Biogas is a renewable energy source that is made from organic matter of thicker and slimmer that is fermented through bacteria. This process goes on when there is no oxygen that leads to the formation of flammable gases such as methane (CH4) and carbon dioxide (CO2). Biogas can be produced from many feedstock such as animal waste, crop residues, post-industrial food processing byproducts, and sewage.
Source: TanzimSharon