How it Works

There are four key biological and chemical stages of anaerobic digestion:

1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis

In most cases organic material is made up of large organic polymers and in order for the bacteria in the anaerobic digester to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts.

The first stage of the process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. The biological process of acidogenesis is where there is a further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here Volatile Fatty Acids are created along with ammonia, carbon dioxide and hydrogen sulphide as well as other by-products.

The third stage of anaerobic digestion is acetogenesis and it is at this stage simple molecules are created through the acidogenesis phase and further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.

The final stage of the biological process is the methanogenesis stage. Here methanogens use the various products of the previous 3 stages and convert them into typically (60%) methane (CH₄), (40%) carbon dioxide (CO₂), water as well as other trace elements. The remaining, non-digestable material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.

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