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Cylindrical graphite based microbial fuel cell for the treatment of industrial wastewaters and bioenergy generation
Accepted Manuscript Cylindrical graphite based microbial fuel cell for the treatment of industrial wastewaters and bioenergy generation Gunda Mohanakrishna, Ibrahim M. Abu-Reesh, Riyadh I. Al-Raoush, Zhen He PII: DOI: Reference:
S0960-8524(17)31739-X https://doi.org/10.1016/j.biortech.2017.09.174 BITE 19002
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
27 July 2017 23 September 2017 25 September 2017
Please cite this article as: Mohanakrishna, G., Abu-Reesh, I.M., Al-Raoush, R.I., He, Z., Cylindrical graphite based microbial fuel cell for the treatment of industrial wastewaters and bioenergy generation, Bioresource Technology (2017), doi: https://doi.org/10.1016/j.biortech.2017.09.174
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Cylindrical graphite based microbial fuel cell for the treatment of
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industrial wastewaters and bioenergy generation
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Gunda Mohanakrishna1, Ibrahim M. Abu-Reesh1 , Riyadh I. Al-Raoush2, Zhen He3
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2713 Doha, Qatar 2
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Department of Chemical Engineering, College of Engineering, Qatar University, P O Box
Department of Civil and Architectural Engineering, College of Engineering, Qatar University, P O Box 2713 Doha, Qatar
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Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
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*Corresponding author: [email protected]
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Abstract
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Cylindrical graphite microbial fuel cell (MFC) configuration designed by eliminating distinct
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casing and membrane was evaluated for bioelectrogenesis and treatment of real-field
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wastewaters. Both petroleum refinery wastewater (PRW) and Labanah whey wastewater
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(LW) were used as substrates, and investigated for electricity generation and organic removal
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under batch mode operation. PRW showed higher bioelectricity generation (current and
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power generation of 3.35 mA and 1.12 mW at 100 Ω) compared to LW (3.2 mA and 1.02
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mW). On the contrary, higher substrate degradation efficiency was achieved using LW
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(72.76%) compared to PRW (45.06%). Superior function of MFC operation in terms of
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volumetric power density (PRW, 28.27 W/m3; LW, 23.23 W/m3) suggesting the feasibility of
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using these wastewaters for bioelectricity generation. Large sources of wastewater that
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generating in the Middle-East countries have potential to produce renewable energy from the
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treatment, which helps for the sustainable wastewater management and simultaneous
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renewable energy production.
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Keywords: Graphite reactor, Bioelectrochemical treatment, Labanah whey wastewater,
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Petroleum refinery wastewater
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2
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1.0 Introduction
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Substantial progress towards valorization of wastewater and waste materials for bioenergy
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production from the treatment is gaining high priority in recent years. Microbial fuel cells
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(MFCs) are one of the widely studied technologies that have potential for waste valorization
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into energy in the form of bioelectricity generation (Pant et al., 2010; Kook et al., 2016). In
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spite of significant advancement happening in this field with respect to microbiology,
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material science, chemistry, electrochemistry, etc., process economics and process
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sustainability were found to be among the most important factors to move the field to the next
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level (Santoro et al., 2017; Mohanakrishna et al., 2010). Unlike the conventional biological
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processes for wastewater treatment, MFC found to have complex design due to two different
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redox reactions such as anodic oxidation and cathodic reduction at distinct locations
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(Mohanakrishna et al., 2012; Liu and Logan, 2004). The chamber that contains anode
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electrode and leads oxidation reaction is designated as an anode chamber. Similarly, the
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cathode chamber harbors reduction reaction with cathode electrodes. In MFCs, proton
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exchange membrane (PEM) or cation exchange membrane (CEM) is also one of the key
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components (Mohan et al., 2008; Franks et al., 2009). All these components have important
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roles in MFC design and scaling up of this technology as a commercial entity. Theoretically,
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irrespective of the volume, a maximum of 1.1 V can be achieved from a single MFC unit
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(Logan et al., 2006), and stacking up of MFCs with efficient and suitable design can increase
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the voltage output.
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Recent studies using different electrode materials as well as MFC reactor configurations in
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the range of 200 mL to few hundred liters were evaluated towards development of pilot-scale
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MFC systems (Janicek et al., 2014). Transforming this technology from bench to commercial
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scale will bring it a step forward towards realization of commercial application of
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bioelectricity generation. Among the several materials used for the electrodes, carbon based
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materials such as graphite and carbon with metal based impregnations or coatings were found
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to be cost-effective and efficient for bioelectricity generation (Mohanakrishna et al., 2012;
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Xie et al., 2010). Carbon based materials are also non corrosive and inert at wide range of
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environmental conditions that can be prevailing due to the application of industrial
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wastewater as substrate (Mohan et al., 2008).
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CAPEX and OPEX, which are also known as capital expenditure and operational
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expenditure, are used to determine the commercial potential of a technology. The
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manufacturability of MFCs can be deployed by extrusion, pultrusion, lamination processes to
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facilitate a considerable reduction of CAPEX on mass production. Similarly, OPEX can be
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reduced by choosing a simple design with higher durability (Premier et al., 2012). Many
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MFC designs were used in the laboratory scale such as H-type (Mohan et al., 2008),
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cylindrical type (Houghton et al., 2016), tubular (Zhang et al., 2013; Tee et al., 2016), flat
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plate type (Cheng et al., 2011; Zhuang et al., 2012), anaerobic digester (AD) type
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(Mohanakrishna et al., 2010), etc. Stack MFC configurations typically consist of insulator to
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be electrically isolated from the adjacent or surrounded MFC units. The insulator materials
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are inert which can be selected from a range of polyvinyl chloride (Zhang et al., 2013),
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polypropylene (Kim et al., 2010), nylon, plastic mesh, etc. (Janicek et al., 2014).
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Technical feasibility of MFC technology must be examined with treating actual wastewater,
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including both municipal and industrial wastewaters. Many of the prior studies have focused
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on municipal or low strength wastewater treatment in MFCs, and industrial wastewaters,
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which are highly diverse, require more attention. Downstream processing of oil industry
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produces huge amount of wastewater that contains organic material, mostly consist of
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petroleum hydrocarbons and dissolved solids comprising from sulfides, ammonia, nitrates
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and heavy metals (Chen et al., 2008; Srikanth et al., 2016). Composition of petroleum
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refinery wastewater (PRW) may vary based on the process and origin of crude petroleum
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reserves (Benyahia et al., 2005). Approximately 3.5 to 5 m3 of wastewater is generating from
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one ton of oil processed. Food wastewater also represents a strong interest for resource
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recovery and efficient treatment. Labanah whey wastewater (LW) is produced during labanah
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production which is a popular type of fermented milk product in the Middle-East countries.
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High amount of milk nutrients (more than 50%) retains in the LW and pH of 3.5, is leading to
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energy intensive biological treatment process (aerobic or anaerobic) (Abu-Reesh, 2014).
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Valorizing the waste organics present in wastewaters for energy generation can create
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sustainable economy along with the treatment (Iskander et al., 2016).
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In this study, two cylindrical MFCs that fabricated with graphite were used to evaluate two
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different wastewaters for valorization by bioelectricity generation: PRW has low
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biodegradable organic material (chemical oxygen demand, COD) and high dissolved solids
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(TDS) concentration, and LW from dairy industry having high COD and low pH. As graphite
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is one of the most used non-corrosive electrode material that exhibit good electron transfer
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and relatively low cost, it has been selected as suitable material for MFC fabrication. As both
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wastewaters used were distinct by characteristics, operation of MFC for bioelectricity
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generation demonstrates versatile performance of the process under batch-mode operation
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and mild operating conditions.
