Waste Management xxx (2015) xxx–xxx
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation K. Chandrasekhar 1, K. Amulya, S. Venkata Mohan ⇑,1 Bioengineering and Environmental Sciences (BEES), CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 19 December 2014 6 May 2015 Accepted 3 June 2015 Available online xxxx Keywords: Solid-state fermentation Bioelectricity Biohydrogen Bioethanol Electrofuels Microbial fuel cell
a b s t r a c t A novel solid state bio-electrofermentation system (SBES), which can function on the self-driven bioelectrogenic activity was designed and fabricated in the laboratory. SBES was operated with food waste as substrate and evaluated for simultaneous production of electrofuels viz., bioelectricity, biohydrogen (H2) and bioethanol. The system illustrated maximum open circuit voltage and power density of 443 mV and 162.4 mW/m2, respectively on 9th day of operation while higher H2 production rate (21.9 ml/h) was observed on 19th day of operation. SBES system also documented 4.85% w/v bioethanol production on 20th day of operation. The analysis of end products confirmed that H2 production could be generally attributed to a mixed acetate/butyrate-type of fermentation. Nevertheless, the presence of additional metabolites in SBES, including formate, lactate, propionate and ethanol, also suggested that other metabolic pathways were active during the process, lowering the conversion of substrate into H2. SBES also documented 72% substrate (COD) removal efficiency along with value added product generation. Continuous evolution of volatile fatty acids as intermediary metabolites resulted in pH drop and depicted its negative influence on SBES performance. Bio-electrocatalytic analysis was carried out to evaluate the redox catalytic capabilities of the biocatalyst. Experimental data illustrated that solid-state fermentation can be effectively integrated in SBES for the production of value added products with the possibility of simultaneous solid waste remediation. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In the present energy-based society, the value of any energy-rich matter is raising. Thus, high organic load in wastewaters is no longer seen as waste, but considered as a valuable energy resource. Today, methane is primarily obtained from the biogas of traditional anaerobic digesters, but its conversion into electricity via combustion is not very efficient (Motte et al., 2013). Direct electricity generation from wastewater treatment using bioelectrochemical systems (BES) is advantageous, because thermodynamic conversion step is not necessary (Venkata Mohan et al., 2007, 2010; Huang and Logan, 2008). BES is a hybrid bioelectrochemical device, which directly transforms energy stored in chemical bonds of substrate to electrical energy via bioelectrochemical reactions mediated by microorganisms as biocatalyst (Rahimnejad et al., 2011; Wu et al., 2012). Microorganisms extract energy required to build biomass (anabolic process) from redox reactions (catabolism) through electron donor/acceptor conditions. In recent years BES has emerged as a promising yet ⇑ Corresponding author. Tel./fax: +91 40 27191765. 1
E-mail address: [email protected] (S. Venkata Mohan). Address: Academy of Scientific and Innovative Research (AcSIR), India.
challenging technology for the recovery of energy from waste besides its treatment and is gaining importance due to its sustainable nature (Min et al., 2005; Moon et al., 2006; He et al., 2007; Kumlanghan et al., 2007; Sevda et al., 2013; Chandrasekhar and Venkata Mohan, 2012). The process provides dual benefits of wastewater treatment and access to cheap and environmental friendly energy. One third of the food produced globally for human consumption is wasted yearly (Gustavsson et al., 2011). The overall amount of food wasted and lost worldwide corresponds to approximately 1.3 billion tonnes. This quantity includes all sorts of food, such as roots and tubers, oilseed and pulses, cereals, fruits and vegetables, meat, seafood, milk and eggs (Gustavsson et al., 2013). Food based waste generated from canteen operations is one of the most prominent and highly biodegradable solid waste (Venkateswar Reddy et al., 2011). Regardless of the food wasted, it is either treated or disposed in landfill sites in order to prevent environmental inconvenience. Elimination of food waste by dumping into landfill sites is inapt, as it causes serious health problems in densely populated areas (Cuéllar and Webber, 2010; Zilbermann et al., 2013). Several studies depicted the possibilities of energy production from food waste by anaerobic digestion and incineration, or
http://dx.doi.org/10.1016/j.wasman.2015.06.001 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chandrasekhar, K., et al. Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.001
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K. Chandrasekhar et al. / Waste Management xxx (2015) xxx–xxx
Nomenclature AB acidogenic bacteria BES bio-electrochemical systems CA chronoamperometry CD current density CDP cell design point CHP cumulative H2 production COD chemical oxygen demand CV cyclic voltammetry DPV differential pulse voltammetry DSW designed synthetic wastewater e electron GPES general purpose electrochemical system Hydrogen (H2) biohydrogen HPLC high performance liquid chromatography I current LSV linear sweep voltammetry MFC microbial fuel cell
OCV PD Q RDAP RE RE RI Rp SBES SSF TDS TS TSS UASB VFA
open circuit voltage power density charge relative decrease in anode potential external resistance reference electrode internal resistance polarization resistance solid state bio-electrochemical system solid state fermentation total dissolved solids total solids total suspended solids up-flow anaerobic sludge blanket reactor volatile fatty acids
utilization of food waste as feed for pigs and cattle in order to close the nutrient loop (Sayeki et al., 2001; Shin and Youn, 2005; De Gioannis et al., 2013; Li et al., 2010; Kim et al., 2013; Dahiya et al., 2015). This waste can also act as good substrate for anaerobic digestion process due to its highly biodegradable organic content (Lim et al., 2000; Kim et al., 2006; Venkateswar Reddy et al., 2011; Ruggeri et al., 2013). Solid state fermentation (SSF) showed promising results in the production of various types of products including food and food ingredients, agro-industrial products and pharmaceutical products (Pandey, 2000). For some specific products, SSF offers higher yields and better product spectra (Ishida et al., 2000; De Vrije et al., 2001). Biodegradable organic matter present in food waste can be effectively utilized for energy recovery in the form of bioelectricity by integrating SSF with BES (Venkata Mohan and Chandrasekhar, 2011a). Nevertheless, within a short period after start-up of BES operation, edible oil forms a thin layer between substrate and electrode (anode) surface hindering direct contact between anode and anodophilic biocatalyst in turn interfering with electricity generation (Venkata Mohan and Chandrasekhar, 2011a). If properly designed BES can assist direct conversion of solid waste through SSF and consequently make the process sustainable for successful waste management. Therefore, the present study was designed to evaluate the function of solid state bio-electrochemical system (SBES) for the recovery of bio fuels in the form of bioelectricity, biohydrogen (H2) and bioethanol simultaneously by integrating SSF of food waste. Based on the output obtained from the previous study (Venkata Mohan and Chandrasekhar, 2011a), in the present study, electrodes (anode and cathode) were vertically placed to avoid interference. Bioelectricity, H2 and ethanol production with the function of time were evaluated. The fate of individual volatile fatty acids (VFA) was studied during the SBES operation. Cyclic voltammetry (CV) was applied to assess the electron discharge properties of the biocatalyst. Bio-electrocatalytic assessment was done through Tafel analysis and the results were interpreted in terms of redox Tafel slopes and polarization resistance, during SBES operation.
