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.

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.

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.

Agricutlural Biogas Casebook 2004 Update:  The Great Lakes Regional Biomass Energy Program has updated the 2002 casebook to include updates on the 19 farms originally profiled in 2002, with an additional profile of a new project in the region. The information contained in the casebook is incredibly useful information detailing operations that are operational or in the construction phase on anaerobic digestion. Costs, biogas and electrical output, and operation experience are included when available.

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.

Anaerobic Digester Lagoon with Methane Gas Recovery :  First year Management and Economics Perdue University's case study of anaerobic digester lagoons, including detailed technical information. Also includes practical advice on steps toward planning for and building your own lagoon.

AURI:  The Agricultural Utilization and Research Institute (AURI) has a very useful "Self Screening Checklist" for evaluating if a digester is right for you.

Benefits, Costs and Operating Experience at Seven New Agricultural Anaerobic Digesters:  A 1998 publication, covering plug flow, complete mix, and covered lagoon digesters.

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.

Blue Spruce Farm, Bridport, Vermont :  1,200 dairy cow operation that received a USDA Rural Development grant to install an anaerobic digester.

Digester case studies:  Case studies from the Clean Energy Resource Teams

Digesters in Ireland :  From the Practically Green web site.

Economics and Feasibility of Electricity Generation using Manure Digesters on Small and Mid-Size Dairy Farms  :  University of Wisconsin analysis containing bottom line economics for digesters installed and operated at different farm sizes.

Fact Sheet: Resources on Anaerobic Digestion,:  Where to get the information you need to get started.

Final Report Swine USA :  Useful case study of an Iowa hog farm that installed a digester in 1999.

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.

Midwest Rural Energy Council:  Contains many useful links, as well as information on the Wisconsin Biogas Development Group.

National Food and Energy Council :  A wealth of information about agricultural methane recovery.

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.

Vermont Public Service  :  Quarterly report on Vermont's Farm Methane Project.