MICROBIAL FUEL CELL: A NEW APPROACH OF WASTEWATER TREATMENT WITH POWER GENERATION

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1 MICROBIAL FUEL CELL: A NEW APPROACH OF WASTEWATER TREATMENT WITH POWER GENERATION M.M. GHANGREKAR 1 AND V.B. SHINDE 2 ABSTRACT: Application of Microbial Fuel Cells (MFCs) may represent a completely new approach to wastewater treatment with production of sustainable clean energy. In recent years, researchers have shown that MFCs can be used to produce electricity from water containing glucose, acetate or lactate. Studies on electricity generation using organic matter from the wastewater as substrate are in progress. The performance of the MFCs as reported in literature with different substrate is presented in this paper. Under different conditions such as inoculum culture (pure or mixed), substrate sources, and external loads, the MFCs are reported to produce power density in the range 0.3 to 3600 mw/m 2. Performance of the laboratory scale membrane less MFC using synthetic wastewater was evaluated for its effectiveness for organic matter removal and electricity production. It was observed that this membrane less MFC was capable to give COD reduction greater than 90 percent, with maximum power production 6.73 mw/m 2. If power generation in MFC can be increased, this technology may provide a new method to offset wastewater treatment plant operating costs, with less excess sludge production. KEYWORDS: Microbial Fuel Cell, Wastewater Treatment, Electric power production INTRODUCTION The high energy requirement of conventional sewage treatment systems are demanding for the alternative treatment technology which will be cost effective and require less energy for its efficient operation. In past two decades, high rate anaerobic processes are finding increasing application for the treatment of domestic as well as industrial wastewaters. The major advantages these systems offer over conventional aerobic treatment are no energy requirement for oxygen supply, less sludge production, and recovery of methane gas. While treating sewage, particularly in small capacity treatment plant recovery of methane may not be attractive, because most of the methane produced in the reactor is lost through effluent of the reactor. The methane concentration of about 16 mg/l (equivalent COD 64 mg/l) is expected in the effluent of the reactor due to high partial pressure of methane gas inside the reactor 1. Hence, while treating low strength wastewater major fraction of the methane gas may be lost through effluents, reducing the energy recovery. In addition, due to global environmental concerns and energy insecurity, there is emergent interest to find out sustainable and clean energy source with minimal or zero use of hydrocarbons. Electricity can be produced in different types of power 1 Assistant Professor, Department of Civil Engineering, Indian Institute of Technology, Kharagpur India. (Phone No (O) (R); ghangrekar@civil.iitkgp.ernet.in) 2 M.Tech. (Environmental Engineering & Management) Student. Department of Civil Engineering, Indian Institute of Technology, Kharagpur India.

2 plant systems, batteries or fuel cells. Bacteria can be used to catalyze the conversion of organic matter into electricity 2,3,4,5,6,7. Fuel cells that use bacteria are classified as two different types: biofuel cells that generate electricity from the addition of artificial electron shuttles (mediators) and microbial fuel cells (MFCs) that do not require the addition of mediator 8. Unlike a battery, a fuel cell does not store energy. Instead, it converts energy from one form to another (much like an engine) and will continue to operate as long as fuel is fed to it. However, unlike internal combustion generators, fuel cells convert chemical energy directly into electricity without an intermediate conversion into mechanical power. Fuel cells, if used for wastewater treatment, can provide clean energy for people, apart from effective treatment of wastewater. The benefits of using fuel cells include: clean, safe, quiet performance; high energy efficiency; low emissions; and ease in operating. Biofuel cells use biocatalysts for the conversion of chemical energy to electrical energy. It is a device that directly converts microbial metabolic or enzyme catalytic energy into electricity by using conventional electrochemical technology 2. The biocatalysts participate in the electron transfer chain between the fuel-substrate (organic or inorganic) and the electrode surfaces 9. That is, microorganisms or redoxenzymes facilitate the electron transfer between the fuel substrate and the electrode interface, thereby enhancing the cell current. It has been shown that, direct electron transfer from microbial cells to electrodes occurs only at very low efficiency 10. Most of the redox-enzymes lack in direct electron transfer with conductive supports and a variety of electron mediators (electron relays) are used for electrical contacting of the biocatalysts and the electrode. Since, most microbial cells are electrochemically inactive; electron transfer from microbial cells to the electrode is facilitated by the help of mediators such as thionine, methyl viologen, humic acid, etc. Enzyme catalyzed generation of NADH (dihydro-nicotinamide adenine dinucleotide) from alcohol, lactic acid, amino acids, formate or other abundant substrates could provide the bio-transformations that activate the anodic compartment of the fuel cell 9. Theoretically, any organic or inorganic compound or a mixture can serve as a fuel, provided it is oxidized by the appropriate organism, e.g., the reaction for glucose is 11 : C 6 H 12 O 6 + 6H 2 O 6CO e H +.(1) Mediator Less MFC It has recently been shown that certain metal-reducing bacteria, belonging primarily to the family Geobacteraceae can directly transfer electrons to electrodes

