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How Anaerobic Digesters Work

Anaerobic digesters use naturally occurring microorganisms to break down organic materials, called feedstocks, in an oxygen-free environment. On dairies, the primary feedstock is manure, though some systems also accept other organic materials. As microbes decompose this material, they produce biogas and leave behind a nutrient-rich material called digestate.

  • Manure enters the digester. Manure is moved into an anaerobic digester, which may be a sealed tank, plug-flow system, mixed reactor, or covered lagoon.
  • Microorganisms break down organic material. Inside this oxygen-free system, microorganisms break down manure and other feedstocks.
  • Biogas is produced. Biogas is primarily methane, the energy-rich part of the gas, and carbon dioxide. After cleaning or upgrading, biogas can be used to produce heat, electricity, or renewable natural gas.
  • Liquid and solid byproducts (called digestate) are produced. Farms may apply the liquid to fields as a nutrient source, separate the solids for bedding or soil amendments, or use additional technologies to recover nutrients or water.

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The digestion process depends on consistent conditions. Temperature, retention time, feedstock quality, and system management all affect how well the digester breaks down manure, captures methane, and produces usable outputs.

Digesters are often part of a broader manure management system rather than a standalone technology. Additional equipment for gas cleaning, solids separation, nutrient recovery, or water treatment can play a major role in how the system performs and whether it is economically viable.

Learn about the benefits and tradeoffs of anaerobic digesters.

Image of steps of biorefinery, which are illegible at this size.
Dairy manure bio-refinery with associated processes and products. Graphic by Nicholas Kennedy and Jingwei Ma. Photos courtesy of American Biogas
Council, Regenis, DVO Inc., Keith Bowers, Craig Frear, Jim Jensen, and Rita Hummel. Source: The Dairy Manure Biorefinery, Washington State University Extension.

Resources

These WSU Extension publications introduce anaerobic digesters and associated technologies: