The decomposition of organic matter in the absence of air would be elicited by the use of physical or chemical processes at high temperature and/or pressure, or the use of microorganisms at near ambient temperature and atmospheric pressure. it is referred to as biogas if generated as a result of the action of microorganisms on the organic wastes. This is why biogas is now defined as “a by-product of the biological breakdown, under oxygen-free conditions of organic wastes such as waste, plants, crop residues, wood and bark residues, and human and animal manure and is known by such other names as swamp gas, marsh gas, ‘will o’ the wisp’ or gobar gas” , digester gas, natural gas, landfill gas (LFG), and sewage gas . The gas is colourless, relatively odourless and flammable: it is also stable and non-toxic. It burns with a blue flame and has a calorific value of 4500–5000 kcal/m3 when its methane content ranges from 60% to 70%