.68 kg COD/m3day. Relatively, SBRAx showed higher substrate degradation (3.34 kg COD/m3-day) compared to SBRAe (2.9 kg COD/m3-day). To further improve treatment efficiency, the effluents from SBR process were fed to BET reactors. BETAx depicted higher SDR (1.92 kg COD/m3-day) with simultaneous power generation (17.12 mW/m2) followed by BETAe (1.80 kg COD/m3-day; 14.25 mW/m2). Integrating both the processes documented significant improvement in COD removal efficiency due to the flexibility of combining multiple microenvironments sequentially. Results were supported with GC–MS and FTIR, which confirmed the increment in biodegradability of wastewater. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Industrial wastewaters tend to carry a huge load of organic and inorganic pollutants of which organic pollutants not only appear at high concentrations but also exhibit a wide diversity with respect to their molecular structures (Crowe et al., 2002; Botalova et al., 2009). ⇑ Corresponding author. Tel./fax: +91 40 27191807. E-mail address: [email protected] (S. Venkata Mohan).

More specifically, wastewaters originating from petrochemical industries are characteristically less biodegradable in nature with diverse pollutants containing high concentrations of salt and carbon. Various structural chemicals, viz., polycyclic aromatic, aliphatic hydrocarbons, cyanides, octanols, formaldehyde, phenols, organic acids, sulfides, etc., persisting in petrochemical wastewater (PCW) warrants treatment prior to disposal (Malmasi et al., 2010; Papadimitriou et al., 2009; Verma et al., 2006). Various treatment methods are available for treating PCW viz., physical, biological,

http://dx.doi.org/10.1016/j.biortech.2015.02.014 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Yeruva, D.K., et al. Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.02.014

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D.K. Yeruva et al. / Bioresource Technology xxx (2015) xxx–xxx

chemical etc. Physical treatment of PCW can be carried out using advanced membrane bioreactor (MBR), but the process has certain limitations due to its high operational and investment costs (Cheryan and Rajagopalan, 1998). Similarly, biological treatment methods also face significant challenges like, poor availability of hydrocarbons to the microorganisms due to their complex structure and water insoluble nature especially when salinity is higher in wastewaters (O’Neill et al., 2000; Lai et al., 2012; Balapure et al., 2015). Hence, in order to overcome these limitations, conventional wastewater treatment technologies require additional conversion processes to enhance the treatment efficiency. Sequencing batch reactor (SBR) also known as periodic discontinuous batch reactor (PDBR) facilitates the integration of diverse redox microenvironments in a single reactor, which provides the possibility to achieve enhanced treatment (Wilderer et al., 2001; Buitron et al., 2004; Rao et al., 2005; Hosseini Koupaie et al., 2013; Venkata Mohan et al., 2007a, 2013; Naresh Kumar et al., 2014). Simultaneous provision of feast and famine conditions during SBR cycle operation enforces controlled short-term unsteady state conditions leading to stable steady state conditions in the long run operation. This enhances the robustness of consortia towards the treatment of complex wastewater (Ong et al., 2010; Venkata Mohan et al., 2005, 2007b, 2009b, 2013; Xiao et al., 2014). It also imposes selective pressure that can select a defined population of organisms, which can degrade complex compounds due to its unique flexibility to combine multiple metabolic functions during operation (Buitron et al., 2004; Venkata Mohan et al., 2007c). Bioelectrochemical treatment (BET) system, which is a microbial catalyzed electrochemical system, can facilitate the direct conversion of substrate to electricity through a cascade of redox reactions (Venkata Mohan et al., 2009a; Luo et al., 2009; Huang et al., 2009). It has been reported in literature that BET offers dual benefits of power generation coupled with the improved treatment efficiency, particularly with complex wastewaters (Venkata Mohan et al., 2014; Velvizhi and Venkata Mohan, 2015; Lovley, 2010). BET which facilitates wastewater treatment with simultaneous power generation has similar characteristics of both fuel cell and biological treatment process (Mishra et al., 2001; Huang et al., 2009; Mohana Krishna et al., 2010). Due to its inherent advantage of coupling the diverse processes, BET is emerging as a viable process for treating complex pollutants in various wastewaters viz., pharmaceutical, diary, textile dye wastewaters, etc., (Venkata Mohan and Chandrasekhar, 2011; Velvizhi and Venkata Mohan, 2011; Vamsi Krishna et al., 2014). Integration of two multiple processes aids in achieving enhanced treatment efficiency, more specifically with complex wastewaters viz., petrochemical wastewater. Hence, by considering the advantage of both the processes discussed above, the present study is designed by integrating the SBR process with BET to achieve enhanced treatment efficiency of complex PCW. Initially, PCW treatment was evaluated by SBR process at two different microenvironments viz., aerobic and anoxic conditions and the resulting effluents of the two SBR outlets were fed into two BET systems to increase the treatment efficiency with simultaneous bioelectricity generation. The process performance of SBR and BET was evaluated with respect to COD and pollutants removal. In addition, the bio-electrochemical parameters viz., bioelectrogenic activity, TDS removal, polarization resistance and anode potential were also assessed for BET systems.

