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What is anaerobic digestion?

What is anaerobic digestion? This article discusses what it is, the biochemical reactions which form the AD process and the end products.
A biogas plant
© University of York/BioYorkshire

So…had you ever heard of AD or anaerobic digestion?

It’s a long-established and important technology that helps us manage our waste – things like food waste, sewage and animal manure – while also producing renewable energy and fertiliser.

AD uses microorganisms to break down waste organic matter without oxygen being present. Anaerobic digesters come in a number of different sizes from food waste treatment plants processing industrial quantities of waste to on-site plants on farms to micro-digesters that could fit in a container or even in your back garden!

When the feedstock is added to the digester the organic matter goes through four biochemical reactions which occur simultaneously:

  • Hydrolysis – Enzymes released by bacteria split long chain organic compounds (fats and proteins) into simple organic compounds (sugars and amino acids)

  • Acidogenesis – Products produced by hydrolysis are converted by bacteria into organic acids as well as forming acetate, hydrogen and carbon dioxide.

  • Acetogenesis – Bacteria convert the organic acids and alcohols into acetic acid, hydrogen and carbon dioxide.

  • Methanogenesis – Bacteria convert the products produced by the other processes into methane and carbon dioxide.

The process also produces a nutrient-rich biofertiliser called digestate.

Are there any AD facilities in your local area? Share your comments about AD below.

© University of York/BioYorkshire
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