Haubenschild Farms Final Report - The Minnesota Project

The Minnesota Project August 2002
Benefits of Anaerobic Digesters
Anaerobic digesters offer many potential benefits to farmers and the environment,
including:
• Odor and fly control. Anaerobic digesters consume odor-causing compounds in
manure as it moves through the digester, reducing odor problems (note that odors
will still exist at normal levels until the manure enters the digester). One study
showed that anaerobic digestion reduced odor by 97 percent over fresh manure. 10
For some projects, odor control is a primary reason for installing a digester,
especially covered lagoon systems. Fly propagation is also extremely limited in
digested manure compared to fresh manure.
• Renewable energy production. Not all digester systems are used to produce
energy; in some cases odor is removed and the gas produced is simply flared.
However, using the gas to produce energy may offer significant economic payback
depending on farm scale. Most commonly the gas is burned in an engine-generator
to produce electricity, and the waste heat can be used to produce hot water for
heating the digester and other applications, such as space heating.
• Distributed generation of electricity. The electricity generated by an anaerobic
digester, as opposed to a large central station power plant, is a distributed form of
electricity generation. This offers potential benefits to the electric utility, including
increased generation capacity (especially valuable during periods of peak electric
demand), voltage support, deferred transmission and distribution line construction,
and less loss of power through transmission. The benefits of distributed generation
to the utility have been estimated to be from $100 to $800 a year per kilowatt of
capacity. 11
• Potential increase in value as a fertilizer. Manure is already widely spread on
fields as a soil amendment. For many farmers, anaerobic digestion may increase
the value of their manure as a fertilizer. The digestion process converts organic
nitrogen into a mineralized form (ammonia or nitrate nitrogen) that can be taken up
more quickly by plants than organic nitrogen. 12 Timing of the plant uptake of
ammonia and nitrate nitrogen, similar to that used in commercial fertilizers, is more
predictable than the plant uptake of organic nitrogen from raw manure. However,
nitrogen in ammonia form can easily be lost to the air (called volatilization), where
it is a pollutant (see below). Therefore, care must be taken to handle the digested
manure in such a way as to minimize nutrient leaching and volatilization.
In addition, some research suggests that the mircroflora present in digested manure
may lead to increases in crop yields. One study found yields to increase an average
10 As reported in Philip Lusk, “Methane Recovery from Animal Manures: The Current Opportunities
Casebook,” Department of Energy, National Renewable Energy Lab (DOE/NREL), 1998.
11 Philip Lusk, 1998 (see reference 10).
12 See, for example, Andrew Wheatley, “Anaerobic Digestion: A Waste Treatment Technology,” Elsevier
Applied Science: New York, 1990.
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