CASE STUDY: Haubenschild Dairy Manure Digester, Princeton, MN
The following is from the Executive Summary of “Final Report: Haubenschild Farms Anaerobic Digester, August 2002,” written by The Minnesota Project
Haubenschild Farms is a 1000-acre, family owned and operated dairy farm near Princeton, Minnesota. In 1998 the owners were planning to increase the size of their operations, and considered the possibility of installing an anaerobic manure digester. They knew that this type of system could result in environmental benefits while offering a return on their investment. Some of the key expected benefits of an anaerobic digester are:
- Odor control
- Renewable energy production
- Pathogen reduction
- Greenhouse gas reduction
- Reduction in total oxygen demand of the treated manure (total oxygen demand is a measure of potential impact on aquatic systems)
Haubenschild Farms applied for and was selected as an AgSTAR “Charter Farm,” one of 13 such farms selected nationwide to demonstrate farm-scale anaerobic digestion technologies. AgSTAR is a joint program of the U.S. Environmental Protection Agency, U.S. Department of Energy and U.S. Department of Agriculture, designed to promote the use of anaerobic digestion systems. In addition to the AgSTAR program, the Haubenschild Farms project received assistance from the Minnesota Department of Agriculture, Minnesota Department of Commerce and the Minnesota Office of Environmental Assistance. With financing complete, construction of the digester was started in the summer of 1999 and completed in October of the same year. Total construction cost of the digester and generator system was about $355,000.
The Haubenschild Farms digester is a covered 350,000-gallon concrete tank installed in the ground, with suspended heating pipes to heat the manure inside the digester where bacteria breaks down the manure, creating methane. A 135-kilowatt engine-generator set is fueled with methane captured from the digester. The hot water to heat the digester is recovered from the engine-generator’s cooling jacket. Barn floor space is also heated with the recovered heat. The digested effluent, odor reduced, flows to a lined storage pond where it is kept until it can be injected or broadcast spread on fields for crop production.
When the digester was started, it was processing manure from about 425 dairy cows, which was about half of its total design capacity of 1000 cows. In 2000, Haubenschild Farms built a second free stall barn and has expanded to a current size of about 750 cows. Since startup in the fall of 1999, the biogas output of the digester steadily increased to about 65,000 cubic feet by May 2000. Currently, more biogas is being produced than can be used by the engine-generator, so it is hard to estimate exactly how much biogas is being produced. The Haubenschilds are considering adding generation capacity to utilize the excess biogas. Approximately 70,000 cubic feet/day of biogas is used by the engine generator; the rest is currently flared. With 425 cows, the biogas output per cow was almost twice projections – with 750 cows, the output per cow has come down somewhat to about 40 percent above projections. Haubenschild’s cows are producing about 50 percent more manure per cow than the digester was engineered for, which somewhat explains the high biogas production per cow.
The sale of the electricity generated is an important benefit of the project. Before the digester was built, Haubenschild Farms entered into a power purchase contract proposed by the local electric cooperative, East Central Energy, who greeted the project with enthusiasm and offered Haubenschild Farms a very favorable contract. Since the expansion of the milking herd size from 425 to about 750 cows in the summer of 2000, the digester has been producing enough electricity to provide all the electric needs on-farm, plus enough surplus electricity to power about 75 additional homes.
Construction and operation of the Haubenschild Farms project has offered several key lessons for future digesters:
- Payback of 5 years on investment is possible
- A good time to install a digester is when changing or expanding operations
- Electric utility cooperation is important
- Active management is crucial for stable digester and engine operation
- Digester design and engineering expertise is key
- There are barriers to financing digester systems
- Cooperative agency participation reduces the barriers to a project’s success
- Manure collection method and collection frequency are important.