Nothing is a waste until it is wasted. Waste generated from one process can be an important input to another process. This is an idea behind zero-waste agriculture. Wastes used to generate useful energy such as biogas are biodegradable in nature; examples include, Animal dung, Poultry waste, Plant residue (duckweed or rice straw), Vegetable waste, Wheat offal, Cassava Peels, Human Waste, Meat Processing waste, etc. Biogas is an important gas for cooking and generation of electricity. Many homes in countries like India, China, Kenya, Australia, etc. have been using biogas generated from their wastes for cooking thereby reducing their cost on other energy sources. Biodegradable wastes are being generated on a daily basis from households, Livestock farms, Crop farms, food and fruit processing factory, etc. Many unused wastes are major contributors to environmental pollution and green house effect. However, it is important that we maximize our wastes effectively and efficiently in order to save our cost on energy and of course create a clean and safe environment for ourselves.        
Biogas Production
Biogas is mixture of gases which is generated when biomass (organic matter) are fermented in the absence of air/oxygen (Anaerobic Fermentation). This gas mixture is mainly made of carbon dioxide (CO2) and methane (CH4). Methane is a combustible gas, which means it can be burned. It can be used as a fuel for cooking and lighting. Methane is produced artificially in a container called Digester. During digestion, 30-60% of the digestible solids are converted into biogas. Feedstock (biomass used) for the production of biogas includes: livestock effluents, meat processing waste, organic components of landfills, any other source of biomass (e.g. wastewater treatment sludge or food and beverage industry wastes), and household waste. On average, biogas contains: 55-80% methane (CH4); 20-40% carbon dioxide (CO2); trace gases, including toxic hydrogen sulphide and nitrous oxide.
Digester comprises the inlet, the fermentation chamber, the gas, the gas storage bag or tank, and the outlet and the exit pipe through which the gas is removed. Organic matter (biomass) is brought into the fermentation chamber through the inlet. The process of anaerobic fermentation will take place here to generate biogas. It will also produce a substrate rich in nutrients which can be used as organic fertilizer or fish feed. The processing of manure, organic rubbish and wastewater in the plant helps to keep the environment clean. There is no longer any bad smell from sewage or livestock manure. Cooking by biogas is much cleaner than cooking over a wood fire, and there is no smoke to cause lung problems and eye diseases.
Begin by loading the fermentation chamber with the materials to be fermented (manure or other wastes). You should begin with an initial load of 300 - 500 kg of materials for each cubic meter of the fermentation chamber. This needs to be supplemented by an additional 8- 10 kg each day for each cubic meter of the fermentation chamber. The gas output will be 250 - 400 liters for each cubic meter of the chamber. Expressed another way, 1 kg of manure will have a gas yield of 30 - 60 liters/day, for several days. The gas yield from 1 kg of water hyacinth will be 40 - 50 liters/day, for several days. One cubic meter of gas (= 1000 liters) is enough to cook the day's food for a 6 - 7 member family, or provide lighting for 4 - 5 hours. It could replace one liter of petrol to operate a 400W electric generator for two hours.