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SEMINAR 2

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SEMINAR 2

  1. 1. 1 Chapter 1 INTRODUCTION In an era of climate change, alternate energy sources are desired to replace oil and carbon resources. Subsequently, climate change effects in some areas and the increasing production of biofuels are also putting pressure on available water resources. A microbial fuel cell (MFC) or biological fuel cell is a bio-electrochemical system that drives a current by using bacteria and mimicking bacterial interactions found in nature. MFCs can be grouped into two general categories, those that use a mediator and those that are mediator-less. The first MFCs, demonstrated in the early 20th century, used a mediator: a chemical that transfers electrons from the bacteria in the cell to the anode. Mediator-less MFCs are a more recent development dating to the 1970s; in this type of MFC the bacteria typically have electrochemically active redox proteins such as cytochromes on their outer membrane that can transfer electrons directly to the anode.[1][2]Since the turn of the 21st century MFCs have started to find a commercial use in the treatment of wastewater.[3] Microbial Fuel Cells have the potential to simultaneously treat wastewater for reuse and to generate electricity; thereby producing two increasingly scarce resources. While the Microbial Fuel Cell has generated interest in the wastewater treatment field, knowledge is still limited and many fundamental and technical problems remain to be solved Microbial fuel cell technology represents a new form of renewable energy by generating electricity from what would otherwise be considered waste, such as industrial wastes or waste water etc. A microbial fuel cell [Microbial Fuel Cell] is a biological reactor that turns chemical energy present in the bonds of organic compounds into electric energy, through the reactions of microorganism in aerobic conditions.It is achieved by catalytic reaction of microorganisms.A current is produced by the interaction of bacteria.Generally the fuel cell consist of anode and cathode .These two are separated by using a membrane which is positively charged called cations. Different operations takes place in both the compartments.Microorganisms oxidise the fuel inorder to generate crbondioxide along with protons and electrons in the anode compartment.These electrons and protons are too transferred to cathode compartment.An external circuit is required to transfer the electrons.The membrane is required to transfer protons.Protons and electrons in the cathode compartment mix with oxygen to produce water.
  2. 2. 2 Chapter2 HISTORY The idea of using microbial cells in an attempt to produce electricity was first conceived in the early twentieth century. M. Potter was the first to perform work on the subject in 1911.[4]A professor of botany at the University of Durham, Potter managed to generate electricity from E. coli, but the work was not to receive any major coverage. In 1931, however, Barnet Cohen drew more attention to the area when he created a number of microbial half fuel cells that, when connected in series, were capable of producing over 35 volts, though only with a current of 2 milliamps.[5] More work on the subject came with a study by DelDuca et al. who used hydrogen produced by the fermentation of glucose by Clostridium butyricum as the reactant at the anode of a hydrogen and air fuel cell. Though the cell functioned, it was found to be unreliable owing to the unstable nature of hydrogen production by the micro- organisms. Although this issue was later resolved in work by Suzuki et al. in 1976 the current design concept of an MFC came into existence a year later with work once again by Suzuki.[6] By the time of Suzuki’s work in the late 1970s, little was understood about how microbial fuel cells functioned; however, the idea was picked up and studied later in more detail first by MJ Allen and then later by H. Peter Bennetto both from King's College London. People saw the fuel cell as a possible method for the generation of electricity for developing countries. His work, starting in the early 1980s, helped build an understanding of how fuel cells operate, and until his retirement, he was seen by many as the foremost authority on the subject. It is now known that electricity can be produced directly from the degradation of organic matter in a microbial fuel cell. Like a normal fuel cell, an MFC has both an anode and a cathode chamber. The anoxic anode chamber is connected internally to the cathode chamber via an ion exchange membrane with the circuit completed by an external wire. In May 2007, the University of Queensland, Australia completed its prototype MFC as a cooperative effort with Foster's Brewing. The prototype, a 10 L design, converts brewery wastewater into carbon dioxide, clean water, and electricity. With the prototype proven successful,plans are in effect to produce a 660 gallon version for the brewery, which is estimated to produce 2 kilowatts of power.[7]
  3. 3. 3 Chapter 3 MICROBIAL FUEL CELLS 3.1Definiton A microbial fuel cell is a device that converts chemical energy to electrical energy by the catalytic reaction of microorganisms.[8] A typical microbial fuel cell consists of anode and cathode compartments separated by a cation (positively charged ion) specific membrane. In the anode compartment, fuel is oxidized by microorganisms, generating CO2, electrons and protons. Electrons are transferred to the cathode compartment through an external electric circuit, while protons are transferred to the cathode compartment through the membrane. Electrons and protons are consumed in the cathode compartment, combining with oxygen to form water.[9] 3.2 Construction and working of microbial fuel cells: Figure 3.1 Microbial fuel cell
  4. 4. 4 Microbial fuel cell consists of anode and cathode, connected by an external circuit and separated by Proton Exchange Membrane. Anodic material must be conductive, bio compatible, and chemically stable with substrate. Metal anodes consisting of noncorrosive stainless steel mesh can be utilized, but copper is not useful due to the toxicity of even trace copper ions to bacteria. The simplest materials for anode electrodes are graphite plates or rods as they are relatively inexpensive, easy to handle, and have a defined surface area. Much larger surface areas are achieved with graphite felt electrodes.The most versatile electrode material is carbon, available as compact graphite plates, rods, or granules, as fibrous material (felt, cloth, paper, fibers, foam), and as glassy carbon.Proton Exchange Membrane is usually made up of NAFION or ULTREX.[10] Microbial Fuel Cells utilise microbial communities to degrade organics found within wastewater and theoretically in any organic waste product; converting stored chemical energy to electrical energy in a single step.[11] Oxygen is most suitable electron acceptor for an microbial fuel cell due to its high oxidation potential, availability, sustainability and lack of chemical waste product, as the only end product is water. If acetate is used as substrate, following reaction takes place: Anodic reaction: CH3COO- + H2O → 2CO2 + 2H+ +8e- Cathodic reaction: O2 + 4e- + 4 H+ → 2 H2O Electrons produced by bacteria from these substrates are transferred to anode (negative terminal) and flow to the cathode ( positive terminal) linked by a conductive material. Protons move to cathodic compartment through Proton Exchange Membrane and complete the circuit. Microbial fuel cells use inorganic mediators to tap into the electron transport chain of cells and steal the electrons that are produced. The mediator crosses the outer cell lipid membranes and plasma wall; it then begins to liberate electrons from the electron transport chain that would normally be taken up by oxygen or other intermediates.
