Methane is the main constituent of biogas. Other gases present with it are hydrogen, carbon dioxide and hydrogen sulphide. Biogas is prepared in a fixed dome type plant as follows:
Fixed dome type biogas plant: It consists of underground tank T called digester tank which is made of bricks. It has a dome-shaped roof made of cement and bricks. The dome of the digester tank acts as a gas holder. There is a gas outlet through a valve at the top of the

Fig. Fixed dome type biogas plant

dome. On the left side of the digester tank is a sloping inlet chamber and on the right side is a rectangular outlet chamber, both made of cement and bricks. The inlet chamber is connected to a mixing tank while the outlet tank is connected to the overflow tank.

Working: Cattle dung and water are mixed in equal proportions in the mixing tank M to prepare slurry. The slurry is fed into digester tank T through the inlet chamber I. The tank is filled only up to the cylindrical level so that enough space is available for the biogas that would be formed. The tank is closed for 50 to 60 days. During this period animal wastes etc. undergo fermentation by anaerobic microorganisms in the presence of water. Gases such as methane, carbon dioxide, hydrogen, hydrogen sulphide etc. (biogas) start collecting in the dome. As more and more biogas collects in the dome, it exerts pressure on the slurry in the digester tank and forces it to go into the overflow tank F through the outlet chamber. The biogas collected can be used through the outlet valve at the top of the dome. The spent slurry obtained in the overflow tank can be used as manure as that is rich in nitrogen and phosphorus compounds.
Once the biogas plant becomes operative, more and more slurry may be fed into it to give a continuous supply of biogas.
Instead of animal dung, domestic sewage or human excreta may also be used in the biogas plant. The above fixed dome type plant is cheap, easy to construct and is also called Janta gobar gas plant.
The main advantage of fixed dome type plant is that it cannot be corroded and thus used over the years whereas the gas holder used in floating dome is made of steel and is corroded over the time and needs replacement.