Describe the construction and working of the biogas plant with the figure.

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.

 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.

Two uses of biogas are as follows:

1. As a fuel for cooking food.

2. As a fuel for lightning.