Biogas is produced by the bacterial decomposition of animal or plant substances in a biogas plant. These substances can be, for example, biogenic residues, such as the waste from the organic waste garbage can or liquid manure from agriculture. The fermentation of these substances produces, among other things, biomethane. The resulting biomethane can be used as an energy source in the same way as natural gas and can be fed into the natural gas grid, for example. There are various ways to produce biogas and not all variants are ecologically sustainable.

Substrates, also called feedstocks, are the biogenic raw materials that are introduced into a biogas plant to produce gas. Here, a rough distinction can be made between three classes:

• Biogenic residues (e.g. biowaste from the organic waste garbage can, plant residues, residues from agriculture such as harvest and slaughter waste, liquid manure, slurry and dung).
• Sewage sludge and sewage gases from wastewater treatment plants, industrial processes, or commercial production (e.g., from biogenic residues in paper recycling).
• Renewable raw materials or energy crops (e.g., corn or through-grown silphia).

A biogas plant usually consists of a fermentation tank with gas storage, the so-called fermenter, as well as a preliminary pit, a secondary fermenter and fermentation residue storage.

The fermenter, the core of a biogas plant, is filled with the substrates (e.g. NawaRo and liquid manure from the preliminary pit). In the fermenter, the actual biogas formation takes place under exclusion of sunlight and oxygen. Depending on the initial substrate, biogas can be produced by wet or dry fermentation (= digestion). In wet fermentation, a high water content in the fermentation substrate makes the mass stirrable and flowable. Dry fermentation or solid-state fermentation is carried out with stackable organic biomass without mixing. However, it is rather rare in Germany.

The substrate is heated to around 35 to 55 degrees Celsius to accelerate the methane formation process. On average, the substrate remains in a fermenter for 30 days before degassing in the secondary fermenter for another month. The resulting biogas consists of 50 to 70 percent methane, plus carbon dioxide, hydrogen and hydrogen sulfide.

Depending on the end use (on-site power generation, feeding into the natural gas grid, etc.), the biogas is further upgraded. The remaining substrate mixture from the fermenter (digestate) is then reused by farmers as high-quality fertilizer for their fields.