Due to the growing energy demands of a fast modernising world, it has become necessary to look for alternate sources of energy that are clean, renewable and available at low cost. The latest developments in the field of microbiology have helped scientists create microbial fuel cells which exploit metabolic reactions of bacteria on waste products to produce electricity and hydrogen gas. Both of these can act as source of energy.
Another concern of proper waste management to maintain a clean environment also gets answered with this invention. The microbes act on wastewater, garbage or agricultural and industrial wastes to produce hydrogen and electricity and in the process remove the pollutants and recycle the resource. Hence, MFCs find wide applicability in breweries, wastewater treatment plants, etc.
This paper will deal with newer discoveries in the field of bioremediation and how these are being used to maximise the electricity output. The paper will also review the application of MFCs and how the developed and developing nations put these to maximum use. The purpose of the paper is to discover the fastest moving field of research in harnessing hydrogen from MFCs. This will also help in the formulation of a relevant design study as part of course work.
Keywords: microbial fuel cell, literature review on MFCs, clean renewable fuel, bioremediation, alternative fuel, wastewater management technology
Introduction
We are all living in a world where the energy requirements are huge, to support our ever expanding population, but the resources are very limited. So an intense search is on to discover renewable sources of energy that will be sustainable, cheap and relatively free from other non-biodegradable by-products. This has led to the discovery of microbial fuel cells which exploit the biochemical activity of microbes during the process of fermentation in order to obtain a clean fuel - hydrogen gas as well as produce electricity.
Microbial fuel cells are devices which convert chemical energy into electrical energy by the metabolic activities of bacteria. This field of study though comparatively new, was first observed by M.C. Potter in 1911 at the University of Durham (Mercer, 2012). It is of great significance in present times as the use of microorganisms to produce electricity from waste means that there is an unending supply of both the substrates and the organisms. There are many other benefits as well. The waste water gets purified by the action of the microorganisms as they break down the pollutants to produce among others, hydrogen and electrons, which are capable of generating electricity in an anion-cation compartmentalised cell.
The concept of bioremediation comes from the ability of microorganisms to break down organic matter and chemicals polluting our environment. Hence, this concept has been employed for cleaning up soils, surface and ground water resources from chemical pollutants like petroleum products, etc. as an economically viable and environmentally friendly technique (Crawford & Crawford, 1998). What MFCs have done is that it has made electricity generation possible along with bioremediation.
Microbial fuel cells can act as batteries powered by microorganisms. It is a renewable form of electricity generation. The main principle of the MFCs are the biochemical reactions of certain strains of bacteria called electricigen like Shewanella species that can be used to generate electricity from organic waste (Hou. et al, 2009). What that means is that it is a source of "green energy" along with additional benefits like removing impurities from water and making it suitable for industrial consumption.
Although, the MFC devices that have been developed so far have shown low output, this line of study presents immense potential for use in the near future. With greater use of semi-conductors and similar advancements in technology it will be very possible to use bacteria produced electrical energy to overcome the internal resistance of the wires that could even support popular electrical appliances.
Research in this field is being carried out on several fronts. There is research work being done to improve the electricity output by building better MFC devices. There are yet other microbiologists who are on the lookout for newer strains exhibiting greater electrochemical activity. Different substrates are being experimented with and under different environmental conditions to improve the efficiency of these hydrogen cells.
History
The idea of using microbes to generate electricity was first conceived by M.C. Potter. He was the first to construct a microbial fuel cell that wasn't very efficient but the idea could not be developed due to lack of knowledge about bacteria. However, it was M.J. Allen and H.P. Bennetto from Kings College, London, who in the 1980s, improved upon the design to create the modern MFC using the recent understanding of electron transport chains to provide a sustainable form of clean fuel. The first designs required mediators to transfer the electrons from the electron transport chain to the anode. B.H. Kim in the 1990s from Korean Institute of Science and Technology discovered electrochemically active bacteria that eliminated the need for costly mediators (Mercer, 2012).
