Agricultural and municipal waste

Agricultural waste can represent a significant source of CH4. The anaerobic decomposition of livestock and poultry manure, common to manure heaps and slurry tanks, often leads to large amounts of CH4 production due to its large organic carbon content. Similarly, the processing of industrial and domestic wastewater and sewage can produce significant amounts of CH4.

In total, such waste accounts for 14-25 Tg CH4/year globally. Historically, emissions from this source are likely to have been much lower due to lower livestock numbers and densities. Where traditionally animal manure would have been spread over a wide area and decomposed aerobically, intensive livestock rearing methods mean high concentrations of manure build-up in relatively small areas. In a similar way, greater human numbers and population densities have led to larger concentrations of wastewater and sewage in collection areas such as sewage works.

The trapping of CH4 from strong sources, such as slurry tanks, has already proved a very successful way of reducing emissions to the atmosphere. The recovered CH4, often called 'biogas', can be simply flared off or can be used as a fuel. Pilot studies have shown that such biogas capture may provide an alternative to petroleum as fuel for transport.

Other CH4 mitigation options include reducing demand for meat and dairy products and a move away from intensive rearing methods, with an increase in grazing time for animals and so a greater dispersal of their manure. For human wastewater and sewage, ensuring aerobic decomposition using aeration methods is an oft-employed strategy, though CH4 capture and subsequent flaring is practised at some sewage treatment sites.

emissions from biomass burning can be overshadowed by the large amounts of CO2 that are also produced, but in many cases the subsequent regrowth, and CO2 uptake, of previously burned woodland and savannah areas means that CH4 emissions have a disproportionately greater climate-forcing effect.

Burning of agricultural waste also produces significant amounts of CH4 due to its generally high water content. Additionally, wood burning as a domestic fuel source and for charcoal production can release significant amounts of CH4.

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