The dominant process in most landfills is the third process whereby anaerobic bacteria decompose organic waste to produce biogas, which consists of methane and carbon dioxide together with traces of other compounds.[3] Despite the heterogeneity of waste, the evolution of gases follows well defined kinetic pattern. Formation of methane and CO2 commences about six months after depositing the landfill material. The evolution of gas reaches a maximum at about 20 years, then declines over the course of decades.[2]