Which renewable-energy process uses microorganisms to break down waste without oxygen?

Anaerobic digestion uses microorganisms to break down waste without oxygen.

In an anaerobic digester, bacteria and other microorganisms decompose biodegradable material in an oxygen-free environment. Typical feedstocks include animal manure, sewage sludge, food scraps, and energy crops. The process produces biogas and a nutrient-rich residual material called digestate.

Biogas is chiefly a mixture of methane and carbon dioxide. Operators can burn it in an engine or turbine to make electricity and heat, or clean it into biomethane. Digestate may be used as a soil amendment when regulations and contamination controls permit.

Anaerobic digestion differs from composting, which generally requires oxygen and produces mainly heat, carbon dioxide, and stabilized organic matter. It also differs from pyrolysis and gasification, which use high temperatures rather than living microbial communities as the main conversion mechanism.