(23,232 mg COD/l) and salt concentration (16,730 mg TDS/l) with low biodegradable nature (BOD5/COD: 0.36) [pH 12.4; nitrates 98 mg/l; phosphates 369 mg/l; sulfates 15.5 mg/l, TSS 23.5 mg/l]. Prior to usage, the effluent was stored at 4 °C. Before feeding, the pH of PCW was adjusted to 7.0 ± 0.2 using 0.1 N HCl. 2.2. Bioreactors Two sequencing (periodic discontinuous) batch reactors (SBR/ PDBR) were designed with a total volume of 1.1 l and interchangeable water volume of 0.99 l (diameter, 7.5 cm; length, 22.5 cm). SBR-aerobic rector (SBRAe) was operated by sparging air through pump. Single chambered BET systems were also fabricated with perspex material with a total/working volume of 1.2/1.0 l with a dimension of 14  11  4 cm (Venkata Mohan et al., 2013). BET was operated without membrane using non-catalyzed graphite plate as cathode (280 cm2) and stainless steel mesh as anode (280 cm2) with a distance of 3.5 cm. The anode was completely submerged in the anolyte and cathode was partially submerged (bottom portion) and top portion was exposed to atmospheric air. Proper provisions were made in design for feeding, decanting, recirculation and air supply operations. 2.3. Biocatalyst The aerobic consortium procured from a full scale effluent treatment plant (ETP) was used as parent inoculum for both the bioreactors (SBRAe/SBRAx). In the case of bioelectrochemical treatment (BET) process, anaerobic consortium procurred from a full scale anaerobic effluent treatment plant was used as parent inoculum. Prior to inoculation, the parent biomass was washed (5000 rpm, 20 °C) twice with phosphate buffer (50 mM) followed by enrichment in designed synthetic wastewater (DSW) [glucose: 3 g/l; NH4Cl: 0.50 g/l, KH2PO4: 0.25 g/l, K2HPO4: 0.25 g/l, MgCl2:0.30 g/l, CoCl2:25 mg/l, ZnCl2:11.50 mg/l, CuCl21: 0.50 mg/l, CaCl2:5 mg/l, MnCl2:15 mg/l, NiSO4:16 mg/l, FeCl3:25 mg/l] at required microenvironments. 2.4. Experimental design and operation Experiments were designed by integrating SBR with BET to enhance the remediation of PCW. SBR operation was carried out in two different microenvironments, viz., aerobic (SBRAe) and anoxic (SBRAx) with an organic loading rate of 9.6 kg COD/m3-day with sequential operation comprised of 15 min of filling phase (FILL), 2820 min of react phase and recirculation (REACT) phase, 30 min of settling phase (SETTLE) and 15 min of decant phase (DECANT). During REACT phase of SBRAe operation DO concentration was maintained (4 ± 1.5 mg/l) by sparging air. In SBRAx air was sparged for 10 min for every 4 h of cycle period during REACT phase, by maintaining DO within 0.75 ± 0.25 mg/l (Venkata Mohan et al., 2013). The outlet of SBR reactors were subsequently fed into the two BET systems viz., BETAe and BETAx, with the respective OLR of 6.33 ± 0.56 kg COD/m3-day and 5.81 ± 0.47 kg COD/ m3-day. Bioreactors were operated in suspended growth configuration with a cycle period (retention time) of 48 h at ambient room temperature. 2.5. Analysis

2. Methods 2.1. Petrochemical wastewater (PCW) Real field petrochemical wastewater (PCW) was used in the present study. The effluent has high concentration of carbon load

