Researchers at Penn State have developed a microbial fuel cell that generates power by cleaning domestic wastewater:
The single-chambered microbial fuel cell is essentially a Plexiglass cylinder about the size of a soda bottle. Inside are eight graphite anodes (or negative electrodes), upon which the bacteria attach, and a hollow central cathode (or positive electrode). Electrons flow along a circuit wired from the anode to the cathode.A steady flow of wastewater pumped into the chamber feeds the bacteria. Bacterial digestion of the wastewater's organic matter unleashes electrons into the electrical circuit and positively charged hydrogen ions into the solution. Those ions reduce the solution's oxygen demand, a key goal of wastewater treatment. The hydrogen ions also pass through a proton-exchange membrane to reach the cathode. Meanwhile, a hollow tube within the cylinder contains the cathode, which is exposed to air. At the cathode, oxygen from the air, hydrogen ions coming through the membrane and the electrons coming down the circuit combine to create water.
In other microbial fuel cells, microbes have been fed glucose, ethanol and other fuels, but, according to Bruce Logan, the Penn State professor of environmental engineering who leads the project, "Nobody has ever tried this with domestic wastewater. We're using something thought to be completely useless."
The single-chamber design is important, he said, because it facilitates a "continuous flow-through system," a design consistent with existing treatment systems.
This is still in its early stages. The microbial fuel cell doesn't generate much power -- between 10 and 50 milliWatts -- but with time and further study, output could well be boosted to usable levels. It would be of particular value in the developing world, where water treatment can be too expensive for wide use. Wastewater treatment facilities that actually generate power while cleaning the water would be enormously attractive.
