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Anaerobic Digestion Process
Last Updated on:  06/25/2015 06:30 AM

Also see:  Biomass    Biogas    Algae    Wood

Anaerobic digestion is a biological process.  Anaerobic digesters provide an environment for the oxygen-free decomposition of organic materials by bacteria.   It produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas in an oxygen-free environment. 

These gases are produced from organic wastes such as livestock manure, food processing waste, etc.  The manure or waste is broken down in an airtight container called digester by using bacteria and water. This process releases the methane.

Methane is the principle constituent in natural gas.  Biogas is composed of 55%-80% pure methane.  The remaining porting is carbon dioxide.  State-of-the-art systems report producing biogas that is more than 95 percent pure methane.

Anaerobic Digestion - Turning waste into watts

We are more familiar with aerobic decomposition that takes place in compost heaps that are “aerated” by frequent turning to expose materials to oxygen-rich air.   In contrast, anaerobic digestion requires a sealed and often heated environment that has no oxygen present—similar to conditions in your own stomach.

Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant.

The process of anaerobic digestion consists of three steps.

  • The first step is the decomposition (hydrolysis) of plant or animal matter.
  • The second step is the conversion to organic acids.
  • The third step converts acids to methane gas.

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Links to Anaerobic Digestion Resources

AgSTAR :  AgSTAR is a cooperative effort by several U.S. government agencies to promote and provide accurate information on manure digester systems. Many useful documents can be downloaded, including the AgSTAR handbook (a comprehensive guide to those considering a digester). There is also a software package available for analyzing the economics of digesters. AgSTAR also staffs a hotline -- 1-800-AgSTAR, you need to leave a message and they will call you back.

AgSTAR's guide to operational systems :  Information on 40 digesters across the United States.

Biogas Works:  An extremely comprehensive site. Includes a history of anaerobic digestion; recent news items about AD; reports, studies, and presentations (including policy information); resources; and yet more useful links.

Julian Barham farrow-to-wean Farm, Zebulon, North Carolina  :  Ambient temperature anaerobic digester and greenhouse for swine waste treatment and bioresource recovery.

Langerwerf Dairy in Durham, California :  A 400-cow dairy plug-flow digester, in operation for over 16 years.

Methane Recovery from Animal Manures A Current Opportunities Casebook:  This is a 1998 publication from the National Renewable Energy Laboratory (NREL) at the Department of Energy. Contains much useful information on anaerobic digesters, and descriptions of the operating experience of over 20 digesters installed across the United States.

Vermont Farm Methane Project  :  a state initiative to develop techniques and technologies to make anaerobic digestion available to a variety of farm sizes and management styles.

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