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Secondary treatment is designed to substantially degrade the biological content of the sewage such as are derived from human waste, food waste, soaps and detergent, but physicochemical processes may also be used to eliminate chemical pollutants. Many municipal and industrial plants treat the settled sewage liquor using aerobic biological processes. On the opposite, a biomethanation system doesn't require any oxygen, since biomethanation is an anaerobic digestion process. In all these methods, bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc.
This physical-chemical process generates methane from a mixture of various gases out of the sludge fermentation. The main components are carbon monoxide and hydrogen. Anaerobic treatment of wastewater has a number of advantages over aerobic treatment -no energy for oxygenation, lower production of sludge per unit mass of organic matter stabilized, low nutrient requirement for biological growth and production of bio-gas, a valuable source of renewable energy.