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Anaerobic Digestion

Anaerobic digestion is a biological process in which bacteria digest biomass in an oxygen-free environment and produce a gas principally composed of methane (CH4) and carbon dioxide (CO2), otherwise known as biogas. This process is often used for sewage treatment or for managing animal waste. However, almost any organic material can be processed in this manner. This includes biodegradable waste materials such as waste paper, grass clippings, leftover food, sewage and animal waste.

The process of anaerobic digestion consists of three basic steps. The first step is the decomposition or hydrolysis of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to biogas.

The biogas can be used to co-generate heat and electricity, or it can be scrubbed  (separated into purified streams of gases) and compressed into a liquid fuel or used as an industrial chemical base for creating other products.  In addition to biogas,  anaerobic digesters can produce raw material by-product streams that can be further refined into higher value products such as fertilizer.   

In Washington State, anaerobic digestion of dairy wastes is receiving a lot of attention. Currently there are approximately 600 operating dairy farms in the state, that manage nearly 250,000 dairy cows. These dairies are often identified as sources of odor, water and air pollution, including greenhouse gas emissions. The use of anaerobic digesters to help manage dairy wastes can alleviate many of these environmental problems, while providing farmers with a new source of revenue. 

Washington State University’s Center for Sustaining Agriculture and Natural Resources has taken the lead in developing anaerobic digestion technologies for dairy operations in Washington. Through the Climate Friendly Farming initiative, WSU is investing in research, technology development, demonstration and educational activities to advance the use of this technology throughout the state. For more information on anaerobic digestion and WSU activities related to this technology, please visit the Climate Friendly Farming website.

State support for anaerobic digestion applications also extends beyond WSU activities. The Department of Ecology’s Beyond Waste program is looking at a variety of sustainable, waste management technologies including anaerobic digestion. In addition, the Energy Freedom Fund, passed by the 2006 Washington State legislature, has awarded funding to two anaerobic digestion projects located near the cities of Sunnyside and Monroe, Washington. 

Additional Resources:

Washington State University Climate Friendly Farming

Washington State Department of Ecology Beyond Waste Program

EPA AgSTAR Program

Northwest CHP Application Center

For dairy farmers and others interested in exploring the idea of using anaerobic digestion as a waste control measure, the Northwest CHP Application Center has developed a factsheet to help identify key questions prior to implementing an anaerobic digestion program. This factsheet can be found at the CHP Application Center website.

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