What is AD?

Anaerobic Digestion (AD) is a process of decay of biomass through the activity of anaerobic (non air-breathing) bacteria, unlike composting in which fungi and lower-order creatures as well as aerobic (air-breathing) bacteria are all responsible for the degradation process. AD occurs naturally wherever high concentrations of organic matter accumulate in the absence of oxygen, for example, in the sediment of lakes and ponds, swamps and peat-bogs, and the intestines of animals.

The overall process of AD occurs through the symbiotic action of a complex consortium of bacteria. This process can be broken down into three distinct stages:

1. Hydrolytic micro-organisms break down the complex organic wastes, which are then fermented into short-chain fatty acids (e.g. glycerol), soluble acids, alcohols, carbon dioxide, hydrogen gas and neutral compounds.
2. Acetogenic micro-organisms convert the complex mixture of short-chain fatty acids to acetic acid and release further carbon dioxide and hydrogen gas.
3. Methanogenic micro-organisms convert the acetic acid and the carbon dioxide and hydrogen gases into the commonly-used term biogas. The biogas is made of predominantly methane plus carbon dioxide and numerous trace compounds.

It is one of the oldest biological processes in the world and has been used for the treatment of sewage sludge for decades and is now included in most modern sewage works. It is also used widely on the continent for the treatment of biodegradeable wastes.