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.
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.