In this study, a single chambered solid state bio-electrochemical system (SBES) with air-cathode was designed and fabricated in the laboratory (Fig. 1). SBES was operated with non-catalyzed graphite electrodes (5 5 cm; 0.5 cm thick; surface area 60 cm2) as anode and cathode. The design was modified by placing the electrodes vertically, instead of placing horizontally (Venkata Mohan and Chandrasekhar, 2011a) in order to avoid the interference of residual oil content. One side of the cathode was vertically immersed in food waste (electrolyte) and the other side was exposed to air, while the anode was completely dipped in the substrate. Copper wires attached to the electrodes through epoxy sealant were used to provide connection. Provisions were made in the design for wire input, sampling ports and inlet and outlet ports. The reactor had a total/working volume of 400/300 ml and was operated at room temperature (30 ± 1 °C) in fed-batch mode, with a total cycle period of 30 days. The fuel cell was closed to maintain strict anaerobic microenvironment throughout the process. Reactor was fed with solid state food based canteen waste (270 ml) along with 30 ml (10% v/v) of tap water to maintain moisture content and to enhance the conductivity of anolyte (substrate). Prior to feeding, pH of the food waste was 6.8 ± 0.1 and was adjusted to 7 1 N NaOH solution.
2. Materials and methods
2.3. Anaerobic consortia
2.1. Composite food waste
Anaerobic mixed culture from full scale hydrogen producing UASB reactor was used as biocatalyst in SBES (Venkateswar Reddy et al., 2011). Prior to inoculation, the mixed culture was washed twice in saline buffer (6000 rpm, 22 °C) and enriched in designed synthetic wastewater [DSW (in g/l), Glucose, 3.0;
Food waste collected from the institute canteen was grinded and used as solid feed for the operation of SBES. The solid food waste was composite in nature majorly comprising of boiled rice
followed by vegetable peelings, cooked vegetables, un-cooked vegetables (spoiled), cooking oil, etc. with a water content varying between 10% and 15%. The physic–chemical characteristics of the food waste prior to feeding were evaluated and it was noticed that it had high organic content (pH, 6.8 ± 0.4; TSS, 31 ± 0.6 g/l; TS, 42 ± 0.9 g/l; TDS, 11.2 g/l; VFA, 8.4 g/l; COD, 380 g/l; carbohydrates (total), 68.5 g/l; oil content, 38 g/l). Prior to feeding the oil fraction of the waste was separated by gravity-separation mechanism (Venkata Mohan and Chandrasekhar, 2011a). Pre-characterized substrate was stored in airtight container and preserved in a refrigerator at 4±.5 °C to avoid spoilage. 2.2. Bio-electrochemical system architecture and operation
Please cite this article in press as: Chandrasekhar, K., et al. Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.001
K. Chandrasekhar et al. / Waste Management xxx (2015) xxx–xxx
3
Head space Volume 100 ml
Fig. 1. Schematic details of the solid state bio-electrochemical system.
NH4Cl, 0.5, KH2PO4, 0.25; K2HPO4, 0.25 l; MgCl2, 0.3; CoCl2, 0.025; ZnCl2, 0.0115; CuCl2, 0.0105; CaCl2, 0.05; NiSO4, 0.016; MnCl2, 0.015; FeCl3, 0.025] under anaerobic microenvironment at pH 7.0 (100 rpm; 48 h) (Amulya et al., 2014; Venkata Mohan and Chandrasekhar, 2011b). The anaerobic mixed consortia was subjected to repeated pretreatment (three times) changing between heat-shock, acid-shock and chemical-shock methods to selectively enrich the H2 producing acidogenic bacteria (AB) (Chandrasekhar and Venkata Mohan, 2014b).
2.4. Analysis The power output and H2 production efficiency were considered as the two key parameters to evaluate the performance of SBES. Open circuit voltage (OCV)/potential difference and current (I; measured in series at 100 X) were recorded manually on auto-range digital multi-meter. Power (mW) was derived from P = IV equation. Power density (PD) (mW/m2) and current density (CD) (mA/m2) were considered with the function of anodic surface area (m2). Polarization was carried out manually to evaluate polarization behavior of SBES using a variable resistor set-up with a range of 30 kO to 50 O. To evaluate the bio-electrochemical fuel cell behavior, electrochemical techniques such as cyclic voltammetry (CV), chronoamperometry (CA) and differential pulse voltammetry (DPV) were performed using a potentiostat-galvanostat system (Autolab, PGSTAT12, Ecochemie) over the voltage range from +0.5 to 0.5 V (CV), 0 to +0.3 V (CA) and +0.5 to 0.5 V (DPV), respectively. During analysis, substrate (food waste) was used as electrolyte with anode and cathode of SBES as working and counter electrodes, respectively against saturated Ag/AgCl as reference electrode (RE). Scan rate of 30 mV/s was optimized for the reactor, which showed major interfacial electron-transfer kinetics. The system was operated through a PC using GPES software. Cell potentials (anode) were measured against RE. SBES performance with respect to treatment efficiency was monitored by analyzing pH, VFA and COD according to the Standard Methods (APHA, 1998). Quantitative estimation of VFA was carried out by High performance liquid chromatography (HPLC; Shimadzu
LC10A) with UV–Vis detector at 210 nm and C18 reverse phase column (250 4.6 mm diameter and 5 l particle size). Before analysis, samples were diluted with deionized water to reduce concentration and filtered through a polystyrene membrane filter (0.45 lm). The mobile phase used for separation was 40% acetonitrile in 1 mN H2SO4 (pH, 2.5–3.0) with flow rate of 0.5 ml/min. A sample containing ethanol was initially acidified with H2SO4 (20%, 10 ll/ml of sample), filtered through a polystyrene membrane filter (0.45 lm) and subsequently analyzed by HPLC as described by Crespo et al. (2012). Samples were analyzed immediately after collection. The analytical system (HPLC) was calibrated with standard chemicals from Sigma Aldrich in the range of concentration of the samples analyzed. 3. Results and discussion 3.1. Electrogenesis SBES was operated under anaerobic conditions and strictly monitored for a period of 30 days. The experimental results clearly depicted the influence of substrate (electrolyte) on the performance of this air-cathode SBES. Bioelectricity generation was evident by SSF of food waste in SBES. Immediately after start-up, relatively low open circuit voltage (OCV, 17 mV) and power density (PD, 0.32 mW/m2) were observed (Fig. 2a). With time, marked improvement in OCV and PD was observed accounting for a maximum of 443 mV and 162.4 mW/m2, respectively on 9th day of SBES operation. SBES documented stable performance from 9th to 20th day of operation with marginal variations (Fig. 2a) followed by decreasing trend till 25th day of operation. From 26th day of operation a visible drop in both OCV (286 mV) and PD (21.3 mW/m2) was observed, which reduced further on 30th day (OCV, 67 mV and PD, 0.57 mW/m2). Relatively higher power output observed on 9th day operation might be due to the availability of simple sugars around anodic surface area for anodophilic bacteria. Gradual decrement in the power generation after 26th day of operation might be due to less availability of simple sugars (monomers) and lower metabolic functions of electrochemically active biocatalyst.
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Fig. 2. (a) Voltage and current generation, (b) polarization behavior, (c) cell potentials and (d) RDAP during SBES operation.
