Anaerobic Digestion (AD) is a biological process that happens naturally when bacteria breaks down organic matter in environments with little or no oxygen. It is effectively a controlled and enclosed version of the anaerobic breakdown of organic waste in landfill which releases methane.
Almost any organic material can be processed with AD, including waste paper and cardboard (which is of too low a grade to recycle, e.g. because of food contamination), grass clippings, leftover food, industrial effluents, sewage and animal waste.
The material to be processed can be shredded to increase the surface area available to microbes in the digesters and hence increase the speed of digestion. The AD process takes place in an airtight container, known as a digester.
The first stage of AD is a 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.

This is followed by three biological processes:

1.    acidogenesis - further broken down by acidogenic bacteria by into simpler molecules, volatile fatty acids (VFAs) occurs, producing ammonia, CO2 and hydrogen  sulfide as byproducts.

2.    acetogenesis - the simple molecules from acidogenesis are further digested by bacteria called acetogens to produce CO2, hydrogen and mainly acetic acid.

3.    methanogenesis - methane, CO2 and water are produced by bacteria called methanogens.

The pH level should be kept between 5.5-8.5 and the temperature between 30-60°C, in order to maximise digestion rates. Biogas produced in anaerobic digesters consists of methane (50–80%), carbon dioxide (20–50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The biogas production rates according to the implemented technology, however some typical values vary from 70 to 120 m3 per ton of feed. In addition, the Lower Heating Value (LHV) varies according to the methane consistence and ranges between 18-22 MJ/Nm3.