The process performance of all the bioreactors (SBR/BET) was assayed by evaluating the chemical oxygen demand (COD; 5220C; closed-reflux (titrimetric) method), pH (4500-H+B) and TDS (2540 C) according to standard methods (APHA, 1998). Voltage and current were measured using a digital multimeter. Anode potential was measured with reference to Ag/AgCl (S) employing

Please cite this article in press as: Yeruva, D.K., et al. Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.02.014

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Aerobic

25200

19600 16800

0

96

192

3.1.2. Bioelectrochemical treatment (BET) SBR processes were found to have limitations in the reduction of complexity of PCW. Both the SBR treated effluents, were fed to BET system in order to treat the residual carbon as an integrated approach. BET is a hybrid microbial catalyzed electrochemical system which could integrate several cascade redox reactions in a defined fuel cell type setup with the presence of electrode assembly (Venkata Mohan et al., 2010, 2014). The inherent advantage of coupling diverse processes could facilitates the treatment of complex pollutants with simultaneous bioelectricity generation (Velvizhi and Venkata Mohan, 2015; Yang et al., 2012). BET systems were operated with two different OLRs (BETAx 6.33 kg COD/

BET

SBR

BET

SBR

BET

SBR

384

480

576

b

Anoxic

3.2

2.4

1.6

0

96

192

288

384

480

BET

SBR

BET

SBR

BET

SBR

BET

SBR

BET

SBR

0.0

BET

0.8 SBR

SDR (kg COD/m3-day)

3.1.1. Sequencing batch reactor (SBR) Initially, both the SBRs (SBRAe/SBRAx) were operated with DSW using glucose (3000 mg COD/l) as a carbon source showed treatment efficiency of 95% (SBRAe) and 92% (SBRAx). After achieving stabilized performance in terms of COD removal, reactors were fed with PCW (BOD5/COD: 0.36). To avoid load-shock, first two cycles were operated with OLR of 4.8 kg COD/m3-day by diluting the PCW with 50% tap water (data not shown). Subsequently, the bioreactors were operated with full strength organic load of 9.68 kg COD/m3-day. Performance data illustrated comparatively higher treatment efficiency with anoxic SBR operation with COD removal efficiency of 34.6% (SBRAx - 3.34 kg COD/m3-day) compared to SBRAe (2.9 kg COD/m3-day; 30%) (Fig. 1 and Table 1). Initially both the bioreactors showed marginal variations in the treatment efficiency and substrate degradation [2nd cycle: SBRAe - 2.86 kg COD/m3-day; SBRAx - 2.86 kg COD/m3-day]. With the course of operation (repeated batch cycles), both the bioreactors attained stabilized performance (6th cycle). SBRAx showed higher BOD5 removal efficiency (21.5%; 6620 mg/l) than the corresponding SBRAe operation (17.5%; 6970 mg/l). Though the inoculated cultures were same in both the biosystems, the variations in the substrate degradation might be due to the prevailing microenvironment during operation. In SBRAx operation the limited oxygen availability (microaerophilic) resulted in an integrated oxidation– reduction behavior which controls the distribution and physiological state of the microorganisms makes them robust and thus enhances the treatment processes. Oscillation between anoxicaerobic microenvironments (microaerophilic) shifts the bacterial metabolism towards reduction facilitating dual metabolic functions under single reactor operation (Venkata Mohan et al., 2013; Papadimitriou et al., 2009). Facultative consortia thriving on microaerophilic conditions can switch between anoxic oxidation and fermentation processes based on the terminal electron acceptor availability (Velvizhi and Venkata Mohan, 2015; Feng et al., 2008).

288

Time (h)

576

Time (h) 60

COD removal efficiency (%)

3.1. COD removal

BET

BET

SBR

SBR

11200

SBR

14000

Aerobic

3. Results and discussion

a

Anoxic

22400

BET

COD removal (mg/l)

anode as working electrode at varying resistances (30–0.05 kO). Polarization curve was recorded by varying resistances from 30–0.05 kO. Changes in bioelectrocatalytic behavior of all the bioreactors (SBR/BET) were studied by employing cyclic voltammetry (CV) using potentiostat–galvanostat system (Autolab-PGSTAT12, Ecochemie). Voltammograms were recorded by applying a potential of +0.5 to 0.5 V with a scan rate of 30 mV/s. Bioelectrochemical assays were performed using platinum wire as working electrode and carbon rod as counter electrode against reference electrode (Ag–AgCl(S)) in wastewater (anolyte). In case of BET reactors, anode was used as working electrode, cathode as counter electrode and Ag–AgCl(S) as reference electrode (Velvizhi and Venkata Mohan, 2015).