3.1.1. Electron losses To estimate the power outputs, polarization curves were drawn during stable phase of SBES operation (on 20th day). The cell design point (voltage change region) of a particular fuel cell system can be obtained by considering the point at which highest PD noticed on the polarization curve intersects with voltage and CD (Barbir, 2005). Polarization curve documented e discharge at 5 kO on 20th day of operation (Fig. 2b). Cell design point (CDP) for SBES was observed at 300 O accounting for a power density (PD) of 40.01 mW/m2. It is a standard exercise to run the fuel cell on the left side of the PD peak and at an elevated voltage or low current density (Kiran Kumar et al., 2012; Velvizhi et al., 2012; Venkata Mohan et al., 2014). Hence, SBES with food waste can be operated beyond 300 O. This indicated the stabilized performance of fuel cell after 20th day of operation in terms of enhanced e discharge supporting the metabolic efficiency of microorganisms in the anodic compartment. The polarization curve assists to analyze the e losses during transfer from the biocatalyst to the anode and then to the cathode (Fig. 2b). Electron transfer (direct electron transfer or mediated electron transfer) from biocatalyst toward the electrode (anode) is normally hindered by anodic over-potentials, which impact the power generation efficiency of microbial fuel cell (MFC) (Kiran Kumar et al., 2012). There are three different losses viz., activation, ohmic and concentration losses that are normally possible during fuel cell operation. Oxidation of substrate at the electrode (anode) surface or reduction of a
compound on the biocatalyst surface or in the bacterial cell requires activation energy, which incurs a potential loss usually termed as activation losses (Chandrasekhar and Venkata Mohan, 2012). It is obvious that activation losses are measured in the lower range of current densities and are vital. There is a possibility that many metabolic functions may occur at the same time in anaerobic mixed consortia, resulting in high activation losses. This also facilitates e quenching, which normally arises due to rivalry between the metabolic pathways. SBES operated with food waste also showed higher activation losses due to the various metabolic functions occurring in bacteria (Chandrasekhar and Venkata Mohan, 2012). Ohmic losses caused by the electrical resistances of the membrane, electrodes material, electrolyte, etc., can be reduced by high electrolyte conductivity. During SBES operation, significantly less ohmic losses were observed due to higher e transfer, which also improved power output. Huge oxidative force of the anode creates concentration losses where substrate oxidation is quicker at the anode generating more e than that can be transported to the anode surface and to the cathode, especially at lower resistances (Chandrasekhar and Venkata Mohan, 2012; Lee et al., 2009). 3.1.2. Cell potentials Variations in cell, anode and cathode potentials were evaluated against the external resistance (RE) range from 50 X to 30 kX on 20th day of SBES operation. Wide range of variations (542–44 mV)
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3.1.3. Sustainable power SBES system is considered to be in steady state, when the power generated is equal to the power consumed for an extensive period of time. With the function of applied RE, relative decrease in anode potential (RDAP) is employed to assess highest sustainable power, which can be calculated using the initial anodic potential (Eo,anodic) and anodic potentials at each applied RE as shown in Eq. (1).
RDAPð%Þ ¼ ½ðEo;anodic Eanodic Þ=Eo;anodic 100
ð1Þ
The linear fit at higher RE represents a region at which the resistance controls the power. When the RE was high, RDAP amplified linearly due to limited e delivery to the cathode. Nevertheless, at lower RE, the e delivery to the cathode was limited by kinetic and/or mass transfer (or internal resistance (RI)). At conditions where RE and RI limitations were the same among these two lines, a parallel line from the intersection was drawn to estimate RE, which allows the measurement of sustainable power. SBES showed maximum sustainable resistance of 8 KO with PD of 3.7 mW/m2 (Fig. 2d). Observed sustainable resistance during SBES operation indicated a decrease in the kinetic and/or mass transfer or internal resistances and increase in transfer of e to cathode which resulted in high sustainable power. This also supports longer duration of power sustainability observed during operation of SBES (30 days). 3.2. Biohydrogen (H2) Experimental data also depicted the feasibility of fermentative H2 production through food waste as substrate. The biodegradability of food waste is mainly related to carbohydrate material, which is the main substrate for H2 production (De Gioannis et al., 2013). H2 production rate differed significantly with varying time. H2 production was observed from 2nd day of operation and maximum H2 production rate of 21.52 ml/h was achieved on 19th day. This is an indication for the lag in the metabolic activities due to higher substrate concentration and the necessity of additional time for the conversion of organic matter into H2 (Chandrasekhar and Venkata Mohan, 2014b). Production rate was observed to fall gradually after attaining a maximum value. The accumulation of volatile fatty acids as by-products leads to drop in pH and this might be reason for the observed decrement in H2 production after 26th day of operation (Chandrasekhar and Venkata Mohan, 2014b). Cumulative H2 production (CHP) increased till the end of the SBES operation (Fig. 3). CHP was observed to increase with time and showed 5400 ml H2 at end of the operation. CHP was observed to stabilize after 26th day of SBES operation. 3.3. Bioethanol Production and accumulation of VFA and ethanol as secondary metabolites takes place due to anaerobic SSF in bacteria and yeast (Raghavarao et al., 2003). During SBES operation, biocatalyst mediated catalytic functions lead to biotransformation of VFA into acetaldehyde and CO2. Acetaldehyde is further transformed to ethanol as end product in the presence of biocatalyst. Samples were collected at frequent intervals viz., 10th, 20th and 30th days of SBES operation to evaluate ethanol production efficiency. SBES
CHP
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were observed in cathode potential under applied RE indicating that the current generated during SBES operation was not restricted by the anodic reactions alone (Fig. 2c). Considerable drop in anode potential was observed during operation when RE decreased. Anode potential regulates the kinetics of e transfer from the exo-electrogenic biocatalyst to the electrode surface (anode). Significant drop in potential was observed from or below 10 kX suggesting the feasibility of effective e discharge below 10 kX during SBES operation.
5000
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operation documented feasibility of ethanol production parallel to substrate utilization. During SBES operation, ethanol production of 1.45% w/v was observed at initial stages (10th day), followed by an increase and attained a maximum value on 20th day (4.85% w/v) of operation. After 20th day a decrease in ethanol production (2.15% w/v) was observed till 30th day of operation. 3.4. Process monitoring 3.4.1. Bio-electrochemical evaluation Bio-electrochemistry involves the understanding of anodic biocatalyst mediated redox catalytic phenomena coupled with charge separation and charge transfer, which can take place evenly in solution, or heterogeneously on electrode surfaces. Electrode reactions generally take place in the interfacial region between solution and electrode where charge delivery differs from that of the bulk phases. The electrode acts as e acceptor and can be affected by the structure and nature of electrode. Nevertheless, the change in charge distribution during fuel cell operation is due to the substrate metabolism and nature of the biocatalyst (microorganism). Few microbial species have the capability to discharge reducing powers (e and H+) generated during the metabolic activity via bacterial (biocatalyst) membrane bound organelles or soluble mediators. In situ evaluation of e discharge employing CV (Vs Ag/AgCl (S)) with time helps to understand the metabolic shifts and the carriers involved in e transfer. Electrochemical evaluation visualized marked variation in e discharge properties with respect to time (Fig. 4a). SBES documented higher catalytic currents (oxidation, 130.26 mA; reduction, 18.39 mA) at 20th day of operation followed by 10th day (oxidation, 78.07 mA; reduction, 19.12 mA), 0th day (oxidation, 32.18 mA; reduction, 16.21 mA) and 30th day (oxidation, 16.72 mA; reduction, 14.17 mA) in both oxidation and reduction sweeps. Nevertheless, oxidation was quite higher than reduction in SBES operation. This is a direct evidence of the association of electrochemically active bacteria (biocatalyst) with the native mixed consortia avoiding the e losses prior to reaching the electrode (anode). During the operation, SBES was subjected to chronoamperometry (CA) analysis by maintaining a steady potential (1.2 V). CA enumerates the utmost possible sustainable current during the SBES operation. Gradual increment in current was observed in CA profiles from 0th day to 15th day of SBES operation where decreasing trend was observed from 20th day to till the end of operation
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The stored energy in a capacitor (fuel cell) is equal to the work done to charge it (fuel cell). Considering a capacitor of capacitance C, holding a charge Q and dW = QC dQ upon integration gives following equation.
a
i/A
W charge ¼
Z
Q Q 2 CV2 dQ ¼ ¼ C 2C 2
ð3Þ
where W represents work done (J), Q is the charge (C) and C is the capacitance (F). Comparatively higher energy output was observed on 20th day (31.25 mJ) followed by 10th day (20.80 mJ), 0th day (12.48 mJ) and 30th day (5.83 mJ) of SBES operation. Observed results correlated well with enhanced performance of the system in terms of electricity generation on 20th day of operation.
