Microbial Fuel Cell
In the contemporary world, germs or bacteria are strongly repelled at all costs. Children are advised to wash their hands with disinfectants and housekeepers advised to disinfect every item in the home to keep bacteria away. Industries also take caution of germs and are required to adhere to ISO standards of cleanliness. This means bacteria are never welcomed anywhere and is a wish for many that they cease to exist. However, bacteria are useful in a number of ways, and one of them is being harnessed to produce electricity. The production of electricity is effected through microbial fuel cell (MFC).
MFC is a budding technology with a promising future. It is effected through the use of bacteria from waste products to break down the waste products to generate power. The power realized from MFCs is clean and sustainable, and can be produced at a low cost. MFCs have other advantages, whereby apart from generating electric power, they assist in the reduction of pollution, as well aiding in water treatment procedures (Davis & Higson, 2007). Currently, MFCs are harnessed for water treatment and production of electric power concurrently. This paper will explore this technology in details.
Meaning of Microbial Fuel Cell
Microbial fuel cells utilize the power of bacteria, whereby the technology involves converting the energy from metabolic reaction into electric energy. The cell consists of two electrodes divided by a semi-permeable membrane. The electrodes are submerged in an electrolyte maintained at a certain level. A typical setup MFC used in the laboratory for experiments is shown in the diagram below:
Figure 1: A typical setup MFC (Logan, 2008, p. 2)
The two electrodes are connected using a wire and the negative electrode or anode has some bacteria growing on it. The bacteria at the anode break down waste products from food to yield electricity. The process that takes place in an MFC cell is self-sustaining because bacteria are the main agent of the process. As long as food for nourishing the bacteria on the anode are available, they will continue to replicate, and this implies that the system is self-sustaining and simple to maintain. At the same time, MFCs are very effective because they do not require specialized fuels. Simple food and sewage waste products are used as a source of fuel. Mostly a catalyst is applied to speed the reaction. Based on the mode of working, an MFC is, therefore, a system that facilitates the conversion of chemical energy to electric energy in the presence of a catalyst (Davis & Higson, 2007).
How a Microbial Fuel Cell Works
To comprehend how an MFC system works, the diagram in figure 2 will be used, as shown below:
Figure 2: how MFC system works (Logan, 2008, p. 4)
From the diagram above, the MFC is divided into aerobic and anaerobic chamber separated by a semi-permeable membrane. The aerobic chamber consists of a positive electrode, and has oxygen gas bubbled through it; in the same way a fish tank works. The other aerobic section of the chamber has no oxygen and allows the negative electrode to be an electron receiver for bacterial process. The semi permeable membrane prevents oxygen from reaching the anaerobic chamber, but allows hydrogen ions to pass through (Logan, 2008).
To understand how the system works, the notations in diagram in figure 2 will be used to explain the working principle of the system. The notations 1, 2, 3, 4, and 5 illustrate each process that occurs in the sequence during the working process of the battery. Each step is defined below:
- Anode bacteria breakdown organic matter to release electron and hydrogen ions
- Electrons migrate from bacteria to the anode assisted by a mediator molecule.
- The electrons flow to the cathode from the anode through the wire. During the process of flow of electrons, electricity is generated. The generated electricity can be used to perform work, and thus, it is harnessed to the load.
- The hydrogen ions from the anode flow through a semi-permeable membrane to the cathode. This process is facilitated by electrochemical gradient, whereby the high concentration of hydrogen ions near the anode naturally flows to the cathode region where there is a low concentration.
- The electrons released from the cathode link with dissolved oxygen and hydrogen ions to form pure water (Logan, 2008).
In the anaerobic compartment, bacteria food in solution is circulated. Such food consists of compounds found in sewage and food waste, including glucose and acetate. The bacteria’s main role is to breakdown the food into useful products that are used in the production of electric power. In the first place, the part of the food molecule is broken down into carbon dioxide, hydrogen ions, and electrons (Logan, 2008). The procedure is displayed in the figure below:
Figure 3: The electron transport chain (Mercer, 2014, p. 25)
From the diagram, it can be seen that bacteria utilizes electrons to produce energy through electron transport chain. The electron transport chain is disrupted by a mediator molecule as a way of shuttling electrons to the anode. This means MFCs are an extension of a chain of electron transport, whereby the final step takes place outside the cell of the bacteria to facilitate harvesting of energy. This final step involves the formation of water from the combination of electrons, oxygen, and hydrogen ions (Mercer, 2014).
The electron transport chain shown in figure 3 can be explained using the notations 1, 2, 3, 4, and 5 as shown on the diagram. Each notation is explained below:
- The electron transport chain process commences with a biological transport molecule labeled NADH on the diagram. NADH releases an electron (e-) from a high energy level. It also releases a proton inform of hydrogen ion (H+).
- During the second stage, the electrons flow through a path denoted by a red line on the diagram. The red path passes through the large blobs (colored black in the diagram) in the membrane of the mitochondria.
- During the process of electron flow through each blob, hydrogen ions are pumped through the membrane.
- In a typical cell of bacteria, the electron proceeds along the red dotted line in the figure above and combines with oxygen to make water, but this is not the case in a microbial cell.
- In MFCs, the electron proceeds along the solid red path indicated in the diagram in figure 3, whereby it is picked by a mediator molecule and transported to the anode. This completes the process (Davis & Higson, 2007).
The Chemical Reaction
The reaction producing electricity in MFCs can be represented by an equation. First, it ought to be noted that microorganism produces water and carbon dioxide when they consume sugar in the presence of oxygen. However, in the absence of oxygen, the microorganism produces electrons, protons, and carbon dioxide. The process can be represented by the equation given below:
C12H22O11 + 13H2O -> 12CO2 + 48H+ + 48e–
From the reaction above, it can be seen that the metabolic reaction process produces free electrons and hydrogen ions. The hydrogen ions created are united with oxygen to form water. The free electrons produced, on the other hand, are responsible for the producing of electricity in the cell. Microbial Fuel Cell utilizes mediators, explained in the previous section, to redirect the electrons produced into the conductor (Mercer, 2014).
History of a Microbial Fuel Cell
Professor M. C. Potter from the University of Durham is credited with the MFC concepts. As a botany professor, Potter was obsessed with the idea of obtaining electric energy from bacteria in 1911. Potter discovered that the electric energy was produced when organic matter decomposed and thus sort to investigate the phenomena in details. Potter, therefore, looked for ways of harnessing this energy for commercial use. In addition, little was recognized about the metabolic process of bacterial. Therefore, Potter came up with a primitive MFC that was later improved (Davis & Higson, 2007).
Since 1911, when MNC was first discovered, there was little improvement in the design for harnessing the energy for the commercial process until 1980. In 1980, Peter Bennetto and MJ Allen from Kings College in London improved the original MFC design by Potter. Bennetto and Allen were motivated by the quest to provide an inexpensive source of power to the developing nations. Therefore, they incorporated new development in the electron transport chain to understand the process. Also, they depended on the progression in technology to improve the physical design of the Microbial Fuel Cell, and the design they produced is still in the market today (Logan, 2008).
Although the advancement in MFCs was meant to improve developing countries, the technology is still in its pilot state in most countries. The main problem has been ways of simplifying the design to enable the rural population to use it. Nevertheless, there is an amplified interest in the growth of the technology among scientists (Mercer, 2014).
When scientists started working on the microbial fuel cell, one main issue that occupied their minds was how to transfer electrons from the electron transfer chain to the anode. When focusing on this issue, B-H Kim, a scientist from the Institution of Technology and Science in Korea realized that some bacteria species were active electrochemically. Such bacteria did not require one to use mediator molecule to facilitate the transportation of electrons to the electrodes. As a result, a new Microbial Fuel Cell was discovered. This MFC was cheap and eliminated the use of toxic and expensive mediators (Logan, 2008).
The current efforts in microbial fuel cells design and development is focused on optimizing electrode materials, bacteria combination, and the transfer of electrons in the cell. Although this technology was discovered 100 years ago, effort to utilize it on a commercial scale only started in 2000s. At the same time, the full understating of the process involved in the technology is a 21st century innovation (Logan, 2008).
Types of Microbial Fuel Cell
Based on the historical account of MFCs and other the working principle of the technology, it can be seen that MFCs are divided into two categories; mediator-less and mediator microbial fuel cells.
Mediator Microbial Fuel Cell
In this type of microbial fuel cell, the microbial cells used are inactive electrochemically. As a result, mediators are used for the electron transfer. Common mediators used on the market today include humic acid, methyl viologen, thoinine, and neutral red among others. These mediators are toxic and expensive (Logan, 2008).
Mediator-free MFCs use bacteria that are active in terms of the electrochemical transfer. This means electrons are carried directly to the electrode from the respiratory enzyme of bacteria. There are various bacteria species with electrochemical qualities, and they include Aeromonas hydrophila and Shewanella Putrefaciens among others. Studies into mediator-less MFCs are new and still ongoing (Cai, Zheng, Qaisar, & Xing, 2014). Mediator-less MFCs have some limitations. Their optimal operations depend on a number of factors, and the first one is the condition of the system. Conditions, such as high temperature, varying pH and the concentration level of the electrolyte affects the way in which bacteria works. Another way through which mediator-free MFCs are affected is through the strain of bacteria is used. Little information is available on how bacteria can be enhanced to reduce the strain during the operation of the cell. At the same time, the type of membrane used is also an issue because a number of factors, which have not been realized, affect ion exchange through the membrane (Mercer, 2014).
