Anaerobic digestion is the simple, natural breakdown of organic matter into carbon dioxide, methane and water, by two groups of microorganisms, bacteria and archaea. Since many of these are intolerant to oxygen, this process is known as anaerobic.

What are the main stages of the process?

There are four:
Each stage breaks the matter into smaller and smaller parts, until the only remaining substances are methane, carbon dioxide and water, three very simple molecules.

Hydrolysis
breaks down the complex organic matter – carbohydrates, fats and proteins – into simple sugars, fatty acids and amino acids. Carbohydrates, long chains of simple sugars, are broken down into single glucose molecules; proteins, long folded chains of amino acids, become individual amino acids; while fats, made up of head groups and fatty acid chains, have the latter part removed from the head groups and cut into smaller and smaller pieces.

Acidogenesis
sees those single sugar molecules, fatty acids and amino acids broken down further into alcohols and volatile fatty acids (like ethanol and propionic acid), with by-products of carbon dioxide, ammonia and hydrogen sulphide.

Acetogenesis
is the third stage: here, those volatile fatty acids and alcohols are converted again, this time into hydrogen, carbon dioxide, and acetic acid.

Methanogenesis
where methanogenic archaea convert the remaining hydrogen and acetic acid into methane, and more carbon dioxide.

What can we do with the final products?
At the end of the process we have a mixture of methane and carbon dioxide gases (biogas), water and some organic material (digestate).

Biogas can be burned to produce both heat and electricity, while methane can be used as vehicle fuel or injected into the gas grid.

Digestate is a stable, nutrient-rich substance and can be used for a range of products and purposes: most usefully as a fertiliser, rich in nutrients, but also as feedstock for ethanol production, and in low-grade building materials, like fibreboard.

Water, after treatment within the AD process, may be returned to the watercourses.

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