BioCycle November 2005, Vol. 46, No. 11, p. 34
BioCycle Energy
If you thought anaerobic digesters were only possible for large-scale dairy farms, that scene is changing – as The Minnesota Project focuses on 100 to 300 cow operations.
Amanda Bilek

IN THE PAST, most anaerobic digesters have been targeted for large dairies with greater than 500 cows, but a new report by Phil Goodrich of the University of Minnesota aims to serve farms with 100 to 300 milking cows. Titled AD Systems for Mid-Sized Dairy Farms, the publication focuses on less costly methods that control odors, are simpler to build, produce energy, and comply with new emission regulations. Guiding principles for the systems are: Less management time; Shared responsibility to run digester; Building in stages (modularity); Expansion options; Less capital. These systems are designed for odor control, simpler manure collection, limited space, option to handle materials as a solid, using fibers from manure for bedding or compost, reducing phosphorus and applying liquid fraction as irrigation.
As explained by author Philip Goodrich of the University of Minnesota, the controlled anaerobic treatment of manure degrades organics efficiently without releasing many compounds that produce unpleasant odors. Nutrients are conserved in the effluent material for recycling to the next crop. Fibrous material can be separated and composted.
An enclosed container at a temperature of about 100°F promotes growth of microorganisms that produce biogas with about 60 percent methane – suitable for conversion to electricity and heat energy. Large operations have focused on production of energy for sale off-farm. This report stresses benefits of simpler systems to create a healthier environment for farm operators and neighbors.
Systems described in the report generated by The Minnesota Project and made possible through a grant from AgStar Financial, Fund for Rural America, include the following:
Upright Digester Treating Only Liquids After Solids Separation – featuring an insulated 10.5 ft diameter by 16 ft high concrete tank, delivered in five sections for on-site erection. Hydraulic retention time is about seven days in digester, which collects 2,400 cubic ft/day of biogas (40 percent carbon dioxide, 60 percent methane). Environment benefits include methane being captured in digester with less produced in storage tank, solids available for sale as soil amendments, nutrients out of streams, less energy needed to agitate storage tank for pumping into spreader, and less complaints about odors.
Capital costs are estimated at $60,000 for digester tank and materials, $60,000 for solids and liquids separation and composter, $40,000 for engineering design; Annual operating costs come to $16,000, while annual benefits add up to $30,000. Annual cost per cow comes to $20. For additional information about similar digesters, contact Stanley Weeks (e-mail sweeksl997@aol.com; Pro-Dairy, Cornell Coop Extension (phone 607-255-2803).
Upright Mixed Digester Treating All Manure from Herd – featuring manure collection, digester and biogas production, boiler system and heat exchanger, manure separator system and liquid storage. Capital costs for digester are $98,000; liquid storage – $40,000; Annual capital cost – $13,800; Annual benefits – $30,000; Annual cost per cow – $24. For additional information, contact Steve Dvorak, GHD Inc. – www.ghdinc.net/products.aspx; Stanley Weeks (e-mail sweeksl997@aol.com.)
Conventional Plug Flow Digester with Flexible Cover – featuring manure collection, digester and biogas production, boiler system and heat exchanger. Capital costs for digester are $64,000; Solids and liquids separation – $59,000; Engineering designs – $40,000; Annual capital cast is $16,300; Annual benefits – $30,000; Annual cost per cow – $246. For additional information, contact Mark Moser at Resource Conservation Management – contact@rcmdigesters.com and Richard Mattocks – waste2profits.com/htm
Covered Pond Digester with Insulated Sides and Cover – featuring subsystems that include manure collection, digester and biogas production, boiler system and heat exchanger and liquid storage. (Perhaps the system that has least energy output and most potential “to not work very well.”) Farmers who might want to install this digester are those that seek to reduce odors from existing storage, want a very inexpensive system with low inputs and low outputs, and a simple system with few parts. Environmental benefits include less nitrogen lost to atmosphere, solids available for sale as soil amendments, less rainwater collected and mixed with manure. Capital costs for digester are $64,000; Solids and liquid separation – $60,000; Liquid storage excavation – $20,000; Engineering design – $40,000; Annual capital cost – $18,400; Annual benefits – $30,000; Annual cost per cow (includes long-term storage) – $64. For additional information, contact Doug Williams – wmsengr@thegrid.net; Richard Mattocks – waste2profits.com/htm.
Low Cost Plug Flow Digester Utilizing A Plastic Liner Inside A Large Steel Culvert – featuring subsystems that include manure collection, digester and biogas production, boiler system and steam injection, and liquid storage. Biogas would be fed to the low-pressure steam boiler to generate steam that would be injected into manure in the mixing pit. Capital costs for digester are $55,000; Engineering design – $40,000; Pumps, plumbing, etc. – $10,000; Annual capital cost – $15,000; Annual benefits – $15,000; Annual cost per cow – $60. For additional information, contact Philip Goodrich – goodrich@umn.edu.
Community Digester Owned and Operated by 3-4 Farms – featuring farms with a total of 1,000 to 1,200 cows on site within two miles with a system that includes manure collection, digester and biogas production, boiler system and heat exchanger, manure separator system, liquid storage, composting and electrical generation. Each farm would have a short-term storage pit where manure is collected from the barn; minimum volume would be one-week storage. Second long-term storage facility for digested material should be located either at the barn or close to where manure will be spread. Tanker hired by central digester management will transport manure to central digester. Electricity is produced for at least one participating farm and for sale to the grid. Capital costs for digester are $170,000; Solids and liquids separation – $150,000; Engine generator, electrical switch gear, engineering design – $230,000; Annual operating costs – $100,000; Annual benefits – $193,000; Annual cost per cow – $38. For additional information, contact Port of Tillamook Bay MEAD project – www.potb.org/methane energy.htm.
ENGINE GENERATORS AND MICROTURBINES
An engine generator set (genset) connected to an induction generator can be interfaced to the power grid. The heavy-duty gas fired engines are not normally made in small sizes that match the output of the small digesters serving 100 cows. But for 300 cows, this is more reasonable. An engine generator set will cost approximately $100,000 including installation. These can be obtained from suppliers who also sell backup power systems.
A microturbine generator coupled to an induction generator can be connected to the grid. These are available in 30kW sizes and will match to a digester serving about 200 cows. A ratio of five to eight cows per kW is reasonable to use in sizing a microturbine or generator system.
Anaerobic digester projects are rapidly developing across the country. Technology is also developing to accommodate the agriculture interest in installing digesters on North American farms. The next step for the Minnesota Project is to install and study a pilot project using one of the systems identified in this report. The pilot project will be partially funded by the Environment and Natural Resources Trust Fund, recommended by the Legislative Commission on Minnesota’s Resources.
“Farmers have always recognized the value of manure,” stresses Bob Lefebvre, Executive Director of the Minnesota Milk Producers. “Anaerobic digestion of manure is another means of tapping into its value, and Dr. Goodrich’s work is key to expanding the utilization of this technology to more farmers. This is beneficial to the farmer and the nonfarmer alike because anaerobic digestion improves the sustainable dairy ecosystem.”
For more information on the report and its findings, contact Amanda Bilek of The Minnesota Project via e-mail – abilek@mn project.org or by phone (651) 645-6159 ext. 5.
Amanda Bilek is an Energy Program Associate with The Minnesota Project based in St.Paul, Minnesota. To read the complete report or request a copy, visit: www.mnproject.org.