Besides operating on wastewater, mediator-less MFCs can derive energy from certain plants. A system called plant MFC is used in this process. A sample system setup for a plant MFC is shown below:
Figure 4: plant MFC (Logan, 2008, p. 4)
A number of species of plants are used for operating this system, and they include tomatoes, algae, reed sweet grass, rice, and cord grass among others. The energy production in this case is in situ-energy, whereby the production of energy is done onsite. This means the process is used to the advantage of the environment (Logan, 2008).
Another type of mediator-less microbial fuel cell is the MEC or the microbial electrolysis cells. MECs works by reversing the process of MFCs, whereby it results in the production of methane rather than water. In this case, carbon dioxide is reduced by bacteria with the aid of electric energy to form compounds of carbon (Logan, 2008).
Application of Microbial Fuel Cell
Microbial Fuel Cell technology is applied in a number of cases as discussed below:
MFCs are used for harnessing electric power in various situations. However, the energy is produced on a low scale. The energy is also applied in a situation, whereby it is expensive to replace batteries in use. For instance, in wireless sensor networks, MFC is applied because it is effective and easy to maintain. MFCs are applied in various situations to replace battery power because they do not need one to recharge them, but are rather self-sustaining (Davis, & Higson, 2007).
Microbial fuel cells that are based on plants are vital education tools. In the first place, their construction process is facilitated through a number of considerations drawn from various disciplines. Such disciplines include geochemistry, microbiology, and electric engineering among others. They are preferred because they can be constructed from the common or local material from the laboratories. Various kits have been designed for classroom use, as well as for corporate use. Most research tools in microbiology employ this technology (Davis & Higson, 2007).
Municipal wastewater is a common nuisance to the municipal governments because they must be treated and released in water. Microbial fuel cells are essential in this process because it involves waste digestion. Microbial fuel cells operate on the principle that is harnessed to measure the concentration of wastewater before being released to the water. As a biosensor, microbial fuel cells are utilized based on the principle that the current generated by the cell is directly proportional to the energy content of the wastewater. In this case, is it possible to ascertain the concentration of the solute in wastewater? The concentration of municipal wastewater is ascertained through the biochemical oxygen demand or BOD (Logan, 2008). A BOD sensor utilizing Microbial Fuel Cell technology is efficient and fast that conventional BOD sensors. According to a study by Zhang, Qiao, Miao, Yang, Li Xu, and Ying (2014), conventional BOD sensors take about five days to ascertain the BOD of waste waster compared to Microbial Fuel Cell BOD sensors, which takes less than five days.
According to a study by Zejie, Taekwon, Bongsu, Chansoo, and Joonhong (2014), wastewater in the sewage plant can be biologically treated using Microbial Fuel Cell. Studies into microbial fuel cells indicate that the system can digest the amount of organic components in the sewage for up to about 80%. The wastewater is cleaned to remove some toxins, and thereafter, taken through the bioreactor for microbial fuel cells treatment process. During the process of treatment, the waste is converted to electric energy and water. The electricity created is used to counterbalance the high cost of treating water and thus the process is made cheaper and justified.
Treatment of Brewery Wastewater
Wastewaters from brewing plants are rich in organic compounds, and thus, it is easy for one to treat it using MFCs. Wastewater from the brewery has constant substrate concentration and thus it is easy use MFCs to clean it. In the setup, all the wastewater is channeled to the chamber containing the system of microbial fuel cells. Electricity is generated from the process, and this ensures that the plant gains twice. In the first place, the plant obtains power to use in the plant. Thereafter, the company also ensures that its wastewater is cleaned for easy disposal. Australia is among the leading nations that have applied microbial fuel cell technology to treat brewery wastewater (Wu et al., 2013).
Production of Hydrogen
Microbial Fuel Cells technology can be applied in the creation of hydrogen that in turn can be used as fuel. However, the process is supported by external source of power for it to be effective. The external power source is used for converting the organic materials into hydrogen gas and carbon dioxide. The system used for producing hydrogen gas has two chambers just like the microbial fuel cell system. However, both chambers are made anaerobic, and enhanced by electric power of about 0.25 volts. According to a study by Li, Cheng, and Wong (2013), it was discovered that more than 90% of electrons and protons generated by bacteria at the anode are converted to hydrogen gas. The conventional methods used to produce hydrogen gas require electricity that is ten times higher than the energy used by microbial fuel cells for generating electricity.
Waste products, especially wastewater from breweries and municipal plants can be a nuisance to the environment. However, this paper has established that such waste products are useful because they are harnessed for producing electricity. Microbial fuel cells are an essential innovation because they have opened up opportunities for researchers in various disciplines. MFCs are simple to construct and are not expensive. MFCs have other advantages, whereby apart from generating electric power, they assist in the reduction of pollution, as well aiding in water treatment procedures.
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