A biochemical process in which bacteria break down organic matter in the absence of oxygen into a mixture of carbon dioxide and methane known as biogas. The main stages involve hydrolysis, acidogenesis, acetogenesis, and methanogenesis. An anaerobic digester can be operated at a steady-state condition through control of temperature for psycrophilic, mesophilic, and thermophilic bacteria, pH, the carbon-to-nitrogen ratio, organic dry matter content, hydraulic retention time, degree of mixing, the availability of nutrients and trace elements, and rate of biogas removal.