Scientists Generate Electricity from Sewage with 'Wired Microbes'

First Posted: Sep 17, 2013 12:26 PM EDT

When you think of electricity, you don't necessarily think of sewage. Yet they may have more in common than you might first think. Scientists have created a new way to generate electricity from sewage using naturally-occurring "wired microbes" that act as mini power plants.

Scientists have long known of the existence of exoelectrogenic microbes. These organisms evolved in airless environments and developed the ability to react with oxide minerals rather than breathe oxygen in order to convert organic nutrients into biological fuel. For years, researchers have tried to find ways to use these microbes as bio-generators, but making this an energy efficient method has proven to be difficult.

The new microbial battery that the researchers designed seems to overcome that issue, though. It's about the size of a D-cell battery and looks like a chemistry experiment with two electrodes plunged into a bottle of wastewater. Inside this murky vial, attached to the negative electrode, an unusual type of bacteria feast on particles of organic waste and produce electricity that is then captured by the battery's positive electrode.

"We call it fishing for electrons," said Craig Criddle, one of the researchers, in a news release.

More specifically, wired microbes cling to carbon filaments at the battery's negative electrode. As they ingest organic matter and convert it to biological fuel, their excess electrons flow into the carbon filaments and across to the positive electrode. Made of silver oxide, this positive electrode attracts electrons. The electrons gradually reduce the silver oxide to silver, storing the spare electrons in the process.

Currently, the microbial battery can extract about 30 percent of the potential energy locked in waste water. That's about as efficient as the best solar cells converting sunlight into electricity. That said, there's far less energy potential in waste water. Even so, the technique could potentially help offset some of the electricity that we use to treat water in sewage plants.

That's not to say that the battery is currently financially feasible. Silver is far too expensive to use on a large scale, which means that the researchers will have to find another way to achieve the same result. In the future, though, this method could help plants filter sewage by generating electricity.

The findings are published in the journal Proceedings of the National Academy of Sciences.

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