A recent review of the operating record of manure digester projects by the Focus on Energy program and published in BioCycle magazine provides hard operational data on manure digesters in Wisconsin.  The review highlights the range of systems in use and provides information on Wisconsin’s renewable biogas capacity.

Anaerobic digestion of dairy manure and wastewater produces diverse saleable products such as electricity, heat, bedding, and fertilizer which can be used on the farm or sold offsite.

Electricity generation capacity

In 2008 Wisconsin farm biogas systems had an electricity generation capacity of 7.3 megawatts. Seventeen anaerobic digesters systems operate in Wisconsin, digesting between 26,000 and 120,000 gallons per day of dairy manure and liquid.

Wisconsin digesters average approximately 70 tons per week of digested solids and 130,000 cubic feet per day of biogas.  Between January 2007 and June 2008:

• Farms with herd sizes under 2,000 produced an average of 150,000 kWh of electricity.
• Farms with herd sizes between 2,000 – 4,500 produced an average of 440,000 kWh of electricity.

Source: BioCycle December 2008

Revenue for farmers

Farms partner with utilities which buy the farm-generated electricity, providing a steady revenue stream for farmers. Digested solids are used onsite or sold offsite as fertilizers or animal bedding.  Some farms sell solids as bedding at $20/ton, representing up to 2/3 of the income from digester by-products.  In addition, waste heat from digesters is used to heat the digesters, milking parlors, other buildings, and homes.

The big picture: anaerobic digesters provide economic and environmental benefits

Farms investing in anaerobic digester systems have benefited from the steady revenue stream from the sale of electricity, commodity by-products, and reduction of onsite expenses in fertilizer, bedding, and heat and electricity bills.

Anaerobic digesters also mitigate some environmental problems associated with large scale dairy farm operations, such as odor and runoff from manure lagoons.

For additional information, please see the articles the November and December 2008 issues of BioCycle for “Farm Digester Progress in Wisconsin” by Joe Kramer and Larry Krom.