Source: National Centre for Biotechnology Education
Structure of a typical MFC
A typical Microbial Fuel Cell contains two chambers with one electrode in each in an electrolyte separated by a semi-permeable membrane or a cation exchange membrane and an external electric circuit to complete the cell. The bacteria are present in the anaerobic anode compartment. The bacterial metabolic action on the wastewater allows electrons to become free and the cell starts conducting electricity as soon as the electrons reach the cathode through the wire and combine with the protons to form hydrogen gas. The protons were formed in the anode but penetrated the semi-permeable membrane to travel to the cathode compartment. In other cells, the cathode compartment is aerobic. The protons combine with oxygen and free electrons to form water molecules. This concept is used in wastewater treatment plants and desalination plants.
Designs of MFCs
Although different architecture of MFCs are possible, the most widely used is the two-chamber MFC built in the shape of an H with two bottles or compartments- anode and cathode- and a tube containing the cation exchange membrane as a separator connecting the two bottles. The membranes can be Nafion or Ultrex or even a simple salt bridge, though the efficiency with this is quite low due to high internal resistance (Min & Logan, 2004). H-shaped MFCs are mostly used to research basic parameters like amount of power generated, viable microbial communities, different compounds to be degraded, etc.
Microbes can transfer electrons to the anode either by using exogenous mediators, bacterial mediators or respiratory enzymes. Mediators help shuttle the electrons from inside the microbial cell to the anode. However mediators can be expensive, non-sustainable and some may inhibit microbial action. But certain bacterial species can produce their own mediators and transfer electrons directly to the electrode. This is a more efficient and sustainable method and the cell is called mediator-less MFC (Logan et al., 2006). The bacterial species capable of working in mediator-less MFCs include Shewanella putrefaciencs, Geobacter sulfurreducens (Reguera et al., 2006), Geobacter metallireducens and Rhodoferax ferrireducens (Min & Logan, 2004). Mixed cultures have shown even greater results and can be used on organic waste and marine sediments.
Source: Mercer, 2012
The anode can be any chemically stable, biocompatible and conductive material like stainless steel mesh. However, graphite rods, plates or felt cloth provide sufficient surface area and are relatively inexpensive as well as easy to handle (Logan et al., 2006).
The cathode can be potassium ferricyanide (Prabu et al., 2012) since ferricyanide is a strong electron acceptor and gets reduced to ferrocyanide (Min & Logan, 2004). At high concentrations it is 1.5 to 1.8 times more powerful than platinum catalyst cathode and oxygen in solution, although this may only be used for lab studies due to non-sustainability (Logan et al., 2006).
The factors affecting the power output of MFCs are the rate of bacterial metabolism, which depend on the species of microorganisms/ microbial culture, composition of substrates or metabolites present in solution, electrode potential determined by the biofilm on electrode surface (Zhao et al., 2009) and nature of semi-permeable membrane as also the surface area of cathode to that of anode and surface area of the membrane (Logan et al., 2006). Various researches are being conducted to arrive at an efficient MFC that can be put to immediate large scale commercial use.
The various kinds of substrates that are being investigated for greater electricity outputs are carbohydrates like glucose, starch, etc. volatile fatty acids like acetate, butyrate, etc., alcohols like ethanol and methanol, amino acids, proteins and inorganic compounds like sulfides, etc. With large scale MFCs that are used at wastewater treatment plants, breweries, paper recycling industries, etc. the output is ten times lower than that achievable with pure substrates. However, a combination of anodophylic cultures may produce higher outputs (Cheng, 2009). The possibility of energy production, along with waste treatment, is an added bonus and presents enormous prospects in the near future for a green fuel for nations to install MFCs in organic waste treatment plants.
