Scientists understand the term “fermentation” as the microbial degradation of organic materials without the influence of oxygen for the purpose of energy production.

Agricultural biogas plants normally use animal excrements (liquid and solid manure) and energy plants as a substrate. In non-agricultural plants, material from the biological waste bin or waste products from food production are used. As a by-product, a fertilizer referred to as fermentation residue is produced or the residue is passed on for recycling by means of subsequent composting (cascade utilization). In most biogas plants, the generated gas is used locally in a combined heat and power unit (CHP) to produce electricity and heat. Other biogas plants process the generated gas and feed it into the natural gas grid.