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How Anaerobic Digestion Works
Anaerobic digesters are typically large reactors constructed of either concrete or steel. The volume of the reactor depends on the volume of wastes to be processed in the system. With most conventional digesters, a holding time of 20 - 30 days is required to convert manure waste into methane. Methane gas can be utilized onsite, serve as fuel for an electricity generator, or purified and supplied to public natural gas lines.
In cogeneration, heat produced by the electricity generator is captured and utilized to meet digester heating requirements to 35oC. Cogeneration has been the most common use for methane produced by anaerobic digestion. Recently, there is a growing interest in purification of biogas for resupply to natural gas lines due to high maintenance requirements for generators. This requires that all gas components aside from methane are removed. Of note is that hydrogen sulfide should be removed from biogas for cogeneration due to its corrosive nature. This can be done by simply passing the biogas through iron filings.
Anaerobic digestion requires that feed material be of low solids content, less than 15% solids by weight. The microorganisms that convert organic materials into methane are very sensitive, requiring a pH near 7. In addition, the organisms work best at high temperature, around 35oC. For each 11oC decrease, gas production will be cut by approximately one half or will take twice as long. In the digester, organics are removed as they are converted to methane while nutrients are conserved. The end product is a low odor, high nutrient, stabilized waste suitable for land application.
Updated Thursday, May 17, 2012