c

45

30

15

0 SBRAe

BETAe

SBRAe +BETAe SBRAx

BETAx

SBRAx +BET Ax

Fig. 1. Substrate degradation with respect to reactor configuration, (a) COD removal (mg/l), (b) substrate degradation rate (SDR) and (c) COD removal efficiency (%).

m3-day; BETAe 6.77 kg COD/m3-day) with a retention time of 48 h. During the initial cycles of operation, BETAx/BETAe reported a removal efficiency of 17.3%/16.6% thus accounting for SDR of 1.12/1.09 kg COD/m3-day (Fig. 1). With the course of operation, BETAx showed relatively higher SDR (1.92 kg COD/m3-day; 30.4%) than corresponding BETAe system [1.8 kg COD/m3-day; 27%]. Significant improvement in substrate degradation was observed with subsequent increment in cycles operation in both BET systems due to the acclimatization of microbes. The presence of electrode assembly facilitates the development of in situ potential in system, that leads to direct and/or mediated anodic oxidation (Venkata Mohan et al., 2009a). The organic pollutants which are adsorbed on the anode surface by direct anodic oxidation gets cleaved through anodic electron transfer reactions. In case of mediated oxidation processes, the oxidants formed on the electrochemically active surfaces oxidize the organic matter thus enhancing the overall treatment efficiency. The cumulative performance with the integrated processes showed higher COD removal efficiency in SBRAx–BET (5.27 kg COD/m3-day; 55%) followed by SBRAe–BET (4.73 kg COD/ m3-day; 49%). The observed increment in substrate (COD) degradation indicates the significance of BET

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Table 1 Comparative performance of SBR and BET systems. Parameters 3

OLR (kg COD/m -day) COD removal (%) SDR (kg COD/m3-day) TDS removal (%) OCV (mV) Power density (mW/m2) Anode potential (mV)

SBRAe

BET

SBRAe + BET

SBRAx

BET

SBRAx + BET

9.68 29.9 2.9 — — — —

6.77 27.07 1.8 28 248 14.25 330

— 48.94 4.73 — — — —

9.68 34.57 3.34 — — — —

6.33 30.39 1.92 24 280 17.12 357

— 54.45 5.27 — — — —

Note: — not applicable.

operation as tertiary treatment unit to enhance the treatment efficiency along with bioelectricity generation. Thus in this integrated approach SBR facilitates as a pre-treatment process yielding simplified molecules, which can be further treated in the BET systems more effectively. Placing electrode assembly in BET system facilitates bio-potential/bioelectrogenic activity, which helps to enhance the biodegradability of complex compounds as well as the intermediates that are produced in the SBR processes. The presence of electrode offers a potential difference between anodic oxidation and cathodic reduction reactions which has positive influence on pollutant removal. Also, anode acts as an alternative electron acceptor and promotes degradation of aromatic hydrocarbon (Huang et al., 2009; Zhang et al., 2010; Venkata Mohan and Chandrasekhar, 2011). 3.2. GC–MS and FTIR analysis GC-MS analysis was carried out for raw PCW, SBR outlet and BET outlet samples to elucidate the specific pollutants (Sfig. 1). The chromatogram of raw PCW showed distinct peaks arranged from low to high in order of boiling point. Each peak corresponded to a certain organic compounds viz., iso butyl alcohol [Mass to Charge ratio (m/z) (atomic mass units (amu)]: 1-Butanol (74), Butanoicacid (88), 2,5-Dimethyl-N-nitrosopyrrolidine (128), 3,7-Dimethyloctyl propyl phosphonofluoridate (266), 3,7-Dimethyloctyl iso propyl phosphonofluoridate (266), Octanoic acid (254), 8-methyle 6-nonenoic acid (170), 2,3-Dipropyle-cyclopropanecarboxilic acid ethyle ester (198) and 2-cyclo hexen-1-one,5-methyle-2-(1-ethyle) (152), with molecular weights ranging from 100 to 300 atomic mass units (amu) (Stable 1). In raw PCW the organics (benzene and its analogs) both biodegradable and non-biodegradable (BOD5/COD: 0.36) that constitute 43.9% of total peak area were observed. In addition to this, alcohols (31.37%) and alkanoic acids (24.67%) were also identified. This indicates that raw PCW contained complex to simple organic compounds. In the case of SBRAx outlets the total peak area of alkanoic acids increased to 36.75%, suggesting the reduction of complex organic compounds to easily biodegradable intermediate compounds. This also correlated well with the increment in biodegradability (BOD5/COD: 0.45) of the wastewater. In SBRAe outlet, the total peak area of alkanoic acids decreased to 17.67% indicating that these simpler compounds were degraded effectively. This degradation of simpler compounds can be attributed to the oxidation behavior during aerobic processes. To enhance the residual removal of organic/inorganic compounds, the effluent of SBR reactors was fed to BET systems. After treatment with BET, long-chain alkane, alcohols, alkanoic acids, benzene and its analogs were observed to disappear completely. In addition, formation of new peaks were also observed indicating the presence of amines, esters and aldehydes. The degradation of complex pollutants in the BET system can be attributed to the presence of electrodes that help in the enrichment of electrochemically active bacteria, which release more number of electrons due to the potential gradient developed in the system. The intermediates formed during the treatment of

