Microbial Fuel CellsMar 13, 2014
The use of microbes by humans dates back thousands of years to the production of fermented beverages and foods, but it is only recently in the last century, and primarily in the last 30 years, that the concept of Microbial Fuel Cells (MFCs) has been developed due to the rise in demand of new technologies to replace the use of fossil fuels . It is of particular interest due to it being carbon-neutral and producing electricity while contributing to the reduction of organic waste. Therefore, one of the primary applications being suggested is utilizing it in wastewater treatment. Unfortunately, there are currently energy output bottlenecks for a large-scale commercial application, and more research is required to decrease cost and increase efficiency before it is a viable replacement to current renewable energy methods.
MFCs are not to be confused with methanogenic anaerobic digesters, which use chemical processes with bacteria to produce methane gas that is then later burned or, more efficiently, processed to extract hydrogen for fuel cells. Instead, they use special types of bacteria to do a direct conversion to electricity or hydrogen from organic matter. The way it works is through a simple two-step process. Think of how a battery works, with chemical processes moving electrons from one end (anode -) to the other end of the battery (cathode +). First, the bacteria remove electrons from the organic matter, a process called oxidation. Then, the bacteria transfer these electrons to a molecule that accepts them, usually oxygen or a nitrate, this process is called reduction. Once these molecules are electrically charged, the electrons move through a wire to the cathode to combine with protons and oxygen where water is the end-product. The flow of these electrons from the anode to the cathode is what generates the current and voltage that generates electricity. There are currently a few notable examples of recent advances in this technology:
Ecobot I IV: The Bristol Robotics Laboratory, a collaboration between the University of the West of England and the University of Bristol, has produced multiple iterations of a self-sustainable robot that utilizes different types of natural raw materials to function. The goal of the project is to mimic real digestion and respiration using multiple microbial fuel cells, 24 at the moment, and for the robot to collect its energy through nutrients from the environment and getting rid of waste. 
Cambrian Innovations EcoVolt: This Company uses microbial fuel cells in a process called electromethanogenesis, which converts the CO2 and electricity generated from the microbes consumption of organic matter into near-pipeline-quality methane gas and water, which provides companies that have a lot of organic waste, mainly in the food and beverage industry, with sustainable energy and waste management through a combined heat and power system. The Bear Republic Brewery, a company that has installed one of its reactors, believes it will be able to eliminate 80 percent to 90 percent of the [organic material] of its wastewater with the EcoVolt and reuse 10 percent of its water it can cover 50 percent of its electricity needs. . This company has also received $8 million in in DoD and National Science Foundation grants to produce a self-powered wastewater treatment facility for off-grid locations and developing nations.
In-water sensors: The Naval Research Laboratory has produced a prototype sensor for marine research that uses Benthic Microbial Fuel Cells to function. This removes the need for maintenance of the sensors and battery replacement since they are energy self-sufficient from the organic matter in sediments of lakes and oceans. It is limited to 1-watt time average power. 
MFCs for on-site filtration of water: Researchers at Penn State and King Abdullah University of Saudi Arabia developed a prototype hybrid MFC Membrane Bioreactor for wastewater treatment and immediate consumption of water, achieving high levels of filtration with a low-energy input. This could be useful for developing areas without a wastewater treatment infrastructure.
Aside from these projects, laboratories in MIT, Penn State, and the Georgia Institute of Technology, among others, are researching this technology and new developments are made every year. Some of the challenges they are facing are the low power output, the limited types of biomass that the microbes or enzymes can consume, and the fact that the system can be interrupted by impurities and other factors. Even so, just recently the Georgia Institute of Technology has developed a hybrid Solar-MFC that deals with these challenges by introducing a catalyst that reacts to light or heat. Since this new procedure is chemically stable, it can handle multiple types of organic compounds and is not interrupted by impurities. They report a power density of up to 0.72 milliwatts per square centimeter, on par with the best current MFCs, and believed to be increased five to ten times with optimization. 
The future of this technology seems to be very promising, and one can only imagine of the potential applications if this technology is improved upon. Some of the proposed future uses are MFCs implanted in the human body to power medical devices with nutrients supplied by the human body (after health/safety concerns are dealt with, of course) and a centralized waste reactor in a home that recycles human waste into electricity, potable water, and fertilizer. This ability to make complete use of what was once considered simply waste, could eventually even be used in manned spaceships, according to a NASA-funded research team led by Dr. Bruce Rittmann from Northwestern University. 
 Zhuwei Du, Haoran Li, Tingyue Gu. A state of the art review on microbial fuel cells: A promising
technology for wastewater treatment and bioenergy. Web. 23 May 2007.
 Bristol Robotics Laboratory. Ecobot Project Overview. Web. 14 June 2013. http://www.brl.ac.uk/researchthemes/bioenergyselfsustainable/ecobotprojectoverview.aspx
 Naval Research Laboratory. Continuous Sustainable Power Supply: Benthic Microbial Fuel Cell. Navy.mil. Web. 25 February 2014. http://www.nrl.navy.mil/techtransfer/available-technologies/energy/benthic-fuel-cell.
 Lamonca, Martin. Bio-Energy Box Coverts Beer Waste to Electricity. IEEE Spectrum. Web. 10 February, 2014. http://spectrum.ieee.org/energywise/green-tech/fuel-cells/bioenergy-box-coverts-beer-wastewater-to-electricity.
 Georgia Institute of Technology. Hybrid Fuel Cell Yields Electricity Straight from Biomass. Biomass Magazine. Web. February 20, 2014. http://www.laboratoryequipment.com/news/2014/02/hybrid-fuel-cell-yields-electricity-straight-biomass.
 Miller, Karen. Waste Not. NASA.gov. Web. 4 June 2004. http://www.nasa.gov/vision/earth/technologies/18may_wastenot.html.
Lilian Malaeb, Krishna P Katuri, Bruce E Logan, Husnul Maab, Suzana P Nunes, Pascal E Saikali. A Hybrid Microbial Fuel Cell Membrane Bioreactor with a Conductive Ultrafiltration Membrane Biocathode for Wastewater Treatment. Magazine: Environmental Science & Technology (Impact Factor: 5.26). 09/2013; DOI:10.1021/es4030113 Source: PubMed