Two types of biogas plants are being used in our country; they are

1. The fixed-dome type

2. The floating gas-holder type the main raw material used in these plants is animal dung (cow-dung, dung of horse, elephant, goat, etc). Plant wastes like vegetable skins, fruit pulp and human excreta may be added to the animal dung.

Biogas is produced by the anaerobic decomposition of animal wastes like animal dung in the presence of water. The decomposition which takes place in the absence of oxygen is called anaerobic decomposition. This decomposition is carried out by anaerobic micro-organisms called anaerobic bacteria in the presence of water but in the absence of oxygen. Animal dung and plant wastes contain a lot of carbon compounds like carbohydrates, proteins and fats. The anaerobic bacteria decomposes these carbon compounds to form methane gas, which is the main constituent of bio-gas. Some other gases like carbon dioxide, hydrogen and hydrogen sulphide are also formed.