Bacteria: Energy Source of the Future
With the price of oil skyrocketing and the world facing a serious energy crisis, alternative sources of energy are in high demand. In the wake of the global warming issue, renewable energy is even a more desirable power source. Microbial fuel cells use bacteria to generate electricity and clean wastewater at the same time. Although microbial fuel cells did not receive much attention because the amount of energy they generate is too low to be of use, improvements over the years make this potential energy source a valuable research topic in the field of biotechnology.
What is a Microbial Fuel Cell (MFC)?
Microbial fuel cells, or biological fuel cells, use bacteria to convert chemical energy in biodegradable materials such as wastewater pollutants into electricity. The bacteria consume the pollutants, releasing electrons which flow through a circuit and generate electricity. In this process of power generation, pollutants are broken down, and clean water is produced.
How does it work?
A MFC follows the basic principle of a fuel cell. It consists of battery-like terminals: an anode and a cathode electrode. An external circuit connects the two electrodes, and an electrolyte solution helps conduct electricity. The anode and cathode are separated by a proton or cation exchange membrane. There is a difference in voltage between the two electrodes. And when electrons flow through the circuit, electric power is generated.
In a MFC, bacteria live at the anode and convert organic substrates in wastewater into CO2, protons and electrons. Under aerobic conditions, oxygen or nitrate act as terminal electron receptors to produce water. However, oxygen is not available in a MFC, so the bacteria need an insoluble receptor like the anode. The electrons are transferred by membrane-associated components, electron shuttles, or nano-wires and then flow through the external circuit to the cathode, generating electricity. The protons flow across the proton exchange membrane to reach the cathode. At the cathode, oxygen is reduced as the electron receptor to produce water.
Rabaey, K., and W. Verstraete. (2005) Microbial fuel cells: novel biotechnology for energy generation. Trends in Biotechnology 23: 291-298
Different Types of MFCs
- Research Type MFCs – used to study specific microbial processes or new materials used. It is easy to construct but not suitable or long-term operation. The electrochemical design is not optimal for maximum energy output.
- Continuous MFCs – designed for long-term MFC operation. Substrates (glucose, sucrose, acetate, wastewater) are continuously fed into the MFC to generate electricity. It can be used to study the microbial community or the electrochemical parameters. It is designed to generate high energy output.
Applications of MFCs
- serve as a portable energy system that can also generate reusable water for developing countries or remote areas
- save energy and thus operation cost when incorporated into water and wastewater treatment facilities (5% of the electricity in the U.S. is used to water pumps or treatment plants).
- used to measure biochemical oxygen demand of the wastewater in real-time, as the electricity generated is proportional to the concentration of organic substrates in the water
Pros & Cons.
- cleans up pollutants while producing electricity
- can handle a variety of water-based organic fuels, unlike traditional fuel cells, which rely on hydrogen gas as the fuel source
- small-scale MFCs are better than traditional battery because it generate renewable energy, does not require to be recharged, and operate at neutral pH
- The energy output is not enough to solve the energy crisis (a cubic meter of bioreactor only generates 1,010 watts of electricity, enough to power 16 60-watt light bulbs).2
- The potential of microbial metabolism can not be fully utilized due to limitations such as internal resisntance, slow proton transfer through the aqueous phase, oxygen reduction efficiency rate at the cathode, etc.
- Most materials used for the mediator that transfers the electrons from the bacterial cells to the anode are toxic.
Researches of MFC in Progress
Making modification to MFCs can significantly improve the amount of hydrogen produced and utilize a wide range of pollutants, no longer limited to carbohydrate-based biomass. The scientists at Pennsylvania State University is developing a new type of MFC called BioElectrochemically-Assisted Microbial Reactor (BEAMR). By using 10% of the voltage generated to give the bacteria a “boost”, dead-end fermentation products such as acetate can also be turned into hydrogen. This quadruples the amount of hydrogen production and cleans up industrial, agricultural and human wastewater in the process.1 Scientists at Oregon State University seeks to increase the power output of MFC by adding a cloth layer between the anode and cathode, reducing the internal resistance to improve the power density.
Arizona State University (2008, January 7). Fuel Cell That Uses Bacteria To Generate Electricity. ScienceDaily. Retrieved March 19, 2008, from http://www.sciencedaily.com /releases/2008/01/080103101137.htm
Kim B.H., I. S. Chang and G. M. Gadd (2007) Challenges in microbial fuel cell development and operation. Appl. Microbiol. Biotechnol. 76, 485-494
Logan, B.E. and J.M. Regan. 2006. Feature Article: Microbial fuel cells-challenges and applications. Environ. Sci. Technol. 40(17):5172-5180.
Oregon State University (2007, August 29). New Microbial Fuel Cell Design Boosts Electricity Production. ScienceDaily. Retrieved March 19, 2008, from http://www.sciencedaily.com /releases/2007/08/070823155306.htm