Anaerobic Digestion

What is AD?

Anaerobic Digestion (AD) uses naturally occurring micro-organisms to break down plant and animal material in a sealed tank or ‘digester’ in the absence of air. The end result of the process is mainly methane biogas and an organic nutrient rich fertiliser known as ‘digestate’. This process has been naturally taking place since the world began and is the source of the natural gas we use today.

The UK has been harnessing the power of AD since the 1800s and it is already widely used in the sewage industry to render thousands of tons of waste into safe and virtually odour-free fertilliser. In the meantime, AD technology has been developed into an extremely efficient process and it now plays a major role in supplying renewable energy for many counties.

Extending its use to process waste from UK homes, farms, factories, supermarkets and schools will increase the amount of clean, renewable energy we are able to produce and help to reduce landfill.

AD can be classified as:

  • Mesophilic – Operating at a range of 25 – 40 °C
  • Thermophilic – Operating at 50 – 60 °C (faster though put and biogas production but with a higher energy and capital requirement).
  • Wet or Dry Digestion – Feedstocks are pumpable with wet digestion and more easily mixed than dry digestion. There are higher running costs with wet digestion but lower capital outlay.
  • Continuous flow or batch digestion – Most systems are based on a continuous flow of mixed feedstocks giving a controlled and continuous production of biogas.
  • Single tank or multiple tank digestion – Multiple stage digesters are the most efficient design to ensure efficient gas production and breakdown of organic matter but will increase capital outlay

Anaerobic digestion diagram


AD plant facilities allow the capturing the methane gas from agricultural and food wastes that would otherwise escape into the atmosphere during breakdown adding greatly to greenhouse gas emissions and potential climate change issues. Most food and farm waste currently has no control on its methane emissions. This causes environmental damage and is a huge renewable fuel resource being lost.

AD is a sustainable and non-intermittent renewable energy source that also improves energy security and increases food production. It allows farmers to become energy self-sufficient and energy producers while increasing the long term profitability and financial security of their farms.


Biogas is a mixture of approximately 60% methane and 40% carbon dioxide with traces of hydrogen sulphide and ammonia – although the exact make-up depends on the type of waste which has been used in the digestion process.

Biogas and its derivative, upgraded biomethane, can be used for:

  • heat production
  • electricity production
  • heat and power production
  • transport fuel in vehicles designed to run on compressed gas
  • injection into the mains gas grid for domestic or industrial use.


Digestate is made up of left over organic material and dead micro-organisms. It contains mineralised, bioavailable nutrients − including 25% more plant available nitrogen. Digestate has a reduced volume of about 90 – 95% of the original feedstock and and is a highly effective fertiliser.

Digestate is weed and pathogen-free and eliminates pre-spreading manure storage times. It can be piped directly onto fields, thereby reducing soil compaction.  It also significantly reduces and/or eliminates the farmer’s reliance on bought-in fossil fuel fertiliser. This enables farmers to produce environmentally-friendly food at no increased cost to the consumer.