Microbes in the Production of Biogas (Gobar Gas)

Biogas is a mixture of gases generated when organic matter is broken down anaerobically (without oxygen) by microbial action; it can be produced from agricultural waste, animal manure, municipal solid waste, plant material, sewage and food waste. It consists mainly of methane (about 63%), along with carbon dioxide and some hydrogen, and burns with a smokeless blue flame, without any characteristic odour. The methane is produced by methanogens, of which Methanobacterium is a common example; methanogens are naturally present in anaerobic sludge and also in the rumen of cattle, where they help break down cellulose from plant fodder. The excreta of cattle is commonly called 'gobar' (dung), and biogas generated by the anaerobic decomposition of cattle dung is popularly called gobar gas — it too consists mainly of methane and CO2, with traces of hydrogen, nitrogen and other gases. In a rural biogas plant (Fig. 9.4), dung slurry is prepared in a mixing tank and fed through an inlet pipe and inlet tank into an airtight cylindrical digester (built of concrete, brick and cement, or steel), where anaerobic digestion proceeds through three stages — solubilisation, acidogenesis and methanogenesis. The generated gas collects in a dome-shaped gas-storage tank and is piped out through a gas control valve for use in cooking and lighting, while the digested slurry drains through …