wastewaters also act as electron acceptors, thereby increasing the treatment efficiency. The biodegradability of PCW (BETAx/BETAe: 0.5/0.47) was also observed to increase with treatment in BET system. These results indicate that the SBR process majorly degrades the complex organic pollutants to simple intermediates, which are further degraded to simpler compounds in the BET system, thus increasing the biodegradability of PCW. FT-IR analysis also showed distinctive peaks of various functional groups present in PCW (Sfig. 2). The strong absorption peak at 3414 cm 1 corresponds to the presence of ANH stretching of aromatic amines coupled with stretching of AOH groups (phenols). In addition, multiple peaks between 2958, 1425, 1030 and 880 cm 1 confirmed the presence of CAH stretching (alkyl structures), aromatic C@C, C@O in amides (I), ketone and quinine and aromatic ACAHA bonding groups. With SBRAx operation (after 48 h) decrement of peak intensity between 2958 and 1030 cm 1 in addition to the new peak at 1650 cm 1 were observed (corresponding to ketone and quinine groups) (Lai et al., 2012; Botalova et al., 2009). In the case of SBRAe operation, an intermediate peak was observed at 1344 cm 1 corresponding to nitro-based compounds (nitrates/nitrites). Subsequent feeding of SBR outlet to BET systems resulted in significant decrement of peak intensity followed by appearance of new peaks at 470 and 534 cm 1 (alkyl halides (CABr)) that might be the intermediates accumulated in the process. These results indicate that SBR acts as an effective pre-treatment process that provides simpler intermediates to BET systems, which further degrade these intermediates. Thus this sequential integration processes documented its potential to treat complex PCW with simultaneous bioelectricity generation. 3.3. Salts removal Interestingly, BET operation showed considerable reduction in TDS concentration at the end of the cycle operation. Initial cycles showed lower TDS removal (2nd cycle: BETAx/BETAe: 14/12%). With the course of cycle operation, BETAx documented higher TDS removal (28%) than BETAe (24%) (Table 1). The bioelectrochemical mediated disassociation of salt splitting mechanism might be possible due to the in situ biopotential developed during BET operation (Venkata Mohan et al., 2009a). The degradation of organic substrate results in the generation of electrons and protons that create an external potential gradient, which influences salt reduction. The primary oxidants get adsorbed on the active sites of anode surface and further get oxidizes into simpler compounds (free radicals). The radicals formed during the reactions will react with salts (free chloride ions). The lower life span and high oxidation potential of radicals decompose into other inorganic compounds (Velvizhi and Venkata Mohan, 2015; Yang et al., 2012). 3.4. Redox conditions The redox microenvironment of bio systems based on pH and volatile fatty acids (VFA) showed a distinct trend (Fig. 2). The

Please cite this article in press as: Yeruva, D.K., et al. Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.02.014

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Aerobic

Anoxic

a

7.6

pH

7.4

7.2

7.0

0

96

192

288

384

BET

SBR

BET

SBR

BET

SBR

BET

SBR

BET

SBR

6.6

BET

SBR

6.8

480

576

Time (h) Aerobic

8000

Anoxic

b

VFA mg/l

7200

6400

0

96

192

288

384

480

BET

SBR

BET

SBR

BET

SBR

BET

SBR

BET

SBR

SBR

4800

BET

5600

576

Time (h) Fig. 2. Variations in redox microenvironments of SBR and BET systems, (a) pH, (b) VFA.