  5. 5. 5 The now-reduced mediator exits the cell laden with electrons that it shuttles to an electrode where it deposits them; this electrode becomes the electro-generic anode (negatively charged electrode). The release of the electrons means that the mediator returns to its original oxidised state ready to repeat the process. It is important to note that this can only happen under anaerobic conditions, if oxygen is present then it will collect all the electrons as it has a greater electronegativity than the mediator. Organic substrates are utilized by microbes as their energies are transferred to electron acceptor( molecular oxygen) in absence of such electron acceptors micro-organisms shuttle electron into anode surface with help of mediators. However few micro-organisms are able to transfer electrons directly to electrode. This type of system is called as Mediator Less Microbial Fuel Cell. Examples of such micro-organisms which are currently available are : shwanella, geobacter etc. Mediator Less Microbial Fuel Cell have more commercial potential as mediators are expensive and sometimes toxic to microorganisms. 3.3 Thermodynamics and the electromotive force Electricity is generated in an Microbial Fuel Cell only if the overall reaction is thermodynamically favorable. The reaction can be evaluated in terms of Gibbs free energy expressed in units of Joules (J), which is a measure of the maximal work that can be derived from the reaction calculated as, ∆G r = Gr 0 + RT(lnπ) Where, Gr (J) is the Gibbs free energy for the specific conditions, G0 r (J) is the Gibbs free energy under standard conditions usually defined as 298.15 K, 1 bar pressure, and 1 M concentration for all species, R (8.31447 J mol-1 K-1) is the universal gas constant, T (K) is the absolute temperature, and π is the reaction quotient calculated as the activities of the products divided by those of reactants. The standard Gibbs free energy is calculated from tabulated energies of formation for organic compounds in water. For Microbial Fuel Cell calculations, it is more convenient to evaluate the reaction in terms of the overall cell electromotive force (emf), Eemf (V), defined as the potential difference between the cathode and anode. This is related to the work, W(J), produced by cell, or
  6. 6. 6 W = EemfQ = ∆ Gr Where, Q = nF is the charge transferred in the reaction, expressed in coulomb (C), which is determined by the number of electrons exchanged in the reaction, n is the number of electrons per reaction mole and F is Faraday’s constant(9.64853×104 C/mol). Combining these two equations, we have, Eemf = ∆ Gr∕ nF If all reactions are evaluated at standard conditions, π = 1, then E0 emf = G0 r ∕ nF where E0 emf (V) is the standard cell electromotive force. We can therefore use the above equations to express the overall reaction in terms of the potential as Eemf = E0 emf –RT∕nF ln(π) The advantage of above equation is that it is positive for a favorable reaction , and directly produces a value of the emf for the reaction. This calculated emf provides an upper limit for the cell voltage; the actual potential derived from the Microbial Fuel Cell will be lower due to various potential losses.
  7. 7. 7 Chapter 4 MICROBIAL FUEL CELL DESIGN Many different configurations are possible for Microbial Fuel Fells. A widely used and inexpensive design is a two chamber Microbial Fuel Fell built in a traditional “H” shape, consisting usually of two bottles connected by a tube containing a separator which is usually a cation exchange membrane (CEM) such as Nafion or Ultrex, or a plain salt bridge. The key to this design is to choose a membrane that allows protons to pass between the chambers (the CEM is also called a proton exchange membrane, PEM), but optimally not the substrate or electron acceptor in the cathode chamber (typically oxygen). In the H-configuration, the membrane is clamped in the middle of the tubes connecting the bottle . However, the tube itself is not needed. As long as the two chambers are kept separated, they can be pressed up onto either side of the membrane and clamped together to form a large surface. An inexpensive way to join the bottles is to use a glass tube that is heated and bent into a U-shape, filled with agar and salt (to serve the same function as a cation exchange membrane), and inserted through the lid of each bottle . The salt bridge Microbial Fuel Fell, however, produces little power due the high internal resistance observed. H-shape systems are acceptable for basic parameter research, such as examining power production using new materials, or types of microbial communities that arise during the degradation of specific compounds, but they typically produce low power densities. The amount of power that is generated in these systems is affected by the surface area of the cathode relative to that of the anode and the surface of the membrane . The power density (P) produced by these systems is typically limited by high internal resistance and electrode-based losses. When comparing power produced by these systems, it makes the most sense to compare them on the basis of equally sized anodes, cathodes, and membranes. Using ferricyanide as the electron acceptor in the cathode chamber increases the power density due to the availability of a good electron acceptor at high concentrations. Ferricyanide increased power by 1.5 to 1.8 times compared to a Pt-catalyst cathode and dissolved oxygen (H-design reactor with a Nafion CEM) . The highest power densities so far reported for MFC systems have been low internal resistance systems with ferricyanide at the cathode . While ferricyanide is an excellent catholyte in terms of system performance, it must be chemically regenerated and its use is not sustainable in practice. Thus, the use of ferricyanide is restricted to fundamental laboratory studies.
