Landfill gas production results from chemical reactions and microbes acting upon the waste as the putrescible materials begins to break down in the landfill. The rate of production is affected by waste composition and landfill geometry, which in turn influence the bacterial populations within it, chemical make-up, thermal characteristics, entry of moisture and escape of gas. 

The spatially heterogeneous nature of most landfills means that there will be a wide range of physical conditions and biological ecosystems co-existing simultaneously within most sites. This heterogeneity, together with the frequently unclear nature of the contents, makes landfill gas production more difficult to predict and control than standard industrial bioreactors for sewage treatment.

Due to the constant production of landfill gas, the increase in pressure within the landfill (together with differential diffusion) causes the gas's release into the atmosphere. Such emissions lead to important environmental, hygiene and security problems in the landfill.Several accidents have occurred, for example at Loscoe, England in 1986,where migrating landfill gas which was allowed to build up destroyed the property. An accident causing two deaths occurred from an explosion in a house adjacent to Skellingsted landfill in Denmark in 1991.Due to the risk presented by landfill gas there is a clear need to monitor gas produced by landfills. In addition to the risk of fire and explosion, gas migration in the subsurface can result in contact of landfill gas with groundwater. This in turn can result in contamination of groundwater by organic compounds present in nearly all landfill gas.

Landfill gas is approximately forty to sixty percent methane, with the remainder being mostly carbon dioxide. Landfill gas also contains varying amounts of nitrogen, oxygen, water vapor, sulfur, and other contaminants. Most of these other contaminants are known as "non-methane organic compounds" or NMOCs. Some inorganic contaminants (for example mercury) are also known to be present in landfill gas. There are sometimes also contaminants (for example tritium) found in landfill gas. The non-methane organic compounds usually make up less than one percent of landfill gas. In 1991, the EPA identified ninety-four non-methane organic compounds including toxic chemicals like benzene, toluene, chloroform, vinyl chloride, and carbon tetrachloride. At least forty one of the non-methane organic compounds are halogenated compounds (chemicals containing halogens: typically chlorine, fluorine, or bromine). General options for managing landfill gas are: flaring, boiler (makes heat), internal combustion engine (makes electricity), gas turbine (makes electricity), fuel cell (makes electricity), convert the methane to methyl alcohol, clean it enough to pipe it to other industries or into natural gas lines.

The Environmental Protection Agency estimates that there are approximately six thousand landfills in the United States. Most of these landfills are composed of municipal waste, and therefore producing methane. These landfills are the largest source of anthropogenic methane emissions in the United States. These landfills will contribute an estimated four hundred and fifty to six hundred and fifty billion cubic feet of methane per year (in 2000).

http://en.wikipedia.org/wiki/Landfill_gas