Poop Power

July 24, 2008 -- Researchers have identified a climate-energy win-win, but it may put them in deep doo-doo.

That's what happens when you study poop for a living. Michael Webber and Amanda Cuéllar of the University of Texas at Austin estimate that digesting all of the nation's livestock manure to produce methane to burn for energy could supply more than 2 percent of the country's electricity needs.

Meanwhile, the process would avoid the greenhouse gas emissions created by burning the equivalent amount of coal, and it would prevent the release of the super-potent greenhouse gases methane and nitrous oxide released by normal manure degradation, allowing for a double-whammy of greenhouse gas reduction.

The combined savings could reduce U.S. greenhouse gas emissions from electricity by about 4 percent.

"We wanted to look at what would happen if we took all the manure in the nation, which is currently an environmental liability, and turned it into a commodity as a source of energy," Webber said. The pair's work is published today in Environmental Research Letters.

Livestock in the United States unload more than a billion tons of manure each year, most of which ends up in lagoons or other outdoor locations where it decomposes, emitting methane and nitrous oxide, greenhouse gases 21 and 310 times more potent at warming than CO2, respectively.

The proposed approach would send the manure to anaerobic digesters where microorganisms would produce biogas rich in methane that could be captured and burned for energy, releasing the less-potent greenhouse gas CO2. The remaining solids could be used for fertilizer.

This approach would reduce other problems associated with manure ponds, including odor, air pollution, and water pollution from runoff or groundwater contamination, Webber said.

Digesters exist in the United States, and many more are used in Europe. But this is the first time anyone has studied the total potential of biogas production in the United States, he added. "The numbers are big enough that they're worth paying attention to."

"This isn't new technology. It's not hard to implement," he added. "It's decades-old technology that's ready to go right now. We just haven't done it. We don't have incentives."

But now, the rising cost of energy and increasing drive to reduce greenhouse gases and generate renewable energy make this a better time than ever to consider this approach, Webber said.

"There are a lot of biofeedstocks that are receiving a lot of criticism," Webber told Discovery News. "This one seems controversy-free. It's a waste stream right now. It doesn't fall into the trap of competing with food."

"They paint a very rosy picture of biogas potential by considering all the animals in the country," said Christopher Weber of Carnegie Mellon University in Pittsburgh, Pa., who has studied greenhouse gas emissions from livestock.

Webber agrees that not all animals are kept in conditions where it would be easy to collect the manure. Large feedlots would be the best starting point. But his analysis provides an upper limit for what might be possible, he said.

Meat, especially red meat, has received recent attention as a less "green" diet choice than vegetables or fish because of the greenhouse gas emissions associated with its production, including from manure. Reducing the greenhouse gas emissions from manure improves the calculation, but it does not even the playing field, Weber said.

"It would do nothing about the carbon dioxide and nitrous oxide in the production of the grain to feed the animals, which is another large chunk of the greenhouse gases associated with red meat production," he said. Cows would continue to burp methane as part of their digestion, which is the largest source of methane in beef production.

"All in all, for grain-fed beef, I would think manure is responsible for a total of 20 percent of the life cycle emissions or so; a good start but certainly not enough to make beef on par with vegetables or fish in greenhouse gas production."

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Related Links:

How Stuff Works: Why do cows produce methane?

Info on Biogas

Planet Green

Discovery Earth Live