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(Fig. 4b). On 15th day of operation higher current of 6.5 mA was observed initially that showed a drastic drop for a few seconds and then stabilized near 2.5 mA after 250 s. The differential behavior of CA pattern indicated effective performance of SBES from 10th day to 20th day of operation where e generated constantly facilitated power generation. This behavior of SBES might be due to the efficient metabolic function of electrochemically active biocatalyst around electrode (anode) surface. Observed negligible electrochemical activity shown by the biocatalyst on 30th day of operation might be due to the less availability of simple substrate. Charge (Q) represents the number of e present at particular instance on the electrode (anode) surface throughout oxidation. Charge distribution was observed to vary with time and metabolic functions of the biocatalyst used. Voltammetric profiles showed higher charge on the 20th day (125 mC) followed by 10th day (102 mC), 0th day (79 mC) and 30th day (54 mC) of SBES operation. The trend indicated higher availability of e on the anode surface resulting in higher power output. Charge separation can also be represented by capacitance and the complexity of charge transfer by its resistance. Capacitance is the capability of a cell to hold an electrical charge and is a measure of the sum of electrical energy stored at an applied electric potential given by
C ¼ Q =V max
ð2Þ
where C is the capacitance in Faradays, Q is the amount of charge obtained (C) and V is the maximum applied potential (Vmax). Higher capacitance observed on 20th day (251 mF) followed by 10th day (204 mF), 0th day (158 mF) and 30th day (108 mF) of SBES operation supports the observed higher power generation during 20th day of SBES operation.
3.4.2. Electro-kinetic evaluation Tafel analysis was performed to understand the electro-kinetics in terms of redox behavior and polarization resistance (RP) of the biocatalyst during the operation of SBES. Low value of the slope connected to the oxidation process indicated the requirement of lower activation energy that makes oxidation more favorable (Chandrasekhar and Venkata Mohan, 2012). The same theory is applicable in the case of reduction slope. SBES operation documented higher oxidation reactions over reduction reactions. During SBES operation, lower oxidation slope was observed on 20th day of operation (0.126 V/dec) followed by 10th day (0.154 V/dec), 0th day (0.184 V/dec) and 30th day (0.114 V/dec) (Fig. 5). Reduction reaction showed minor variations with respect to time. Lower reduction slope was observed on 20th day (0.217 V/dec) followed by 10th day (0.221 V/dec), 0th day (0.228 V/dec) and 30th day (0.234 V/dec). Observed lower oxidation slope during SBES operation on 20th day indicated favorable conditions for substrate oxidation by the biocatalyst which was well supported by the observed higher H2 production. Values of reduction slopes were relatively higher than that of oxidation slope. This might be due to rapid consumption of H+ for H2 production by H2 producers in the presence of dehydrogenase, which facilitated the accessibility of H+ through redox reactions. Observed results correlated well with the improvement in H2 production. The resistance to e transfer could be explained in terms of polarization resistance (Rp). High Rp was observed on 30th day of SBES operation. Higher the resistance in SBES, lower is the current generated which indicated a reduced amount of metabolic functions of the biocatalyst. SBES operation showed higher Rp on 30th day (272 O) followed by 0th day (202 O), 10th day (179 O) and 20th day (102 O). It is evident from Rp values that SBES operation was more favorable for effective electron transfer on 20th day of operation. The observed lower Rp on 20th day of SBES operation might be due to improved metabolic functions of the biocatalyst and also rapid substrate oxidation/consumption by the biocatalyst, which in turn lead to higher bioelectricity generation. 3.4.3. Differential pulse voltammetry (DPV) In the present study, whole-microorganism voltammetry was used to evaluate the electrochemical behavior of the biocatalyst (mixed consortia) during SBES operation. DPV is the most suitable voltammetry technique for the biological system, which can monitor redox species at small concentrations (Liu et al., 2003). Although CV is the most informative method used to investigate redox carriers involved in substrate oxidation by anodophilic microorganism, it has low detection limit, and subtraction of ohmic (capacitive) current is necessary to reveal minute details (Velvizhi et al., 2012). Furthermore, the catalytic redox reactions occurring at the electrodes comprise of complex e transfer chains and to study such reaction mechanisms CV necessitates substantial
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E/V Fig. 5. Tafel plots derived from cyclic voltammograms during SBES operation [against reference electrode (Ag/AgCl (s))].
time and appropriate post-processing of data (Marsili et al., 2008). In comparison, pulse methods have the potential to disclose characteristic peaks while cancelling out capacitive current, even at elevated scan rates, and are often used as complementary techniques to CV (Liu et al., 2003). DPV (vs. Ag/AgCl) was performed in SBES with similar electrode arrangements as described above for CV. As depicted in Fig. 6, DPV commonly revealed a consistent broad peak at a formal potential of 0.065 ± 0.005 V vs. Ag/AgCl3 during operation of SBES although the maximum peak concentrations were achieved on 20th day (Fig. 6). This indicated that elevated biocatalytic behavior was observed on 20th day of SBES operation due to enhanced metabolic reactions of the electrochemically active bacteria. Observed bioelectrogenesis and CV results correlated well with the observed higher efficiency of SBES operation on 20th day of operation. 3.4.4. Substrate degradation The organic fraction of the composite food waste functioned as an electron donor in the anode compartment of SBES resulting in power generation apart from substrate degradation. At the end of the cycle, COD analysis was performed to understand the substrate degradation efficiency of SBES by sampling waste around anode surface. Based on COD analysis, prior to feeding, substrate COD was noticed as 380 g/l. At the end of the SBES operation (on 30th day), substrate COD was noticed as 106 g/l. Overall SBES operation documented 273 g/l of substrate degradation with 72% substrate removal efficiency. Above results enumerated the function of SBES as a treatment system apart from value added product recovery. 3.4.5. pH and VFA The pH and VFA profiles of the SBES were also recorded at frequent intervals in order to realize the metabolic functions of the biocatalyst. Anaerobic fermentation of organic substrate for H2 production was accompanied with VFAs and alcohols production. VFAs are generally regarded as indicators for monitoring the dark fermentative H2 production process (Dong et al., 2009; Chandrasekhar and Venkata Mohan, 2014a; Venkata Mohan et al., 2009). During SBES operation, samples were collected at frequent intervals viz., 0th, 10th, 20th and 30th days of SBES operation to evaluate the changes in VFAs formation. VFA concentrations showed gradual rise with respect to time till the end of operation. Initially on 0th day, VFA was observed as 8450 mg/l followed by increasing trend on 10th day (15,400 mg/l), 20th day (31,500 mg/l) and 30th
Fig. 6. Differential pulse voltammetry profile during SBES operation [against reference electrode (Ag/AgCl (s))].
