Anaerobic Digestion

Types of anaerobic digestion systems

Anaerobic digestion systems can be categorized into the continuous process, batch process and semi-continuous process according to the way of flow of substrate in the digester.

Continuous process
In a continuous process, the substrate is added to and removed from the digester continuously. Since fresh substrate is added continuously, all reactions involved in biogas generation will occur at a fairly constant rate. This results in a fairly constant biogas production rate. Usually, two digesters are used in the continuous process and the substrates are digested in two stages. The advantage of this process is that the digesters can be used as storage devices.

Batch process
In the batch process the substrate is put in the digester and then the digester is sealed for the entire period without adding additional substrate until the decomposition process is near completion. Most of the digested substrate is then emptied and the digester is filled with new substrates, and then the digestion process starts again. In a batch process, the production of biogas is non-continuous. Gas production will peak at the middle of the process and will be low at the beginning and at the end of the process. Typically, in order to ensure a more steady supply of biogas, a number of batch digesters with substrates at different stages of anaerobic digestion are operated in parallel.

Semi-continuous process
In a semi-continuous process, the digester initially works under the batch process. A small fraction of the digester contents is then released. After the release process, fresh material is injected to make up the contents and the digestion process is allowed to run for a while until the next release, followed by subsequent injection again.