Anaerobic digestion is a process in which anaerobic bacteria break down or "digest" organic material in the absence of oxygen to produce "biogas" as a byproduct of their metabolism. The process occurs naturally in water-logged soils, deep water bodies, and in the digestive systems of termites and large animals (that includes you).
In anaerobic digesters, naturally-occurring biological processes are exploited in an engineered system to treat and dispose of waste materials, stabilize end products, destroy pathogens, and generate biogas, a valuable product. 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, water vapor, and hydrogen sulfide.