day (37,650 mg/l) (Fig. 7). During SBES operation, pH followed a decreasing trend from 1st day (7) to 26th day (4.4) of operation indicating quick metabolic functions in the fuel cell. From 26th day, significant change in pH was not observed due to low substrate availability (Chandrasekhar and Venkata Mohan, 2014b). The fall in pH might be directly related to power output and H2 production where acidic pH (
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Waste Management journal homepage: www.elsevier.com/locate/wasman
Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation K. Chandrasekhar 1, K. Amulya, S. Venkata Mohan ⇑,1 Bioengineering and Environmental Sciences (BEES), CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 19 December 2014 6 May 2015 Accepted 3 June 2015 Available online xxxx Keywords: Solid-state fermentation Bioelectricity Biohydrogen Bioethanol Electrofuels Microbial fuel cell
a b s t r a c t A novel solid state bio-electrofermentation system (SBES), which can function on the self-driven bioelectrogenic activity was designed and fabricated in the laboratory. SBES was operated with food waste as substrate and evaluated for simultaneous production of electrofuels viz., bioelectricity, biohydrogen (H2) and bioethanol. The system illustrated maximum open circuit voltage and power density of 443 mV and 162.4 mW/m2, respectively on 9th day of operation while higher H2 production rate (21.9 ml/h) was observed on 19th day of operation. SBES system also documented 4.85% w/v bioethanol production on 20th day of operation. The analysis of end products confirmed that H2 production could be generally attributed to a mixed acetate/butyrate-type of fermentation. Nevertheless, the presence of additional metabolites in SBES, including formate, lactate, propionate and ethanol, also suggested that other metabolic pathways were active during the process, lowering the conversion of substrate into H2. SBES also documented 72% substrate (COD) removal efficiency along with value added product generation. Continuous evolution of volatile fatty acids as intermediary metabolites resulted in pH drop and depicted its negative influence on SBES performance. Bio-electrocatalytic analysis was carried out to evaluate the redox catalytic capabilities of the biocatalyst. Experimental data illustrated that solid-state fermentation can be effectively integrated in SBES for the production of value added products with the possibility of simultaneous solid waste remediation. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction In the present energy-based society, the value of any energy-rich matter is raising. Thus, high organic load in wastewaters is no longer seen as waste, but considered as a valuable energy resource. Today, methane is primarily obtained from the biogas of traditional anaerobic digesters, but its conversion into electricity via combustion is not very efficient (Motte et al., 2013). Direct electricity generation from wastewater treatment using bioelectrochemical systems (BES) is advantageous, because thermodynamic conversion step is not necessary (Venkata Mohan et al., 2007, 2010; Huang and Logan, 2008). BES is a hybrid bioelectrochemical device, which directly transforms energy stored in chemical bonds of substrate to electrical energy via bioelectrochemical reactions mediated by microorganisms as biocatalyst (Rahimnejad et al., 2011; Wu et al., 2012). Microorganisms extract energy required to build biomass (anabolic process) from redox reactions (catabolism) through electron donor/acceptor conditions. In recent years BES has emerged as a promising yet ⇑ Corresponding author. Tel./fax: +91 40 27191765. 1
E-mail address: [email protected] (S. Venkata Mohan). Address: Academy of Scientific and Innovative Research (AcSIR), India.
challenging technology for the recovery of energy from waste besides its treatment and is gaining importance due to its sustainable nature (Min et al., 2005; Moon et al., 2006; He et al., 2007; Kumlanghan et al., 2007; Sevda et al., 2013; Chandrasekhar and Venkata Mohan, 2012). The process provides dual benefits of wastewater treatment and access to cheap and environmental friendly energy. One third of the food produced globally for human consumption is wasted yearly (Gustavsson et al., 2011). The overall amount of food wasted and lost worldwide corresponds to approximately 1.3 billion tonnes. This quantity includes all sorts of food, such as roots and tubers, oilseed and pulses, cereals, fruits and vegetables, meat, seafood, milk and eggs (Gustavsson et al., 2013). Food based waste generated from canteen operations is one of the most prominent and highly biodegradable solid waste (Venkateswar Reddy et al., 2011). Regardless of the food wasted, it is either treated or disposed in landfill sites in order to prevent environmental inconvenience. Elimination of food waste by dumping into landfill sites is inapt, as it causes serious health problems in densely populated areas (Cuéllar and Webber, 2010; Zilbermann et al., 2013). Several studies depicted the possibilities of energy production from food waste by anaerobic digestion and incineration, or
http://dx.doi.org/10.1016/j.wasman.2015.06.001 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Chandrasekhar, K., et al. Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.001
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Nomenclature AB acidogenic bacteria BES bio-electrochemical systems CA chronoamperometry CD current density CDP cell design point CHP cumulative H2 production COD chemical oxygen demand CV cyclic voltammetry DPV differential pulse voltammetry DSW designed synthetic wastewater e electron GPES general purpose electrochemical system Hydrogen (H2) biohydrogen HPLC high performance liquid chromatography I current LSV linear sweep voltammetry MFC microbial fuel cell
OCV PD Q RDAP RE RE RI Rp SBES SSF TDS TS TSS UASB VFA
open circuit voltage power density charge relative decrease in anode potential external resistance reference electrode internal resistance polarization resistance solid state bio-electrochemical system solid state fermentation total dissolved solids total solids total suspended solids up-flow anaerobic sludge blanket reactor volatile fatty acids
utilization of food waste as feed for pigs and cattle in order to close the nutrient loop (Sayeki et al., 2001; Shin and Youn, 2005; De Gioannis et al., 2013; Li et al., 2010; Kim et al., 2013; Dahiya et al., 2015). This waste can also act as good substrate for anaerobic digestion process due to its highly biodegradable organic content (Lim et al., 2000; Kim et al., 2006; Venkateswar Reddy et al., 2011; Ruggeri et al., 2013). Solid state fermentation (SSF) showed promising results in the production of various types of products including food and food ingredients, agro-industrial products and pharmaceutical products (Pandey, 2000). For some specific products, SSF offers higher yields and better product spectra (Ishida et al., 2000; De Vrije et al., 2001). Biodegradable organic matter present in food waste can be effectively utilized for energy recovery in the form of bioelectricity by integrating SSF with BES (Venkata Mohan and Chandrasekhar, 2011a). Nevertheless, within a short period after start-up of BES operation, edible oil forms a thin layer between substrate and electrode (anode) surface hindering direct contact between anode and anodophilic biocatalyst in turn interfering with electricity generation (Venkata Mohan and Chandrasekhar, 2011a). If properly designed BES can assist direct conversion of solid waste through SSF and consequently make the process sustainable for successful waste management. Therefore, the present study was designed to evaluate the function of solid state bio-electrochemical system (SBES) for the recovery of bio fuels in the form of bioelectricity, biohydrogen (H2) and bioethanol simultaneously by integrating SSF of food waste. Based on the output obtained from the previous study (Venkata Mohan and Chandrasekhar, 2011a), in the present study, electrodes (anode and cathode) were vertically placed to avoid interference. Bioelectricity, H2 and ethanol production with the function of time were evaluated. The fate of individual volatile fatty acids (VFA) was studied during the SBES operation. Cyclic voltammetry (CV) was applied to assess the electron discharge properties of the biocatalyst. Bio-electrocatalytic assessment was done through Tafel analysis and the results were interpreted in terms of redox Tafel slopes and polarization resistance, during SBES operation.
In this study, a single chambered solid state bio-electrochemical system (SBES) with air-cathode was designed and fabricated in the laboratory (Fig. 1). SBES was operated with non-catalyzed graphite electrodes (5 5 cm; 0.5 cm thick; surface area 60 cm2) as anode and cathode. The design was modified by placing the electrodes vertically, instead of placing horizontally (Venkata Mohan and Chandrasekhar, 2011a) in order to avoid the interference of residual oil content. One side of the cathode was vertically immersed in food waste (electrolyte) and the other side was exposed to air, while the anode was completely dipped in the substrate. Copper wires attached to the electrodes through epoxy sealant were used to provide connection. Provisions were made in the design for wire input, sampling ports and inlet and outlet ports. The reactor had a total/working volume of 400/300 ml and was operated at room temperature (30 ± 1 °C) in fed-batch mode, with a total cycle period of 30 days. The fuel cell was closed to maintain strict anaerobic microenvironment throughout the process. Reactor was fed with solid state food based canteen waste (270 ml) along with 30 ml (10% v/v) of tap water to maintain moisture content and to enhance the conductivity of anolyte (substrate). Prior to feeding, pH of the food waste was 6.8 ± 0.1 and was adjusted to 7 1 N NaOH solution.
