Anaerobic digestion - the process

13th December 2016

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.

Overview

Overview

In this guide we detail the basic steps in the AD process. Although the exact process may vary for different types of AD or be altered to favour intended outcomes, most current AD operations follow this process.

To read the steps, please use chapters below:

The process

 

Related

Related

To find information related to that contained in this report, please use the following links:

In vessel composting (IVC) >>
Open windrow composting >>
Anaerobic digestion >>
Thermophillic aerobic digestion >>

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.

To find more information on the uses of digestate, please use the following links:

Biofertiliser management: Best practice for agronomic benefit and odour control >>

Bioremediation case studies >>
Biofertiliser management >>

Good practice guide for PAS 100 compost >>
BSI PAS 110 Producing quality anaerobic digestate >>
Using compost and digestate >>
Using compost and digestate good practice guide >>
Compost use in landscape and regeneration Good Practice Guide >>
Digestate use in landscape and regeneration >>
New markets for digestate >>
Landscape and regeneration case studies >>

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. 

To find more information on the production and use of biogas, please use the following links:

UK AD & Biogas >>
AWM gas to grid guide >>

Where next?

Want to know more about anaerobic digestion? Take a look at our reports, guides, tools and case studies for further information:

Reports >>
Guides >>
Tools >>
Case studies >>