Research on MFC devices
The MFC devices advanced so far are not very suited to high throughput screening to uncover high powered microorganisms. Therefore, a research team consisting of group of students (Hou et al.) have come up with an MFC array that has been microfabricated to identify and classify electrochemically active microorganisms. The reason for trying to find high electrochemically active microorganisms is that there is loss in the output of the MFC in case it has to be mediated with substances like theonine, methyl blue, neutral red, etc. (*) for electrochemically less active microbes.
The device consists of 24 anode and cathode compartments that work like mini MFCs and can be used to compare electrochemical activities of microorganisms. Shewanella oneidensis loaded array shows less than 8% difference in electrical energy generation profiles.
MFCs consist of anode and cathode chambers maintained in anaerobic conditions and these are separated by a proton exchange membrane but connected externally by a wire. The conducting wire also carries a load to run a device (Logan, 2008). "Electricigens" or electrochemically active bacteria are present in the anode chamber. The oxidation of the organic compounds results in the release of electrons at the MFC anode and flow to the cathode through the external circuit while protons diffuse through the PEM to join the electrons, now at the cathode chamber, to complete the circuit. The newness of this technology is that not only does it provide a self-sustaining clean fuel and a renewable source of energy, this technique can be used in bioremediation devices, in waste water treatment facilities, to power autonomous sensors for long term operations in remote areas, support civilian or combat operations in war torn areas, etc. However, commercial applications of MFCs are limited due to low power output.
Also the need to identify newer and more electrochemically active microorganisms for greater power output necessitated the development of a compact, micro-fabricated MFC array prototype that can compare and analyse the electricity generation capabilities of microorganisms in parallel. This device is less bulky than the conventional two bottle MFCs and can help speed up the research on electrochemically active microbes. The anode used was carbon cloth versus gold in traditional MFC devices both displaying similar open circuit voltage outputs. The array developed consisted of 24 integrated cathode and anode chambers functioning as 24 tiny MFCs. One species identified is Shewanella, which produces double the power than other strains in both conventional and microfabricated array designs.
According to another research paper submitted by Hale and Fong (2005) Ion Power Inc. and Penn State environmental engineers have been able to develop a new kind of MFC known as the BEAMR (BioElectrochemically-Assisted Microbial Reactor) capable of generating four times the amount of hydrogen as generally obtained by fermentation. The microbes can act on any domestic or industrial waste water with high biodegradable, organic content in it. In addition to reducing wastewater treatment costs, it can help in utilising the hydrogen to meet some energy requirements. The research paper states how by providing a small amount of electricity can help bacteria overcome the "fermentation barrier" and help them catalyse the fermentation end products further into hydrogen and carbon dioxide. The bacteria through the metabolic reactions transfer electrons from hydrogen atoms to the anode, while the protons diffuse into the solution. The electrons travel through the wire to the cathode and combine with the protons to release hydrogen gas. Since the voltage supplied to the bacteria is less than one-tenth of that required for electrolysis of water, it can prove to be a sustainable means of producing hydrogen gas to be used as hydrogen fuel for transportation and other applications.
Research on higher yields with changes in the environment (getting more high yielding substrates/ changes in the environmental surroundings)
Although much of the recent research is centred around increasing the power output density by increasing the surface area at the anode, however little work has been done to be able to use them at a large scale. Voltage output or power density is a function of concentration of the substrate was proved by using Monod-type equation on a Fortran program with Newton- Raphson non-linear equations.
The paper describes the three methods of carrying out the catalytic reactions (Zielke, 2006)-
1. using exogenous mediators like potassium ferricyanide, thionine or neutral red
2. using mediators produced by the microbes, or
3. by direct electron transfer from the enzymes like cytochromes to the electrode.
Some bacterial species like S. putrefaciens, Geobacter metallireducens, G. sulfurreducens and Rhodoferax ferriducens can work in mediator less MFC.
