Anaerobic digestion (AD) is a biological process similar to composting, but without air. As in composting, micro-organisms break down organic matter into simpler smaller compounds and reduce its bulk or “mass”.
However, unlike composting, which overall always consumes energy, anaerobic digestion can be used to create energy. That is because it produces a gas known as biogas, just over half of which is methane. Everyone that lives in a developed nation and has cooked anything on a kitchen stove, using gas piped in from the street, has used methane, and knows how cleanly it burns, and how wonderfully hot the flame is, because “natural gas” is pure methane.
In addition to biogas, the process also produces “digestate”, which is a solid fibrous residue, and any liquid which is left over.