Methane is generated from solid waste and wastewater through the anaerobic decomposition of the organic component of the waste stream. Each of these sources is believed to account for about 10% of human-induced methane emissions, although there are large uncertainties in the precise emission levels (IPCC 1996a, WGII, Section 22.4.4.1). In the future, methane emissions from these sources are expected to grow, with the largest increases expected in developing countries. Population and economic growth are the major drivers behind the rising emission levels of developing countries. In addition, current efforts to improve solid waste and wastewater management will affect future emission levels. If developing countries pursue solid waste disposal in large landfills and anaerobic wastewater treatment without including methane recovery technologies, emissions could increase significantly. There are a variety of technologies and approaches available that require less capital, use less complicated technologies and avoid GHG emissions. These technologies may be particularly appropriate in developing countries, but could also be more widely used in industrialised countries. The key challenge will be ensuring that the opportunities are identified and implemented.

Solid waste disposal sites (landfills and open dumps) are reported to emit about 20-40 Mt of methane (or 110-230 Mt Carbon-equivalent, based on a 100-year GWP of 21), with the largest share of emissions coming from Annex I countries. Emissions from most Annex I countries are expected to remain stable or decline over the next 10-20 years (IEA, 1996). A recent review of national communications and the Second Compilation and Synthesis report, for example, estimated that overall Annex I emissions from solid waste disposal sites were 24 Mt (138 MtCe) in 1990 and are estimated to fall slightly to 23 Mt (132 MtCe) in 2010 (USEPA, 1999a). The principal reasons cited are increasing attention on reducing the amount of organic material disposed of in landfills and the expanded use of methane collection systems. In contrast, methane emissions from developing countries are expected to increase in the future (IEA, 1996). Key factors are population growth, additional waste generation associated with economic development, and the continuing priority of many developing countries to reduce unmanaged dumping and develop larger, solid waste disposal sites, which typically have higher methane emissions. If, however, the use of locally appropriate technologies that avoid methane generation is expanded, and those solid waste landfills that remain essential are modified to include methane recovery systems, then methane emissions could be significantly reduced.

Methane emissions from domestic and industrial wastewater disposal are estimated to be about 30-40 Mt (170-230 Mt Ce) annually (IPCC 1996a, WG II, Section 22.4.4.1). Industrial processes, principally food processing and pulp and paper, are the major contributor, accounting for up to 95% of total emissions from this source. Domestic and commercial wastewater emissions are believed to emit only about 2 Mt of methane annually (USEPA, 1997a). Most countries only report domestic wastewater emissions in their National Communications; these emissions are expected to decline slightly in Annex I countries between 1990 and 2010. Emissions are believed to be significantly higher in developing countries, where domestic sewage and industrial waste streams are often unmanaged or maintained under anaerobic conditions without methane control. Future emissions are expected to grow, primarily driven by population and industrial growth. Emission levels are highly uncertain, however, and depend heavily on the rate at which wastewater infrastructure is developed. To the extent that developing countries successfully expand wastewater management services and reduce the proportion of their wastewater managed anaerobically without methane control, emissions would be lower than under a "business as usual" case.

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