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2.0 Materials and Methods
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2.1 Wastewaters
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The present study used two different types of real field wastewaters namely petroleum
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refinery wastewater (PRW) and Labanah whey wastewater (LW). PRW collected from local
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petroleum refinery wastewater treatment plant in Doha, Qatar. Grab sample of PRW was
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collected from the feed point to the wastewater treatment plant (COD, 2150 mg/L; pH, 7.45).
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Labanah whey is the yellow greenish liquid portion of the milk that separated from the curds
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during Labanah manufacture. It is collected from Dandy Company, Doha (COD, 18546
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mg/L; pH, 6.78). The wastewater collected were immediately shifted to the laboratory and
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stored at 4 °C to preserve the nature of the wastewater and to avoid decomposition of
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organics by the indigenous bacteria of the wastewater. As per the experimental plan, only the
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required amount of wastewater was collected and used for experimentation.
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2.2 Graphite based MFC
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2.2.1 Configuration
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A MFC designed with cylindrical anode and cathode electrodes fabricated with graphite
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material was used in this study. Cylindrical cathode electrode with dimensions of 75 mm
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height and 34 mm diameter was epicentrically placed in a cup shaped anode electrode with
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the dimensions of 75 mm height and 62 mm diameter. As the anode is having closed type of
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design at one end with a wall thickness of 8 mm, concentrically placed cathode is projected
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outside by 8 mm as showed in Fig 1. Bottom of the cathode was closed with a plastic cap to
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avoid electrolyte direct contact between anolyte and catholyte. Similarly the open portion
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between anode and cathode (at top) was closed with a plastic lid. Top portion of the cathode
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was open and facilitated to supply air for dissolved oxygen in catholyte. This design presents
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a dual chambered membraneless MFC having apparent anode volume of 50 mL and cathode
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volume of 35 mL. Similarly the apparent/geometric surface area of anode and cathode were
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calculated as 146.61 cm2 and 65.34 cm2 respectively. The design was also achieved a
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minimum distance of 6 mm between anode and cathode, which is a favorable factor for the
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MFC performance (Srikanth and Mohan, 2012). Titanium wire was used as connector for
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regular electrochemical analysis such as voltage and current reading by multimeter and CV.
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Outer side of the anode chamber was covered with parafilm layer to avoid exposure to
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atmospheric air, which can interfere with the electrochemical activity in the system. The
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plastic lid of the anode was provided with a re sealable opening of 0.7 mm, to feed and
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replace/sampling anolyte using a syringe.
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Fig 1
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2.2.2 Biocatalyst and electroactive biofilm development
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Mixed microbial consortia developed from sewage water was used as the biocatalyst for the
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bioelectrogenic activity from the wastewater treatment in MFCs. Sewage wastewater was
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collected from sewage collection tank at a sewage treatment plant in Doha (COD, 780 mg/L;
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pH, 7.66). Prior to inoculation, one liter of sewage water was settled for 30 minutes at room
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temperature from which 900 mL of supernatant was discarded and the remaining 100 mL of
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settled mass along with the liquid portion was used as the inoculum for the MFC operation.
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Acetate was added at a concentration of 3 g/L to the settled sewage to provide simple
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substrate that improved the growth rate of bacteria. This media was added to the graphite
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anode compartment that develops anodic biofilm on the walls and operated for 4 days under
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anaerobic conditions. The same media was replenished from the anode chamber for 5 cycles
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for considerable biofilm growth. Subsequent to anodic biofilm formation, cathode chamber
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was introduced without disturbing anodic biofilm as per the design described in section 2.2.1.
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This stage was considered to shift the MFCs to the next proposed experiment with real field
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wastewater, PRW and LW.
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2.2.3 Operation
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Two different MFCs were assembled according to the described configuration in section
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2.2.1 and anodic biofilm was developed as explained in section 2.2.2. These MFCs were 7
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operated using two different wastewaters, one with PRW and another with LW. 50 mL PRW
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having average COD of 1950 mg/L, along with the nutrient solution (2 mL/L) was added to
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the anode chamber. In the cathode chamber, 35 mL of 100 mM phosphate buffer solution
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with neutral pH (pH 7) was added and this chamber was continuously supplied with
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atmospheric air to provide abundant oxygen required for cathodic reduction reaction.
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Similarly, the second MFC was also operated with LW at an average inlet COD of 3100
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mg/L (diluted Lananah whey) and 2 mL/L concentration of nutrient solution. Prior to the
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introduction of wastewater to the anode chambers, pH of both PRW and LW were adjusted to
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7.0. The feed wastewater was also supplied with nitrogen gas for 30 minutes to eliminate
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dissolved oxygen. Both MFCs were operated in batch mode at 22 ± 2 ºC (ambient
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temperature). After four days of operation (hydraulic retention time, HRT), the anodic feed
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was carefully removed with syringe and fresh feed was added to start the next batch cycle.
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Outlet liquid sample was preserved in the refrigerator for analysis to evaluate the system
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performance. Each reactor was operated for five consecutive batch cycles at the same
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operational conditions.
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2.3 Measurements and analysis
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Performance evaluation with respect to output parameters of MFCs such as potential
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difference/open circuit voltage (OCV), voltage and current (I; measured in series at 100 Ω)
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were recorded using auto-range multi-meter. Derived parameters such as current density
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(mA/m2), power (mW) and volumetric power density (W/m3) were calculated by relating
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with the specific design parameters of the present MFCs as described elsewhere
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(Mohanakrishna et al., 2012, 2017; Chiranjeevi et al., 2012). Cyclic voltammetry (CV) for
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behavior of bioanode/biocatalyst and electrode interface were studied using Bio-Logic
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potentiostat system (SP150, Bio-Logic Science Instruments, France). CV graphs were
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achieved at an applied potential ramp (scan rate) of 20 mV/s in the scan range of -1.0 V and
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+0.5 V vs open circuit potential and cathode was considered as counter electrode. The
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wastewater used in the system was acted as electrolyte. Polarization behavior of the MFC
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system was evaluated by current density across the range external resistance of 50 Ω to 30
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KΩ, during stable performance period of each MFC during 1st and 5th cycles of operation.
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Liquid samples from the anode chamber was collected at the end of each cycle of operation
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and preserved for analysis of pH and TDS according to APHA (1998). COD was analyzed
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with LANGE, UK testing kit. Power yield (W/Kg CODR) is a derived parameter that obtained
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by relating maximum power with the total amount of COD removed in each cycle.
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3.0 Results and discussion
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3.1
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Active biofilm on anode electrode is one of the important criteria for bioelectricity
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generation. The bacterial consortia present on the electrode surface anaerobically oxidize the
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organic matter (COD) present in the wastewater/anolyte and the generated electrons will be
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delivered to the anode that develops anodic potential (Torres et al., 2008; Mohanakrishna et
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al., 2017). When anodic biofilm was developed (by providing 3 g/L acetate as substrate and
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sewage as a source of bacteria) and operated for 4 days under anaerobic conditions, clear
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biofilm was formed gradually on the walls of the anode. The biofilm formation phenomenon
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was also evidenced by the decrease in the COD of the given wastewater. COD removal
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efficiency was increased with every cycle of operation. The first cycle was evidenced a drop
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in COD from 3815 mg/L to 2149 mg/L (CODR 43.67%). The subsequent cycles were clearly
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evidenced gradual improvement in COD removal efficiency up to the 4th cycle of operation
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(2nd cycle, 49.95%; 3rd cycle, 70.96%; 4th cycle, 81.81%). To confirm the effective biofilm
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formation, the 5th cycle of operation was operated without sewage wastewater (bacterial
MFC adaptation
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source) which confines bacteria only from the biofilm and eliminates what is in the
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suspension; this was also evidenced by good substrate removal of 64.69%. Subsequently,
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cylindrical cathode electrode was fixed for MFC operation, and acetate was added to anode
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as the sole carbon source and operated for 3 cycles, which also exhibited good COD removal
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efficiency along with bioelectricity generation. At initial stage, 0.35 mA of current was
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recorded which was gradually increased with time and recorded maximum currents of 2.45
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and 2.38 mA, for the second and third MFCs respectively. In the case of COD removal
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efficiency, it was fluctuated in the narrow range of 62.22 % to 70.50% confirming the stable
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biofilm exhibiting bioelectrogenesis. These MFCs were further shifted to real field
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wastewaters, PRW and LW.