  8. 8. 8 Chapter 5 METABOLISM IN MICROBIAL FUEL CELLS To assess bacterial electricity generation, metabolic pathways governing microbial electron and proton flows must be determined. In addition to the influence of the substrate the potential of the anode will also determine the bacterial metabolism. Increasing MFC current will decrease the potential of the anode, forcing the bacteria to deliver the electrons through more-reduced complexes. The potential of the anode will therefore determine the redox potential of the final bacterial electron shuttle, and therefore, the metabolism. Several different metabolism routes can be distinguished based on the anode potential: high redox oxidative metabolism; medium to low redox oxidative metabolism; and fermentation. Hence, the organisms reported to date in MFCs vary from aerobes and facultative anaerobes towards strict anaerobes. At high anodic potentials, bacteria can use the respiratory chain in an oxidative metabolism. Electrons and, concomitantly, protons can be transported through the NADH dehydrogenase, ubiquinone, coenzyme Q or cytochrome. The use of this pathway was investigated. They observed that the generation of electrical current from an MFC was inhibited by various inhibitors of the respiratory chain. The electron transport system in their MFC used NADH dehydrogenase, Fe/S (iron/sulphur) proteins and quinines as electron carriers, but does not use site 2 of the electron transport chain or the terminal oxidase. Processes using oxidative phosphorylation have regularly been observed in MFCs, yielding high energy efficiencies of up to 65%. Examples are consortia containing Pseudomonas aeruginosa, Enterococcus faecium and Rhodoferax ferrireducens.If the anode potential decreases in the presence of alternative electron acceptors such as sulphate, the electrons are likely to be deposited onto these components. Methane production has repeatedly been observed when the inoculum was anaerobic sludge , indicating that the bacteria do not use the anode. If no sulphate, nitrate or other electron acceptors are present, fermentation will be the main process when the anode potential remains low. For example, during fermentation of glucose, possible reactions can be: C6H12O6 + 2 H2O →4H2 + 2CO2 + 2C2H4O2 C6H12O6 → 2 H2 + 2CO2 + C4H8O2 This shows that a maximum of one-third of a hexose substrate electrons can theoretically be used to generate current, whereas two thirds remain in the produced fermentation
  9. 9. 9 product such as acetate and butyrate.The one-third of the total electrons are possibly available for electricity generation because the hydrogenases, which generally use the electrons to produce hydrogen gas, are often situated at places on the membrane surface that are accessible from outside by mobile electron shuttles or that connect directly to the electrode. As repeatedly observed, this metabolic type can imply a high acetate or butyrate production. This pathway is further substantiated by the significant hydrogen production observed when MFC enriched cultures are incubated anaerobically in a separate fermentation test.
  10. 10. 10 Chapter 6 MICRO-ORGANISMS USED IN MICROBIAL FUEL CELLS 6.1 Axenic bacterial cultures Some bacterial species in MFCs, of which metal-reducing bacterial are the most important, have recently been reported to directly transfer electrons to the anode. Metal- reducing bacteria are commonly found in sediments, where they use insoluble electron acceptors such as Fe (III) and Mn (IV). Specific cytochromes at the outside of the cell membrane make Shewanella putrefaciens electrochemically active in case it is grown under anaerobic conditions. The same holds true for bacteria of the family Geobacteraceae, which have been reported to form a biofilm on the anode surface in MFCs and to transfer the electrons from acetate with high efficiency. Rhodoferax species isolated from an anoxic sediment were able to efficiently transfer electrons to a graphite anode using glucose as a sole carbon source. Remarkably, this bacterium is the first reported strain that can completely mineralize glucose to CO2 while concomitantly generating electricity at 90% efficiency. In terms of performance, current densities in the order of 0.2-0.6mA and a total power density of 1-17 W/m 2 graphite have reported for Shewanella putrefaciens, Geobacter sulfurreducens and Rhodoferax ferrireducens at conventional (woven) graphite electrodes. However, in case woven graphite in the Rhodoferax study was replaced by highly porous graphite electrodes, the current and power output was increased up to 74 mA/m2 and 33 mW/m2 , respectively.[12] Although these bacteria generally show high electron transfer efficiency, they have a slow growth rate, a high substrate specificity (mostly acetate or lactate) and relatively low energy transfer efficiency compared to mixed cultures. Furthermore, the use of a pure culture implies a continuous risk of contamination of the MFCs with undesired bacteria.
