1-tonne (or approximately 2,200-pounds) of degradable trash produces about 400-500 cubic metres (106,000-132,000 gallons (approximately) of landfill gas. As a result the gas emissions from landfill sites must be constantly monitored for many years after the sites have stopped accepting rubbish.
Landfill gas is a complex mix of various different gases created by the action of micro-organisms within a landfill.
Landfill gas is approximately 40% to 60% percent methane (CH4), with the remainder being mostly carbon dioxide (CO2). Trace amounts of other volatile organic compounds comprise the remainder (<1%). These trace gases include mainly simple hydrocarbons.
Landfill gases are the result of three primary processes:
- Evaporation of volatile organic compounds (e.g., solvents)
- Chemical reactions between waste components, and
- Microbial action, especially methanogenesis.
The first two processes depend to a great extent on the nature of the waste.
The primary process in most landfills is the third process whereby anaerobic bacteria decomposes organic waste (i.e.anaerobic digestion) to produce biogas, which consists of methane and carbon dioxide together with traces of other compounds.
Despite the heterogeneity of waste, the evolution of gases follows well defined patterns. Formation of methane (CH4) and Carbon Dioxide (CO2) takes approximately six months after depositing the landfill material. The evolution of gas reaches a maximum after about 20 years, and then declines over the course of time.