2. Materials and methods
2.3. Anaerobic consortia
2.1. Composite food waste
Anaerobic mixed culture from full scale hydrogen producing UASB reactor was used as biocatalyst in SBES (Venkateswar Reddy et al., 2011). Prior to inoculation, the mixed culture was washed twice in saline buffer (6000 rpm, 22 °C) and enriched in designed synthetic wastewater [DSW (in g/l), Glucose, 3.0;
Food waste collected from the institute canteen was grinded and used as solid feed for the operation of SBES. The solid food waste was composite in nature majorly comprising of boiled rice
followed by vegetable peelings, cooked vegetables, un-cooked vegetables (spoiled), cooking oil, etc. with a water content varying between 10% and 15%. The physic–chemical characteristics of the food waste prior to feeding were evaluated and it was noticed that it had high organic content (pH, 6.8 ± 0.4; TSS, 31 ± 0.6 g/l; TS, 42 ± 0.9 g/l; TDS, 11.2 g/l; VFA, 8.4 g/l; COD, 380 g/l; carbohydrates (total), 68.5 g/l; oil content, 38 g/l). Prior to feeding the oil fraction of the waste was separated by gravity-separation mechanism (Venkata Mohan and Chandrasekhar, 2011a). Pre-characterized substrate was stored in airtight container and preserved in a refrigerator at 4±.5 °C to avoid spoilage. 2.2. Bio-electrochemical system architecture and operation
Please cite this article in press as: Chandrasekhar, K., et al. Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.001
K. Chandrasekhar et al. / Waste Management xxx (2015) xxx–xxx
3
Head space Volume 100 ml
Fig. 1. Schematic details of the solid state bio-electrochemical system.
NH4Cl, 0.5, KH2PO4, 0.25; K2HPO4, 0.25 l; MgCl2, 0.3; CoCl2, 0.025; ZnCl2, 0.0115; CuCl2, 0.0105; CaCl2, 0.05; NiSO4, 0.016; MnCl2, 0.015; FeCl3, 0.025] under anaerobic microenvironment at pH 7.0 (100 rpm; 48 h) (Amulya et al., 2014; Venkata Mohan and Chandrasekhar, 2011b). The anaerobic mixed consortia was subjected to repeated pretreatment (three times) changing between heat-shock, acid-shock and chemical-shock methods to selectively enrich the H2 producing acidogenic bacteria (AB) (Chandrasekhar and Venkata Mohan, 2014b).
2.4. Analysis The power output and H2 production efficiency were considered as the two key parameters to evaluate the performance of SBES. Open circuit voltage (OCV)/potential difference and current (I; measured in series at 100 X) were recorded manually on auto-range digital multi-meter. Power (mW) was derived from P = IV equation. Power density (PD) (mW/m2) and current density (CD) (mA/m2) were considered with the function of anodic surface area (m2). Polarization was carried out manually to evaluate polarization behavior of SBES using a variable resistor set-up with a range of 30 kO to 50 O. To evaluate the bio-electrochemical fuel cell behavior, electrochemical techniques such as cyclic voltammetry (CV), chronoamperometry (CA) and differential pulse voltammetry (DPV) were performed using a potentiostat-galvanostat system (Autolab, PGSTAT12, Ecochemie) over the voltage range from +0.5 to 0.5 V (CV), 0 to +0.3 V (CA) and +0.5 to 0.5 V (DPV), respectively. During analysis, substrate (food waste) was used as electrolyte with anode and cathode of SBES as working and counter electrodes, respectively against saturated Ag/AgCl as reference electrode (RE). Scan rate of 30 mV/s was optimized for the reactor, which showed major interfacial electron-transfer kinetics. The system was operated through a PC using GPES software. Cell potentials (anode) were measured against RE. SBES performance with respect to treatment efficiency was monitored by analyzing pH, VFA and COD according to the Standard Methods (APHA, 1998). Quantitative estimation of VFA was carried out by High performance liquid chromatography (HPLC; Shimadzu
LC10A) with UV–Vis detector at 210 nm and C18 reverse phase column (250 4.6 mm diameter and 5 l particle size). Before analysis, samples were diluted with deionized water to reduce concentration and filtered through a polystyrene membrane filter (0.45 lm). The mobile phase used for separation was 40% acetonitrile in 1 mN H2SO4 (pH, 2.5–3.0) with flow rate of 0.5 ml/min. A sample containing ethanol was initially acidified with H2SO4 (20%, 10 ll/ml of sample), filtered through a polystyrene membrane filter (0.45 lm) and subsequently analyzed by HPLC as described by Crespo et al. (2012). Samples were analyzed immediately after collection. The analytical system (HPLC) was calibrated with standard chemicals from Sigma Aldrich in the range of concentration of the samples analyzed. 3. Results and discussion 3.1. Electrogenesis SBES was operated under anaerobic conditions and strictly monitored for a period of 30 days. The experimental results clearly depicted the influence of substrate (electrolyte) on the performance of this air-cathode SBES. Bioelectricity generation was evident by SSF of food waste in SBES. Immediately after start-up, relatively low open circuit voltage (OCV, 17 mV) and power density (PD, 0.32 mW/m2) were observed (Fig. 2a). With time, marked improvement in OCV and PD was observed accounting for a maximum of 443 mV and 162.4 mW/m2, respectively on 9th day of SBES operation. SBES documented stable performance from 9th to 20th day of operation with marginal variations (Fig. 2a) followed by decreasing trend till 25th day of operation. From 26th day of operation a visible drop in both OCV (286 mV) and PD (21.3 mW/m2) was observed, which reduced further on 30th day (OCV, 67 mV and PD, 0.57 mW/m2). Relatively higher power output observed on 9th day operation might be due to the availability of simple sugars around anodic surface area for anodophilic bacteria. Gradual decrement in the power generation after 26th day of operation might be due to less availability of simple sugars (monomers) and lower metabolic functions of electrochemically active biocatalyst.
Please cite this article in press as: Chandrasekhar, K., et al. Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.001
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Resistance (kΩ Ω)
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Fig. 2. (a) Voltage and current generation, (b) polarization behavior, (c) cell potentials and (d) RDAP during SBES operation.
