Anaerobic Digestion

Anaerobic digestion (AD) is the degradation of organic matter by microorganisms under oxygen-free conditions. The process produces biogas – mainly methane and carbon dioxide – that can be used to produce heat and electricity, and even to power vehicles.

AD is a natural process which occurs in wetlands and can be likened to the digestive system of a cow. The microorganisms’ breakdown the matter and produce biogas and digestate, which can be further separated into fibre and liquor and used as animal bedding, peat-alternative and fertiliser on the land.

Useable Substrates 

By-products and wastes from industries involved in the production, preparation and processing in agriculture, horticulture, meat industry and aquaculture, sugar processing, dairy, baking and confectionary, drinks, wood processing, paper production, leather, fur and textile industry, packaging, waste water treatment plants.  There may be an element of pre-treatment in using these substrates to mix, remove inhibitory compounds and the addition of water for example.


Co-digestion, the mixing and digestion of two or more substrates is required when a substrate lacks nitrogen. Co-digestion aids solid material digestion, improves biogas production and can dilute inhibitory compounds.

Advantages of AD

  • Kills pathogens in feedstock used, for example human sewerage, because anaerobic bacteria out-compete the pathogens.
  • Reduces odours (compared to directly spraying AD substrate on the land)
  • Alternative to composting and landfill, which create greenhouse gases, leachate, odour, as well as needing space and transportation and as such are an expense.
  • Reduces need for fossil fuels
  • Reduces the need for fertilisers and therefore the costs and environmental impacts associated with this.
  • Releases nutrients such as nitrogen and phosphorus increasing the soil fertility.  It also conditions the soil* raising the soil organic content, which reduces irrigation demand, surface run-off and eutrophication and soil erosion. It is also an alternative to peat*, therefore reducing the environmental impacts of peat harvesting.
  • Allows for energy sustainability, reduction in energy bills and revenue generation.

*Before AD digestate can be used on the land is subject to meeting quality and fit for purpose legislation set out by SEPA & WRAP.

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