Anaerobic digestion is a biochemical process during which complex organic components decompose in absence of oxygen, by various types of anaerobic microorganisms. The process of anaerobic digestion is common in many natural environments such as marine water sediments, stomach of ruminants or peat bogs. In a biogas plant, the result of the process of anaerobic digestion is biogas and compost.

When the substrate for anaerobic digestion is a homogeneous mixture of two or more types of raw materials (animal slurries and organic waste from the food industries) then we have the so-called 'merger' which is common with many of its applications of biogas today.

The complete biological degradation of organic matter to biogas, in conditions of absence of oxygen (anaerobic conditions), is a complex process that involves the interaction of different groups of microorganisms. Each of these groups is responsible for performing a different part of the overall process. Thus, the material can be a waste for a group of microorganisms it can be a substrate (feed of the microorganisms) for another team. Compared with the aerobic digestion of organic matter (decomposition of organic matter in the presence of air-oxygen), the growth of anaerobic bacteria is significantly lower than that of aerobic bacteria. Consequently, the end-product obtained from the anaerobic digestion is less per unit weight of the organic material in relation to the aerobic. For example, while the aerobic degradation of 1 kg of organic material (substrate) results the production of 0,5 kg biomass, the corresponding value for the anaerobic digestion is only 0,1 kg. This feature of anaerobic digestion, that the significant reduction of the final volume is achieved, makes it particularly attractive as a method of treatment of organic waste.