Anaerobic digestion - the process

Anaerobic digestion processing systems operate in different ways. For example, material may be fed into a reactor in distinct batches, or in a continuous flow. 

Pre-treatment

Feedstock processing begins with pre-treatment. This involves mixing the various feedstock elements together to ensure the right consistency and C:N ratio and may also involve the addition of water.

The material should also be screened for contaminants, such as plastic and grits at this stage. 

Packaged food waste will also be extracted from its packaging at this stage.

Digestion

The feedstock is then fed into a digestor.

Digestion usually occurs under a slow mixing process and the biogas produced is collected.

Use of the digestate

The digestate produced is stored until required, and can be separated into liquid and solid fractions.

Solid fractions can be processed further on site by being put into a composting operation for further processing or used directly on land.

The liquid may also be used on the land as a biofertiliser. 

The Anaerobic Digestion Quality Protocol (ADQP) has also now been published, providing information about the end of waste criteria for digestate. The ADQP is available from the Environment Agency website for download.

Use of biogas

The biogas produced will be stored before being either refined further into biomethane for vehicle fuel or for injection into the gas grid or burned in a combined heat and power engine to produce electricity and heat, or burned in a gas boiler to produce heat for local use.