Degradation of organic material in the absence of oxygen is called anaerobic. This limits the occurrence of anaerobic biomass degradation in the natural environment. This environment is known, for example, from bogs or stagnant waters with a lot of biomass in the sediment. One product of anaerobic degradation processes is biogas (methane gas). Due to this fact, anaerobic degradation or digestion is used in wastewater technology. Digestion specifically produces methane gas and converts it into electricity and heat. Since the speed of the degradation process is temperature-dependent, the digestion process is operated on a technical scale at higher temperatures than in the environment. A distinction is made between mesophilic (20 °C - 40 °C) and thermophilic (40 °C - 57 °C) temperature ranges. In nature, these processes mostly take place in the psychrophilic (< 20 °C) range.

Anaerobic degradation follows different stages.

First, particulate and high-molecular substances such as fats, carbohydrates and proteins are degraded. This is done by facultative anaerobic bacteria and is called the hydrolysis phase.

In a further step, the acidification phase, the decomposition products from the hydrolysis phase are converted into hydrogen, carbon dioxide, organic acids and alcohols.

This is followed by the conversion of these substances to acetic acid and already small amounts of methane from the reaction of H₂ and CO₂. Since the main part is converted to acetic acid, this stage is also called the acetogenic stage.

At the end of the process, the methanogenic phase breaks down the acetic acid to methane.
However, this process does not occur spatially separated in the digester, but takes place simultaneously over simultaneously.


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