Anaerobic Digestion

AD How it works

Anaerobic Digestion (AD) is a series of biological processes by which microorganisms break down biodegradable material in the absence of oxygen the end product of which is biogas. The process occurs naturally in swamps and bogs and also takes place in landfill sites.
AD is a 4-stage process: hydrolysis, acidification, acetogenesis & methanogenesis. These are long words which describe why cows have four stomach compartments.

An Anaerobic Digester is a device for optimizing and controlling the digestion process to produce:

biogas (made up of around 60% methane, 40% CO2 and small amounts of hydrogen, hydrogen sulphide, nitrogen, and water vapour).

biofertiliser (digestate).

gg cows walking sAD has been applied for nearly a century in the water industry as the favoured method for the stabilisation of sewage sludge, with biogas production viewed as a bonus. There are thousands of digesters in rural China, India and Nepal where village-scale plants are important for the recycling of nutrients in farm manure and producing biogas for cooking and lighting.

The technology has been widely adopted across Europe. In Denmark there are a number of farm co-operative AD plants which produce electricity and district heating for local villages. In Sweden biogas plants have been built to produce vehicle fuel while in Germany and Austria, encouraged by favourable government renewable energy policies, there are several thousand on-farm digesters treating mixtures of manure, energy crops and restaurant waste, with the biogas used to produce electricity.