Biogas is a combustible mixture of gases produced by micro-organisms when livestock manure and other biological wastes are allowed to ferment in the absence of air in closed containers(Dream Farms). The major constituents of biogas are methane (CH4, 60 percent or more by volume) and carbon dioxide (CO2, about 35 percent); but small amounts of water vapour, hydrogen sulphide (H2S), carbon monoxide (CO), and nitrogen (N2) are also present. The composition of biogas varies according to the biological material. The methane content of biogas produced from night soil (human excreta), chicken manure and wastewater from slaughterhouse sometimes could reach 70 percent or more, while that from stalk and straw of crops is about 55 percent. The concentration of H2S in biogas produced from chicken manure and molasses could be as high as 4 000mg/m3, and from alcohol wastewater even higher at 10 000 mg/m3. Biogas is mainly used as fuel, like natural gas, while the digested mixture of liquids and solids ‘bio-slurry’ and ‘bio-sludge’ are mainly used as organic fertiliser for crops. But there are numerous other uses for biogas, bio-slurry and bio-sludge in China.

China is one of countries in the world to have used biogas technology early in its history. By the end of the nineteenth century, simple biogas digesters had appeared in the coastal areas of southern China.The main reason for using anaerobic digestion, which generates biogas as a by-product, is to treat wastes.

According to the government’s Chinese Ecological White Paper issued in 2002, the total amount of livestock and poultry wastes generated in the country reached 2.485 billion tonnes in 1995, some 3.9 times the total industrial solid wastes.