raw PCW effluent is alkaline in nature (pH, 12.5) and to avoid process inhibition the pH was adjusted to 7 ± 0.2 (before feeding) using 1N HCl. In the case of SBRAx operation, an increment in pH (7.15) was observed until 6 h followed by a drop to 6.2 at 48 h. Whereas, SBRAe operation resulted in gradual increment in the pH till the end of the cycle operation (7.8). pH profiles was well supported by VFA profiles. Significant increment in the VFA concentration was documented under SBRAx operation (2800 mg/l) due to prevailing anoxic conditions. These VFA were produced as the metabolic by-products by the facultative microbes persisting in the reactor. In case of aerobic SBR operation, the VFA generation was significantly low (300 mg/l) due to prevailing oxidative microenvironment. The pH varied in the range of 7.0–7.2 in both the BET systems. Initial cycles of operation documented higher VFA concentration (BETAx/BETAe 6.8/6.5 g/l), which decreased and stabilized with further operation (6th cycle: 5.5/5.1 g/l). This might be due to the dissociation of salts, which act as a buffer to balance the redox reactions. 3.5. Bioelectrogenesis of BET Besides waste remediation, BET can be a source for harnessing bioelectricity. Bioelectrogenic activity increased with each cycle operation, gradually. BETAx operation showed marginal higher

bioelectrogenic activity (254 mV; 16.9 mW/m2) compared to BETAe (243 mV; 13.54 mW/m2) during initial phase of operation (Fig. 3). Subsequently, performance stabilized at 6th cycle of operation (BETAx 280 mV; 17.12 mW/m2 and BETAe 250 mV; 14.25 mW/ m2). Marginally higher bioelectrogenic activity of BETAx might probably attribute to low concentration of toxic intermediates present in the effluent of SBRAx. Further, polarization study of both the BET systems was evaluated with a range of resistors (30–0.05 kO). The polarization curve depicts a change in voltage with change in resistance, wherein the maximum cell voltage recorded for BETAx and BETAe was 278 and 245 mV, respectively. A maximum power density was noticed with BETAx (10 mW/m2) operation compared to BETAe (6.4 mW/m2) with a maximum current density of 67.5 and 55.7 mA/m2, respectively. Interestingly, the cell design point (CDP) for both the reactors was observed at 100 O. However, in both BET systems similar kind of internal losses were documented. Where activation loss was found to be prominent in both the BET systems. To understand the electron delivery, anode half-cell potential was measured against a standard reference electrode (sat. Ag/AgCl). Change in anode potential was observed with application of variable resistances. As the load was reduced, Ean decreased indicating the possibility for electron flow until 15 kO, thereafter Ean dropped to a minimum value. This behavior of anode potential with respect to electron delivery is governed by the kinetics and mass transfer

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a

Power Density (mW/m2)

14.4 13.2 12.0

BETAe

17.1 17.0 16.9

BETAx

16.8 0

48

96

144

192

240

288

Time (h)

BET Ae V

12

b

BET Ae PD

10

Voltage (mV)

240

8

200

6 160 4 120

Power density (mW/m2)

BETAx V BET Ax PD

280

2 80

0 0

10

20

30

40

50

60

70

80

2

Current density (mA/m )

Anode potential (mV)

0

BETAe

BETAx

c

-100

-200

-300

-400 0

5

10

15

20

25

30

Resistance (KΩ) Fig. 3. Bioelectrogenic activity of BET system evaluated through (a) power density, (b) polarization profiles and (c) anode potential.

(Fig. 3). The anode potential of BETAx was maximum ( 357 mV) followed by BETAe ( 330 mV). The disparity in performance can be attributed to the biofilm developed on the anode surface, which comprises of the catalytic components that are involved in exocellular electron transfer mechanisms. 3.6. Bio-electrochemical behavior of SBR and BET The bio-electrochemical behavior of SBR and BET systems was evaluated by employing cyclic voltammetry (CV) analysis, an electrochemical technique to characterize the electron transfer interactions between microorganisms or microbial metabolites by applying an external potential. Voltammograms were recorded against Ag/AgCl(S) by applying a potential ramp (+0.5 to 0.5 V) at a scan rate of 30 mV/s employing platinum wire and carbon rod as working and counter electrodes for SBR systems and

