(a) What is biogas? Name the major component of biogas.
(b) What are the raw materials used for making biogas?
(c) Describe the construction and working of a biogas plant with the help of a labelled diagram.
(d) Write any two uses of biogas.
(e) Write any two advantages of using biogas.

(a) Biogas is a mixture of gases, like methane, carbon dioxide, hydrogen and hydrogen sulphide which is produced by the anaerobic degradation of biomass (animal and plant wastes) in the presence of water but in the absence of oxygen.

The major component of biogas is methane, as it contains up to 75% methane gas.

b) The raw materials used for making biogas are animal wastes and plant wastes in the presence of water.

A biogas plant consists of a wall-shaped underground tank known as 'digester' which is made of bricks and cement and has a dome-shaped roof 'D'. The dome acts as a storage facility for biogas. On the top of the dome, there is a gas outlet 'S' that has a valve 'V'. There is a sloping inlet chamber 'I' which is connected to a mixing tank 'M' on the left side and a rectangular outlet chamber 'O' is connected to the overflow tank 'F' on the right side.

First of all, a slurry is made by mixing cow dung and water in equal proportions in the mixing tank 'M'. This slurry is fed into the digester tank 'T' through the inlet chamber 'I' to fill the tank up to the cylindrical level, and the dome is left free for the collection of biogas. Then, in 50-60 days, the cow dung undergoes degradation by anaerobic bacteria, and the gradual evolution of biogas get collected in the dome. Now, the collected biogas in the dome exerts pressure on the slurry which forces the spent slurry to go into the overflow tank 'F' through outlet chamber O, from where it is removed.

(d) Two uses of biogas are as follows:

1. As a fuel for cooking food.

2. As a fuel for lightning.

(e) Two advantages of using biogas are as follows:

1. It is a clean fuel (smoke-free) with a high calorific value.

2. It utilises biowaste and sewage at a large-scale for producing biogas.