AeroThermal Group PLC

Anaerobic Digestion (AD)


Anaerobic Digestion (AD) is the preferred stabilisation process for the treatment of wastewater sludge and organic wastes. The process provides volume and mass reduction and delivers valuable renewable energy with biogas production. Anaerobic digestion is a simple process that can greatly reduce the amount of organic matter that might otherwise be destined for landfill or burnt in an incinerator.


Anaerobic digestion is a naturally occurring biological process that comprises the following key steps:

  • Hydrolysis – large molecules/polymers such as polysaccharides and proteins are converted to smaller compounds.
  • Acidogenesis – conversion of small compounds to Volatile Fatty Acids (VFA) by anaerobic digestion.
  • Methanogenesis – Volatile Fatty Acids are broken down by methanogenic bacteria to CH4 and CO2.

With the exception of high lignin compounds, almost any organic materials, such as waste paper, food, sewage and animal waste can be processed with anaerobic digestion. By using an autoclave process as a pre-treatment, where the material is hydrolysed even the lignin barrier reduces, ensuring more material is amenable to biodegradation.

A by-product from this anaerobic digestion process is biogas, which is largely composed of methane and carbon dioxide. It contains a calorific value of approximately 21 MJ/m3 and can easily be used as a fuel to create renewable energy, attracting Renewable Obligation Credits (ROC) in many parts of the world.

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