Agricultural waste can represent a significant source of methane. The anaerobic decomposition of livestock and poultry manure, common to manure heaps and slurry tanks, leads to large amounts of methane production due to its large organic carbon content. Similarly, the processing of industrial and domestic waste water and sewage can also produce significant amounts of methane.

In total, such waste accounts for between 14 and 25 million tonnes of methane emission per year globally. Historically, methane emission from this source is likely to have been much lower due to lower livestock numbers and relatively low livestock densities. Today though consumer demand, for meat and dairy products in particular, requires ever higher numbers and densities of livestock.

Human Impact

Clearly, man is directly responsible for methane emission from livestock manure and human sewage. Where animal manure would, in the wild, be spread over a wide area and decompose aerobically in the natural environment, intensive livestock rearing methods mean high concentrations of manure build up in relatively small areas. Such conditions lead to a predominance of, methane producing, anaerobic decomposition of the manure.

In a similar way, greater human numbers and population densities have led to larger concentrations of waste water and sewage in collection areas, such as sewage works. Human waste processing can also produce large amounts of methane if anaerobic decomposition is allowed to predominate.

Potential for control

The trapping of methane from strong sources of livestock manure methane, such as slurry tanks, has already proved a very successful way of reducing methane emissions to the atmosphere from this source. The recovered methane, often called 'biogas', can be simply flared off as carbon dioxide or can potentially be used as a fuel.

Other options include a move away from such intensive rearing methods, with an increase in grazing time for animals and so a greater dispersal of their manure. For human waste water and sewage, ensuring aerobic decomposition using aeration methods is an oft employed strategy, though methane trapping and subsequent burning is practiced at some sites.