Anaerobic digestion is a biological treatment process in which microscopic bacteria break down organic materials (e.g. livestock wastes) into gases and liquids. The word anaerobic means without oxygen, as opposed to digestion using microorganisms that need oxygen (aerobic digestion). The anaerobic process is carried out in a tank or other enclosure (digester) which eliminate or largely restrict the entrance of oxygen.

When the digester becomes too acidic, this means the acid forming bacteria was producing faster than the methane formers. The "acid formers" produce too much acid for the "methane formers" to digest, causing the balance to be more acidic. This sours the digester and prevents the formation of methane. One method to correct this situation is to dump sodium bicarbonate into the digester. This counteracts the acid and brings the balance back to normal.