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3.2 Electricity generation from PRW and LW
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Bioelectricity generation from the operation of graphite MFC has visualized the function of
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COD present in wastewater for bioenergy energy generation (Fig 2). The trend of
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bioelectricity production from both wastewaters was found to be similar. However, current
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generation was found to be depending on the type of wastewater. Among the two
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wastewaters, PRW showed higher performance than LW. In the case of PRW, the first cycle
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exhibited maximum current of 3.1 mA (100 Ω) (Fig 2a). When the maximum bioelectrogenic
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activity was observed, an apparent improvement was observed with subsequent cycles of
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operation (2nd cycle, 3.15 mA; 3rd cycle, 3.20 mA; 4th cycle, 3.17 mA), and showed
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maximum performance during the 5th cycle (3.35 mA). In the case of LW, the first cycle
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registered maximum current of 3.03 mA (100 Ω) (Fig 2b). Unlike PRW, LW showed almost
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stable performance in subsequent operating cycles (2nd cycle, 2.89 mA; 3rd cycle, 3.05 mA;
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4th cycle, 3.18 mA; 5th cycle, 3.20 mA). The stable performance with LW and negligible
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fluctuations with PRW were attributed to the maximum performance of the respective
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wastewaters in this MFC configuration and operational conditions.
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Fig 2
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Polarization curves were plotted by using the current that recorded at different external
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resistances in the range of 100 Ω to 30 KΩ, during the 1st and 5th cycles of operation using
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PRW and LW as substrates (Fig 3). At higher external potential, negligible amount of current
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was recorded and the voltage was found to be almost equals to the OCV. As the external
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resistance decreased, current generation found to be increased, which also resulted in a drop
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in voltage (Ieropoulos et al., 2010). Similar trend was observed with both PRW and LW at
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the 1st and 5th cycles of operation. Higher electron discharge is possible with lower
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resistances that results in higher current generation and lower closed circuit potentials.
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Voltage stabilization was observed to be relatively rapid at higher resistances compared to
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that of lower resistances. This phenomenon also can be explained by the electron discharge
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capacity at different external resistances. Cell design point can be determined by the
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polarization behavior, according to which the maximum volumetric power density (VPDMax)
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of LW was found at 750 Ω (for both 1st and 5th cycle of operation). However, the 5th cycle
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showed higher power density of 3.85 W/m3 than the 1st cycle (2.64 W/m3), that illustrating the
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better adaptability of the system with number of cycles operation. On the contrary to the
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power density, voltage was found to be higher during the 1st cycle (154 mV; 750 Ω) of
265
operation than the 5th cycle (116 mV; 750 Ω). The electron discharge capability of anodic
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biofilm also improves with time where selection of electroactive bacterial consortia will be
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dominant. In the case of PRW as substrate, the cell design point was also observed at 750 Ω.
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Unlike LW, PRW shows similar variations in the maximum power density through
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polarization curve from the 1st cycle of operation (PDMax, 3.41 W/m3 at 500 Ω) to the 5th cycle
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of operation (PDMax, 2.90 W/m3 at 750 Ω). In the case of PRW higher voltage was exhibited
271
during the 5th cycle (121 mV ; 750 Ω) compared to the 1st cycle (108 mV; 750 Ω). This
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phenomenon exhibits higher electron discharge of anodic biofilm in the 5th cycle compared to
273
the1st cycle.
274
Fig 3
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3.3 Organic carbon conversion
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Microbial oxidation of organic matter present in the anolyte or wastewater is feeding the
277
electrons that are required for bioelectricity generation in MFCs. Both the wastewaters, PRW
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and LW used in the present study were contributed for the electricity generation during their
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treatment. Among both substrates, LW showed higher substrate degradation compared to
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PRW which might be due to the nature of the components present. Majority of COD in the
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PRW is contribution from the petroleum hydrocarbons, which has lower biodegradability
282
than LW (Fig 4). Organics present in LW are from the dairy based biological processes that
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naturally exhibit high biodegradability. From five cycles of operation with PRW, the average
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substrate degradation (CODR) percentage was recorded as 38.84% which is considerably
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lower than that of LW (Fig 4a). The minimum and maximum percentage of substrate
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degradation were noted as 32.48% (1st cycle) and 45.06% (5th cycle). From the five cycles of
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operation, CODR of LW was observed in the range of 63.35% (1st cycle) and 72.76% (3rd
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cycle) with an average value of 68.53% (Fig 4b). The pattern of CODR of PRW and LW
289
showed an apparent improvement with each cycle of operation. The increased substrate
290
degradation during later cycles explains the functional adaptability of the system for the
291
applied operating conditions. Volumetric power density observed with both the wastewaters
292
can also be related to the change in substrate degradation efficiency from the 1st to the 5th
293
cycle (Fig 4). Higher substrate degradation by the anodic biofilm facilitates higher electron
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discharge, which in turn influences the power density of the system. Such improvement was
295
significant with LW rather than PRW.
296
Fig 4
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Considering MFC as the technology for wastewater treatment, substrate degradation is one of
298
the important criteria along with bioenergy generation for measuring the performance of
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MFC. Previous studies on PRW treatment in microbial desalination cell (MDC) configuration
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with anodic working volume of 1.13 L showed substrate degradation rate of 0.71 kg
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COD/m3/day (Sevda et al., 2017). Present study with anodic chamber working volume of
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0.05 L showed, 2 to 3 times higher degradation rate using PRW as a substrate. Average
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substrate degradation rate of PRW was registered as 1.90 kg COD/m3/day (maximum, 2.27
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kg COD/m3/day in the 5th cycle; minimum, 1.52 kg COD/m3/day in the 2nd cycle). Higher
305
substrate degradation rate was due to the relatively small volume of the anode chamber and
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larger surface area available for biofilm that improves the biological oxidation of COD. In the
307
present configuration, cathode chamber is intruding into volume of anode that distinctly
308
making the design very compact and simple. It was also providing more surface area per
309
volume of anode chamber. The nutrients availability from the bulk liquid is higher for such a
310
configuration, which also facilitates more substrate degradation, which in turn improves the
311
electron transfer rate.
312 313
3.4 Electrochemical response of anodic biofilm
314
Cyclic voltammetry is a technique that evaluates the electrochemical response of the biofilm
315
across a range of applied potentials. The electrochemical response of both reactors during the
316
1st and 5th cycles of operation showed significant variation in the electrochemically active
317
biofilm (Fricke et al., 2008; Mohanakrishna et al., 2012). Electron discharge pattern for LW,
318
with respect to forward sweep (FS, 2.44 mA) and reverse sweep (RS) evidenced significant 13
319
improvement from the 1st cycle to the 5th cycle (Fig 5). Thickness and age of the biofilm on
320
the anode surface having potential of matured electron transfer rate than younger and thin
321
biofilm. In the case of LW, the phenomenon was clearly visible by the oxidation (FS) and
322
reduction sweeps (RS) of CV. From the 1st to the 5th cycle, oxidation peak was shifted from
323
1.98 mA to 2.44 mA. In case of reduction peak, it was shifted from -1.04 mA to -2.40 mA.
324
The electrochemical response of the MFC with PRW was contrary to the MFC with LW.
