During the process of anaerobic digestion, organic matter is almost entirely broken down into biogas by a range of different microorganisms. The final gas mixture primarily consists of methane (50-75%) and carbon dioxide (25-50%). Small quantities of hydrogen, ammonia, hydrogen sulphide and traces of other gases are also included in the mixture.
The four main stages of this anaerobic process are:
At the end of the process, the main elements of the gas mixture can be utilised for a number of different causes.
While biogas tends to be burned to produce both electricity and heat, methane can be injected into the gas grid or be utilised as vehicle fuel. The digestate, on the other hand, is commonly used as a fertiliser or feedstock as it is very rich in nutrients. The remaining water can be returned to the watercourses after the AD process has been completed.