Anaerobic Digestion

Unlike aerobic digestion, anaerobic digestion is the decomposition of biomass by bacteria in the absence of oxygen. Biogas, or methane, is the primary product produced with digestion effluent being sold as fertilizer and bedding materials. Anaerobic digestion is governed by temperature, retention time, chemical composition of the influent, and presence of toxicants, and as each influence changes, so does the quantity and quality of biogas produced. Fluctuation from a high temperature gradient to a lower one during digestion has shown to produce 25 percent more biogas than maintaining a single steady heat (Mathews 2004). Retention times typically range from 25 to 35 days and pH levels must remain relatively steady at around 7 (Simons 2004).

Not all waste streams are appropriate for anaerobic digestion. The higher the fat content in the feedstock the more biogas can be produced. Other feedstocks, such as lignin, require longer retention times and/or higher concentrations of bacteria in the substrate (Simons 2004). New technologies are looking to increase yields and decrease time by adding ultrasound to the process. Called sonication, the waves disintegrate the solids in the influent, increasing surface area and allowing for more complete digestion (Yoshitani 2003).

Anaerobic Digestion Pathway