The United States Environmental Protection Agency describes the biorefinery process as follows:
Anaerobic digestion is a biological process that occurs when organic matter (in liquid or slurry form) is decomposed by bacteria in the absence of oxygen (i.e., anaerobic). As the bacteria “work,” biogas is released, which consists of approximately 60% methane and 40% carbon dioxide. This gas can be recovered, treated and used to generate energy in place of traditional fossil fuels. The effluent remaining after controlled anaerobic decomposition is low in odor and rich in nutrients.
Methane emissions occur whenever animal waste is managed in anaerobic conditions (i.e., free from oxygen). Manure deposited on fields and pastures, or otherwise handled in a dry form, produces negligible amounts of methane. Through the use of liquid manure management systems such as ponds, anaerobic lagoons, and holding tanks, anaerobic digesters provide an ideal, oxygen-free environment for methane production through bacterial decomposition (the resulting biogas is a mixture of about 60 percent methane and 40 percent CO2). A cover and gas collection system is an essential part of the process; otherwise, methane produced by the system would simply escape to the atmosphere.
Anaerobic digestion systems have four components:
- Manure collection system—A system to collect manure and transport it to the digester. Existing liquid/slurry manure management systems can readily be adapted to deliver manure to the anaerobic digester.
- Anaerobic digester—Commonly in the form of covered lagoons or tanks, digesters are designed to stabilize manure and optimize the production of methane.
- Biogas-handling system—A system to collect, treat and pipe the biogas (a product of the decomposition of the manure, typically comprising about 60 percent methane and 40 percent carbon dioxide) to a device that can put the gas to use.
- Biogas use device—Biogas can be used to generate electricity, as a boiler fuel for space or water heating, upgraded to commercial natural gas quality, or for a combination of these and other uses. Flares may also be installed to burn off extra gas and as a back-up mechanism for the primary gas use device.
Effluent storage is also needed to contain the treated effluent until it is applied to land. Solids separators, which remove the digested solids for uses such as bedding materials or soil amendments, are common components in dairy applications.
Anaerobic digestion systems differ from conventional manure storage lagoons because they separate the treatment and storage functions. This design provides several benefits, including:
- Lower total volume requirements, which reduce excavation costs and the land area required for the waste management system; and
- Lower cover costs for covered lagoons because of smaller lagoon surface areas.
The separation of treatment and storage areas also improves environmental performance. Visit AgSTAR’s Anaerobic Digestion 101 webpage for more descriptions and photos of anaerobic digestion systems.