Question

Describe a fixed dome type biogas plant with the help of a labelled diagram.

Answer 1

Fixed dome type biogas plant :
It consists of follows parts :

• Digester tank: It is an underground tank. Its base and walls are made of bricks and cement. Its roof is made in the form of dome either with cement or bricks or with concrete. The dome collects the biogas. It is provided with an outlet for biogas. The outflow of the gas is controlled by a gas valve.
• Slurry mixing tank: On the left-hand side of the digester is constructed a brick-lined tank above the ground level. The bottom of this tank connects the digester through a brick-lined channel near the base.
  The fresh slurry of animal dung and water in equal proportions is poured into the slurry mixing tank. This slurry slowly flows into the digester.

• Spent slurry tank: On the right-hand side of the digester is constructed spent slurry tank, below the ground level. The base of this tank is in line with the lowest point of the dome of the digester. The base of this tank is connected to the base of the digester through a brick-lined channel.
• Working of fixed dome type biogas plant: Everyday the slurry of animal dung and water in equal proportions is poured into the mixing tank. This slurry flows into the digester. In the digester anaerobic fermentation starts with the release of biogas. The biogas formed collects in the dome.
  As more and more slurry flows into the digester every day, the level of slurry rises up. It takes about 30 to 60 days for the slurry to fill the digester depending upon the temperature within the digester. During this time the biogas collects in the dome under pressure.
  When the pressure of biogas exceeds a certain limit, the biogas forces the spent slurry into an overflow tank. The biogas is withdrawn from the dome through the outlet pipe. Its flow is regulated by a gas valve. The spent slurry is periodically removed and is allowed to dry. It is then used as manure.
  Once the biogas plant gets operative, it is a continuous process. The everyday fresh slurry is added in the mixing tank and spent slurry is removed from the overflow tank.