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1. MICROBIAL FUEL CELL
In a microbial fuel cell (MFC), bacteria do not directly transfer their electrons to their characteristic terminal electron acceptor, but these electrons are diverted towards an electrode, i.e. an anode. The electrons are subsequently conducted over a resistance or power user towards a cathode and thus, bacterial energy is directly converted to electrical energy. To close the cycle, protons migrate through a proton exchange membrane. Simplified, a MFC can be compared to a car battery, in which the anode contains bacteria as catalyst to liberate electrons. Microbial fuel cells could be applied for the treatment of liquid waste streams.
2. STRUCTURE OF MICROBIAL FUEL CELLS: -
The structure of microbial fuel cell is as follows.
Basic Microbial Fuel Cell
Most microbial cells are electrochemically inactive. The electron transfer from microbial cells to the electrode is facilitated by mediators such as thiamine, methyl viologen (methyl blue), neutral red etc, and of the mediators available is expensive and toxic. Microbial fuel cells produce power by use of a microbial cell-permeable chemical mediator, which in the oxidized form intercepts a proportion of NADH (nicotinamide adenine dinucleotide) within the microbial cell and oxidises it to NAD+. The now reduced form of mediator is also cell-permeable and diffuses away from the microbial cell to the anode where, the reduced redox mediator is then electro-catalytically re-oxidized. In addition, cell metabolism produces protons in the anodic chamber, which may migrate through a proton selective membrane to the cathodic chamber. In the latter, they are consumed by ferricyanide (Fe3-(CN)6) and incoming electrons (via the external circuit) reducing it to ferrocyanide (Fe4-(CN)6 ). The oxidized mediator is then free to repeat the cycle. This cycling continually drains off metabolic reducing power from the microbial cells to give electrical power at the electrodes. Our mediator is pyocyanin and this has been shown to improve performance of fuel cells.
3. WORKING PRINCIPLE: -
Researchers in Belgium are using a mixture of natural selection and chemical insight to improve the performance of microbial fuel cells. Bacteria in these cells transfer electrons generated during respiration to an anode, rather than passing them along the bacterial electron transfer chain. The electrons travel through the chosen circuit to a cathode, where they reduce oxygen to form water.
Korneel Rabaey, Willy Verstraete and co-workers at the University of Ghent have optimised the transfer of electrons to the anode to achieve a considerably higher output than previously reported, up to 4.31 Watts per square metre of electrode surface (664 mV, 30.9 mA). "We started from the idea that bacteria that could grow onto an electrode would be adapted to communicate with it," Korneel Rabaey told The Alchemist.
The operating principles of a microbial fuel cell. Electrons can flow to the anode via chemical mediators
4. ADVANTAGES OF MFCs
There are several advantages of microbial fuel cell in the day today life. They are listed below.
4.1. 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 biowaste 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.
4.2. 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.
4.3. Sludge production
In an aerobic bioconversion process, the growth yield is generally estimated to be about 0.4 g Cell Dry Weight / g Chemical Oxygen Demand removed. Due to the harvesting of electrical energy, the bacterial growth yield in a MFC is considerably lower than the yield of an aerobic process.
4.4. 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 be discharged, provided that no large quantities of H2S or other odorous compounds are present in the gas, and no aerosols with undesired bacteria are liberated into the environment.
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.
5. FUTURE APPLICATIONS OF MICROBIAL FUEL CELLS
Microbial fuel cells have still some way to go to reach large scale commercialization. The largest reactors reported thus far had an anode internal volume of 0.388 L (Liu et al. 2004). Two types of implementation into practice offer perspectives within a reasonable time scale: MFCs for treatment of wastewater and MFCs converting renewable biomass in batteries.
5.1. MFCs for wastewater treatment
If a MFC is used with an open air cathode, no aeration is needed. The putative energy of the input organic matter amounts to 8,950 kWh per day. The costs for sludge processing will be lower, since no aerobic cell yields can be attained. For methanogenesis, the cell yield is about 0.05 g CDW / g substrate; for MFC 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 3,150 kWh per 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 the same order, the comparison illustrates a significant difference in operational costs. Hence, if large scale MFCs can be built at an acceptable price, this will be a viable technology.
Membrane Less Microbial Fuel Cell used in the study Wastewater
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/m2. 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 s a cost of wastewater treatment at present.
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