  11. 11. 11 6.2 Mixed bacterial cultures MFCs that make use of mixed bacterial cultures have some important advantages over MFCs driven by axenic cultures: higher resistance against process disturbances, higher substrate consumption rates, smaller substrate specificity and higher power output. Mostly, the electrochemically active mixed cultures are enriched either from sediment (both marine and lake sediment) or activated sludge from wastewater treatment plants.[13] By means of molecular analysis, electrochemically active species of Geobacteraceae, Desulfuromonas, Alcaligenes faecalis, Enterococcus faecium, Pseudomonas aeruginosa, Proteobacteria, Clostridia, Bacteroides and Aeromonas species were detected in the before-mentioned studies.Also reorted the presence of nitrogen fixing bacteria (e.g., Azoarcus and Azospirillum) amongst the electrochemically active bacterial populations.[14]
  12. 12. 12 Chapter 7 LIST OF SUBSTRATES USED IN MICROBIAL FUEL CELLS Substrates not only provides energy for the bacterial cell but also influences the economic viability and overall performance such as power density and coloumbic efficiency of MFCs.The composition , concentration and type of the substrate also effects the microbial community and power production.Many organic substrates including carbohydrates, proteins volatile acids, cellulose and waste water have been used as a feed in MFC studies.It can range from simple pure low molecular sugar to complex organic matter containing waste water to generate electricity.In most of the MFCs, acetate is commonly used as a substrate due to its inertness towards alternative microbial conversions(fermentations and methenogenisis) that lead to high coloumbic efficiency and power output.Power generated to b higher when compared with other substrate.Following table presents list of substrate used in MFCs. Substrate type Concentrations Current density (m A/cm2) Acetate 1g/L 0.8 Lactate 18mM 0.005 Glucose 6.7Mm 0.7 Sucrose 2674mg/L 0.19 Glucaronic acid 6.7mM 1.18 Phenol 400mg/L 0.1 Sodium fumerate 25mM 2.05 Starch 10g/L 1.3 Cellulosic particles 4g/L 0.02 Xylose 6.7mM 0.74 Domestic wastewater 600mg/L 0.06 Brewery wastewater 2240mg/L 0.2 Table 7.1 Substrates and current density
  13. 13. 13 Chapter 8 TYPES OF MICROBIAL FUEL CELLS More broadly, there are two types of microbial fuel cell: mediator and mediator-less microbial fuel cells. 8.1 Mediator microbial fuel cell Most of the microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thionine, methyl viologen, methyl blue, humic acid, and neutral red.[15][16] Most of the mediators available are expensive and toxic. 8.2 Mediator free microbial fuel cell Figure 8.1 A plant microbial fuel cell [PMFC] Mediator-free microbial fuel cells do not require a mediator but use electrochemically active bacteria to transfer electrons to the electrode (electrons are carried directly from the bacterial respiratory enzyme to the electrode). Among the electrochemically active bacteria are, Shewanella putrefaciens,[17] Aeromonas hydrophila, and others. Some bacteria, which have pili on their external membrane, are able to transfer their electron production via these pili. Mediator-less MFCs are a more recent area of research and, due to this, factors that affect optimum efficiency, such as the strain of bacteria used in the system, type of ion-exchange membrane, and system conditions (temperature, pH, etc.) are not particularly well understood.Mediator-less microbial fuel cells can, besides
  14. 14. 14 running on wastewater, also derive energy directly from certain plants. This configuration is known as a plant microbial fuel cell. Possible plants include reed sweetgrass, cordgrass, rice, tomatoes, lupines, and algae. Given that the power is thus derived from living plants (in situ-energy production), this variant can provide additional ecological advantages. 8.3 Microbial electrolysis cell A variation of the mediator-less MFC is the microbial electrolysis cells (MEC). Whilst MFC's produce electric current by the bacterial decomposition of organic compounds in water, MECs partially reverse the process to generate hydrogen or methane by applying a voltage to bacteria to supplement the voltage generated by the microbial decomposition of organics sufficiently lead to the electrolysis of water or the production of methane.A complete reversal of the MFC principle is found in microbial electrosynthesis, in which carbon dioxide is reduced by bacteria using an external electric current to form multi- carbon organic compounds.[18] 8.4 Soil-based microbial fuel cell Figure 8.2 A soil based MFC Soil-based microbial fuel cells adhere to the same basic MFC principles as described above, whereby soil acts as the nutrient-rich anodic media, the inoculum, and the proton- exchange membrane (PEM). The anode is placed at a certain depth within the soil, while the cathode rests on top the soil and is exposed to the oxygen in the air above it. Soils are naturally teeming with a diverse consortium of microbes, including the electrogenic microbes needed for MFCs, and are full of complex sugars and other nutrients that have accumulated over millions of years of plant and animal material decay.
  15. 15. 15 Moreover, the aerobic (oxygen consuming) microbes present in the soil act as an oxygen filter, much like the expensive PEM materials used in laboratory MFC systems, which cause the redox potential of the soil to decrease with greater depth. Soil-based MFCs are becoming popular educational tools for science classrooms.[19] 8.5 Phototrophic biofilm microbial fuel cell Phototrophic biofilm MFCs (PBMFCs) are the ones that make use of anode with a phototrophicbiofilm containing photosynthetic microorganism like chlorophyta, cyanophyta etc., since they could carry out photosynthesis and thus they act as both producers of organic metabolites and also as electron donors A study conducted by Strik et al. reveals that PBMFCs yield one of the highest power densities and, therefore, show promise in practical applications. Researchers face difficulties in increasing their power density and long-term performance so as to obtain a cost-effective MFC. The sub-category of phototrophic microbial fuel cells that use purely oxygenic photosynthetic material at the anode are sometimes called biological photovoltaic systems.[20] 8.6 Nanoporous membrane microbial fuel cells The United States Naval Research Laboratory (NRL) developed the nanoporous membrane microbial fuel cells which operate the same as most MFCs, but use a non-PEM to generate passive diffusion within the cell.The membrane used instead is a nonporous polymer filter (nylon, cellulose, or polycarbonate) which generates comparable power densities as Nafion (a well-know PEM) while remaining more durable than Nafion. Porous membranes allow passive diffusion thereby reducing the necessary power supplied to the MFC in order to keep the PEM active and increasing the total output of energy from the cell.[21] MFCs that do not use a membrane can deploy anaerobic bacteria in aerobic environments however, membrane-less MFCs will experience cathode contamination by the indigenous bacteria and the power-supplying microbe. The novel passive diffusion of nanoporous membranes can achieve the benefits of a membrane-less MFC without worry 0f cathode contaminatio.Nanoporous membranes are also ten times cheaper than Nafion (Nafion-117, $0.22/cm2 vs. polycarbonate, <$0.02/cm2).
