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Biogas plant has a dome-shaped digestor in which cow dung is kept. It is completely isolated and thus does not have oxygen in them. In this anaerobic conditions, methanogens (methane producing bacteria) perform their function & produces methane (CH₄) by digesting the cow dung. Along with methane, carbon-dioxide (CO₂), hydrogen sulphide (H₂S) is also produced. These gases so produced is recovered from the gas outlet while the remaining slurry can be used as fertiliser.

two advantage of using animal dung for making biogas over using it as a fuel in the form of dried dung cakes are:-

1) biogas is a very clean fuel i.e. there is no hydrocarbon emission while using biogas but at the same time this cow dung is very polluting and can even cause various lungs related problems.

2) the remains of biogas i.e. the cow dung slurry can be used as a fertiliser so there will be no waste production while using biogas. But using dry dung cakes produces ashes which further creates a problem of its disposal.