325
Oxidation peak was unchanged from the 1st to the 5th cycle (0.61 mA). However, significant
326
improvement was identified with respect to the reduction peak (1st cycle, 1.42 mA; 5th cycle,
327
1.50 mA). Substrate degradation and bioelectrogenesis can be clearly correlated with the
328
bioelectrochemical activity of both MFCs.
329
Fig 5
330
3.5 Substrate conversion to electricity
331
In the case of LW, substrate degradation rate was identified in the range of 4.85 kg COD/m3/
332
day (4th cycle) and 5.37 kg COD/m3/day (5th cycle) that are much higher than PRW. This
333
might be due to the higher inlet COD value and higher biodegradability nature of the LW
334
wastewater. It can also be confirmed from the higher current density and power density in the
335
present study. For the two types of wastewater, the maximum current density (LW, 219
336
mA/m2; PRW, 229 mA/m2) and volumetric power density (LW, 20.48 W/m3; PRW, 22.45
337
W/m3) were identified with PRW that attributing it as the more favorable conditions for
338
bioelectricity generation. High concentration of total dissolved solids (TDS) present in PRW
339
might be the factor for high current generation. TDS present in the anolyte helps in the
340
improved electron transfer (Mohan et al., 2010; Zhang et al., 2011). The specific power yield
341
(SPY) that can be calculated from the maximum power produced in relation to the substrate
342
removed during particular cycle of operation. SPY of both wastewaters are also supporting
343
similar observation. PRW showed good SPY of 1.945 W/kg CODR, which is 3 times higher 14
344
than LW (0.593 W/kg CODR). The volumetric power yield (VPY) is the relative
345
representation which is derived by dividing the maximum power produced with volume of
346
anode chamber. Higher VPY of 28.27 W/m3 was observed with PRW compared to LW
347
(22.23 W/m3). Both SPY and VPY are the two key expressions that elucidate commercial or
348
upscaling feasibility of MFC technology.
349 350
3.6 pH variation
351
Neutral pH was considered for the feed wastewater to provide an optimum conditions for the
352
bacterial activity. The effluent of MFCs or outlet of anolyte at the end of each cycle was
353
analyzed for the pH and monitored for the influence of the bioelectrogenesis and wastewater
354
remediation on the redox conditions. In the case of LW, the pH of the wastewater was found
355
to decrease towards mild acidic conditions (Fig 6). A fluctuation in the pH was found in the
356
range of 5.6 (2nd cycle) and 6.2 (4th cycle) with an average outlet pH of 5.96. Oxidation of
357
complex organic molecules present in the dairy based wastewater contains lactose, lactic acid
358
and glucose, produces organic acids. These acids, also called volatile fatty acids (VFAs)
359
causes acidic conditions in the anode chamber (Mohan et al., 2010). MFC operating with
360
PRW depicted negligible change in the pH of the outlet water. Maximum change in the pH
361
was observed in the 1st and 2nd cycle (7.4) and the minimum pH observed was in the 3rd and
362
4th cycles of operation. Average outlet pH was identified as 7.27. PRW contains
363
hydrocarbons which demonstrate complex metabolites on oxidation ranging from C8 to C20
364
hydrocarbons (Li et al., 2016).
365
Fig 6
366 367 368
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369
4.0 Conclusions
370
Cylindrical membrane-less graphite based MFC operated with petroleum refinery wastewater
371
(PRW) and Labanah whey wastewater (LW) were found to be feasible for bioelectricity
372
generation along with treatment. Availability of huge amount of PRW and LW and stringent
373
guidelines for the discharge of treated wastewaters opens opportunities for renewable energy
374
production. Good strength of graphite and simple design eliminates casing for MFC
375
fabrication that minimizes the capital expenditure for set-up. The power densities recorded
376
were on the lower side. However, enough scope is available for improved energy recovery by
377
adapting efficient electron transfer and treatment methods with these wastewaters.
378 379
Acknowledgements
380
This publication was made possible by NPRP grant # 6-289-2-125 from the Qatar national
381
research fund (a member of Qatar Foundation). The statements made herein are solely the
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responsibility of the authors.
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Captions for Figures
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Fig 1: Schematic diagram of MFC fabricated with graphite in which anode and cathode were concentrically arranged to build complete reactor. Fig 2: Bioelectricity production from the treatment of (a) Petroleum refinery wastewater and (b) Labanah whey wastewater in cylindrical graphite MFCs. Fig 3: Polarization behaviour of MFC with (a) Petroleum refinery wastewater and (b)
493
Labanah whey wastewater during first (open symbol) and fifth (solid symbol) cycles
494
of operation.
495 496 497
Fig 4: Substrate degradation pattern of (a) Petroleum refinery wastewater and (b) Labanah whey wastewater during bioelectricity generation. Fig 5: Cyclic voltammetric response of (a) MFC with petroleum refinery wastewater and (b)
498
MFC with Labanah whey wastewater during first (red colour) and fifth cycle (blue
499
colour) of operation.
500 501
Fig 6: Change in anolyte pH with petroleum refinery wastewater and Labanah whey wastewater during MFC operation for bioelectricity prodution.
502 503
22
504
505 506 507
Fig 1
508
23
509
Current at 100 Ohms (mA)
3.5
a
Petroleum Refinery wastewater
3.0 2.5 2.0 3.5
b
Labanah whey wastewater
3.0 2.5 2.0 0
100
200
300
Time in Hours 510 511 512
Fig 2
513
24
400
500
514 515
400
4
300
3
200
2
100
1
3
Petroleum Refinery wastewater
0 0
50
100
150
0 200
4
400
3
Voltage (mV)
2
200
1
100
0
50
100
3
Labanah whey wastewater
0
150
Current density (mA/m2)
Fig 3
25
Volumetric power density (W/m )
b
300
518
250
Current density (mA/m2)
516
517
Volumetric power density (W/m )
Voltage (mV)
a
200
0 250
519
3000
a
Petroleum refinery wastewater
b
Labanah whey wastewater
COD Inlet COD Outlet COD Removal
COD Concentration (mg/L)
2000
1000
0 4000
3000
2000
1000
0 C1
C2
C3
Operating cycle 520 521
Fig 4
522 523
26
C4
C5
524
vs. Ewe 11 02 15 h type cell control abiotic 50 mM bic_02_CV_C11.mpr, cycle 2 #
a
0.5 /mA
12 02 15 h type cell control abiotic 1_02_CV_C11.mpr, cycle 4
0 -0.5 -1
Petroleum refinery wastewater
-1.5 -1
-0.5
0
0.5
Ewe/V
525
vs. Ew e 13 02 15 h type cell cv-ca biotic 1 EXP1_01_CV_C11.mpr, cycle 3 #
b
2 /mA
13 02 15 h type cell cv-ca biotic 1 EXP1_02_CV_C11.mpr, cycle 3
1 0 -1
Labanah whey wastewater
-2 -1 526 527
-0.5
0 Ewe/V
Fig 5
528 529
27
0.5
530
Petroleum refinery wastewater Labanah whey wastewater
8
Outlet pH
6
4
2
0 C1
C2
C3 Operating cycle
531 532
Fig 6
533
28
C4
C5
534
Research Highlights
535
Cylindrical graphite microbial fuel cell evaluated for bioelectricity generation.
536
Wastewater from petroleum refinery and Labanah whey processes are suitable
537 538
substrates. The design is simple, cost-effective and feasible for scaling up MFC technology.