3 using electrochemically active redox enzymes, such as cytochromes on their outer membrane 12,13. These microbial fuel cells does not need mediator for electron transfer to electrodes and are called as mediator less MFCs. Mediator less MFCs are considered to have more commercial application potential, because mediators used in Biofuel cells are expensive and can be toxic to the microorganisms 7. The schematic diagram of mediator less MFC is shown in the Figure 1. In a MFC, two electrodes (anode and cathode) are placed in water in two compartments separated by a proton exchange membrane (PEM). Most studies have used electrodes of solid graphite 7, graphite-felt 14, carbon cloth 15 and platinum coated graphite cathode electrode 16. Microbes in the anode compartment oxidize fuel (electron donor) generating electrons and protons. Electrons are transferred to the cathode compartment through the external circuit, and the protons through the membrane. Electrons and protons are consumed in the cathode compartment reducing oxygen to water. Rate Limiting Steps: (1) Oxidation of Fuel, (2) Electron transfer from the microbial cells to the electrode, (3) Electric load in the circuit, (4) Proton supply into the cathode compartment, (5) Oxygen supply and reduction at the cathode [Gil et al. 6 ]. Figure 1: Mediator Less Microbial Fuel Cell with rate limiting steps In addition to microorganisms that can transfer electrons to the anode, the presence of other organisms appears to benefit MFC performance. It is reported that, a mixed culture generated a current that was six fold higher that that generated by a pure culture 17. Hence, the microbial communities that develop in the anode chamber may have a similar function as those found in methanogenic anaerobic digesters, except that microorganisms that can transfer electrons to the electrode surface replace methanogens. Rabaey et al. 18 referred to such microbial communities as adapted anodophilic consortia. Anodophilic bacteria from different

4 evolutionary lineages from the families of Geobacteraceae, Desulfuromonaceae, Alteromonadaceae, Enterobacteriaceae, Pasteurellaceae, Clostridiaceae, Aeromonadaceae, and Comamonadaceae were able to transfer electrons to electrodes 19. Methanogens also reported to have a capacity to transfer electrons 20. Because the power output of MFCs is low relative to other types of fuel cells, reducing their cost is essential, if power generation using this technology is to be an economical method of energy production. The overall limiting steps to enhance the power production are showed in the Figure 1. Further research is required to enhance the power production by overcoming these limitations. The main disadvantage of a two chamber MFC is that the solution cathode must be aerated to provide oxygen to the cathode 8. The power output of an MFC can be improved by increasing the efficiency of the cathode, e.g. power is increased by adding ferricyanide to the cathode chamber 21. The effects of operational conditions of a microbial fuel cell were investigated and optimized for the best performance of a mediator-less microbial fuel cell by Gil et al. 6. The optimal ph reported was 7. The resistance higher than 500 Ω was the ratedetermining factor by limiting electron flow from anode to cathode. At the resistance lower than 200 Ω, proton and oxygen supplies to the cathode were limited. For the construction of an efficient microbial fuel cell, a non-compartmentalized fuel cell with an electrode having a high oxygen reducing activity should be developed. Since the concentration of fuel determines the amount of electricity generation from the fuel cell, the device can be used as a BOD sensor. It is possible to design a MFC that does not require the cathodes to be placed in water. In hydrogen fuel cells, the cathode is bonded directly to the PEM so that oxygen in air can directly react at the electrode 22. This technique was successfully used to produce electricity from wastewater in a single chamber MFC 15. However, a maximum of 788 mw/m 2 power density was reported by Park and Zeikus 21 with a Mn 4+ graphite anode and a direct air Fe 3+ graphite cathode. Membrane Less MFC In a meditor less MFC, the membrane separates the anode from the cathode as in other MFCs, and the membrane functions as an electrolyte that plays the role of an electric insulator and allows protons to move through 16. However, the use of membrane can limit the application of MFC to wastewater treatment. Proton transfer