anode/cathode as working/counter electrodes, respectively in BET systems using wastewater as anolyte. CV profiles illustrated significant variations in the electron discharge properties and redox catalytic currents [oxidation current (OC) and reduction currents (RC)] with the function of reactor configuration and microenvironment. Higher redox catalytic currents were depicted by SBRAx operation in comparison to SBRAe, during the course of operation with DSW and PCW as the substrate. SBRAe-DSW operation showed increment in the oxidation currents (OC) till 24 h (0.29 lA) followed by a marginal decrement (0.26 lA) by the end of cycle period (48 h). Whereas the reduction currents (RC) showed marginal variations ( 0.14 lA) (Fig. 4). SBRAx-DSW operation showed comparatively higher OC and RC (0.38/ 0.25 lA). Though the biocatalyst (aerobic) inoculated in both the conditions were same, OC was dominant under SBRAe operation, while the RC was higher under anoxic operation. This might

Please cite this article in press as: Yeruva, D.K., et al. Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.02.014

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be due to the shift in the metabolic activities of the biocatalyst with the function of available terminal electron acceptor (TEA). Subsequently, the reactors fed with PCW resulted in significant variations in the redox currents. SBRAx operation with PCW showed relatively higher redox catalytic currents ( 0.22/0.2 lA) than SBRAe ( 0.1/0.11 lA). Interestingly, SBRAx resulted in higher RC at initial hours (6 h: 0.28 lA) followed by a decrement ( 0.16 lA) at 12 h which remained more or less similar ( 0.07 lA) till the end of cycle period. This might be attributed to the initial higher electron discharge ability of the biocatalyst against the toxicity of PCW, followed by a decrement due to the accumulation of metabolites as well as its intermediates. While in the case of OC, a gradual increment was observed (0.28 lA) till 48 h. Higher OC observed during SBRAx is attributed to the rapid substrate degradation capabilities of the biocatalyst due to the prevailing partial aerobic and anaerobic (microaerophilic) conditions. In the case of BET system, relatively higher redox currents were recorded in comparison to SBR. Maximum redox catalytic currents were observed with BETAx operation (9.54/ 7.16 mA) followed by

BETAe (7.2/ 4.32 mA) (Fig. 4). Voltammetric analysis documented simultaneous redox behavior and higher catalytic currents with BET systems than SBR systems which elucidates the significance of electrode assembly in reducing the complexity of the wastewater thereby simultaneously generating bioelectricity (PCW degradation in the present study). The electrochemical activity and kinetic features characterize the e transfer interactions between microbial biofilm and solid electrodes in BET system. Oxidation currents were slightly higher than reduction currents in both the systems, which depicts the biocatalyst capabilities in degrading the substrate rapidly. In addition, the simultaneous redox reactions results in enhanced substrate degradation as well as the degradation of complex wastewater under the self-induced bio-potential conditions (Velvizhi and Venkata Mohan, 2015; Luo et al., 2009). Moreover, presence of electrodes also creates an electron accepting condition, which helps to promote the anaerobic degradation of organic contaminants. In case of SBR operation, the catabolized substrate remains as a fermented end product and still requires an additional energy in the absence of electrode assembly.

0.40u

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Fig. 4. Voltammogram profiles with the function of reactor configuration and wastewater composition.

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Tafel analysis helps to visually understand the losses occurring during the process and also helps to interpret the biocatalytic activity based on the derived kinetic parameters viz., oxidative Tafel slope (ba), reductive Tafel slope (bc) and polarization resistance (Rp). The potential ramp of 0.5 to +0.5 V was used for linear regression analysis to minimize the mass transfer limitation and the best fit of multiple linear regression within the chosen potential range was used to calculate the kinetic parameters (Venkata Mohan et al., 2013). Tafel slope is inversely proportional to the electrocatalytic activity of the biocatalyst, thus lower Tafel slopes indicates higher bioelectrocatalytic activity and electron transfer efficiency. Lower redox Tafel slopes were found in SBRAx operation (ba/bc: 0.32 ± 0.21/0.24 ± 0.19 V/dec) followed by SBRAe system (0.67 ± 0.04/0.1 ± 0.02 V/dec) depicting the higher biocatalytic activity in SBRAx (Fig. 5). Initially with DSW operation, both the systems resulted marginal variations in terms of redox Tafel slope (SBRAe/SBRAx, 0.24 ± 0.08/0.21 ± 0.06 V/dec). This might be due to the seeding of same inoculum in both the bioreactors. Thereafter, loading with PCW resulted variations in the process performance, SBRAx depicted lower redox slopes (0.21/0.12 V/dec) than SBRAe operation (0.29/0.19 V/dec). This is due to the enrichment of facultative bacteria in SBRAx that might have survived in the oxygenated or deoxygenated microenvironments. In addition, reduction slope

