Biogas Plant: Structure, Diagram, and Working Principle

A typical dome-shaped biogas plant consists of the following main parts:

• Mixing Tank: Where cow dung and water are mixed to form a slurry.
• Inlet Pipe: Carries the slurry from the mixing tank to the digester.
• Digester Tank: A large, sealed, underground brick tank where oxygen is completely absent. This is where the actual breakdown of waste occurs.
• Gas Holder (Dome): The curved roof of the digester where the generated gas collects.
• Outlet Chamber & Overflow Tank: Where the spent slurry (digestate) is pushed out and collected to be used as fertilizer.

• The slurry of dung and water is fed into the digester.
• Since the digester is completely sealed off from oxygen, specific anaerobic bacteria start decomposing the complex organic compounds present in the cow dung.
• This decomposition process takes a few days. As the waste breaks down, it releases a mixture of gases.
• The gas accumulates in the dome, creating pressure that forces the spent slurry out into the overflow tank. The gas is drawn out through a pipe for use.

Biogas is an excellent fuel because it contains up to 75% Methane ($CH_4$). The rest is mostly Carbon Dioxide ($CO_2$), along with tiny traces of Hydrogen ($H_2$) and Hydrogen Sulphide ($H_2S$).