How Anaerobic Digestion (AD) Works

Anaerobic Digestion is a natural biological process where organic matter is broken down by bacteria in the absence of oxygen to produce biogas for renewable energy.

The AD process takes place in an airtight container, known as a digester. The material to be processed is macerated to increase the surface area available to microbes in the digesters and hence increase the speed of digestion.

The first stage of AD is a natural chemical reaction called hydrolysis, where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups.

isometric view

This is followed by three biological processes:

  • acidogenesis – further broken down by acidogenic bacteria into simpler molecules, volatile fatty acids (VFAs) occurs, producing ammonia, CO² and hydrogen sulphide as by-products.
  • acetogenesis – the simple molecules from acidogenesis are further digested by bacteria called acetogens to produce CO2, hydrogen and mainly acetic acid.
  • methanogenesis – methane, CO² and water are produced by bacteria called methanogens.

After this natural process we have 2 key products:

‘biogas,’ which we use to produce our green energy. It is carbon neutral, because the CO2 released from ‘biomass’ when it is digested is reabsorbed by plants grown to replace it; and:

‘bio-fertiliser,’ which is used on farmland. The bio-fertiliser contains Nitrogen, Phosphates and Potash and is used to replace expensive mineral fertilisers. The bio-fertiliser also contributes to the organic matter content of the soil, enhancing biological activity, availability of nutrients and water retention.

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