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Biogas
Biogas is the mixture of gases (about 60% methane) produced by anaerobic bacterial digestion of organic matter. Biogas consists mainly of methane (CH4; ca. 60%) and CO2 (ca. 40%) plus traces of hydrogen (H2) and several other gases. It is produced by anaerobic degradation of a variety of organic materials, ranging from simple sugars to polymers like cellulose and nucleic acids by a community of organisms. Anaerobic digestion occurs in nature in the sediments of lakes and ponds, and in rumen of cattle; it is carried out by man in a variety of reactors called digesters and even in land-fill sites under nonseptic conditions. A land-fill site is a location at which a pit was filled with, usually, domestic refuse, covered with soil and, generally, landscaped.

The substrate

The substrate usually employed for biogas generation is a waste product of industrial, agricultural, animal husbandry, or domestic and municipal origin. Therefore, the waste would contain a variable proportion of nonbiodegradable matter in form of plastics, inorganic materials, lignin etc. Lignin is virtually nondegradable under anaerobic conditions. Generally, the wastes are divided on the basis of their organic dry matter or total solids content into the following four categories: (i) low (0.2-1% organic dry matter), (ii) medium (1-5%), (iii) high (5-12%) strength and (iv) solid (20-40%) wastes. The decision of digester design will depend mainly on whether the waste is completely soluble or it has particulate matter as well. The wastes may originate mainly from the following 3 sources.

The digester

The digester design will mainly depend on the type of waste to be handled and the level of operation, i.e. small rural or large industrial operations. The digesters may be run in a batch or a continuous mode, the latter being more desirable for large scale operations.
 
The micro-organisms

Several hundred species of micro-organisms are involved in the anaerobic digestion and biogas production. These bacteria can be divided into the following 4 trophic groups: (i) hydrolytic and fermentative bacteria, (ii) syntrophic H2 producing bacteria, (iii) methanogenic bacteria and (iv) acetogenic bacteria.

The process of biogas production


The process of biogas production is explained using ‘gobar gas’ as an example. ‘Gobar gas’ plants are based on excreta of cattle and other farm animals, which contains about 20% inorganic particles or ‘rajkans’  (meaning dust particles). The level of dust particles is reduced to about 10% by mixing the dung with water in 1:1 ratio. The feeding rate of a typical dung based biogas plant is at the rate of 3,500 kg dung/day. Generally, spent slurry at about 2% (v/v) of the fresh dung slurry is added back to maintain the microbial population. Calcium ammonium nitrate at the rate of 1% (w/w) of the dung is added to the slurry. In addition to cowdung, human excreta (upto 3% of slurry) and kitchen waste can also be used. Addition of human excreta markedly increases biogas output, perhaps due to its higher nitrogen content which supports microbial growth.

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