All kinds of biodegradable garbage — from sewage to leftover food — could yield valuable hydrogen fuel, an alternative to fossil fuels, with the aid of microbes cultivated in special reactors.

When hydrogen is burned, it yields just energy and water. That being an attractive sort of fuel, researchers globally are investigating ways to generate hydrogen en masse in hopes of replacing fossil fuels, the burning of which releases the global warming gas carbon dioxide.

Unfortunately, most of the hydrogen available today for use is actually generated from fossil fuels.

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Now environmental engineers at Pennsylvania State University are perfecting a way of generating hydrogen from biodegradable garbage — that is, organic matter from plants, animals and other organisms.

The idea, first announced in 2005 but improved upon in newer work, is to take liquid waste, such as effluent from sewers, breweries or food processing plants, and feed it to soil- or wastewater-derived bacteria raised in reactors designed to foster their growth.

These microbes then break down the organic matter, releasing hydrogen gas.

"We could use all sorts of wastewaters, turning them into hydrogen instead of using energy to treat the wastewater," Penn State researcher Bruce Logan told LiveScience.

These microbes do need a low voltage supplied by researchers to generate the hydrogen, which Logan and colleagues discovered in 2005.

Still, burning some of the hydrogen the bacteria produce can help generate the electricity the germs require to make the gas.

Back in 2005, the researchers envisioned the process largely as a way to cut down on the cost of dealing with sewage. Now, they say the reactors can prove significantly efficient as hydrogen producers.

For example, when given acetic acid — a common leftover of fermentation — the bacteria in the reactors generated hydrogen at up to nearly 99 percent of the theoretical maximum yield.

The reactors also worked when stuffed with cellulose, found in plants.

"This could really make a hydrogen economy work from renewable energy sources," Logan said.

Future research will focus on improving the rates of hydrogen production and lowering the cost of reactor materials.

"We hope to see pilot tests of this soon," Logan said. "We have been contacted by several companies, but so far no plans — yet — for a demonstration project."

Logan and his colleague Shaoan Cheng detailed their findings online Nov. 12 in the Proceedings of the National Academy of Sciences.

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