Although not much is yet known about the mechanisms of electron transfer from microbes to the anodes, Reguera & et al. (2006) have studied the significance of electrically conductive pilli in transfer of electrons to the biofilms. Extensive genetic studies have shown that the pilli form an electronic network through the biofilm and increase electricity production ten times. The pilli stack up the cells on the anode surface and hence, not just an increase in surface area of the anode but increase in the concentration of cells can contribute towards greater power density.
Discussion
The literature review enables us to get a better idea about the subject of bioremediation and how microbes can be engaged to clean up polluted resources as well as provide us with a clean fuel. The research being undertaken envisages developing better and cheaper microbial fuel cells that finds greater applicability in commercial setups. The designing of microarrays with electrodes of different components like graphite felt, platinum or gold enable the researchers to identify newer and more electrochemically effective strains for greater electricity production. The microarrays allow the use of different strains in parallel to compare and locate isolates or mixed communities of microbes.
Computer applications are used to derive the specifics of microbial fuel cells. Monod equations are applied (Zielke, 2006), to ascertain the correct size of MFCs according to the size of the plant they are to be used in. The first commercial application in Fosters' Brewery in Yatala, Queensland, Australia, sets the right example for commercial application of MFC. The brewery has 12 modules of MFCs to recycle the waste product (Rabaey & Keller, 2008). However, more recent developments have allowed beer brewing facility at home with capability of electricity generation (Wilson, 2007). The on-going research is now directed towards the kinds of locations MFCs can be installed in. Other applications of MFCs include simulation of colonic conditions and understanding ruminant activities and other biochemical functions in living organisms.
Advantages of fuel cells include higher efficiency rates achievable than combustion engines since the electricity is produced directly from chemical energy without conversion to mechanical or other forms of energy. They are long lasting and no undesirable products like NOx, SOx or particulate emissions are produced (O'Hayre et al., 2008). Also the possibility of producing fertilizers like ammonium nitrate, phosphate and sulphate using microbial cells in large farms with simple ingredients like wood chips and nitrogen from air can be economical and environmentally friendly (Pure Energy Systems, 2011).
The other advantages include scalability of microbial fuel cells and also the recyclability as compared to ordinary batteries that have to be thrown away or plugged in to recharge. However, there are a few disadvantages as well. These include heavy implementation or initial costs and low power density. Others are susceptibility to environmental poisons (O'Hayre et al., 2008). However, these can be overcome with regular check-ups and greater technological advancements. The pros of a sustainable, clean fuel far outweigh the cons and initial investments can easily be recovered by later savings on energy.
Ieropoulos, Greenman and Melhuish (2012) have even reported electricity generation from urine with the production of nitrogen, phosphorus and potassium without causing environmental pollution.
Conclusion
The review of literature on microbial fuel cells has provided a lot of information on bioremediation. Microbes can be used to recover resources that have been polluted due to man's excessive use of them in the last few centuries. However, what the microbial fuel cells have enabled us to do is clean up the environment as well as produce a much more important energy resource. Microbial fuel cells help in generating electricity and hydrogen gas, both of which can be used as an important fuel. The production of hydrogen gas takes a little bit of external electricity to overcome the fermentation barrier (Hale & Fong, 2005) whereas the water purification and desalination plants would require the cathode to be connected to oxygen supply for the protons to react with the electrons and oxygen to form water molecules.
Microbial fuel cells can not only prove to be a viable energy resource it can also be up-scaled to be fitted with breweries and cereal manufacturing companies to work on their waste products containing high organic content and cut down on costs by saving them a part of the cost of electricity. This has been applied to Foster's brewery in Queensland, Australia (Rabaey & Keller, 2008) which has been fitted with 12 three metres high microbial fuel cell modules. There are ways of installing MFCs at home (Wilson, 2007), and although, present day limitations do not allow for a lot of electricity to be produced, the on-going research in this field will bring results in the near future. This will help nations to produce electricity or hydrogen as well as recover the nature's resources using the metabolic activities of microscopic organisms.
