A fuel cell that runs on wastewater
What's even better than a fuel cell that runs on hydrogen? How about a fuel cell that runs on wastewater? Bruce Logan has not only dreamed of such a device. He's built one, and shown that it works.
For Logan, the Kappe professor of environmental engineering at Penn State, the idea came naturally. "All living things oxidize organic matter," he explains. "They eat stuff and their bodies burn it to make energy." It's the same as what happens in a conventional fuel cell, when incoming hydrogen is split into protons and electrons.
Logan and Liu
To make a microbial fuel cell, Logan continues, "you take bacteria, give it food but no oxygen, and add two conductive electrodes, an anode and a cathode. The bacteria oxidize the organic material, and transfer electrons to the anode," or negative electrode. Then the electrons flow from the anode through a wire to the cathode, "and you have current," he says. On the cathode side, the electrons recombine with the protons and with oxygen to form water.
Other researchers have shown that microbial fuel cells can produce electricity from organic material, he adds, but always using glucose, or other high-energy carbodydrates. "We showed that you can do it with any biodegradable material." Even the wastewater that's flushed down drains and toilets.
Logan's single-chambered cell, a Plexiglass cylinder the size of a soda can, houses eight graphite rods which function as anodes, providing plenty of surface area for bacteria to attach. In the center is the cathode, protected by a hollow plastic tube whose ends are open to the air.
In a trial run early last year, domestic wastewater skimmed from the settling pond of a local sewage treatment plant was pumped into the cell. Bacteria already present in the water, feeding on the organic matter also present, produced a tiny but measurable amount of electricity: from 10 to 50 milliwatts per square meter of electrode surface. Subsequent modifications, including removing the platinum catalyst on the anode—"If you have bacteria, you don't need platinum there," Logan explains—have both made the device much cheaper and boosted the power of newer devices to 494 milliwatts, enough to power a small fan. Logan's goal is 1,000 milliwatts per square meter using wastewater, a goal they have already exceeded using sugar.
The real beauty of the MFC, however, is that while it produces electricity, it also cleans the wastewater it uses. As Logan explains it, the air flow into the cathode tube causes oxidation, providing up to 80 percent of the cleaning effect normally accomplished by aeration. Since four to five percent of all U.S. electricity is used to treat wastewater, the potential savings are huge.
Last month, Logan and collaborators at Penn State and Ion Power, Inc. unveiled still another twist. By applying a small electrical current to boost the action of the bacteria in a microbial fuel cell, they reconfigured the cell to produce not electricity from wastewater, but hydrogen. Significantly, the rate of production was four times higher than that achievable by standard fermentation.
"We can theoretically use our MFC to obtain high yields of hydrogen from any biodegradable, dissolved, organic matter—human, agricultural, or industrial wastewater—and simultaneously clean the wastewater," Logan said.
"This has implications not just for the hydrogen economy," he adds. "One billion people in the world lack sanitation. Even if somebody built them a wastewater treatment plant, they couldn't afford to keep it running. But if a wastewater treatment plant can be a power plant—that's a potential paradigm shift."
