Anaerobic Digestion is the breakdown of organic matter by living processes (bugs) in the absence of Oxygen. It has been going on since the first blob of primaeval stuff looked at his neighbour and thought "I wonder what he tastes like?".
The first reliable report of deliberate Anaerobic Digestion by humans comes from 10th century BC Assyria where rotting manures were used to heat bathwater. In the 17th century Jan Babtita Van Helmont discovered that rotting animal manures produced flammable gases, and in 1808 Sir Humphrey Davy determined that the gas methane (CH4) was the flammable constituent.
In 1859 a leper colony in Bombay, India built the first digestion plant, and in 1895, Exeter in Britain, became the first town whose gas lights were powered by sewage gas (now referred to as Biogas).
It was not until the 1930s that the link to anaerobic bacteria was established.
There are three identifiable stages to the Anaerobic Digestion process:
Carbohydrates, cellulose, proteins and fats are broken down by enzymes produced by hydrolytic bacteria. During this phase Carbon Dioxide (CO2) is given off and about 20% Hydrogen (H2). There is considerable work being carried out attempting to isolate this H2, but commercialisation is some way in the future.
Organic acids formed in the previous hydrolysis phase are converted by acetogenic bugs to acetic acid (vinegar) and other organic acids. Production of CO2 and H2 drops to nearly zero during this phase.
Methanogenic bacteria proliferate and using the organic acids produced in the Acidogenesis phase they produce methane at around 60% and CO2 at around 40%.