Which of the following is needed for the formation of biogas from cow-dung and which not?
Water, Oxygen

Biogas is is the mixture of gases formed by the anaerobic degradation of organic matters (cow dung) in the presence of water but in the absence of oxygen. It comprises primarily methane (CH4) and carbon dioxide (CO2) and a trace amount of hydrogen sulphide (H2S), ammonia (NH3), oxygen and nitrogen. It is a clean and renewable energy source.