3.1.1. Electron losses To estimate the power outputs, polarization curves were drawn during stable phase of SBES operation (on 20th day). The cell design point (voltage change region) of a particular fuel cell system can be obtained by considering the point at which highest PD noticed on the polarization curve intersects with voltage and CD (Barbir, 2005). Polarization curve documented e discharge at 5 kO on 20th day of operation (Fig. 2b). Cell design point (CDP) for SBES was observed at 300 O accounting for a power density (PD) of 40.01 mW/m2. It is a standard exercise to run the fuel cell on the left side of the PD peak and at an elevated voltage or low current density (Kiran Kumar et al., 2012; Velvizhi et al., 2012; Venkata Mohan et al., 2014). Hence, SBES with food waste can be operated beyond 300 O. This indicated the stabilized performance of fuel cell after 20th day of operation in terms of enhanced e discharge supporting the metabolic efficiency of microorganisms in the anodic compartment. The polarization curve assists to analyze the e losses during transfer from the biocatalyst to the anode and then to the cathode (Fig. 2b). Electron transfer (direct electron transfer or mediated electron transfer) from biocatalyst toward the electrode (anode) is normally hindered by anodic over-potentials, which impact the power generation efficiency of microbial fuel cell (MFC) (Kiran Kumar et al., 2012). There are three different losses viz., activation, ohmic and concentration losses that are normally possible during fuel cell operation. Oxidation of substrate at the electrode (anode) surface or reduction of a
compound on the biocatalyst surface or in the bacterial cell requires activation energy, which incurs a potential loss usually termed as activation losses (Chandrasekhar and Venkata Mohan, 2012). It is obvious that activation losses are measured in the lower range of current densities and are vital. There is a possibility that many metabolic functions may occur at the same time in anaerobic mixed consortia, resulting in high activation losses. This also facilitates e quenching, which normally arises due to rivalry between the metabolic pathways. SBES operated with food waste also showed higher activation losses due to the various metabolic functions occurring in bacteria (Chandrasekhar and Venkata Mohan, 2012). Ohmic losses caused by the electrical resistances of the membrane, electrodes material, electrolyte, etc., can be reduced by high electrolyte conductivity. During SBES operation, significantly less ohmic losses were observed due to higher e transfer, which also improved power output. Huge oxidative force of the anode creates concentration losses where substrate oxidation is quicker at the anode generating more e than that can be transported to the anode surface and to the cathode, especially at lower resistances (Chandrasekhar and Venkata Mohan, 2012; Lee et al., 2009). 3.1.2. Cell potentials Variations in cell, anode and cathode potentials were evaluated against the external resistance (RE) range from 50 X to 30 kX on 20th day of SBES operation. Wide range of variations (542–44 mV)
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3.1.3. Sustainable power SBES system is considered to be in steady state, when the power generated is equal to the power consumed for an extensive period of time. With the function of applied RE, relative decrease in anode potential (RDAP) is employed to assess highest sustainable power, which can be calculated using the initial anodic potential (Eo,anodic) and anodic potentials at each applied RE as shown in Eq. (1).
RDAPð%Þ ¼ ½ðEo;anodic Eanodic Þ=Eo;anodic 100
ð1Þ
The linear fit at higher RE represents a region at which the resistance controls the power. When the RE was high, RDAP amplified linearly due to limited e delivery to the cathode. Nevertheless, at lower RE, the e delivery to the cathode was limited by kinetic and/or mass transfer (or internal resistance (RI)). At conditions where RE and RI limitations were the same among these two lines, a parallel line from the intersection was drawn to estimate RE, which allows the measurement of sustainable power. SBES showed maximum sustainable resistance of 8 KO with PD of 3.7 mW/m2 (Fig. 2d). Observed sustainable resistance during SBES operation indicated a decrease in the kinetic and/or mass transfer or internal resistances and increase in transfer of e to cathode which resulted in high sustainable power. This also supports longer duration of power sustainability observed during operation of SBES (30 days). 3.2. Biohydrogen (H2) Experimental data also depicted the feasibility of fermentative H2 production through food waste as substrate. The biodegradability of food waste is mainly related to carbohydrate material, which is the main substrate for H2 production (De Gioannis et al., 2013). H2 production rate differed significantly with varying time. H2 production was observed from 2nd day of operation and maximum H2 production rate of 21.52 ml/h was achieved on 19th day. This is an indication for the lag in the metabolic activities due to higher substrate concentration and the necessity of additional time for the conversion of organic matter into H2 (Chandrasekhar and Venkata Mohan, 2014b). Production rate was observed to fall gradually after attaining a maximum value. The accumulation of volatile fatty acids as by-products leads to drop in pH and this might be reason for the observed decrement in H2 production after 26th day of operation (Chandrasekhar and Venkata Mohan, 2014b). Cumulative H2 production (CHP) increased till the end of the SBES operation (Fig. 3). CHP was observed to increase with time and showed 5400 ml H2 at end of the operation. CHP was observed to stabilize after 26th day of SBES operation. 3.3. Bioethanol Production and accumulation of VFA and ethanol as secondary metabolites takes place due to anaerobic SSF in bacteria and yeast (Raghavarao et al., 2003). During SBES operation, biocatalyst mediated catalytic functions lead to biotransformation of VFA into acetaldehyde and CO2. Acetaldehyde is further transformed to ethanol as end product in the presence of biocatalyst. Samples were collected at frequent intervals viz., 10th, 20th and 30th days of SBES operation to evaluate ethanol production efficiency. SBES
CHP
6000
Cumulative hydrogen production (ml)
were observed in cathode potential under applied RE indicating that the current generated during SBES operation was not restricted by the anodic reactions alone (Fig. 2c). Considerable drop in anode potential was observed during operation when RE decreased. Anode potential regulates the kinetics of e transfer from the exo-electrogenic biocatalyst to the electrode surface (anode). Significant drop in potential was observed from or below 10 kX suggesting the feasibility of effective e discharge below 10 kX during SBES operation.
5000
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Time (Days) Fig. 3. Cumulative hydrogen production during SBES operation.
operation documented feasibility of ethanol production parallel to substrate utilization. During SBES operation, ethanol production of 1.45% w/v was observed at initial stages (10th day), followed by an increase and attained a maximum value on 20th day (4.85% w/v) of operation. After 20th day a decrease in ethanol production (2.15% w/v) was observed till 30th day of operation. 3.4. Process monitoring 3.4.1. Bio-electrochemical evaluation Bio-electrochemistry involves the understanding of anodic biocatalyst mediated redox catalytic phenomena coupled with charge separation and charge transfer, which can take place evenly in solution, or heterogeneously on electrode surfaces. Electrode reactions generally take place in the interfacial region between solution and electrode where charge delivery differs from that of the bulk phases. The electrode acts as e acceptor and can be affected by the structure and nature of electrode. Nevertheless, the change in charge distribution during fuel cell operation is due to the substrate metabolism and nature of the biocatalyst (microorganism). Few microbial species have the capability to discharge reducing powers (e and H+) generated during the metabolic activity via bacterial (biocatalyst) membrane bound organelles or soluble mediators. In situ evaluation of e discharge employing CV (Vs Ag/AgCl (S)) with time helps to understand the metabolic shifts and the carriers involved in e transfer. Electrochemical evaluation visualized marked variation in e discharge properties with respect to time (Fig. 4a). SBES documented higher catalytic currents (oxidation, 130.26 mA; reduction, 18.39 mA) at 20th day of operation followed by 10th day (oxidation, 78.07 mA; reduction, 19.12 mA), 0th day (oxidation, 32.18 mA; reduction, 16.21 mA) and 30th day (oxidation, 16.72 mA; reduction, 14.17 mA) in both oxidation and reduction sweeps. Nevertheless, oxidation was quite higher than reduction in SBES operation. This is a direct evidence of the association of electrochemically active bacteria (biocatalyst) with the native mixed consortia avoiding the e losses prior to reaching the electrode (anode). During the operation, SBES was subjected to chronoamperometry (CA) analysis by maintaining a steady potential (1.2 V). CA enumerates the utmost possible sustainable current during the SBES operation. Gradual increment in current was observed in CA profiles from 0th day to 15th day of SBES operation where decreasing trend was observed from 20th day to till the end of operation
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The stored energy in a capacitor (fuel cell) is equal to the work done to charge it (fuel cell). Considering a capacitor of capacitance C, holding a charge Q and dW = QC dQ upon integration gives following equation.
a
i/A
W charge ¼
Z
Q Q 2 CV2 dQ ¼ ¼ C 2C 2
ð3Þ
where W represents work done (J), Q is the charge (C) and C is the capacitance (F). Comparatively higher energy output was observed on 20th day (31.25 mJ) followed by 10th day (20.80 mJ), 0th day (12.48 mJ) and 30th day (5.83 mJ) of SBES operation. Observed results correlated well with enhanced performance of the system in terms of electricity generation on 20th day of operation.
