"A dirty little secret in America is that livestock agriculture is a source of a lot of the emission problems that we are facing," Michael Webber of the University of Texas at Austin, US, told environmentalresearchweb. "It has always impressed me just how much manure we have – about one billion tons a year."

Webber and colleague Amanda Cuéllar believe they have completed the first study of the impact this approach could have on US carbon emissions.

"This is not a new or very sophisticated concept – it’s already in place in a lot of locations in Germany – but it’s not widespread in its use in the US and as far as we can tell, no-one’s ever done the analysis to figure out if it’s even worthwhile to pursue in terms of its scale," said Webber.

Most US manure is disposed of in lagoons or left outside to decompose, a process which releases methane and nitrous oxides. These two greenhouse gases have a global warming potential that is 21 times and 310 times that of carbon dioxide, respectively. Webber and Cuéllar propose the use of anaerobic digestion – a well-tested process – to convert the manure to biogas that is 60-70% methane, and an odourless sludge that is suitable for use as a fertiliser.

The technique could produce nearly one quadrillion BTU of renewable energy each year – around 1% of US total energy consumption. Of that quadrillion BTU, about 700 trillion would come from cattle dung, 125 trillion from swine and 125 trillion from poultry manure.

The biogas could be used in cooking or home heating, or if scrubbed to remove corrosive hydrogen sulphide, could be converted to compressed natural gas. But the researchers believe the biogas shows greatest potential for cutting greenhouse gas emissions as a substitute for coal in electricity generation.

According to Webber, the barriers to widespread introduction of the system are economic, cultural, educational and political, rather than technical. Farmers are not generally interested in getting into the power plant business and there are upfront costs to build or buy an anaerobic digester.

Another issue is that "not all the manure is in one spot". So the technique is likely to take off first in locations such as feed lots, concentrated animal operations or pig farms, where manure is already handled in an intensive way, rather than for cattle out to pasture where the manure is lying in the fields and is hard to collect.

"It’s hard to truck the manure around, so you’d probably have to have distributed biogas digesters, which is fine because then each farm can benefit," said Webber. "There are some advantages to distributed generation but it’s not going to have the economies of scale or efficiency of having a large power plant."

Webber believes the approach is promising enough to warrant further investigation. "I suspect the US will move in this direction because US politicians love rural economic development – they love farmers," he said. "Ultimately this will be seen as a way to help farmers either make money or to avoid expensive mitigation of manure or to save money on their electricity bills."

Next, the plan is to look at how converting manure into biogas could cut pollution as well as greenhouse gas emissions. Manure is a source of ammonia, a precursor for the formation of particulates that can cause lung problems such as asthma and cancer. The animal waste also gives off volatile organic compounds and sulphuric compounds.

"If you convert the manure to biogas you remove most of those pollutants, and also reduce pollutants from coal use," said Webber.

The researchers reported their work in the open-access journal Environmental Research Letters.