Anaerobic digestion is a biological process in which bacteria break down organic matter in the absence of oxygen. A biodigester or digester is an airtight chamber in which anaerobic digestion of manure, sewage, food waste, or other organic waste streams occurs [25].

Anaerobic digestion occurs in an aqueous environment, allowing high-moisture feedstock (less than 40% dry matter) to be used without any pretreatment [26].

Digesters have been used commercially for over 30 years and are currently found in the agricultural, wastewater treatment, and food waste management sectors. Anaerobic digestion produces commodities such as biogas (a blend of methane and carbon dioxide), biosolids (used as a soil amendment), animal bedding, and fertilizer. Biogas can be used as a fuel to generate electricity, as a boiler fuel for steam production, space or water heating, or upgraded to natural gas for pipeline injection or for vehicle fuel (compressed natural gas (CNG) and liquefied natural gas (LNG)) [27]. Regardless of the type of device, control of biogas emissions leads to significant reductions in greenhouse gas emissions. Additional benefits of anaerobic digestion include potential water pollution control opportunities, and additional revenue streams or financial savings. Finally, digester projects may be eligible to sell renewable energy credits (RECs) and/or carbon offsets, which can improve project economics. Despite their potential to address pressing environmental concerns and generate revenue, digester use is not widespread in the United States.