  16. 16. 16 8.7 Sediment microbial fuel cells A likely application of microbial fuel cell (MFC) technology is in remote bodies of water where electric energy can be extracted from organic-rich aquatic sediments. For this purpose, researchers have developed sediment MFCs that consist of an anode electrode embedded in the anaerobic sediment and a cathode electrode suspended in the aerobic water column above the anode electrode. Electricigenic bacteria in the sediment transfer electrons produced during the oxidation of organic or inorganic matter to the anode electrode; while oxygen is reduced in the water column by accepting electrons from the cathode electrode. As a result, an electric current is generated. Classically, H-type MFCs have been used to study microbial respiration in the anode. Such MFCs contain a cation exchange membrane to separate the anaerobic anode from the aerobic cathode. A cation exchange membrane is not necessary in sediment MFCs, because the decreasing oxygen gradient over the depth of water and sediment columns creates the necessary potential difference naturally By placing one electrode into a marine sediment rich in organic matter and sulfides, and the other in the overlying oxic water, electricity can be generated at sufficient levels to power some marine devices. Protons conducted by the seawater can produce a power density of up to 28 mW/m2. Graphite disks can be used for the electrodes, although platinum mesh electrodes have also been used. “Bottle brush” cathodes used for seawater batteries may hold the most promise for long-term operation of unattended systems as these electrodes provide a high surface area and are made of noncorrosive materials. Sediments have also been placed into H-tube configured two- chamber systems to allow investigation of the bacterial community. 8.8 Permanganate cathodic electron acceptor MFC Permanganate has been used as an environment-friendly oxidant in industries for many years. Its high redox potential offers the possibility of its application in a fuel cell system to establish a high potential difference between the anode and the cathode. Five-fold more power density can be achieved in a permanganate two-chamber MFC than with other electron acceptors such as hexacynoferrate and oxygen; In a MFC, also a three-fold maximum power density can be produced when using permanganate as the electron acceptor as compared to using hexacynoferrate . It is the outstanding redox potential of the permanganate that enhanced the power output of a MFC. The similar mechanism also
  17. 17. 17 applies to the other high redox potential electron acceptors such as hexacynoferrate which generates higher power by higher redox potentials than dissolved oxygen. Moreover, it is worth pointing out that thispermanganate method has no need for a catalyst, which makes this process simple and economical. But on the other hand, it should be noted that like the other liquid-state electron acceptors this permanganate MFC also requires liquid replacements to compensate its depletion. 8.9 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. However, the use of membrane can limit the application of MFC to wastewater treatment. Proton transfer 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 and used successfully to enrich electrochemically active microbes that converted organic contaminants to electricity. The COD (Chemical Oxygen Demnd) removal rate of 526.67 g/m3 day was reported with maximum power production 1.3 mW/m2 and current density 6.9 mA/m2. 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.
  18. 18. 18 Chapter 9 APPLICATIONS OF MICROBIAL FUEL CELLS 9.1 Waste water treatment Due to unique metabolic assets of microbes, variety of microorganisms are used in Microbial Fuel Cells either single species or consortia. Some substrates (sanitary wastes, food processing waste water,swine waste water and com stovers) are exceptionally loaded with organic matter that itself feed wide range of microbes used in Microbial Fuel Cells. Microbial Fuel Cells using certain microbes have a special ability to remove sulfides as required in waste water treatment. Microbial Fuel Cell substrates have huge content of growth promoters that can enhance growth of bio-electrochemically active microbes during waste water treatment. This simultaneous operation not only reduces energy demand on treatment plant but also reduces amount of unfeasible sludge produced by existing anaerobic production. Microbial Fuel Cells connected in series have high level of removal efficiency to treat leachate with supplementary benefit of generating electricity. Consider a conventional Waste Water Treatment Plant designed for 30000 IE, receiving a daily influent of 5400m3. At a biodegradable chemical oxygen demand (bCOD) concentration 0f 500mg/L, this represents an influx of organic matter of 2700kg dry weight per day. The amount of sludge formed,at a nominal yield of 0.4g cell dry weight per g bCOD converted will be 1080 kg per day. This needs to be disposed off at a cost which can rise up to €5 00 per ton dry matter. The other costs contained in the operational cost are the aeration costs and pump costs for recirculation and processing. If a Microbial Fuel Cell is used with an open air cathode, no aeration is needed. The putative energy of the input organic matter amounts to 8950kWH/day. The costs for sludge processing will be lower, since no aerobic cell yields can be attained . for methanogenesis, the cell yield is about 0.05g CDW/g substrate; for Microbial Fuel Cell the yield can be estimated somewhere in between aerobic and methanogenic conditions. At an energetic efficiency of 35%, which should be attainable on large scale, approximately 3150 kWh/day of useful energy will be produced. This comparison does not take into account the capital cost of both systems. However, if the capital cost is of same order, the comparison illustrates a significant difference in operational costs. Hence,
