In biogas plants, biogas can be produced by using numerous different input substrates. Organic materials are essential as substrates for generating biogas and the key starting point for the production process. The organic material should be free from wood (lignin) because the bacteria in the plant are unable to process this substance. There are seven categories of organic materials which are suitable substrates:
Domestic (communal) organic waste
Organic waste means separate collected fruit and vegetable wastes, flower soil, flowers, eggshells, coffee and tee filters and other organic leftovers. The percentage of dry matter varies between 10 and 30 %.
Green and plant wastes
Loppings and leftovers from cutting gardens or parks which do not contain woody matter (lignin).
Communal sewage sludge
The leftover from communal waste water treatment is sludge, which must be decontaminated to be used for biogas production. This sewage sludge can contain phosphor, nitrate and heavy metals. Experience shows that sludge from waste water treatment is suitable in biogas plants, but is not considered as a high quality substrate. It is illegal to use sludge from non-public sources because of the higher content of heavy metals or genetically modified organisms or hormones. The percentage of dry matter varies between 20 and 30 %.
Liquid or solid manure
Liquid or solid manure from animal farming is one of the main substrates for the digestion process. Cow manure is best, whereas chicken manure for example has a high output, but might contain sand. The dry matter of solid manure varies between 15 and 30% and of liquid manure between 5 and 7 %.
Energy crops are primarily cultivated on otherwise disused fields, such as corn, Sudan grass, millet, white sweet-clover.
Industrial food waste
Industrial food wastes come from food and meat production sites or processes such as whey, potato mash, vegetable leftovers, brewer grains, apple marc. These kinds of substrates are usually very suitable and do not contain harmful substances. Substrates from meat production, like slaughterhouse waste, are good organic materials but need special treatment to reduce health risks and odour.
Maximal potential of gas production per ton of substrate
Spent grains, duff
Domestic organic waste
Industrial food waste
Waste food grease and oils
As mentioned the bacteria are very sensitive. Temperature and pH-value should be precise and constant. Some inputs are toxic for the bacteria and can stop the digestion. Toxic inputs are: