BioCycle December 2008, Vol. 49, No. 12, p. 38
Details on ten dairy digesters in the state are provided in this second article based on a June 2008 report. Part II
Joe Kramer and Larry Krom

THE Wisconsin Agricultural Biogas Casebook was released in June 2008 and includes some history and an operating snapshot of the 17 anaerobic digester systems processing dairy manure in the state. Part I of this article appeared in the November 2008 issue, and covered 7 of the 17 operating digesters. Part II discusses the other 10.
General details on the farms covered in this installment are presented in Table 1, including dairy, location, herd size feeding the digester, manure collection frequency, and type of bedding used. Table 2 provides information on the types of digesters, e.g., plug flow, complete mix, as well as the system designer and when the digester became operational. Table 3 discusses digester ownership and the types of contracts the farms have with the utilities.
Six of the 10 dairy farms in Part II are adding off-farm substrates to their digester (Table 4), including food processing residuals, grease and corn syrup. Two farms covered in Part I received off-farm substrates as well. Some systems, such as those installed by Microgy, have inclusion of off-farm food wastes, and the resulting increase in biogas production, as integral parts of their business model. Figure 1, reprinted from the Casebook, gives an estimate of the biogas production potential from various substrates. Manure is one of the lowest potential biogas producers.
Each system that is generating electricity has a purchase agreement with their servicing utility. Therefore, with the owner’s permission, a consistent measure of electricity generated and sold was obtained from the utility for the Casebook. Figure 2 shows the average herd sizes and the median kWh of electricity generated and sold from biogas over the 18-month period of January 2007 through June 2008. The median measure is an indicator of what “typical” monthly generation sales look like. Medians were calculated based only on months in which the digester and energy generation equipment were operational. (Some systems only came on-line recently and others had significant engine downtimes.)
CASE STUDIES
Green Valley Dairy (Green Valley) is a 2,500 head (2,100 milking) dairy operation that generates about 83,000 to 105,000 gallons/day of manure. When milking parlor wash water is added, the influent stream is about 8 percent solids. The owners installed two Biogas Direct digesters – complete mix, above ground tanks with a flexible dual membrane cover that can expand to accommodate some limited biogas storage. It is a mesophilic system with an operating temperature of 102°F and an HRT of 22 days. Heat recovered from the engine generator is used to preheat the manure entering the digesters. Farm owner Guy Selsmeyer noted the importance of having a backup boiler to provide heat to the digester, especially if the farm is using solids for bedding.
Biogas is dehumidified using a condensate trap and chiller with oxygen addition. Electricity is sold to We Energies under a “sell all” contract; the farm owns a Caterpillar 600 kW engine generator and plans to add generation capacity as part of a summer 2008 expansion. The farm generates about 120 tons/week of digested solids (separated with a Fan unit). About half are used by Green Valley and half are sold to neighboring dairies. The farm was going through an expansion during the summer of 2008, adding 500 cows, a third digester and an additional 600 kW engine generator.
Holsum Dairy (Hilbert) has two separate farms. The Irish Road farm, the older of the two, has 4,000 head of Holsteins and was one of the early dairies in Wisconsin choosing anaerobic digestion for manure treatment. It installed two GHD-designed digesters in 2001-02. These systems are unlike the typical GHD digesters in that they are straight (laid out end to end) rather than U-shaped. The digesters are mixed plug-flow systems using biogas for mixing. The farm receives 1 to 1.5 semiloads/day of by-products from three area food processors and is paid a tipping fee. Manure and other wastes are not pretreated in any way before being loaded into the digesters. The Elm Road dairy has about 4,000 head of Holsteins. The dairy owner decided to install two GHD-designed digesters at this facility as well, which became operational in 2007. These are the U-shaped mixed plug-flow digesters with passive gas-induced mixing. Elm Road also adds about 1 to 1.5 semiloads/day of food processing industry wastes to its influent stream and receives a tipping fee.
Biogas produced at the Irish Road farm is dehydrated via a condensate trap and chiller, then utilized in two 600 kW Guascor engine generators. Holsum Dairy has a contract to sell all the electricity generated to Wisconsin Public Service Corporation. Heat recovered from the engine generator is used for heating the digester, milking parlor, office, shop and holding and transfer areas. Biogas from the digesters at the Elm Road dairy are dehydrated in the same manner and used in two different generators – a Deutz 500 kW and a Caterpillar 200 kW. Electricity is sold to Wisconsin Public Service Corporation under a sell-all agreement. The Irish Road and Elm Road dairies each produce about 16 semiloads/week of digested solids that are separated using Fan screw presses. In both cases, one-third of the solids are used on the farm as bedding and two-thirds are sold to other dairies.
Lake Breeze Dairy (Malone) is a 3,072 head (2,550 milking) Holstein dairy using flush collection for its manure and sand for bedding. About 120,000 gallons/day of material are generated. To remove the sand bedding from the manure prior to treatment, the manure stream flows through one of two alternating sand settling lanes. The lane used is switched daily so the idle one can be scooped out. Sand is stored so bacteria cultures die off and it can be reused. The manure stream goes into a mechanical rotary screen solids separation system; liquid goes into a settling tank and the fine solids that settle out are remixed with the separated solids from the mechanical screen. This combination, with a solids composition of 8 to 9 percent, is fed into the digester while the clarified liquid is sent to a lagoon.
The owners installed two GHD anaerobic digesters side-by-side. Due to temporary manure volume variability, manure is being supplemented with varying amounts of purchased corn syrup from an Oshkosh ethanol plant to support biogas production. Biogas is conditioned using a condensate trap and chiller, then introduced into two Caterpillar 300 kW capacity engine generators. Electricity is sold to We Energies under a sell-all type of contract. Waste heat is used to heat the digester. After digestion, solids are separated with Anderson screw presses. The farm tried using the solids for bedding but had some incidence of mastitis so switched to sand.
Norswiss Farms (Rice Lake) is a 1,240 head dairy farm with mostly Holsteins and some Swiss cows. Manure is scrape-collected to a center gravity-flow system, then pumped to the digester. The farm owner chose to work with Microgy and Dairyland Power to have a digester installed. A 50,000 gallon tank next to the digester is used to store the off-farm food waste received (primarily grease). These wastes are pumped into the digester every half hour and are limited to about 10 percent of the total volume in the digester. Waste milk and footbath water also are added. Norswiss Farms and Microgy established a business partnership similar to the one for Five Star Dairy (described above).
Biogas is scrubbed with a Biothane unit, then utilized to operate a Jenbacher 848 kW engine generator owned, operated and maintained by Dairyland Power. The system is synchronous (but set to shut down in the event of power failure); the utility relies on it to provide voltage support for its distribution system. Dairyland gets renewable energy credits for the electricity generated, and the farm receives carbon credits for methane emissions avoided by using a digester. Carbon credit sales go toward paying down the debt on the digester. Solids are separated with a continually operating Fan screw press; about 55 yards/day are generated. The solids have enabled the dairy to do heavy bedding (6 to 12 inches) on mattresses for better cow comfort and performance. Somatic cell counts have been low, and the farm is avoiding the expense of using sawdust (which costs them $1,800 every six days, and is sometimes hard to find). Use of the liquid fraction on hay, bean and alfalfa fields (which can’t be done with raw manure) has eliminated the need to purchase fertilizer.
Quantum Dairy (Weyauwega) has 1,700 head of milking Holsteins and plans to expand to 2,100 head in the near future. The operation produces about 55,000 gallons/day of manure and liquids for treatment (about 11 percent solids). Manure from the reception pit is pumped to the digester 12 times/day. A GHD digester became operational in 2005.
Biogas goes through a condensate trap and chiller to remove moisture, then is sent to a Caterpillar 300 kW turbocharged engine generator. Electricity is sold to We Energies under a sell-all contract. Heat is recovered and used to heat the digester, two parlors, the engine generator building, the shop and the house. Solids are separated with a Fan screw press; about 133 tons/week (roughly 400 yards) are produced. Three-quarters are used on the farm, and the remainder is sold to other dairies for bedding and occasionally to gardening businesses for $15/ton or $5/yard (picked up). The farm plans to use the existing digester to handle the additional manure from the expansion, but will be installing a new generator (400-450 kW) with funding assistance from Focus on Energy.
Stencil Farm (Denmark) has a herd size of around 1,300 Holsteins. Manure from between 700 and 1,000 head is regularly sent to the digester. The volume, as well as the solids content of the manure sent to the digester, varies depending on which barn it is coming from. The Stencils installed a digester designed by RCM Digesters, Inc. It is a below grade, concrete, straight plug-flow system with a flexible cover that operates in the mesophilic range with a target temperature of 100°F. The system is designed to work best with manure solids concentrations of 9 to 12 percent.
Biogas is used to generate electricity and heat with a Caterpillar 123 kW (biogas rated) engine generator; all electricity is used on the farm. Heat recovered from the engine is used entirely for the digester. Digested solids are separated with a Fan screw press and used for bedding.
Suring Community Dairy (Suring) is a 950 head Holstein operation. Manure volume is about 25,000 gallons. The farm installed a complete mix digester designed by American Biogas Company (AMBICO). The digester – an above ground, stainless steel complete mix tank with a dual membrane flexible cover, resting on a floating concrete pad – operates in the mesophilic range with a target temperature of 100°F, and an HRT of 22 days. Estimated solids content of the influent is 7 to 8 percent. Manure is fed in every two hours; the farm plans to adjust the schedule to pump more in during off hours to take advantage of lower time-of-day electricity rates. Recently, the farm began rerouting effluent from the screw press back into the reception pit for preheating manure prior to digestion. Only manure, wastewater and bedding are sent into the digester, along with small amounts of footbath water.
Biogas is sent through a passive hydrogen sulfide removal system and chilling unit for condensate removal, then fed into a 250 kW Dreyer and Bosse, dual fuel engine generator. The engine uses 20 percent diesel. Electricity is sold to Wisconsin Public Service Corporation under a sell-all agreement. Heat captured from the engine and exhaust is used to heat the digester and the shop building. Between 80 and 100 yards/week of separated, digested solids are produced, with about half used on the farm. The remainder is composted or provided to neighboring dairies that are testing the solids for bedding.
Vir-Clar Farm (Fond du Lac) is a 1,350 head Holstein dairy that produces about 27,000 gallons/day of manure. Two Biogas Direct, LLC digesters were installed to treat the manure; design HRT is 33 days and is currently at about 30 days. (Green Valley Dairy, which also has Biogas Direct digesters, has a shorter design HRT.) Manure is added to the digesters twice a day. Other organics from the farm – including bunker wastes, moldy feed and whatever the cows don’t eat – are also added. Manure going into the digester is mixed with liquid from the solids separators.
Biogas goes through a passive hydrogen sulfide removal system and chilling unit for condensate removal, then is fed into a Caterpillar engine generator that has been modified by the German company SEVA and has a 350 kW generating capacity (the generator is containerized). Electricity is sold to Alliant Energy under a sell-all contract. Heat from the engine and exhaust is captured and used for heating the digester, water for calves and the separator room, and to provide in-floor heating in the calf barn. About 150 tons/week of digested solids are produced; about half are used on the farm with the remainder sold to a small farm and to a potting soil facility that composts it for use in mixes.
Wild Rose Dairy (La Farge) is a 1,050 head dairy (half Holsteins and half Jersey/Holstein). They have 880 head milking; about 33,000 gallons/day of manure are collected. Kiln-dried sawdust is used for bedding. Wild Rose installed a Microgy digester using a business model similar to Five Star Dairy and Norswiss Farms. About 1,100 gallons of manure mixed with food wastes are batched into the digester hourly.
Biogas is treated with a Biothane scrubber, water trap and dehumidifier, then used to operate a Waukesha 750 kW (net) synchronous engine generator owned, operated and maintained by Dairyland Power Cooperative. Heat captured from the engine is used for the digester only. Digested solids (about 5 tons/day) are separated out and sold to other dairies for bedding and to organic farmers for fertilizer at about $20/ton. In contrast, the dairy uses about 12.5 tons/week of sawdust for bedding (nearly 2 tons/day).
Additional information on energy production and owner experiences and innovations is available in the Wisconsin Agricultural Biogas Casebook, which can be downloaded from www.ecw.org/biogascasebook
Joe Kramer is Senior Project Manager with the Energy Center of Wisconsin. Larry Krom is Project Manager, bioenergy and large wind energy, for Wisconsin’s Focus on Energy Renewable Energy Program.