  19. 19. 19 if large scale Microbial Fuel Cells can be built at an acceptable price, this will be a viable technology. Under present investigation, the membrane less MFC was used effectively for synthetic wastewater treatment with COD and BOD removal about 90%. 9.2 Power generation Microbial fuel cells have a number of potential uses. The most readily apparent is harvesting electricity produced for use as a power source. The use of MFCs is attractive for applications that require only low power but where replacing batteries may be time- consuming and expensive such as wireless sensor networks.[22] Virtually any organic material could be used to feed the fuel cell, including coupling cells to wastewater treatment plants. Bacteria would consume waste material from the water and produce supplementary power for the plant. The gains to be made from doing this are that MFCs are a very clean and efficient method of energy production. Chemical processing wastewater and designed synthetic wastewater have been used to produce bioelectricity in dual- and single- chamber mediatorless MFCs (non-coated graphite electrodes) apart from wastewater treatment. Higher power production was observed with biofilm covered anode (graphite).[23]A fuel cell’s emissions are well below regulations.MFCs also use energy much more efficiently than standard combustion engines, which are limited by theCarnot Cycle. In theory, an MFC is capable of energy efficiency far beyond 50%. According to new research conducted by René Rozendal, using the new microbial fuel cells, conversion of the energy to hydrogen is 8 times as high as conventional hydrogen production technologies. However, MFCs do not have to be used on a large scale, as the electrodes in some cases need only be 7 μm thick by 2 cm long. The advantages to using an MFC in this situation as opposed to a normal battery is that it uses a renewable form of energy and would not need to be recharged like a standard battery would. In addition to this, they could operate well in mild conditions, 20 °C to 40 °C and also at pH of around 7. Although more powerful than metal catalysts, they are currently too unstable for long-term medical applications such as in pacemakers (Biotech/Life Sciences Portal). Besides wastewater power plants, as mentioned before, energy can also be derived directly from crops. This allows the set-up of power stations based on algae platforms or
  20. 20. 20 other plants incorporating a large field of aquatic plants. According to Bert Hamelers, the fields are best set-up in synergy with existing renewable plants (e.g., offshore wind turbines). This reduces costs as the microbial fuel cell plant can then make use of the same electricity lines as the wind turbines.[24] 9.3 Secondary fuel production With minor modifications,Microbial Fuel Cells can be employed to produce secondary fuels like hydrogen (H2) as an alternative of electricity. Under standard experimental conditions, proton and electron produced in anodic chamber get transferred to cathode, which then combines with oxygen to form water. H2 generation is thermodynamically not favored or it is a harsh process for a cell to convert aproton into H2. Increase in external potential applied at cathode can be competent to overcome thermodynamic barrier in reaction and used for H2 generation. As a result, proton and electron produced in anodic reaction chamber combine at cathode to form H2. Microbial Fuel Cells can probably produce extra H2 as compared to quantity that pull off from classical glucose fermentation method. Single-chamber membrane-free MECs were designed and successfully produced hydrogen from organic matter using one mixed culture and one pure culture:Shewanella oneidensis MR-1. At an applied voltage of 0.6 V, a hydrogen production rate of 0.53m3/day/m3 was obtained using a mixed bacterial culture by the single-chamber MECs operated at pH 7.0. Higher hydrogen production rate (0.69m3 /day/m3 ) was obtained when the MECs were operated at pH 5.8. High current densities of 9.3 A/m2 (pH 7) and 14 A/m2 Were achieved with the mixed culture in the single-chamber MEC system, attributing to the reduced potential losses associated with membrane. Applied voltages exerted significant influences on MEC’s performance. The performances at 0.6 V were more than two times higher than those at 0.4 V in terms of hydrogen production rate, overall energy efficiency, hydrogen yield, Coulombic efficiency and current density. While 0.3 V was the minimum applied voltage to achieve measurable hydrogen production rate in the MEC system. Hydrogenotrophic methanogens in the mixed culture systems adversely affected hydrogen production. Methanogenesis was avoided by using the pure bacterial culture S. oneidensis in this MEC system. However, the current hydrogen production rates were much lower than those with the mixed culture systems. The current density and volumetric hydrogen production rate of this system have potential to increase significantly by further reducing the electrode spacing and increasing the ratio of electrode surface area/cell volume.
