Anaerobic decomposition is a complex process that occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in an airtight tank called a digester or a covered lagoon, a pond used to store manure, for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, biogas production is a highly useful but secondary benefit.

Anaerobic decomposition occurs in three basic stages as a result of activity of a variety of microorganisms. To form organic acids, a group of microorganisms converts organic material to a form that a second group of organisms utilize to form organic acids. Methane-producing anaerobic bacteria utilize these acids and complete the decomposition process.

At higher temperatures (above 110 °F) decomposition and biogas production occur more rapidly because the process is highly sensitive to disturbances such as changes in feed materials or temperature. While all anaerobic digesters reduce the likelihood of weed seeds and disease-producing organisms, the higher temperatures of thermophilic digestion (113 and 176 °F) result in more complete damage. The process is less sensitive to distress or change in operating regimen, utilizing digesters operating in the mesophilic range (the temperature range of 95 to 105 °F) which must be larger to accommodate a longer period of decomposition within the tank.

Sludge, or waste matter, is the material drawn from the anaerobic digester. It is an excellent soil conditioner because it is rich in nutrients such as ammonia, phosphorus, potassium, and more than a dozen trace elements. The sludge, when dried, can also be used as a livestock feed additive. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the waste matter, and therefore is important to test its content before using it on a large scale.

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