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Is an anaerobic digester in your future?

Kevin A. Janni

Published in Dairy Star December 31, 2009

Last month Agriculture Secretary Tom Vilsack announced an agreement with a U.S. dairy producer organization to accelerate anaerobic digester adoption for manure to energy projects on American dairy farms. Through the agreement, USDA and the Innovation Center for U.S. Dairy will increase the number of anaerobic digesters supported by USDA programs. Anaerobic digestion is a proven technology that can convert organic matter, including dairy manure and food wastes, into biogas for energy. Anaerobic digestion can also reduce methane emissions from dairy manure and are commonly credited with reducing odor emissions too. These multiple benefits make anaerobic digestion a technology worth considering.

digester on farm

A plug-flow anaerobic digester and genset building located on Haubenschild Farms near Princeton, MN

The AgSTAR Program, a voluntary effort jointly sponsored by the U.S. Environmental Protection Agency (EPA), the U.S. Department of Agriculture, and the U.S. Department of Energy, encourages the use of methane recovery (biogas) technologies on livestock farms. The AgSTAR Program reports that there were 140 operational digester systems in 27 states across the United States in October 2009. One hundred and sixteen of these digesters are on dairy farms. Wisconsin has 24 anaerobic digesters on dairy farms, more than any other state, while Minnesota has five anaerobic digesters on dairy farms.

Dr. Norm Scott, a Professor from Cornell University and member of the National Academy of Engineers, presented an invited seminar as part of the Bioproducts and Biosystems Engineering Departmentís centennial celebration in 2009. Dr. Scott made a point that renewable energy systems, including anaerobic digesters, must have three things; an economical and sustainable feedstock, a reliable process for converting the feedstock into energy, and a market for the energy and other by-products. Without all three, a digester does not make economic sense. He also reported that digesters fed animal manure and food wastes outperform digesters fed either separately. In New York State, they were making an effort to have dairy operations partner with food processors to supply anaerobic digesters. The tipping fees that food processors pay for the digester owner to take their food wastes were an important incomes source.

Dairy cow manure provides a very reliable feedstock for digesters. Anaerobic digestion is a proven technology but digesters require careful management to keep the microbes productive.

The challenge for many digester owners is making enough income from selling the energy and other by-products to provide sufficient return on the capital investment for a digester. Anaerobic digesters can cost between $500 and $800 per cow. Many digester owners received grants that paid some initial construction costs.

The biogas from anaerobic digesters, typically 60% methane, 40% carbon dioxide and small amounts of corrosive gases, can be used in multiple ways. The biogas can be used to fuel a boiler. Biogas can be used to power a combustion engine attached to an electrical generator, commonly called a genset, which produces electricity that can be used on the farm or sold and distributed over the electrical grid. Engine heat can be used to heat water for the dairy too. The biogas can also be cleaned and dried to make it pipeline quality for sale as natural gas or converted into hydrogen.

Biogas energy value is related to the cost of other energy sources. Separate digesters on many farms are distributed energy and by-product sources that may need to be handled by an aggregator who negotiates with wholesalers. Digester solids are another valuable by-product. The separated solids can be used as bedding or sold to gardeners and landscapers.

There are numerous challenges to be overcome to increase anaerobic digestion use on dairy farms. Some challenges include the initial investment costs; digesters are part of a manure handling system which can be complicated. Organic bedding is easier to handle in digesters than sand bedding. Anaerobic digesters are a biological process that can be easily upset if not well managed. Many dairy farms do not have people with the skills to operate a digester. Changes in digester feedstocks can disrupt biogas production. In cold weather, some biogas energy is needed to heat the digester and the feedstock, which reduces the energy available for sale. An anaerobic digester and its multiple by-products may become another farm enterprise that requires production, materials handling and marketing skills. Entrepreneurs with creative business models and skills are needed to make anaerobic digesters economically viable business enterprises.

Anaerobic digestion is a proven technology that can produce energy, stabilize nutrients and produce bio-solids for use either as bedding or a soil amendment. There are design and business challenges that need to be overcome before they will be used more widely but an anaerobic digester just might be in your future.

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