  21. 21. 21 Figure 9.1 H2 Generation in MFC 9.4 Bio-Sensors Since the current generated from a microbial fuel cell is directly proportional to the energy content of wastewater used as the fuel, an MFC can be used to measure the solute concentration of wastewater i.e as a biosensor system. The strength of wastewater is commonly evaluated as biochemical oxygen demand (BOD) value.BOD values are determined incubating samples for 5 days with proper source of microbes, usually activate sludge collected from sewage works. When BOD values are used as a real-time control parameter, 5 days' incubation is too long. An MFC-type BOD sensor can be used to measure real-time BOD values. Oxygen and nitrate are preferred electron acceptors over the electrode reducing current generation from an MFC. MFC-type BOD sensors underestimate BOD values in the presence of these electron acceptors. This can be avoided by inhibiting aerobic and nitrate respirations in the MFC using terminal oxidase inhibitors such as cyanide and azide.This type of BOD sensor is commercially available. Bacteria show lower metabolic activity when inhibited by toxic compounds. This will cause a lower electron transfer towards an electrode. Bio-sensors could be constructed, in which bacteria are immobilized onto an electrode and protected behind a membrane. If a toxic component diffuses through the membrane, this can be measured by the change in potential over the sensor. Such sensors could be extremely useful as indicators of
  22. 22. 22 toxicants in rivers, at the entrance of wastewater treatment plants, to detect pollution or illegal dumping, or to perform research on polluted site.[25] MFCs with replaceable anaerobic consortium could be used as a biosensor for online monitoring of organic matter. Though diverse conventional methods are used to calculate organic content in terms of Biological Oxygen Demand(BOD) in waste water, most of them are unsuitable for on line monitoring and control of biological waste water treatment process. A linear correlation between coulombic yield and strength of organic matter in waste water makes MFC a possible BOD sensor. Coulombic yield of MFC provides an idea about BOD of liquid stream that proves to be an accurate method to measure BOD value at quite wide concentration range of organic matter in waste water. A mediator-less microbial fuel cell was tested as a continuous BOD sensor. At a feeding rate of 0.35ml/min (HRT = 1.05 h), BOD values of up to 100mg/l could be measured based on a linear relation. Higher BOD values were then measured using either a model fitting method or a lower feeding rate. About 60min was required to reach a new steady- state current after changing the strength of the AW. When the MFC was starved, the original current value was regained with varying recovery periods depending on the length of the starvation. During starvation, the MFC generated a background level current, probably through an endogenous metabolism. The United States Navy is looking into microbial fuel cells particularly for environmental sensors. The use of microbial fuel cells to power environmental sensors would be beneficial because they would be able to sustain power for a longer amount of time and enable the collection and retrieval of undersea data without using a wire infrastructure. The energy created by these fuel cells was enough to sustain sensors after an initial startup time in research to demonstrate the effectiveness of the fuel cell as a power source for such sensors.Due to undersea conditions (high salt concentrations, fluctuating temperatures, and limited nutrient supply), the U.S. Navy is looking to deploy their MFCs with a mixture of salt-tolerant microorganisms. A mixture would also allow for a more complete utilization of available nutrients to be converted into electricity. Currently, Shewanella oneidensis is their primary microorganism for electrical generation, but their mixture might also include Shewanella spp. as it is very heat- and cold-tolerant.
  23. 23. 23 9.5 Desalination Desalination of sea water and brackish water for use as drinking water has always presented significant problems because of the amount of energy required to remove the dissolved salts from the water.By using an adapted microbial fuel cells this process could proceed with no external energy output. By adding a third chamber in etween the two electrodes of a standard MFC and filling it with sea water,the cell’s positive and negative electrodes attract the positive and negative salt ions in the water and using semi-permeable membrane,filters out the salt from the sea water. Figure 9.2 A Desalination microbial fuel cell 9.6 Educational tool Soil-based microbial fuel cells are popular educational tools, as they employ a range of scientific disciplines (microbiology, geochemistry, electrical engineering, etc.), and can be made using commonly available materials, such as soils and items from the refrigerator. There are also kits available for classrooms and hobbyists.and research-grade kits for scientific laboratories and corporations.[26]
  24. 24. 24 Chapter10 ADVANTAGES OF MICROBIAL FUEL CELLS Microbial fuel cells present several advantages, both operational and functional, in comparison to the currently used technologies for generation of energy out of organic matter or treatment of waste streams: 10.1 Generation of energy out of biowaste/organic matter This feature is certainly the most ‘green’ aspect of microbial fuel cells. Electricity is being generated in a direct way from biowastes and organic matter. This energy can be used for operation of the waste treatment plant, or sold to the energy market. Furthermore, the generated current can be used to produce hydrogen gas. Since waste flows are often variable, a temporary storage of the energy in the form of hydrogen, as a buffer, can be desirable. 10.2 Direct conversion of substrate energy to electricity As previously reported, in anaerobic processes the yield of high value electrical energy is only one third of the input energy during the thermal combustion of the biogas. While recuperation of energy can be obtained by heat exchange, the overall effective yield still remains of the order of 30%. A microbial fuel cell has no substantial intermediary processes. This means that if the efficiency of the MFC equals at best 30% conversion, it is the most efficient biological electricity producing process at this moment. However, this power comes at potentials of approximately 0.5 Volts per biofuel cell. Hence, significant amounts of MFCs will be needed, either in stack or separated in series, in order to reach acceptable voltages. If this is not possible, transformation will be needed, entailing additional investments and an energy loss of approximately 5 %. Another important aspect is the fact that a fuel cell does not –as is the case for a conventional battery- need to be charged during several hours before being operational, but can operate within a very short time after feeding, unless the starvation period before use was too long too sustain active biomass.
  25. 25. 25 10.3 Omission of gas treatment Generally, off-gases of anaerobic processes contain high concentrations of nitrogen gas, hydrogen sulphide and carbon dioxide next to the desired hydrogen or methane gas. The off-gases of MFCs have generally no economic value, since the energy contained in the substrate was prior directed towards the anode. The separation has been done by the bacteria, draining off the energy of the compounds towards the anode in the form of electrons. The gas generated by the anode compartment can hence b discharged provided no larger quantities of H2S and other odorous compounds are present in the gas and no aerosol with undesired bacteria are liberated into the environment. 10.4 Aeration The cathode can be installed as a ‘membrane electrode assembly’, in which the cathode is precipitated on top of the proton exchange membrane or conductive support, and is exposed to the open air. This omits the necessity for aeration, thereby largely decreasing electricity costs. However, from a technical point of view, several aspects need additional consideration when open air cathodes are used. First, the cathode needs to remain sufficiently moist to ensure electrical contact. Preliminary experiments by sceintists indicated that the water formation through oxygen reduction is insufficient to keep the cathode moist. Therefore, a water recirculation needs to be installed, possibly entailing extra energy costs. Secondly, the cathode needs to contain a non-soluble redox mediator to efficiently transfer the electrons from the electrode to oxygen. Generally, platinum is being used as a catalyst, at concentrations up to 40% w/w, representing considerable costs. However, new catalysts need to be developed, which would compensate their possible lower efficiency by a significantly reduced cost and higher sustainability.
