Wherever biodegradable material is deposited in landfill sites, microbial activity will generate landfill gas*. Under optimum conditions, one tonne of waste can produce up to 150-200 cubic metres of gas. The composition of the gas generated depends on the phase of the breakdown process, but methane (about 60%) and carbon dioxide (about 35%) are the major constituents of the gas during the predominant phase of degradation.
Methane is flammable and can be an asphyxiant to humans in confined spaces and to vegetation when it infiltrates the soil. It was the need to minimise these risks, which has led to the development of landfill gas control systems and recovery over the past 20 years.
Methane is also a greenhouse gas that is considered to be 21 times more powerful than carbon dioxide. Its recovery and utilisation from landfill sites therefore reduces methane emissions and generates an energy input from a waste output .
Some people also see the use of landfill gas as a fuel to produce electricity as an environmentally friendly offset to high carbon fuels like coal - but we should remember that this source of fuel depends on a large waste stream containing lots of recyclable and compostable material.
* Waste Management Paper No 27, DoE, 1991.