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was less in comparison to oxidation slope in both the systems which is attributed to the typical reduction behavior of the biocatalyst in SBR process due to the lack of electrode assembly, if present, would help in favoring simultaneous oxidation and reduction reactions. Though the metabolic activities are rapid under aerobic condition the resulting redox equivalents will be neutralized at a faster rate resulting in the electron quenching phenomenon due to the presence of oxygen. Whereas, the facultative bacteria enriched during microaerophilic conditions can be utilized effectively towards the reduction reactions without electron quenching (Venkata Mohan et al., 2008). BET documented relatively low oxidative and high reductive slopes for BETAx (0.28/0.66 V/dec) followed by BETAe (0.44 ± 0.06/ 0.80 ± 0.02 V/dec) (Fig. 5). Among them, BETAx operation resulted higher oxidation by biocatalyst which well corroborated with substrate removal. Besides, the resistance to electron transfer commonly referred as polarization resistance (RP) was found to be higher in BETAe (134 X) than BETAx (99.4 X). This difference in resistivity is related to the bioelectrogenic activity due to the enrichment of electrochemically active bacteria. BET operation depicted low redox slopes in comparison to SBR process. Also, reduction slope was comparatively lower than oxidation slope in SBR system. In addition, polarization resistance (RP) is

0.00 0

24

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Time (h)

Fig. 5. Variations in oxidative, reductive Tafel slopes and polarization resistance with respect to mode of operation and time.

Please cite this article in press as: Yeruva, D.K., et al. Integrating sequencing batch reactor with bio-electrochemical treatment for augmenting remediation efficiency of complex petrochemical wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/j.biortech.2015.02.014

D.K. Yeruva et al. / Bioresource Technology xxx (2015) xxx–xxx

comparatively high during SBR operation. This variation in bioelectrochemical behavior is attributed to the presence/absence of electrode assembly in BET and SBR systems respectively which favors the enrichment of electro-chemically active bacteria in BET system that contributes to the generation of high redox catalytic currents. BET system depicted simultaneous redox behavior whereas, SBR system showed persistent reduction behavior. Besides, the electrode assembly acts as a solid electron acceptor in BET that favors higher redox currents and the substrate (pollutant) degradation. SBR operation imposes selective pressure and therefore, increases the robustness of the bacteria to degrade complex wastewater. In addition, the cyclic feast and famine condition prevailing during operation enforces controlled short-term unsteady state conditions leading to stable steady state conditions helps to remediate complex pollutants in long run operations. The flexibility of combining multiple microenvironments during SBR operation is an added advantage. In the case of BET operation, presence of electrode assembly acts as a solid electron acceptor which develops a potential gradient that helps to induce the cleavage of complex pollutants. The pollutants present in the BET system also act as mediators to enhance the anodic reactions favoring effective transfer of electrons (Venkata Mohan et al., 2009a,b; Rozendal et al., 2008). The electrochemical behavior of BET system favors simultaneous oxidation and reduction to degrade the hydrocarbons present in the PCW. Integrating two advanced biological processes helped in combining multiple microenvironments, which eventually reduced the complexity of petrochemical wastewater to a large extent in a sustainable way. 4. Conclusions Integration of two advanced biological processes helped to treat complex petrochemical wastewater. The varying microbial metabolism prevailed in SBR operation impose selective pressure on the biocatalyst that reduced the organic present in the petrochemical wastewater. The presence of electrode assembly in BET system favored enhanced substrate degradation along with power generation. The added advantage of BET system is reduction of salts due to the self-induced biopotential developed in the system. The study signifies flexibility of combining multiple microenvironments during operation which favored higher degradation of complex wastewater. Acknowledgements The authors wish to thank the Director, CSIR-IICT for support and encouragement in carrying out this work. Funding from Council for Scientific and Industrial Research (CSIR – India) in the form of XII five year network projects [SETCA (CSC-0113)] and Department of Biotechnology (DBT), Government of India in the framework of National Bioscience Award (BT/HRD/NBA/34/01/ 2012(vi)) are gratefully acknowledged. GV acknowledge the CSIR, New Delhi, for providing research fellowship. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.02. 014. References APHA, 1998. Standard Methods for the Examination of Water and Wastewater, AWWA, WEF, 20th ed. American Public Health Association, Washington, DC.

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