Global CH4 emissions from landfill are estimated to be between 30 and 70 Tg/year. Most of this landfill CH4 currently comes from developed countries, where the levels of waste tend to be highest. Landfills provide ideal conditions for methanogen-esis, with lots of organic material and with anaerobic conditions usually prevalent. The huge amounts of waste that are buried in landfill sites can mean that CH4 is produced for years after the site is closed, due to the waste slowly decaying under the ground.

CH4 escapes from landfills either directly to the atmosphere or by diffusion through the cover soil. Highly active communities of CH4-consuming bacteria (methanotrophs) can develop in these overlying soils and these can greatly reduce the amounts of CH4 emitted, as discussed by De Visscher et al. (Chapter 12, this volume).

Our so-called throwaway society in developed countries has led to a large increase in the amounts of organic waste entering landfill sites. Not only does this lead to greater GHG emissions in the form of landfill CH4, but the waste incurs further GHG costs in terms of waste transport. In recent years, landfill CH4 emissions have been identified by several world governments as a GHG source that can be both easily defined and reduced. In addition to the practice of covering landfills with a thick soil layer - to promote CH4 uptake by soil methanotrophs - more proactive strategies are also available.

CH4 recovery systems are now commonly installed at landfill sites and these can reduce emissions to the atmosphere by more than half. Sometimes the recovered CH4 is simply flared off, but often the recovery systems can provide an economically viable energy source, where the collected CH4 is used in electricity generation.

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