  26. 26. 26 Chapter 11 LIMITATIONS OF MICROBIAL FUEL CELLS 11.1 Low power density The major limitations to implementation of MFCs for are their power density is still relatively low and the technology is only in the laboratory phase. Based on the potential difference, ΔE, between the electron donor and acceptor, a maximum potential of nearly 1V can be expected in MFCs, which is not much greater than the 0.7 V that is currently being produced. 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. The maximum power density reported in the literature, 3600mW/m2, 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 11.2 High initial cost A limiting factor to general MFC use is the high cost of materials, such as the nafion membrane commonly used in laboratories as a proton permeable membrane. Attempts are currently underway to produce low cost MFCs constructed from earthen pots for use in India. By removing the proton permeable membrane, utilizing locally produced 400 ml earthen pots, stainless steel mesh cathodes and a graphite plate anode, each MFC unit could be produced for US $1. The earthen pot MFCs used sewerage sludge as an initial inoculum and experiments were conducted using acetate as a carbon source. While producing low levels of power, these devices could potentially be incorporated in large numbers into oxidation ponds for the treatment of concentrated wastewater while generating power. In areas where off grid applications are required, even low power MFC devices may prove useful. The World Bank has provided funding to a company named Lebone (http://www.lebone.org/) to start trials with MFC technology to provide energy to isolated communities. Initial trials will be based in Tanzania and attempt to provide power for high efficiency LEDs and battery powered devices. Current applications are all limited to low power level devices. If power can be increased, or cells engineered for specific applications, then a large range of potential applications have been speculated to be possible
  27. 27. 27 11.3 Activation losses Due to the activation energy needed for an oxidation/reduction reaction, activation losses (or activation polarization) occur during the transfer of electrons from or to a compound reacting at the electrode surface. This compound can be present at the bacterial surface, as a mediator in the solution, or as the final electron acceptor reacting at the cathode. Activation losses often show a strong increase at low currents and steadily increase when current density increases. Low activation losses can be achieved by increasing the electrode surface area, improving electrode catalysis, increasing the operating temperature, and through the establishment of an enriched biofilm on the electrode(s). 11.4 Ohmic losses The ohmic losses (or ohmic polarization) in an MFC include both the resistance to the flow of electrons through the electrodes and interconnections, and the resistance to the flow of ions through the CEM (if present) and the anodic and cathodic electrolytes. Ohmic losses can be reduced by minimizing the electrode spacing, using a membrane with a low resistivity, checking thoroughly all contacts, and (if practical) increasing solution conductivity to the maximum tolerated by the bacteria. 11.5 Bacterial metabolic losses To generate metabolic energy, bacteria transport electrons from a substrate at a low potential through the electron transport chain to the final electron acceptor (such as oxygen or nitrate) at a higher potential. In an MFC, the anode is the final electron acceptorandits potential determines the energy gain for the bacteria. The higher the difference between the redox potential of the substrate and the anode potential, the higher the possible metabolic energy gain for the bacteria, but the lower the maximum attainable MFC voltage. To maximize the MFC voltage, therefore, the potential of the anode should be kept as low (negative) as possible. However, if the anode potential becomes too low, electron transport will be inhibited and fermentation of the substrate (if possible) may provide greater energy for the microorganisms. The impact of a low anode potential, and its possible impact on the stability of power generation, should be addressed in future studies.
  28. 28. 28 Chapter 12 CONCLUSION Development of MFCs was triggered by USA space program in 1960s as a possible technology for a waste disposal system for space flights that would also generate power. MFC technology has been extensively reviewed focusing on recent improvement, practical implementation, anode performance, cathodic limitations, different substrates etc. MFCs have been explored as a new source of electricity generation during operational waste water treatment. In addition, some of the recent modification in MFCs (MEC), in which anoxic cathode is used increased external potential at cathode. Phototropic MFCs and solar powered MFC also represent an exceptional attempt in the progress of MFCs technology for electricity production. MFC is an ideal way of generating electricity since it not only as a renewable source but also it can be used to treat waste. It can also be used for production of secondary fuel as well as in bioremediation of toxic compounds. However, this technology is only in research stage and more research is required before domestic MFCs can be made available for commercialization Microbial fuel cells are evolving to become a simple, robust technology. Certainly in the field of wastewater treatment, middle term application can be foreseen at market value prices. However, to increase the power output towards a stable 1kW per m3 of reactor, many technological improvements are needed. Provided the biological understanding increases, the electrochemical technology advances and the overall electrode prices decrease, this technology might qualify as a new core technology for conversion of carbohydrates to electricity in years to come.
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  30. 30. 30 18.Mohan, V., & raghavulu, v. (2008). influence of anodic biofilim growth on bio electricity production in single chamber meditaor less microbial fuel cells. biosensors and bioelectronics, 24(1), 41-47. 19.Mohan, v., krishnan, M., & srikanth. (2008). harnessing of microbial fuel cell employin aerated cathode through anerobic treatment of chemical wastewater using selectively enriched hydrogen producing mixed consortia. 87(12), 2667-2676. 20.Potter, M. (1911). electrical effects accompanying the decomposition of organic compounds. 84, 260-276. 21.Strik, D. (2008). Green electricity production with living plants and bacteria in a fuel cell. international journal of energy research, 32(9), 870-876.

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