Anaerobic digestion—technology
Produced in partnership with WSP Environmental
Anaerobic digestion—technology

The following Energy practice note produced in partnership with WSP Environmental provides comprehensive and up to date legal information covering:

  • Anaerobic digestion—technology
  • What is anaerobic digestion?
  • Organic Feedstock
  • Biogas
  • Digestate
  • Stages
  • Pre-treatment
  • Digestion
  • Refining of biogas
  • Digestate treatment
  • More...

Anaerobic digestion—technology

What is anaerobic digestion?

Anaerobic digestion (AD) is the creation of digestate and biogas (mainly methane), through the breakdown of an organic feedstock in the absence of oxygen by micro-organisms. This process occurs in an insulated sealed container, heated between 35°C and 55°C.

Organic Feedstock

Almost any biomass can be processed in AD, including food waste, energy crops, crop residues, slurry and manure. Biomass waste from wood cannot be used in AD because the micro-organisms cannot breakdown the lignin.


Biogas is a mixture of methane (60%) and Carbon dioxide (40%) including traces of other contaminant gases. The exact composition depends on the type of feedstock being digested. Biogas can be refined into bio-methane, which can be combusted to provide heat, electricity or both. This is achieved by removing other gases and can be injected into the mains gas grid, burned for heat, generate power, or used as a road fuel.


Digestate is made from left over indigestible material and dead micro-organisms. It contains valuable plant nutrients like nitrogen and potassium. It can be used as a fertiliser and soil conditioner depending on the nature of input material. The volume of digestate will be around 90-95% of what was fed into the digester.


There are four key stages in the AD process. These include:

  1. pre-treatment

  2. digestion

  3. refining of biogas

  4. digestate treatment


The chosen feedstock will usually require some form of pre-treatment

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