Controlled anaerobic, or oxygen free, digestion of animal manure is a way to treat manure to help prevent foul odor production while generating a usable energy product. Under the right conditions, liquid manure will break down into biogas and a low-odor effluent. The digested manure can be stored long term and land applied with significantly less odor than untreated manure. Biogas can be burned to produce heat and/or electricity. However, anaerobic digestion does not reduce the volume or nutrient value of the manure.
An anaerobic digester is a sealed, heated tank, which provides a heated tank, which provides a suitable environment for naturally occurring anaerobic bacteria. Typical digesters have been insulated, squat, silo-like structures or in-ground rectangular or round concrete tanks with rigid or flexible covers. They are designed to hold about 20 days of manure and a small supply of biogas. Manure, added daily to the digester, remains inside for about 20 days, the retention time, before flowing to the storage facility.
Anaerobic digestion occurs naturally in liquid manure systems. The lack of oxygen and abundance of organic matter in liquid manure provide the proper conditions for anaerobic bacteria to survive. Unfortunately, uncontrolled anaerobic decomposition can cause foul odors sometimes associated with liquid manure storage and spreading. However, controlled anaerobic decomposition can reduce the odors in liquid manure systems. The effluent remaining after controlled anaerobic decomposition is liquefied, low in odor, and rich in nutrients. This digested material is biologically stable and resist further breakdown and odor production when stored under normal conditions.
Anaerobic bacteria transform manure into biogas and a liquified effluent during the three stages of biogas production. In the first stage, liquefaction, liquefying bacteria convert insoluble, fibrous materials such as carbohydrates, fats and proteins into soluable substances. In the second stage of anaerobic digestion, acid-forming bacteria convert the soluable organic matter into volatile acids (the organic acids that can cause odor production from traditionally stored liquid manure). Finally, methane-forming bacteria convert those volatile acids into biogas--a gas composed of about 60 percent methane, 40 percent carbon dioxide, and trace amounts of water vapor, hydrogen sulfide, and ammonia.
(Adapted from Ag and Biological Engineering Factsheet G-77 "Anaerobic Digestion: Biogas Production and Odor Reduction from Manure")