539
29
S0960-8524(17)31739-X https://doi.org/10.1016/j.biortech.2017.09.174 BITE 19002
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
27 July 2017 23 September 2017 25 September 2017
Please cite this article as: Mohanakrishna, G., Abu-Reesh, I.M., Al-Raoush, R.I., He, Z., Cylindrical graphite based microbial fuel cell for the treatment of industrial wastewaters and bioenergy generation, Bioresource Technology (2017), doi: https://doi.org/10.1016/j.biortech.2017.09.174
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1
Cylindrical graphite based microbial fuel cell for the treatment of
2
industrial wastewaters and bioenergy generation
3 *
Gunda Mohanakrishna1, Ibrahim M. Abu-Reesh1 , Riyadh I. Al-Raoush2, Zhen He3
4 5 6
1
7 8
2713 Doha, Qatar 2
9 10 11
Department of Chemical Engineering, College of Engineering, Qatar University, P O Box
Department of Civil and Architectural Engineering, College of Engineering, Qatar University, P O Box 2713 Doha, Qatar
3
Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
12 13 14 15 16
*Corresponding author: [email protected]
17 18 19 20 21 22 23 24 25 26
1
27
Abstract
28 29
Cylindrical graphite microbial fuel cell (MFC) configuration designed by eliminating distinct
30
casing and membrane was evaluated for bioelectrogenesis and treatment of real-field
31
wastewaters. Both petroleum refinery wastewater (PRW) and Labanah whey wastewater
32
(LW) were used as substrates, and investigated for electricity generation and organic removal
33
under batch mode operation. PRW showed higher bioelectricity generation (current and
34
power generation of 3.35 mA and 1.12 mW at 100 Ω) compared to LW (3.2 mA and 1.02
35
mW). On the contrary, higher substrate degradation efficiency was achieved using LW
36
(72.76%) compared to PRW (45.06%). Superior function of MFC operation in terms of
37
volumetric power density (PRW, 28.27 W/m3; LW, 23.23 W/m3) suggesting the feasibility of
38
using these wastewaters for bioelectricity generation. Large sources of wastewater that
39
generating in the Middle-East countries have potential to produce renewable energy from the
40
treatment, which helps for the sustainable wastewater management and simultaneous
41
renewable energy production.
42 43
Keywords: Graphite reactor, Bioelectrochemical treatment, Labanah whey wastewater,
44
Petroleum refinery wastewater
45
2
46
1.0 Introduction
47
Substantial progress towards valorization of wastewater and waste materials for bioenergy
48
production from the treatment is gaining high priority in recent years. Microbial fuel cells
49
(MFCs) are one of the widely studied technologies that have potential for waste valorization
50
into energy in the form of bioelectricity generation (Pant et al., 2010; Kook et al., 2016). In
51
spite of significant advancement happening in this field with respect to microbiology,
52
material science, chemistry, electrochemistry, etc., process economics and process
53
sustainability were found to be among the most important factors to move the field to the next
54
level (Santoro et al., 2017; Mohanakrishna et al., 2010). Unlike the conventional biological
55
processes for wastewater treatment, MFC found to have complex design due to two different
56
redox reactions such as anodic oxidation and cathodic reduction at distinct locations
57
(Mohanakrishna et al., 2012; Liu and Logan, 2004). The chamber that contains anode
58
electrode and leads oxidation reaction is designated as an anode chamber. Similarly, the
59
cathode chamber harbors reduction reaction with cathode electrodes. In MFCs, proton
60
exchange membrane (PEM) or cation exchange membrane (CEM) is also one of the key
61
components (Mohan et al., 2008; Franks et al., 2009). All these components have important
62
roles in MFC design and scaling up of this technology as a commercial entity. Theoretically,
63
irrespective of the volume, a maximum of 1.1 V can be achieved from a single MFC unit
64
(Logan et al., 2006), and stacking up of MFCs with efficient and suitable design can increase
65
the voltage output.
66 67
Recent studies using different electrode materials as well as MFC reactor configurations in
68
the range of 200 mL to few hundred liters were evaluated towards development of pilot-scale
69
MFC systems (Janicek et al., 2014). Transforming this technology from bench to commercial
70
scale will bring it a step forward towards realization of commercial application of
3
71
bioelectricity generation. Among the several materials used for the electrodes, carbon based
72
materials such as graphite and carbon with metal based impregnations or coatings were found
73
to be cost-effective and efficient for bioelectricity generation (Mohanakrishna et al., 2012;
74
Xie et al., 2010). Carbon based materials are also non corrosive and inert at wide range of
75
environmental conditions that can be prevailing due to the application of industrial
76
wastewater as substrate (Mohan et al., 2008).
77 78
CAPEX and OPEX, which are also known as capital expenditure and operational
79
expenditure, are used to determine the commercial potential of a technology. The
80
manufacturability of MFCs can be deployed by extrusion, pultrusion, lamination processes to
81
facilitate a considerable reduction of CAPEX on mass production. Similarly, OPEX can be
82
reduced by choosing a simple design with higher durability (Premier et al., 2012). Many
83
MFC designs were used in the laboratory scale such as H-type (Mohan et al., 2008),
84
cylindrical type (Houghton et al., 2016), tubular (Zhang et al., 2013; Tee et al., 2016), flat
85
plate type (Cheng et al., 2011; Zhuang et al., 2012), anaerobic digester (AD) type
86
(Mohanakrishna et al., 2010), etc. Stack MFC configurations typically consist of insulator to
87
be electrically isolated from the adjacent or surrounded MFC units. The insulator materials
88
are inert which can be selected from a range of polyvinyl chloride (Zhang et al., 2013),
89
polypropylene (Kim et al., 2010), nylon, plastic mesh, etc. (Janicek et al., 2014).
90 91
Technical feasibility of MFC technology must be examined with treating actual wastewater,
92
including both municipal and industrial wastewaters. Many of the prior studies have focused
93
on municipal or low strength wastewater treatment in MFCs, and industrial wastewaters,
94
which are highly diverse, require more attention. Downstream processing of oil industry
95
produces huge amount of wastewater that contains organic material, mostly consist of
4
96
petroleum hydrocarbons and dissolved solids comprising from sulfides, ammonia, nitrates
97
and heavy metals (Chen et al., 2008; Srikanth et al., 2016). Composition of petroleum
98
refinery wastewater (PRW) may vary based on the process and origin of crude petroleum
99
reserves (Benyahia et al., 2005). Approximately 3.5 to 5 m3 of wastewater is generating from
100
one ton of oil processed. Food wastewater also represents a strong interest for resource
101
recovery and efficient treatment. Labanah whey wastewater (LW) is produced during labanah
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production which is a popular type of fermented milk product in the Middle-East countries.
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High amount of milk nutrients (more than 50%) retains in the LW and pH of 3.5, is leading to
104
energy intensive biological treatment process (aerobic or anaerobic) (Abu-Reesh, 2014).
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Valorizing the waste organics present in wastewaters for energy generation can create
106
sustainable economy along with the treatment (Iskander et al., 2016).
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In this study, two cylindrical MFCs that fabricated with graphite were used to evaluate two
108
different wastewaters for valorization by bioelectricity generation: PRW has low
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biodegradable organic material (chemical oxygen demand, COD) and high dissolved solids
110
(TDS) concentration, and LW from dairy industry having high COD and low pH. As graphite
111
is one of the most used non-corrosive electrode material that exhibit good electron transfer
112
and relatively low cost, it has been selected as suitable material for MFC fabrication. As both
113
wastewaters used were distinct by characteristics, operation of MFC for bioelectricity
114
generation demonstrates versatile performance of the process under batch-mode operation
115
and mild operating conditions.
116 117
2.0 Materials and Methods
118
2.1 Wastewaters
119
The present study used two different types of real field wastewaters namely petroleum
120
refinery wastewater (PRW) and Labanah whey wastewater (LW). PRW collected from local
5
121
petroleum refinery wastewater treatment plant in Doha, Qatar. Grab sample of PRW was
122
collected from the feed point to the wastewater treatment plant (COD, 2150 mg/L; pH, 7.45).
