By using microbial fuel cells, researchers at Pennsylvania State University have found a way to not only harness energy from wastewater but to clean the wastewater as well. Considering that it costs $25 billion each year to treat about 33 billion gallons of domestic wastewater in the United States, the savings could be considerable.
How Microbial Fuel Cells Work
With the energy prices soaring, there has been a lot of talk recently about fuel cells. A typical fuel cell converts hydrogen and oxygen into water, producing electricity in the process. With a fuel cell, chemicals constantly flow into the cell so it never goes dead and the electricity flows out of the cell. Most fuel cells use hydrogen and oxygen as the chemicals. However, a microbial fuel cell uses the bacteria in wastewater to produce energy.
The idea of microbial fuel cells is not really a new one. Many researchers have dabbled with the design for years now. However what researchers at Penn State are doing is a first. The idea of producing energy and also treating wastewater is that of Bruce Logan, PhD., professor of environmental engineering at Penn State’s University Park campus. “Our contribution has been to put forth the technology as a treatment system and show that it worked,” says Logan. “Bacteria work in our system as they do in any system: They oxidize the organic matter, and they get energy from it and make new cells. The only thing different about our system is that when they’ve oxidized something and removed electrons, instead of sending them to oxygen, which is expensive to dissolve in water, they send those electrons to an electrode.”
In Logan’s model, bacteria are placed in the anode chamber of a fuel cell separated from oxygen. As the bacteria began to digest, they transfer electrons to an enzyme, which then transfers those electrons to an electrode. Because they do not have oxygen, they must transfer the electrons that they obtain from consumption (oxidation) of their food somewhere else other than oxygen; therefore, they transfer them to the electrode. In the microbial fuel cell, these electrons are transferred to the anode, while the counter-electrode (the cathode) is exposed to oxygen. At the cathode the electrons, oxygen, and protons combine to form only water.
Different Microbial Fuel Cell Designs
Whereas many microbial fuel cells use a two-chamber design the one Logan created is a single-chamber microbial fuel cell. The fuel cell consists of an acrylic cylinder with eight graphite anodes (or negative electrodes) inside, to which the bacteria attach, and a hollow central cathode (or positive electrode). Electrons flow along a circuit wired from the anode to the cathode.
Lars Angenent, assistant professor of chemical engineering at Washington University in St. Louis, MO, has designed an upflow microbial fuel cell that is fed continually and, unlike many microbial fuel cells, works with chambers atop each other rather than beside each other.
Angenent uses a carbon-based foam with a large pore size on which biofilm grows, making it possible to connect two electrodes in the anode and cathode chambers with a conductive wire. In a hydrogen fuel cell a membrane separates the anode and cathode chambers. When hydrogen meets the anode electrode, it splits into protons and electrons, with protons going across the membrane to the cathode chamber, and electrons passing over the wire between electrodes to create a current. Oxygen is added to the cathode chamber, and on the electrode there is a reaction of electron plus proton plus oxygen to form water. Catalysts, such as platinum, are needed on both electrodes to promote the reactions.
“We have proved we can generate electricity on a small scale. It will take time, but we believe the process has potential to be used for local electricity generation,” Angenent says in a Washington University press release. “The upflow microbial fuel cell is a promising wastewater treatment process and has, as a lab-scale unit, generated electricity and purified artificial wastewater simultaneously for more than five months.”
Potential Energy Production
Logan reports that with his microbial fuel design he has managed to produce about 500 milliwatts per square meter, or about half a watt per square meter. “Our goal is using wastewater to produce about a watt per square meter, because that’s equivalent to the kind of substrate removal rates that you can get with a trickling filter,” Logan says. “And if we can get that as electron flux instead of oxygen flux then we think we will have a very workable system.”
With his research, David Bagley, associate professor of the department of civil engineering at the University of Toronto, has calculated that the energy potential in wastewater is nearly 10 times the cost to treat it. Logan finds this discovery very promising. “If we use the microbial fuel cell, we’d probably only use about half the energy we use now at the whole treatment plant. That’s like saying if we could just get one-twentieth of the energy out, we’d be breaking even,” Logan says.
Due to cost restraints, Logan has been unable to upscale the microbial fuel cell laboratory design to test it at a wastewater treatment plant. Though he hopes to do this in the future. “The next step is to test this design on a larger scale and to continue to work on increasing power,” Logan says. “We’d like to see demonstrations occurring within the next couple of years and then commercialization a few years after that.”
Nikki Stiles is a freelance writer based in the city of Fairmont, WV.
OW - September/October 2006 |
