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In reply to Zaph Mann's question,  the issue of methane directly produced by animals in their guts ("enteric fermentation") is comment to cattle, goats, sheep, and all the other ruminant mammals, but I don't think it is a problem with other mammals.  I think the amount of methane produced by cattle and goats is not that different between factory farmed animals and animals raised in less-crowded conditions, but I don't have data on that.  On the other hand, methane releases from manure are a problem when you have high concentrations of animals of any kind in factory farms and you have wet manure that gets stored temporarily in large lagoons.    They are not a problem where animals are not crowded and you don't have the manure lagoons that go anaerobic and produce methane.

posted 2 years, 11 months ago
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Source of the EPA numbers:

 

http://www.epa.gov/methane/sources.html

 

EPA numbers show that livestock are the largest sources of human-generated methane in the United States if you combine the "enteric fermentation" (methane from the guts) at 115 Tg CO2 equiv.with manure at 39.1 Tg CO2 equiv.  Landfills produce 131.2 Tg CO2, so both are very large sources.

posted 2 years, 11 months ago
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We have to be careful that providing financial incentives does not make a bad situation worse.  In two of these cases - livestock and landfills - we have a bad situation that could be made better by minimizing the generation of gas in preference to using the gas that is generated.  The last thing we want to do is have the incentives lead to the creation of more gas.

 

In one sense, using livestock manure to generate energy is like strapping a windmill to your SUV to generate power.  You put a lot of energy into the system, and get little out.  With livestock, huge amounts of energy and fertilizer goes into growing the food for the cattle.  Much of that energy comes out of cattle in the form of methane directly from their guts - something that the manure digesters cannot capture.  According to EPA, close to 3/4 of the methane from livestock comes from their guts, and only about 25% comes from the manure.  In addition, huge amounts of greenhouse gases are generated directly from growing the corn and soy to feed cattle, and from the nitrogen fertilizers (released as nitrous oxide - a very potent greenhouse gas).

 

With landfills too, capture of methane by the landfill is not complete, and if only a small amount of methane escapes the landfill, it negates all of the energy captured from the landfill gas that is captured.  This doesn't mean that you shouldn't capture and use landfill gas, but it does mean that you should not purposefully put organic materials into landfills in order to capture the gas.

 

Peter Spendelow, Ph.D.

posted 2 years, 11 months ago
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