Factors Affecting Biogas Production - Biogas yield is measured as m3 gas/kg volatile solids. This depends on the type of waste, temperature during digester operation, the retention time (the period of time a given sample of waste/substrate stays in the digester/fermenter before it flows out) and the presence of inhibitors.

The maximum possible gas yield on complete digestion of carbohydrates (starch, cellulose, glucose) would be 0.8 m3fkg; for fatty acids this value it is about 1.5 m3fkg, and for proteins it is about 0.9 m3/kg. But the organic matter conversion is almost always incomplete.

Agricultural wastes, especially, pig and chicken manure, generate high levels of ammonia, which may inhibit biogas production. Most of the N content of the waste is however retained during anaerobic digestion so that the spent slurry is as good a source of nitrogen as the substrate itself.

Some wastes, e.g., from paper industry, may be rich in sulphate (SO₄^2-). SO₄^2- competes with CO₂ for H₂ thereby reducing methane yields. In addition, antibiotics may be used in animal feed, and detergents/disinfectants, etc. may be present in the waste; these inhibit biogas production to varying degrees.

Anaerobic digestion is mainly used for pollution control, but its use as an energy source is also important. The estimation of net energy yields is rather complex in view of the factors affecting biogas yields. The total world production of biogas is only a tiny fraction of the total energy requirement. It is thought that the biogas technology will develop as a greater emphasis is placed on organic pollution control.