Biological Fundamentals of Biogas Production

Biogas is produced by the decomposition of organic matter under anaerobic conditions. It is therefore an anaerobic degradation process of organic matter, which also occurs in natural cycles and that regarding biogas production is referred to as fermentation.

Organic substances (fats, carbohydrates and proteins) are broken down by microorganisms into low molecular weight components. This anaerobic conversion is done by various bacterial strains, which consecutively degrade each other.  The process takes place in different stages.

The first stage of degradation is called hydrolysis. Here, the used organic materials are broken down into shorter fragments. This is done by means of hydrolytic bacteria.

In the second step (acidification), the fermentative microorganisms take the fragments and bind them in their metabolism. As a final result of this stage fatty acids such as acetic and propionic acid are obtained.

The third stage converts the remaining unused metabolic products in acetic acid. For this reason, this stage is also known as acetic acid formation.

The final step is the formation of methane. In this stage, methane bacteria convert acetic acid, hydrogen and carbon dioxide into biogas.

The produced biogas is about 50-60% in high-energy methane (CH₄) and 30-50% in carbon dioxide (CO₂). In addition, it generates small proportions of hydrogen sulphide and hydrogen (<1%).