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Anaerobic Digestion 101

For an illustration of a manure-fed anaerobic digester system, see AgSTAR's Anaerobic Digester System Flow Diagram (PDF) (1 pg, 826K), and for more in-depth information on how anaerobic digester systems work see Recovering Value from Waste (PDF) (4 pp, 855K).

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. Anaerobic digestion systems are often referred to as “anaerobic digesters,” “biodigesters” or “biogas recovery systems.”

AgSTAR compiled a guide to the major parts of a biogas recovery system.

  • Manure collection systems.
    A system is needed 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 digesters.
    Anaerobic digesters, commonly in the form of covered lagoons or tanks, are designed to stabilize manure and optimize the production of methane. A facility for digester effluent storage is also required.

  • Biogas handling systems.
    Biogas (a product of the decomposition of the manure, typically comprising about 60 percent methane and 40 percent carbon dioxide) is collected, treated, and piped to a gas use device.

  • Gas use devices.
    Biogas can be used to generate electricity, as a boiler fuel for space or water heating, upgraded to natural gas pipeline quality, or for a variety of other uses. Flares are also installed to destroy extra gas and as a back-up mechanism for the primary gas use device.

  • Digester byproducts.
    In addition to the biogas recovery system, the effluent of the anaerobic digestion can be used to create a number of digester byproducts.
Manure source and collection
Manure handling
Anerobic digester Effluent storage Digester byproducts
Biogas handling systemFlare gas
Gas use




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