November 24, 2008 | General

Farm Digester Progress In Wisconsin

BioCycle November 2008, Vol. 49, No. 11, p. 42
Casebook provides information on 17 anaerobic digester systems operating in the state, as of June 2008. Part I
Joe Kramer and Larry Krom

WISCONSIN continues to be one of the leading states in operating farm-based anaerobic digester systems. The Wisconsin Agricultural Biogas Casebook gives a snapshot with some history of the 17 operating anaerobic digester systems in Wisconsin as of June 2008. Information was provided by digester owners, technology suppliers and utilities. General details on the farms covered in Part I are presented in Table 1.
This casebook represents an early step in a larger and ongoing effort to provide coordinated and consistent digester performance information to the general public using uniform methods. The Association of State Energy Research and Technology Transfer Institutions (ASERTTI), USDA Rural Development and EPA AgSTAR program have worked together to produce a standardized performance protocol. Focus on Energy (Focus) has instituted contracting measures in its grant language to enable monitoring and collection of more detailed information for current grant recipients, e.g. system economics, biogas production and electricity production, which can be included in future editions of the Casebook. This two-part article, excerpted from the Casebook, highlights these farm digester operations.
Herd sizes for operational digester systems in Wisconsin range from about 800 to 4,000 head. Several system types are being installed. A total of seven different digester design companies have active systems in Wisconsin. Table 2 lists details on individual systems and design companies for the farms included in Part I.
Table 3 covers digester ownership and utility contracts. Most digester owners utilize the biogas through an engine generator to produce electricity for sale. Farm-scale biogas systems in Wisconsin have a total installed generation capacity of about 7.3 megawatts. In addition to generating electricity, one farm is flaring the biogas and another is selling it to a third party. Owners of these two operations are part of a group that has plans to install a gas pipeline linkage to allow them to upgrade their biogas and inject it into the natural gas distribution system as renewable natural gas. Additional information on these efforts is included in the case studies of Baldwin and Emerald dairies.
Eight farms are adding other substrates to their digesters in addition to the usual manure, bedding and wastewater. 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 models. Table 4 lists farms that have reported addition of other feedstocks.
Biogas generation information was not available for many of the systems in this report due to the lack of gas metering on older systems. Newer systems include biogas metering; this statistic is expected to be included in future editions of this Casebook.
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. 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.)
In general, farms that add additional substrates to be codigested with the manure see proportionally higher energy generation; these are represented on the graph by the yellow dots. There are two exceptions: Holsum Dairy Irish Road has undersized energy generation equipment (relative to its herd size) and is therefore not able to use all the biogas produced for electricity production. Lake Breeze Dairy is adding off-farm wastes to compensate for inconsistent manure addition due to construction projects on the farm. More details on these operations are included in the case studies.
This section includes a summary of the first 7 of the 17 case studies included in the Wisconsin Agricultural Biogas Casebook. Part II will include the other 10 farms. At the time the Casebook was released, all farm-based anaerobic digester systems in Wisconsin were at dairy operations.
Baldwin Dairy (Baldwin) has a current milking herd size of about 1,050 Holsteins. This herd and milking operation produce about 30,000 gallons/day of manure and water at about 8 percent solids. Manure is preheated prior to entering the modified mixed plug flow digester that operates in the mesophilic temperature range with liquid jet mixing. In this system, designed by Komro International, LLC, liquid is sucked out on the bottom and reinjected on the sides. Activated sludge from the last stage is returned to improve digestion efficiency by keeping more active bacteria in the system. Heat from the effluent helps preheat the manure going in. Hydraulic residence time (HRT) is about 21 days; operating digester temperature is between 95° and 100°F. The farm uses a Fan screw press solids separator post digestion.
The owner reports that the system produces nearly 130,000 cubic feet/day (CFD) of biogas. The farm built its own biogas boiler to help heat the digester; remaining gas is flared. The system produces 20 to 30 tons/week of digested solids at about 67 percent moisture. Twenty percent is sold to neighboring farms for bedding. The rest is used by the dairy for its own bedding. The owners are evaluating options for building a biogas pipeline from Baldwin and another local dairy to transport biogas to an upgrading facility near the natural gas pipeline injection point. The facility will use water column technology to yield methane. This project is expected to move forward if financing is approved. During the summer of 2008, construction began on a greenhouse complex that will use biogas from the digester for heat, and to eventually add absorption chilling and possibly electricity generation. The greenhouses may be used for aquaponics, where algae can be grown for biodiesel production and tilapia can be raised.
Clover Hill Dairy (Campbellsport) has a herd size of 1,250 Holsteins. Manure has an average solids content of about 6 percent. The owner installed a mixed plug-flow digester designed by GHD, Inc. The system is a U-shaped, below grade, concrete structure with a fixed concrete cover; manure and wastewater enter and exit on the same end of the digester. The central shared wall holds hot water piping that heats the manure and helps conserve heat in the system by reducing outside surface area of the structure. There are two distinct phases or digestion zones; manure from the first phase flows directly into the second. The digester operates in the mesophilic range (design temperature of about 100°F) and returns activated sludge. Design HRT is 20 days. The biogas is reintroduced into the digester along the bottom; gas percolation through the manure provides passive mixing of the contents.