123
Labanah whey is the yellow greenish liquid portion of the milk that separated from the curds
124
during Labanah manufacture. It is collected from Dandy Company, Doha (COD, 18546
125
mg/L; pH, 6.78). The wastewater collected were immediately shifted to the laboratory and
126
stored at 4 °C to preserve the nature of the wastewater and to avoid decomposition of
127
organics by the indigenous bacteria of the wastewater. As per the experimental plan, only the
128
required amount of wastewater was collected and used for experimentation.
129 130
2.2 Graphite based MFC
131
2.2.1 Configuration
132
A MFC designed with cylindrical anode and cathode electrodes fabricated with graphite
133
material was used in this study. Cylindrical cathode electrode with dimensions of 75 mm
134
height and 34 mm diameter was epicentrically placed in a cup shaped anode electrode with
135
the dimensions of 75 mm height and 62 mm diameter. As the anode is having closed type of
136
design at one end with a wall thickness of 8 mm, concentrically placed cathode is projected
137
outside by 8 mm as showed in Fig 1. Bottom of the cathode was closed with a plastic cap to
138
avoid electrolyte direct contact between anolyte and catholyte. Similarly the open portion
139
between anode and cathode (at top) was closed with a plastic lid. Top portion of the cathode
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was open and facilitated to supply air for dissolved oxygen in catholyte. This design presents
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a dual chambered membraneless MFC having apparent anode volume of 50 mL and cathode
142
volume of 35 mL. Similarly the apparent/geometric surface area of anode and cathode were
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calculated as 146.61 cm2 and 65.34 cm2 respectively. The design was also achieved a
144
minimum distance of 6 mm between anode and cathode, which is a favorable factor for the
145
MFC performance (Srikanth and Mohan, 2012). Titanium wire was used as connector for
6
146
regular electrochemical analysis such as voltage and current reading by multimeter and CV.
147
Outer side of the anode chamber was covered with parafilm layer to avoid exposure to
148
atmospheric air, which can interfere with the electrochemical activity in the system. The
149
plastic lid of the anode was provided with a re sealable opening of 0.7 mm, to feed and
150
replace/sampling anolyte using a syringe.
151
Fig 1
152
2.2.2 Biocatalyst and electroactive biofilm development
153
Mixed microbial consortia developed from sewage water was used as the biocatalyst for the
154
bioelectrogenic activity from the wastewater treatment in MFCs. Sewage wastewater was
155
collected from sewage collection tank at a sewage treatment plant in Doha (COD, 780 mg/L;
156
pH, 7.66). Prior to inoculation, one liter of sewage water was settled for 30 minutes at room
157
temperature from which 900 mL of supernatant was discarded and the remaining 100 mL of
158
settled mass along with the liquid portion was used as the inoculum for the MFC operation.
159
Acetate was added at a concentration of 3 g/L to the settled sewage to provide simple
160
substrate that improved the growth rate of bacteria. This media was added to the graphite
161
anode compartment that develops anodic biofilm on the walls and operated for 4 days under
162
anaerobic conditions. The same media was replenished from the anode chamber for 5 cycles
163
for considerable biofilm growth. Subsequent to anodic biofilm formation, cathode chamber
164
was introduced without disturbing anodic biofilm as per the design described in section 2.2.1.
165
This stage was considered to shift the MFCs to the next proposed experiment with real field
166
wastewater, PRW and LW.
167 168
2.2.3 Operation
169
Two different MFCs were assembled according to the described configuration in section
170
2.2.1 and anodic biofilm was developed as explained in section 2.2.2. These MFCs were 7
171
operated using two different wastewaters, one with PRW and another with LW. 50 mL PRW
172
having average COD of 1950 mg/L, along with the nutrient solution (2 mL/L) was added to
173
the anode chamber. In the cathode chamber, 35 mL of 100 mM phosphate buffer solution
174
with neutral pH (pH 7) was added and this chamber was continuously supplied with
175
atmospheric air to provide abundant oxygen required for cathodic reduction reaction.
176
Similarly, the second MFC was also operated with LW at an average inlet COD of 3100
177
mg/L (diluted Lananah whey) and 2 mL/L concentration of nutrient solution. Prior to the
178
introduction of wastewater to the anode chambers, pH of both PRW and LW were adjusted to
179
7.0. The feed wastewater was also supplied with nitrogen gas for 30 minutes to eliminate
180
dissolved oxygen. Both MFCs were operated in batch mode at 22 ± 2 ºC (ambient
181
temperature). After four days of operation (hydraulic retention time, HRT), the anodic feed
182
was carefully removed with syringe and fresh feed was added to start the next batch cycle.
183
Outlet liquid sample was preserved in the refrigerator for analysis to evaluate the system
184
performance. Each reactor was operated for five consecutive batch cycles at the same
185
operational conditions.
186 187
2.3 Measurements and analysis
188
Performance evaluation with respect to output parameters of MFCs such as potential
189
difference/open circuit voltage (OCV), voltage and current (I; measured in series at 100 Ω)
190
were recorded using auto-range multi-meter. Derived parameters such as current density
191
(mA/m2), power (mW) and volumetric power density (W/m3) were calculated by relating
192
with the specific design parameters of the present MFCs as described elsewhere
193
(Mohanakrishna et al., 2012, 2017; Chiranjeevi et al., 2012). Cyclic voltammetry (CV) for
194
behavior of bioanode/biocatalyst and electrode interface were studied using Bio-Logic
195
potentiostat system (SP150, Bio-Logic Science Instruments, France). CV graphs were
8
196
achieved at an applied potential ramp (scan rate) of 20 mV/s in the scan range of -1.0 V and
197
+0.5 V vs open circuit potential and cathode was considered as counter electrode. The
198
wastewater used in the system was acted as electrolyte. Polarization behavior of the MFC
199
system was evaluated by current density across the range external resistance of 50 Ω to 30
200
KΩ, during stable performance period of each MFC during 1st and 5th cycles of operation.
201
Liquid samples from the anode chamber was collected at the end of each cycle of operation
202
and preserved for analysis of pH and TDS according to APHA (1998). COD was analyzed
203
with LANGE, UK testing kit. Power yield (W/Kg CODR) is a derived parameter that obtained
204
by relating maximum power with the total amount of COD removed in each cycle.
205 206
3.0 Results and discussion
207
3.1
208
Active biofilm on anode electrode is one of the important criteria for bioelectricity
209
generation. The bacterial consortia present on the electrode surface anaerobically oxidize the
210
organic matter (COD) present in the wastewater/anolyte and the generated electrons will be
211
delivered to the anode that develops anodic potential (Torres et al., 2008; Mohanakrishna et
212
al., 2017). When anodic biofilm was developed (by providing 3 g/L acetate as substrate and
213
sewage as a source of bacteria) and operated for 4 days under anaerobic conditions, clear
214
biofilm was formed gradually on the walls of the anode. The biofilm formation phenomenon
215
was also evidenced by the decrease in the COD of the given wastewater. COD removal
216
efficiency was increased with every cycle of operation. The first cycle was evidenced a drop
217
in COD from 3815 mg/L to 2149 mg/L (CODR 43.67%). The subsequent cycles were clearly
218
evidenced gradual improvement in COD removal efficiency up to the 4th cycle of operation
219
(2nd cycle, 49.95%; 3rd cycle, 70.96%; 4th cycle, 81.81%). To confirm the effective biofilm
220
formation, the 5th cycle of operation was operated without sewage wastewater (bacterial
MFC adaptation
9
221
source) which confines bacteria only from the biofilm and eliminates what is in the
222
suspension; this was also evidenced by good substrate removal of 64.69%. Subsequently,
223
cylindrical cathode electrode was fixed for MFC operation, and acetate was added to anode
224
as the sole carbon source and operated for 3 cycles, which also exhibited good COD removal
225
efficiency along with bioelectricity generation. At initial stage, 0.35 mA of current was
226
recorded which was gradually increased with time and recorded maximum currents of 2.45
227
and 2.38 mA, for the second and third MFCs respectively. In the case of COD removal
228
efficiency, it was fluctuated in the narrow range of 62.22 % to 70.50% confirming the stable
229
biofilm exhibiting bioelectrogenesis. These MFCs were further shifted to real field
230
wastewaters, PRW and LW.
