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Written by: Justin Mercer
Written on: May 4th, 2010
Tags: chemical engineering, energy & sustainability, recycling
Thumbnail by: Gilberto Esparza
About the Author
Justin Mercer is a junior majoring in Environmental Engineering. He is from Canyon Country, California, a small town just north of Los Angeles. In his spare time, Justin enjoys running and playing guitar. He is most interested in studying alternative energy production and limiting the impact of human actions on the environment.
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Volume XII Issue II > Microbial Fuel Cells: Generating Power from Waste
Microbial Fuel Cells (MFCs) use bacteria to convert organic waste material into electrical energy. This environmentally-friendly process produces electricity without the combustion of fossil fuels. MFCs have various practical applications such as in breweries, domestic wastewater treatment, desalination plants, hydrogen production, remote sensing, and pollution remediation, and they can be used as a remote power source. Widespread use of MFCs in these areas can take our waste products and transform them into energy.

Introduction

Washing one’s hands with soap is usually accompanied with the satisfaction of killing harmful germs. However, scientists in many research labs around the world seek to put those pesky germs to work generating electricity. Microbial Fuel Cells (MFCs) are an emerging technology that uses bacteria to generate electricity from waste. Bacteria in a Microbial Fuel Cell break down our food and bodily wastes, effectively generating power from the materials that are usually thrown away. By tapping into this previously unharnessed source of power, clean, sustainable energy can be produced at low cost.
MFCs are especially valuable in that there are many applications of their use help to reduce pollution and cut water treatment costs in a sustainable and environmentally-frie​ndly way. Currently, Microbial Fuel Cells are used to produce electricity while simultaneously cleaning wastewater. With future development, MFCs have the potential to produce hydrogen for fuel cells, desalinate sea water, and provide sustainable energy sources for remote areas. The Microbial Fuel Cell, which has historically been used only as a novelty in science fairs, is now a developing reality with great potential for improvements in cleaning techniques and power-generating processes.

What is a microbial fuel cell?

Bruce Logan/Penn State University
Figure 1: A simple microbial fuel cell.
Microbial fuel cells harness the power of bacteria and convert energy released in metabolic reactions into electrical energy. The actual cell consists of two electrodes separated by a semi-permeable membrane submersed in an electrolyte solution.
Fig. 1 depicts a typical MFC set-up in a research laboratory. The electrodes are connected by a wire and the anode (negative electrode) has bacteria growing on it. These bacteria break down food wastes and sewage to generate an electric current. Using microbes to generate electricity implies that the processes in an MFC are self-sustaining; the bacteria replicate and continue to produce power indefinitely as long as there is a food source to nourish the bacteria. Moreover, MFCs are very efficient, do not rely on fossil fuels for energy, and can run effectively on sources like food waste and sewage.

How Does a Microbial Fuel Cell Work?

As shown in Fig. 2, the Microbial Fuel Cell is divided into two halves: aerobic and anaerobic. The aerobic half has a positively charged electrode and is bubbled with oxygen, much like a fish tank. The anaerobic half does not have oxygen, allowing a negatively charged electrode to act as the electron receptor for the bacterial processes. The chambers are separated by a semi-permeable membrane to keep oxygen out of the anaerobic chamber while still allowing hydrogen ions (H+) pass through.
Mercer
Figure 2: A schematic of a microbial fuel cell.
1. The bacteria on the anode decompose organic matter and free H+ ions and electrons.
2. The electrons flow from the bacteria to the anode, sometimes assisted by a mediator molecule.
3. The electrons flow up from the anode, through a wire, and onto the cathode. While flowing through the wire, an electrical current is generated that can be used to perform work.
4. The H+ ions flow through the semi-permeable membrane to the cathode. This process is driven by the electro-chemical gradient resulting from the high concentration of H+ ions near the anode.
5. The electrons from the cathode combine with dissolved oxygen and the H+ ions to form pure H2O.
In the anaerobic chamber, a solution containing food for the bacteria is circulated. This food consists of glucose or acetate, compounds commonly found in food waste and sewage. The bacteria metabolize food by first breaking apart the food molecules into hydrogen ions, carbon dioxide, and electrons. As shown in Fig. 3, bacteria use the electrons to produce energy by way of the electron transport chain. The microbial fuel cell disrupts the electron transport chain using a mediator molecule to shuttle electrons to the anode. In many ways, a microbial fuel cell is an extension of the electron transport chain where the final step of the process (the combination of oxygen, electrons, and H+ to form water) is transferred outside of the bacterial cell from which energy can be harvested.
Mercer/Campbell and Reece/Pearson Education, Inc.
Figure 3: The electron transport chain.
1. The electron transport chain begins with NADH, a biological transport molecule, releasing a high energy electron (e-) and a proton (H+).
2. The electron follows the red path through the proteins (large blobs) in the mitochondrial membrane.
3. As the electron passes through each protein, it pumps hydrogen ions (H+) through the membrane.
4. In a normal bacterial cell, the electron continues along the dotted red path where it combines with oxygen to make water.
5. In a microbial fuel cell, the electron continues along the solid red path, where it is picked up by a mediator molecule and taken to the anode.

A Brief History of the Microbial Fuel Cell

The idea of obtaining energy from bacteria began in 1911 with M. C. Potter, a professor of botany at the University of Durham [1]. In his studies of how microorganisms degrade organic compounds, he discovered that electrical energy was also produced. Potter had the idea of trying to harvest this newfound source of energy for human use. He was able to construct a primitive microbial fuel cell, but not enough was known about the metabolism of bacteria for the design to be improved upon.
In fact, little development occurred on his primitive designs until the 1980s. M. J. Allen and H. Peter Bennetto from Kings College in London revolutionized the original microbial fuel cell design. Spurred by their desire to provide cheap and reliable power to third world countries, Allen and Bennetto combined advancements in the understanding of the electron transport chain and significant advancements in technology to produce the basic design that is still used in MFCs today. However, use of MFCs in third world countries is still in the pilot stages because of the complexities of simplifying the design enough to allow poor rural farmers to build them. The advancements by the Kings College team have shown the scientific community that the microbial fuel cell can be useful technology and generate increased interest in its development.
As scientists all over the world began researching the microbial fuel cell, one major question still remained: how do the electrons get from the electron transport chain to the anode? While researching this problem in the 1990s, B-H. Kim, a researcher from the Korean Institute of Science and Technology, discovered that certain species of bacteria were electrochemically active and didn’t require the use of a mediator molecule to transport electrons to the electrodes. Thus, a new type of microbial fuel cell was born that eliminated the use of the expensive and sometimes toxic mediators.
Currently, researchers are working to optimize electrode materials, types and combinations of bacteria, and electron transfer in microbial fuel cells. Even though the idea of harnessing the energy produced by bacteria has been around for almost 100 years, researchers have just begun to fully understand the MFC and how to bring out its true potential.