5 through the membrane can be a rate limiting factor especially with fouling expected due to suspended solids and soluble contaminants in a large scale wastewater treatment process. In addition, membranes are expensive and hence may limit its application. A membrane-less microbial fuel cell (ML-MFC) was developed by Jang et al. 16 and used successfully to enrich electrochemically active microbes that converted organic contaminants to electricity. The COD removal rate of g/m 3 day was reported with maximum power production 1.3 mw/m 2 and current density 6.9 ma/m 2. The design used in the study showed poor cathode reaction allowing a large quantity of oxygen to diffuse toward the anode. Further studies are required to improve the design of ML-MFC to improve current yield and COD removal efficiency. Power production of MFC and Further Development Required The major limitations to implementation of MFCs for treatment of wastewater are their power density is still relatively low and the technology is only in the laboratory phase. Based on the potential difference, E, between the electron donor and acceptor, a maximum potential of nearly 1 V can be expected in MFCs, which is not much greater than the 0.7 V that is currently being produced 21. However, by linking several MFCs together, the voltage can be increased. Current and power densities are lower than what is theoretically possible, and system performance varies considerably as shown in Table 1. The maximum power density reported in the literature, 3600 mw/m 2, was observed in a dual-chamber fuel cell treating glucose with an adapted anaerobic consortium in the anode chamber and a continuously aerated cathode chamber containing an electrolyte solution that was formulated to improve oxygen transfer to cathode 18. Cost of the MFC can be minimized by using plain graphite electrodes without exogenous electron shuttles and a commercially available membrane 19. Further improvement in the power density is required, and the rates of electron transfer to the anode are the major limiting factor. Optimization of MFCs is required, which involves investigation of a variety of aspects such as, anodophilic consortium selection for efficient electron transfer to electrode; cathode performance, oxygen supply and oxidation of fuel; MFC configuration, external resistance and electrodes (material and surface area); specific inoculum and concentration of bacteria; types of organic material in the wastewater, and electron shuttles.

6 TABLE 1: POWER GENERATION RATES REPORTED IN THE LITERATURE 8 Substrate Description Power (mw/m 2 ) Complex Anaerobic sediments 16 Starch wastewater 19 Starch wastewater 20 Domestic Wastewater 24 Anaerobic sediments 28 Domestic wastewater, CE-PEM 28 Domestic wastewater, CE no PEM 146 Defined Lactate Lactate, Peptone, and yeast extract 788 Acetate (salt Bridge) 0.3 Acetate Glucose Glucose CE-PEM 262 Glucose CE (no PEM) 494 The basic disadvantages of microbial fuel cells are their generally low coulombic yields and low power outputs. Many more improvements will be necessary, before biological fuel cell production and use can be commercialized. A membrane less MFC could improve the economic feasibility of the process to treat wastewater by reducing not only the capital investment, by eliminating costly membrane, but also the operational cost for the membrane maintenance. The objective of this study was to evaluate the electricity production potential and treatment efficiency of mediator less microbial fuel cell without membrane, using synthetic wastewater and mixed anaerobic microbial culture as inoculum. MATERIAL AND METHODS Membrane-less microbial fuel cell Figure 2 shows the schematic diagram of the membrane-less microbial fuel cell (ML-MFC) used in this study. The anode was at the bottom, and the cathode at the top of cylinder-shaped reactor made of polyacrylic having internal diameter of 10 cm. Glass wool (4 cm depth) and glass bead (4 cm depth) was placed at the upper portion of the anode. Graphite rod, as roll form, was used for both anode and cathode. Total height of the reactor was 60 cm, and distance between the anode and cathode was 20 cm, including glass wool and glass bead. The apparent surface area of each anode and cathode was cm 2. The fuel was supplied to the bottom of the anode and the effluent left through the cathode compartment at the top. The electrodes were connected with copper wire through a resistance ranging from 10 Ω

