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Anaerobic digestion ( AD) is the breakdown of organic material by micro-organisms in the absence of oxygen. AD produces biogas, a methane-rich gas that can be used as a fuel, and digestate, a source of nutrients that can be used as a fertiliser. It is a natural process that has been used for many years - the first digester was built in India in 1859 - and it is widely used in the sewage industry to treat effluent. Increasingly it is being used to make the most of our waste by turning it into renewable energy.
There isn't a single type of bacteria that can turn biomass straight into methane. The process of AD occurs in several steps and requires a community of micro-organisms.
- Hydrolysis
Large, complex polymers like carbohydrates, cellulose, proteins and fats are broken down by hydrolytic enzymes into simple sugars, amino acids and fatty acids.
- Acidogenesis
Simple monomers are broken down into volatile fatty acids
- Acetogenesis
The products of acidogenesis are broken down into acetic acid, releasing hydrogen and carbon dioxide.
- Methanogenesis
Bacteria called methane formers produce methane either by cleaving two acetic acid molecules to form carbon dioxide and methane, or by reduction of carbon hydroxide with hydrogen.
You can read more about the process of AD in this report by the NNFCC and Andersons Centre.
The Products of AD
The mixture of gases produced by AD is called biogas. The main gas is methane (CH4) at around 60%, a colourless, odourless gas, and carbon dioxide (CO2) at around 40%. There will also be small amounts of contaminant gases, mostly hydrogen sulphide and ammonia. The precise make up of the gases depends on the type of feedstocks and the type of AD.
Biogas provides a source of renewable energy. It can be used to produce electricity in a generator or combusted to produce heat. It can also be 'upgraded' by cleaning the biogas to remove the CO2 and contaminant gases, leaving pure methane which is often called bio-methane. Bio-methane is effectively renewable natural gas, so it can be used to provide heat and power or compressed for use as a renewable road fuel.
Digestate is the undigested remnants of the feedstocks that the bacteria cannot use and the remains of dead bacteria. It contains valuable plant nutrients like nitrogen, phosphate and potassium and organic humus, so it can be spread on the land as a substitute for synthetic fertiliser.
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