E/V 7.0m
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6.0m
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t/s Fig. 4. (a) Cyclic voltammetry and (b) chronoamperommetry profiles recorded during SBES operation [against reference electrode (Ag/AgCl (s))].
(Fig. 4b). On 15th day of operation higher current of 6.5 mA was observed initially that showed a drastic drop for a few seconds and then stabilized near 2.5 mA after 250 s. The differential behavior of CA pattern indicated effective performance of SBES from 10th day to 20th day of operation where e generated constantly facilitated power generation. This behavior of SBES might be due to the efficient metabolic function of electrochemically active biocatalyst around electrode (anode) surface. Observed negligible electrochemical activity shown by the biocatalyst on 30th day of operation might be due to the less availability of simple substrate. Charge (Q) represents the number of e present at particular instance on the electrode (anode) surface throughout oxidation. Charge distribution was observed to vary with time and metabolic functions of the biocatalyst used. Voltammetric profiles showed higher charge on the 20th day (125 mC) followed by 10th day (102 mC), 0th day (79 mC) and 30th day (54 mC) of SBES operation. The trend indicated higher availability of e on the anode surface resulting in higher power output. Charge separation can also be represented by capacitance and the complexity of charge transfer by its resistance. Capacitance is the capability of a cell to hold an electrical charge and is a measure of the sum of electrical energy stored at an applied electric potential given by
C ¼ Q =V max
ð2Þ
where C is the capacitance in Faradays, Q is the amount of charge obtained (C) and V is the maximum applied potential (Vmax). Higher capacitance observed on 20th day (251 mF) followed by 10th day (204 mF), 0th day (158 mF) and 30th day (108 mF) of SBES operation supports the observed higher power generation during 20th day of SBES operation.
3.4.2. Electro-kinetic evaluation Tafel analysis was performed to understand the electro-kinetics in terms of redox behavior and polarization resistance (RP) of the biocatalyst during the operation of SBES. Low value of the slope connected to the oxidation process indicated the requirement of lower activation energy that makes oxidation more favorable (Chandrasekhar and Venkata Mohan, 2012). The same theory is applicable in the case of reduction slope. SBES operation documented higher oxidation reactions over reduction reactions. During SBES operation, lower oxidation slope was observed on 20th day of operation (0.126 V/dec) followed by 10th day (0.154 V/dec), 0th day (0.184 V/dec) and 30th day (0.114 V/dec) (Fig. 5). Reduction reaction showed minor variations with respect to time. Lower reduction slope was observed on 20th day (0.217 V/dec) followed by 10th day (0.221 V/dec), 0th day (0.228 V/dec) and 30th day (0.234 V/dec). Observed lower oxidation slope during SBES operation on 20th day indicated favorable conditions for substrate oxidation by the biocatalyst which was well supported by the observed higher H2 production. Values of reduction slopes were relatively higher than that of oxidation slope. This might be due to rapid consumption of H+ for H2 production by H2 producers in the presence of dehydrogenase, which facilitated the accessibility of H+ through redox reactions. Observed results correlated well with the improvement in H2 production. The resistance to e transfer could be explained in terms of polarization resistance (Rp). High Rp was observed on 30th day of SBES operation. Higher the resistance in SBES, lower is the current generated which indicated a reduced amount of metabolic functions of the biocatalyst. SBES operation showed higher Rp on 30th day (272 O) followed by 0th day (202 O), 10th day (179 O) and 20th day (102 O). It is evident from Rp values that SBES operation was more favorable for effective electron transfer on 20th day of operation. The observed lower Rp on 20th day of SBES operation might be due to improved metabolic functions of the biocatalyst and also rapid substrate oxidation/consumption by the biocatalyst, which in turn lead to higher bioelectricity generation. 3.4.3. Differential pulse voltammetry (DPV) In the present study, whole-microorganism voltammetry was used to evaluate the electrochemical behavior of the biocatalyst (mixed consortia) during SBES operation. DPV is the most suitable voltammetry technique for the biological system, which can monitor redox species at small concentrations (Liu et al., 2003). Although CV is the most informative method used to investigate redox carriers involved in substrate oxidation by anodophilic microorganism, it has low detection limit, and subtraction of ohmic (capacitive) current is necessary to reveal minute details (Velvizhi et al., 2012). Furthermore, the catalytic redox reactions occurring at the electrodes comprise of complex e transfer chains and to study such reaction mechanisms CV necessitates substantial
Please cite this article in press as: Chandrasekhar, K., et al. Solid phase bio-electrofermentation of food waste to harvest value-added products associated with waste remediation. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.001
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K. Chandrasekhar et al. / Waste Management xxx (2015) xxx–xxx
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E/V Fig. 5. Tafel plots derived from cyclic voltammograms during SBES operation [against reference electrode (Ag/AgCl (s))].
time and appropriate post-processing of data (Marsili et al., 2008). In comparison, pulse methods have the potential to disclose characteristic peaks while cancelling out capacitive current, even at elevated scan rates, and are often used as complementary techniques to CV (Liu et al., 2003). DPV (vs. Ag/AgCl) was performed in SBES with similar electrode arrangements as described above for CV. As depicted in Fig. 6, DPV commonly revealed a consistent broad peak at a formal potential of 0.065 ± 0.005 V vs. Ag/AgCl3 during operation of SBES although the maximum peak concentrations were achieved on 20th day (Fig. 6). This indicated that elevated biocatalytic behavior was observed on 20th day of SBES operation due to enhanced metabolic reactions of the electrochemically active bacteria. Observed bioelectrogenesis and CV results correlated well with the observed higher efficiency of SBES operation on 20th day of operation. 3.4.4. Substrate degradation The organic fraction of the composite food waste functioned as an electron donor in the anode compartment of SBES resulting in power generation apart from substrate degradation. At the end of the cycle, COD analysis was performed to understand the substrate degradation efficiency of SBES by sampling waste around anode surface. Based on COD analysis, prior to feeding, substrate COD was noticed as 380 g/l. At the end of the SBES operation (on 30th day), substrate COD was noticed as 106 g/l. Overall SBES operation documented 273 g/l of substrate degradation with 72% substrate removal efficiency. Above results enumerated the function of SBES as a treatment system apart from value added product recovery. 3.4.5. pH and VFA The pH and VFA profiles of the SBES were also recorded at frequent intervals in order to realize the metabolic functions of the biocatalyst. Anaerobic fermentation of organic substrate for H2 production was accompanied with VFAs and alcohols production. VFAs are generally regarded as indicators for monitoring the dark fermentative H2 production process (Dong et al., 2009; Chandrasekhar and Venkata Mohan, 2014a; Venkata Mohan et al., 2009). During SBES operation, samples were collected at frequent intervals viz., 0th, 10th, 20th and 30th days of SBES operation to evaluate the changes in VFAs formation. VFA concentrations showed gradual rise with respect to time till the end of operation. Initially on 0th day, VFA was observed as 8450 mg/l followed by increasing trend on 10th day (15,400 mg/l), 20th day (31,500 mg/l) and 30th
Fig. 6. Differential pulse voltammetry profile during SBES operation [against reference electrode (Ag/AgCl (s))].
day (37,650 mg/l) (Fig. 7). During SBES operation, pH followed a decreasing trend from 1st day (7) to 26th day (4.4) of operation indicating quick metabolic functions in the fuel cell. From 26th day, significant change in pH was not observed due to low substrate availability (Chandrasekhar and Venkata Mohan, 2014b). The fall in pH might be directly related to power output and H2 production where acidic pH (