7 to 100 Ω, including the resistance of copper wire and a multimeter. The internal resistance of the MFC was ranging from 3 MΩ to 4.5 MΩ. Figure 2: Membrane Less Microbial Fuel Cell used in the study Wastewater The wastewater was applied at the rate of L/d making hydraulic retention time (HRT) in the MFC of 24 h. The cathode compartment was aerated at rate of 60 ml/min for the cathode reaction. A synthetic sewage containing sucrose as a carbon source was used throughout the study. The composition of the synthetic wastewater used is provided in Table 2. The COD of synthetic wastewater used was in the range 312 to 446 mg/l and influent ph was maintained in the range 7.2 to 7.6. TABLE 2: THE COMPOSITION OF THE SYNTHETIC WASTEWATER Component Sucrose NaHCO 3 NH 4 Cl K 2 HPO 4 KH 2 PO 4 CaCl 2.2H 2 O MgSO 4.7H 2 O mg/ L Note: Trace metals were added as FeSO 4.7H 2 O = 10 mg/l, NiSO 4.6H 2 O = mg/l, MnSO 4.H 2 O = mg/l, ZnSO 4.7H 2 O = mg/l, H 3 BO 3 = mg/l, CoCl 2.6H 2 O = 52.6 µg/l, CuSO 4.5H 2 O = 4.5 µg/l, and (NH 4 ) 6 Mo 7 O 24.4H 2 O = 52.6 µg/l. Enrichment of electrochemically active microbes for operating MFC The ML-MFC was inoculated with sludge (1.0 L) collected from septic tank bottom. All experiments were conducted at room temperature ranging from 29 to 33 o C. For the first fifteen days of operation the MFC was operated without any pretreatment of the inoculum sludge. After fifteen days, the sludge form the MFC was removed, heated and inoculated. The sludge was heated at 100 o C for 15 minutes to suppress the methanogens, cooled at room temperature and 1 L volume of sludge was added to the reactor in the anode compartment. No microbial addition was carried out in cathode compartment in both the cases.

8 Analyses The potential was measured using a digital multimeter (MASTECH M-830B, Russia) and converted to power according to P= iv, where P = power (W), i = current (A), and V = voltage (V). The influent and effluent characteristics such as, COD, ph, dissolve oxygen (DO), were monitored according to Standard Methods 23. Biochemical oxygen demand (BOD) was determined for three days at 27 o C. RESULT AND DISCUSSION Enrichment The ML-MFC, inoculated with septic tank sludge, was operated on batch mode for first four days and from fifth day onward with continuous mode. The electric current production of 0.3 ma was observed at external load of 10 Ω for first five days of continuous operation, but latter gradual decrease in current production was observed and it was 0.1 ma after 15 days. This decrease could be attributed to the growth of methanogens in the anode chamber, reducing the availability of electron and proton, hence reducing current. The sludge from the anode chamber of MFC was heated at 100 ºC and re-inoculated. This method was found effective to obtain an enriched culture of hydrogen producers 24. The current slowly increased for the first week, and it was 0.1 ma at external load of 100 Ω. The maximum current of ma was observed at external load of 10 Ω with voltage 0.19 V. The open circuit potential was about 0.3 V after fifteen days of operation. Power density of 6.73 mw/m 2 was observed in this experiment which is higher than the value 1.3 mw/m 2 reported earlier 16. COD Removal Efficiency The ML-MFC was operated at four different external resistance (10 to 100 Ω) and influent COD concentration ranging from 312 mg/l to 446 mg/l as presented in Table 3 and Figure 3. After 55 days of operation the COD and BOD removal efficiency of ML-MFC was 90.86% and 90.67%, respectively. The steady state condition for COD removal efficiency was observed after 55 days of operation with COD removal capacity of kg/m 3.d. The specific organic removal rate, with respect to anode chamber, of 0.08 kg COD/ kg VSS.d was observed. Studies are in progress to increase the loading capacity. The COD removal in the anode compartment was 46.36%. After achieving steady state, the effluent COD was 40.8 mg/l and BOD 3 at 27 o C was 25 mg/l. These values demonstrate the capacity of ML-

9 MFC for wastewater treatment. The COD and BOD values reported are for unsettled effluent containing SS and VSS concentration of 40 mg/l and 6.2 mg/l, respectively. Further improvement in the effluent quality is expected by employing settling tank after MFC. The COD removal efficiency observed is on the higher side of the maximum efficiency reported in literature as 80 to 90 percent 15,16. However, the organic loading rate as high as kg COD/m 3.d is reported with specific organic load as kg glucose/ kg VSS.d for hydrogen production in CSTR type reactor 25. TABLE 3: PERFORMANCE OF THE MEMBRANE LESS MICROBIAL FULE CELL A] COD AND BOD REMOVAL Days External Resistance (Ω) COD (Inlet) (±45.43) (±34.33) (±16.74) (±36.08) COD (middle) (±19.09) (±22.60) (±55.53) (±17.67) B] ELECTRICITY PRODUCTION COD (outlet) (±24.74) 72.4 (±20.90) (±11) 40.8 (±3.26) Efficiency (%) BOD (Inlet) BOD (middle) BOD (outlet) DO (Inlet) DO (outlet) (±15) (±10) (±5) (±1.0) (±0.25) (±20) (±12) (±5) (±1.0) (±0.35) (±18) (±13) (±5) (±1.0) (±0.39) Days Current (ma) (±0.011) (±0.007) (±0.007) (±0.008) Voltage (V) (±0.007) (±0.003) (±0.007) (±0.005) Resistance Power (ohm) (mw/m 2 ) (±0.39) (±0.44) (±0.39) (±0.46) Jule (J/d) HRT (hrs) Effect of External Resistance It was observed that at lower external resistance the current production is higher and vice versa. Even for same wastewater COD concentration the production of current is decreases due to increase in resistance shown in Figure 3. At same COD concentration when resistance was increased to 25 Ω and 50 Ω, the production of current decreased to ma and ma, respectively. These results show that the resistance becomes the rate-limiting step (step 3 in Figure 1). Even at lower resistance, low current production could be attributed to the lower electron consumption rate at the cathode than transfer rate from the external circuit. This might be due to limited supply of proton or oxygen (steps 4 and 5 in Figure 1). The