231 232
3.2 Electricity generation from PRW and LW
233
Bioelectricity generation from the operation of graphite MFC has visualized the function of
234
COD present in wastewater for bioenergy energy generation (Fig 2). The trend of
235
bioelectricity production from both wastewaters was found to be similar. However, current
236
generation was found to be depending on the type of wastewater. Among the two
237
wastewaters, PRW showed higher performance than LW. In the case of PRW, the first cycle
238
exhibited maximum current of 3.1 mA (100 Ω) (Fig 2a). When the maximum bioelectrogenic
239
activity was observed, an apparent improvement was observed with subsequent cycles of
240
operation (2nd cycle, 3.15 mA; 3rd cycle, 3.20 mA; 4th cycle, 3.17 mA), and showed
241
maximum performance during the 5th cycle (3.35 mA). In the case of LW, the first cycle
242
registered maximum current of 3.03 mA (100 Ω) (Fig 2b). Unlike PRW, LW showed almost
243
stable performance in subsequent operating cycles (2nd cycle, 2.89 mA; 3rd cycle, 3.05 mA;
244
4th cycle, 3.18 mA; 5th cycle, 3.20 mA). The stable performance with LW and negligible
10
245
fluctuations with PRW were attributed to the maximum performance of the respective
246
wastewaters in this MFC configuration and operational conditions.
247
Fig 2
248 249
Polarization curves were plotted by using the current that recorded at different external
250
resistances in the range of 100 Ω to 30 KΩ, during the 1st and 5th cycles of operation using
251
PRW and LW as substrates (Fig 3). At higher external potential, negligible amount of current
252
was recorded and the voltage was found to be almost equals to the OCV. As the external
253
resistance decreased, current generation found to be increased, which also resulted in a drop
254
in voltage (Ieropoulos et al., 2010). Similar trend was observed with both PRW and LW at
255
the 1st and 5th cycles of operation. Higher electron discharge is possible with lower
256
resistances that results in higher current generation and lower closed circuit potentials.
257
Voltage stabilization was observed to be relatively rapid at higher resistances compared to
258
that of lower resistances. This phenomenon also can be explained by the electron discharge
259
capacity at different external resistances. Cell design point can be determined by the
260
polarization behavior, according to which the maximum volumetric power density (VPDMax)
261
of LW was found at 750 Ω (for both 1st and 5th cycle of operation). However, the 5th cycle
262
showed higher power density of 3.85 W/m3 than the 1st cycle (2.64 W/m3), that illustrating the
263
better adaptability of the system with number of cycles operation. On the contrary to the
264
power density, voltage was found to be higher during the 1st cycle (154 mV; 750 Ω) of
265
operation than the 5th cycle (116 mV; 750 Ω). The electron discharge capability of anodic
266
biofilm also improves with time where selection of electroactive bacterial consortia will be
267
dominant. In the case of PRW as substrate, the cell design point was also observed at 750 Ω.
268
Unlike LW, PRW shows similar variations in the maximum power density through
269
polarization curve from the 1st cycle of operation (PDMax, 3.41 W/m3 at 500 Ω) to the 5th cycle
11
270
of operation (PDMax, 2.90 W/m3 at 750 Ω). In the case of PRW higher voltage was exhibited
271
during the 5th cycle (121 mV ; 750 Ω) compared to the 1st cycle (108 mV; 750 Ω). This
272
phenomenon exhibits higher electron discharge of anodic biofilm in the 5th cycle compared to
273
the1st cycle.
274
Fig 3
275
3.3 Organic carbon conversion
276
Microbial oxidation of organic matter present in the anolyte or wastewater is feeding the
277
electrons that are required for bioelectricity generation in MFCs. Both the wastewaters, PRW
278
and LW used in the present study were contributed for the electricity generation during their
279
treatment. Among both substrates, LW showed higher substrate degradation compared to
280
PRW which might be due to the nature of the components present. Majority of COD in the
281
PRW is contribution from the petroleum hydrocarbons, which has lower biodegradability
282
than LW (Fig 4). Organics present in LW are from the dairy based biological processes that
283
naturally exhibit high biodegradability. From five cycles of operation with PRW, the average
284
substrate degradation (CODR) percentage was recorded as 38.84% which is considerably
285
lower than that of LW (Fig 4a). The minimum and maximum percentage of substrate
286
degradation were noted as 32.48% (1st cycle) and 45.06% (5th cycle). From the five cycles of
287
operation, CODR of LW was observed in the range of 63.35% (1st cycle) and 72.76% (3rd
288
cycle) with an average value of 68.53% (Fig 4b). The pattern of CODR of PRW and LW
289
showed an apparent improvement with each cycle of operation. The increased substrate
290
degradation during later cycles explains the functional adaptability of the system for the
291
applied operating conditions. Volumetric power density observed with both the wastewaters
292
can also be related to the change in substrate degradation efficiency from the 1st to the 5th
293
cycle (Fig 4). Higher substrate degradation by the anodic biofilm facilitates higher electron
12
294
discharge, which in turn influences the power density of the system. Such improvement was
295
significant with LW rather than PRW.
296
Fig 4
297
Considering MFC as the technology for wastewater treatment, substrate degradation is one of
298
the important criteria along with bioenergy generation for measuring the performance of
299
MFC. Previous studies on PRW treatment in microbial desalination cell (MDC) configuration
300
with anodic working volume of 1.13 L showed substrate degradation rate of 0.71 kg
301
COD/m3/day (Sevda et al., 2017). Present study with anodic chamber working volume of
302
0.05 L showed, 2 to 3 times higher degradation rate using PRW as a substrate. Average
303
substrate degradation rate of PRW was registered as 1.90 kg COD/m3/day (maximum, 2.27
304
kg COD/m3/day in the 5th cycle; minimum, 1.52 kg COD/m3/day in the 2nd cycle). Higher
305
substrate degradation rate was due to the relatively small volume of the anode chamber and
306
larger surface area available for biofilm that improves the biological oxidation of COD. In the
307
present configuration, cathode chamber is intruding into volume of anode that distinctly
308
making the design very compact and simple. It was also providing more surface area per
309
volume of anode chamber. The nutrients availability from the bulk liquid is higher for such a
310
configuration, which also facilitates more substrate degradation, which in turn improves the
311
electron transfer rate.
312 313
3.4 Electrochemical response of anodic biofilm
314
Cyclic voltammetry is a technique that evaluates the electrochemical response of the biofilm
315
across a range of applied potentials. The electrochemical response of both reactors during the
316
1st and 5th cycles of operation showed significant variation in the electrochemically active
317
biofilm (Fricke et al., 2008; Mohanakrishna et al., 2012). Electron discharge pattern for LW,
318
with respect to forward sweep (FS, 2.44 mA) and reverse sweep (RS) evidenced significant 13
319
improvement from the 1st cycle to the 5th cycle (Fig 5). Thickness and age of the biofilm on
320
the anode surface having potential of matured electron transfer rate than younger and thin
321
biofilm. In the case of LW, the phenomenon was clearly visible by the oxidation (FS) and
322
reduction sweeps (RS) of CV. From the 1st to the 5th cycle, oxidation peak was shifted from
323
1.98 mA to 2.44 mA. In case of reduction peak, it was shifted from -1.04 mA to -2.40 mA.
