Anaerobic digestion is a naturally occurring biological process that uses microbes to break down organic material in the absence of oxygen. In engineered anaerobic digesters, the digestion of organic waste takes place in a special reactor, or enclosed chamber, where critical environmental conditions such as moisture content, temperature and pH levels can be controlled to maximize gas generation and waste decomposition rates.

Landfills generating noxious odors are often demonstrations of the impact of organic waste digestion in an enclosed environment with little or no oxygen. However, in the case of anaerobic digestion technology, the gases are captured, so odors are greatly reduced. Commercial anaerobic digestion systems can replicate this natural process in an engineered reactor that produces methane gas much faster (2-3 weeks compared to 30-100 years) then anaerobic conditions in a landfill. 

One of the by-products generated during the digestion process is biogas, which consists of mostly methane (ranging from 55% to 70%) and CO2. The benefit of an AD process is that it is a net generator of energy. The excess energy produced by the AD facility, which is not required for in-plant operations, can be sold off-site in the form of heat, steam or electricity. The level of biogas produced depends on several key factors including the process design, the volatile solids in the feedstock (composition of the feedstock) and the carbon/nitrogen (C:N) ratio. Each of these factors is discussed in more detail below.