10 lower current production means that some electrons are consumed by mechanism(s) other than the cathode reaction. At this point it is not known how this is possible. It is plausible that under the conditions of limited electron disposal through the circuit with a high resistance, the electrons are consumed in the anode to reduce other electron acceptors such as sulfate and nitrate 6, or oxygen diffused from cathode compartment or dissolved oxygen present in the influent. At low resistance the electrons move more easily through the external circuit than at high resistance, oxidizing electron carriers of the microbes in the anode. Higher fuel oxidation by the microbes is expected with high ratio of oxidized electron carriers of the culture at a low resistance to remove organic contaminants at a high rate 16. Influent COD, mg/l COD at outlet of anode chamber, mg/l Efflent COD, mg/l Current,mA Resistance, Ohm Voltage, V 0.23 COD & Resistance (ohm) Current (ma), Voltage (V) Time (days) Figure 3. Time course of performance of membrane less microbial fuel cell Effect of Dissolved oxygen monitoring of cathode compartment The microbial cell was operated with aeration in the cathode compartment. The current and DO concentration measured under different external load are provided in Table 3. When the cathode compartment was aerated, the current increased sharply, and so did DO slowly. When the aeration was stopped the current decreased, whilst the decrease in DO was less significant. These results show that DO is the major limiting factor for the operation of a microbial fuel cell with graphite electrode. Probably, due to poor catalytic activity of graphite to reduce oxygen, which can be explained better by analyzing proton compensation in cathode chamber 6.

11 CONCLUSIONS Under present investigation, the membrane less MFC was used effectively for synthetic wastewater treatment with COD and BOD removal about 90%. The power production of this MFC observed was 6.73 mw/m 2. If power generation in these systems can be increased, MFC technology may provide a new method to offset wastewater treatment plant operating cost, making wastewater treatment more affordable for developing and developed nations. The possibility of direct conversion of organic material in wastewater to bio-electricity is exciting, but fundamental understanding of the microbiology and further development of technology is required. With continuous improvements in microbial fuel cell, it may be possible to increase power generation rates and lower their production and operating cost. Thus, the combination of wastewater treatment along with electricity production may help in saving of millions of rupees as a cost of wastewater treatment at present. ACKNOWLEDGEMENT The grants provided by University Grants Commission, New Delhi (F. No /2003 (SR)) under the head Major Research Project, to undertake this research work, are duly acknowledged. REFERENCES 1. Haandel van Adrianus C. and Gatz Lettinga (1994). Anaerobic Sewage Treatment, John Wiley & Sons, Singapore. 2. Allen R.M., Bennetto H.P.. (1993). Microbial fuel cells: electricity production from carbohydrates. Appl Biochem Biotechnol, 39-40: Wingard L.B., Shaw C.H., Castner J.F.. (1982). Bioelectrochemical fuel cells. Enzyme Microb. Technol., 4: Reimers C.E., Tender L.M., Ferig S, Wang W. (2001). Harvesting energy from the marine sediment-water interface. Environ. Sci. Technol., 35: Kim H.J., Park H.S., Hyun M.S., Chang I.S., Kim M, Kim B.H. (2002). A mediator-less microbial fuel cell using a metal reducing bacterium, Shewanella putrefaciens. Enzyme. Microb. Technol., 30: Gil G.C., Chang I.S., Kim B.H., Kim M, Jang J.K., Park H.S., Kim H.J. (2003). Operational parameters affecting the performance of a mediator-less microbial fuel cell. Biosens. Bioelectron. 18: Bond D.R., and Lovley D.R. (2003). Electricity production by Geobacter sulfurreducens attached to electrodes. App. Environ. Microbiol., 69: Liu H, and Logan B.E. (2004). Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol., 38(14):

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