Biogas is treated with a passive hydrogen sulfide removal system and a chilling unit to remove condensate, then utilized in a 300 kW Guascor engine generator to produce electricity. Waste heat from the water jacket and exhaust is captured and used for heating the digester, milk house, parlor and lanes. The farm uses a Bauer and Ireland brand screw press and produces about 100 tons/week of digested solids. About 70 tons are used for both bedding and field application, and about 30 to 35 tons/week are sold for $20/ton to a dairy farm for bedding.The farm retains ownership of the carbon credits and has signed on with the Pure Farm Energy® Producer Network of farm energy project owners.
Crave Brothers Farm (Waterloo) operates both a dairy farm and specialty cheese production facility. It has 800 head of milking cows producing about 26,000 gallons/day of manure. Crave Brothers partnered with Clear Horizons, LLC to have an above ground, mesophilic, complete mix stainless steel tank system installed. Manure gravity-flows and drops through a slotted floor to a collection pit. About 2,500 gallons/day of whey and other waste products from the cheese operations (with some seasonal variation) also are added to the digester. Target operating temperature is 99°F, with an HRT of 25 days.
Clear Horizons develops, owns, operates and maintains the digester and generation equipment. It has rights to the products and credits associated with the digestion and energy generation. Solids are separated using a Vincent KP-10 screw press solids separator. The farm buys solids back from Clear Horizons for bedding and retains the nutrient-rich liquid for field application. Clear Horizons also produces a trademarked Energro potting mix using digested fiber, perlite and vermiculite, which is marketed in bags. Sales of bedding to the farm and Energro amount to about two-thirds of the income generated from the digester. Biogas is treated with passive hydrogen sulfide removal and a chilling unit for condensate removal, then fed into a 230 kW Deutz engine generator. Electricity is used to power the digester system with excess sold to We Energies. Heat captured from the engine generator is used for digester heating, substrate tank heating, and heating of pumping and separation rooms at the site.
Deere Ridge Dairy/Gordondale Farms (Nelsonville) is an 850 Holstein dairy operation. About 30,000 gallons/day of manure, bedding and milking parlor wastes are generated. Deere Ridge installed the first farm-scale digester designed by GHD, Inc. in 2001. Target influent solids content is 8 to 9 percent. Design hydraulic residence time is 22 days.
Biogas is treated with a water trap, then utilized in an on-site Caterpillar 140 kW (net) engine generator. Electricity is sold to Alliant Energy, which is responsible for operation and maintenance of the energy generation equipment. Captured heat from the engine is used to heat the digester and milking parlor, and for facility water heating. Solids are separated with a Fan screw press; all digested solids are used for bedding on the farm.
Double S Dairy (Markesan) has 1,100 milking Holsteins. About 33,000 gallons/day of manure, wastewater and bedding are generated for treatment. The owners installed a standard GHD, Inc. mixed plug-flow digester (see Clover Hill description). The digester is operating near its designed temperature of 100°F, and has an HRT of about 20 days.
Biogas is dehumidified with a water trap, and utilized in a 200 kW Caterpillar engine generator to produce electricity and heat. Electricity is sold to Alliant Energy through a sell-all purchase agreement that includes ownership of environmental attributes from generation. Captured heat is used for digester, milking parlor and shop heating. Digested solids – separated with a Fan screw press – are used for bedding or landspread on the farm. About three to four semiloads are produced weekly.
Emerald Dairy (Emerald) is a 1,600 head Holstein dairy that produces about 45,000 gallons/day of manure, bedding and wastewater. Manure averages about 8 percent solids. Emerald Dairy replaced an older covered lagoon digester with a GHD, Inc. system in 2005. Influent is pumped into the digester four times per day. Biogas is utilized in a moisture trap and iron sponge to remove hydrogen sulfide, then upgraded into compressed natural gas (CNG) using water column technology. The CNG is shipped using a tube tanker to a pipeline injection point, and put into a natural gas pipeline and sold to 3M, Inc. Pending approval of financing, the owner plans to partner with another large dairy (Jon-De Dairy) to build a gas distribution pipeline to enable biogas to be piped from Baldwin, Emerald and the third dairy to the injection point. The plan is to move the gas upgrading equipment currently at the farm to the injection point so the biogas from all three farms can be processed with it.
About 38 tons/week of digested solids at about 67 percent moisture content are produced. They are separated with a Fan screw press; effluent is stored in a lined lagoon before being land applied. Solids are used for bedding in higher proportions than at typical dairies because Emerald uses deep beds for its herd. Remaining solids are sold to other farms. Additional solids separation is achieved using an Integrated Separation Solutions system that cleans the water to a dischargeable level.
Five Star Dairy (Elk Mound) has 850 milking Holsteins. The dairy entered into an agreement with Microgy, Inc. and Dairyland Power to have an anaerobic digester installed on the farm in 2000. Microgy installed the digester with no cash outlay from the farm owner, and operates and maintains the system. The farm owner pays off the debt on the digester through biogas sales to Dairyland Power, which has an engine generator at the farm.
The Microgy system is a complete-mix above ground, carbon steel tank. It operates in the thermophilic range with a target temperature of 125°F; design HRT is 20 days. As a complete mix system, it has an inherent retention of activated sludge. The Microgy business model includes codigestion of off-farm food wastes, preferably high fat wastes such as greases and oils, to increase biogas production. The Five Star Dairy system includes a storage tank for delivered food processing wastes. A mixture of manure and about 10 percent food wastes is batched into the digester every half hour. Solids are separated out after digestion. Most are used for bedding and some are given to local gardeners.
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. Part II of this article will appear in the December 2008 issue of BioCycle.

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