324
The electrochemical response of the MFC with PRW was contrary to the MFC with LW.
325
Oxidation peak was unchanged from the 1st to the 5th cycle (0.61 mA). However, significant
326
improvement was identified with respect to the reduction peak (1st cycle, 1.42 mA; 5th cycle,
327
1.50 mA). Substrate degradation and bioelectrogenesis can be clearly correlated with the
328
bioelectrochemical activity of both MFCs.
329
Fig 5
330
3.5 Substrate conversion to electricity
331
In the case of LW, substrate degradation rate was identified in the range of 4.85 kg COD/m3/
332
day (4th cycle) and 5.37 kg COD/m3/day (5th cycle) that are much higher than PRW. This
333
might be due to the higher inlet COD value and higher biodegradability nature of the LW
334
wastewater. It can also be confirmed from the higher current density and power density in the
335
present study. For the two types of wastewater, the maximum current density (LW, 219
336
mA/m2; PRW, 229 mA/m2) and volumetric power density (LW, 20.48 W/m3; PRW, 22.45
337
W/m3) were identified with PRW that attributing it as the more favorable conditions for
338
bioelectricity generation. High concentration of total dissolved solids (TDS) present in PRW
339
might be the factor for high current generation. TDS present in the anolyte helps in the
340
improved electron transfer (Mohan et al., 2010; Zhang et al., 2011). The specific power yield
341
(SPY) that can be calculated from the maximum power produced in relation to the substrate
342
removed during particular cycle of operation. SPY of both wastewaters are also supporting
343
similar observation. PRW showed good SPY of 1.945 W/kg CODR, which is 3 times higher 14
344
than LW (0.593 W/kg CODR). The volumetric power yield (VPY) is the relative
345
representation which is derived by dividing the maximum power produced with volume of
346
anode chamber. Higher VPY of 28.27 W/m3 was observed with PRW compared to LW
347
(22.23 W/m3). Both SPY and VPY are the two key expressions that elucidate commercial or
348
upscaling feasibility of MFC technology.
349 350
3.6 pH variation
351
Neutral pH was considered for the feed wastewater to provide an optimum conditions for the
352
bacterial activity. The effluent of MFCs or outlet of anolyte at the end of each cycle was
353
analyzed for the pH and monitored for the influence of the bioelectrogenesis and wastewater
354
remediation on the redox conditions. In the case of LW, the pH of the wastewater was found
355
to decrease towards mild acidic conditions (Fig 6). A fluctuation in the pH was found in the
356
range of 5.6 (2nd cycle) and 6.2 (4th cycle) with an average outlet pH of 5.96. Oxidation of
357
complex organic molecules present in the dairy based wastewater contains lactose, lactic acid
358
and glucose, produces organic acids. These acids, also called volatile fatty acids (VFAs)
359
causes acidic conditions in the anode chamber (Mohan et al., 2010). MFC operating with
360
PRW depicted negligible change in the pH of the outlet water. Maximum change in the pH
361
was observed in the 1st and 2nd cycle (7.4) and the minimum pH observed was in the 3rd and
362
4th cycles of operation. Average outlet pH was identified as 7.27. PRW contains
363
hydrocarbons which demonstrate complex metabolites on oxidation ranging from C8 to C20
364
hydrocarbons (Li et al., 2016).
365
Fig 6
366 367 368
15
369
4.0 Conclusions
370
Cylindrical membrane-less graphite based MFC operated with petroleum refinery wastewater
371
(PRW) and Labanah whey wastewater (LW) were found to be feasible for bioelectricity
372
generation along with treatment. Availability of huge amount of PRW and LW and stringent
373
guidelines for the discharge of treated wastewaters opens opportunities for renewable energy
374
production. Good strength of graphite and simple design eliminates casing for MFC
375
fabrication that minimizes the capital expenditure for set-up. The power densities recorded
376
were on the lower side. However, enough scope is available for improved energy recovery by
377
adapting efficient electron transfer and treatment methods with these wastewaters.
378 379
Acknowledgements
380
This publication was made possible by NPRP grant # 6-289-2-125 from the Qatar national
381
research fund (a member of Qatar Foundation). The statements made herein are solely the
382
responsibility of the authors.
383
16
384 385 386 387
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Captions for Figures
487 488 489 490 491 492
Fig 1: Schematic diagram of MFC fabricated with graphite in which anode and cathode were concentrically arranged to build complete reactor. Fig 2: Bioelectricity production from the treatment of (a) Petroleum refinery wastewater and (b) Labanah whey wastewater in cylindrical graphite MFCs. Fig 3: Polarization behaviour of MFC with (a) Petroleum refinery wastewater and (b)
493
Labanah whey wastewater during first (open symbol) and fifth (solid symbol) cycles
494
of operation.
495 496 497
Fig 4: Substrate degradation pattern of (a) Petroleum refinery wastewater and (b) Labanah whey wastewater during bioelectricity generation. Fig 5: Cyclic voltammetric response of (a) MFC with petroleum refinery wastewater and (b)
498
MFC with Labanah whey wastewater during first (red colour) and fifth cycle (blue
499
colour) of operation.
500 501
Fig 6: Change in anolyte pH with petroleum refinery wastewater and Labanah whey wastewater during MFC operation for bioelectricity prodution.
502 503
22
504
505 506 507
Fig 1
508
23
509
Current at 100 Ohms (mA)
3.5
a
Petroleum Refinery wastewater
3.0 2.5 2.0 3.5
b
Labanah whey wastewater
3.0 2.5 2.0 0
100
200
300
Time in Hours 510 511 512
Fig 2
513
24
400
500
514 515
400
4
300
3
200
2
100
1
3
Petroleum Refinery wastewater
0 0
50
100
150
0 200
4
400
3
Voltage (mV)
2
200
1
100
0
50
100
3
Labanah whey wastewater
0
150
Current density (mA/m2)
Fig 3
25
Volumetric power density (W/m )
b
300
518
250
Current density (mA/m2)
516
517
Volumetric power density (W/m )
Voltage (mV)
a
200
0 250
519
3000
a
Petroleum refinery wastewater
b
Labanah whey wastewater
COD Inlet COD Outlet COD Removal
COD Concentration (mg/L)
2000
1000
0 4000
3000
2000
1000
0 C1
C2
C3
Operating cycle 520 521
Fig 4
522 523
26
C4
C5
524
vs. Ewe 11 02 15 h type cell control abiotic 50 mM bic_02_CV_C11.mpr, cycle 2 #
a
0.5 /mA
12 02 15 h type cell control abiotic 1_02_CV_C11.mpr, cycle 4
0 -0.5 -1
Petroleum refinery wastewater
-1.5 -1
-0.5
0
0.5
Ewe/V
525
vs. Ew e 13 02 15 h type cell cv-ca biotic 1 EXP1_01_CV_C11.mpr, cycle 3 #
b
2 /mA
13 02 15 h type cell cv-ca biotic 1 EXP1_02_CV_C11.mpr, cycle 3
1 0 -1
Labanah whey wastewater
-2 -1 526 527
-0.5
0 Ewe/V
Fig 5
528 529
27
0.5
530
Petroleum refinery wastewater Labanah whey wastewater
8
Outlet pH
6
4
2
0 C1
C2
C3 Operating cycle
531 532
Fig 6
533
28
C4
C5
534
Research Highlights
535
Cylindrical graphite microbial fuel cell evaluated for bioelectricity generation.
536
Wastewater from petroleum refinery and Labanah whey processes are suitable
537 538
substrates. The design is simple, cost-effective and feasible for scaling up MFC technology.
539
29