Anaerobic digestion (AD) is a naturally occurring biological process that uses microorganisms to break down organic material in the absence of oxygen. In engineered AD systems, the breakdown takes place within specially designed reactors or chambers. Critical environmental conditions, such as moisture content, temperature, and pH levels, are measured and controlled within the reactor to maximize biogas generation and waste decomposition rates. In an engineered system for municipal solid waste (MSW) source separated organics (SSO) digestion, the digestion process generally occurs during a two- to six-week period. Chapter 6 describes specifics on engineered AD systems and equipment currently being used for MSW organics processing.
Perhaps the most important byproduct of the AD process is biogas because it can be used as fuel, so provides a renewable energy source. Biogas consists primarily of methane (CH4) and carbon dioxide (CO2), but can also contain significant concentrations of hydrogen sulphide (H2S) and may also contain trace quantities of siloxanes and various volatile organic compounds (VOCs).