Animal and plant waste contains high percentage of protein, fat and carbohydrates. In anaerobic decomposition of such substances (which can basically be divided into four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis) a considerable amount of gas is produced. This gas has a high percentage of methane (45-65% volume), called biogas (with calorific value between 16.2 and 19.8 MJ/Nm3). The process of decomposition occurs in the mesophilic temperature zone (35 – 42 ºC), with long retention time (30 to 60 days).
The main part of the process takes place in fermenters (digesters), where from, you get the largest share of biogas, but the biological process continues inside lagoons, where the digestate is being stored and the processes end.
If the lagoons are covered in the last phase, and the remaining gas collected, additional amount of biogas can be received (3-8% based on the total amount).
To make an interesting comparison, we prepared an image where u can see the parallel between a cow and a biogas power plant.