BioCycle April 2010, Vol. 51, No. 4, p. 49
As of fall 2009, there are 22 farms with 31 operating anaerobic digesters, making Wisconsin the leader in U.S. farm-based digester installations.
Joe Kramer and Larry Krom

THE USE of anaerobic digestion (AD) to treat livestock manures and other feedstocks continues to grow. Wisconsin remains at the forefront of U.S. farm-based digester installations. As of fall 2009, there were 22 farms with 31 operating anaerobic digesters in the state, as described in the second edition (2009) of the Wisconsin Agricultural Biogas Casebook. Funded by Focus on Energy, the casebook includes brief case studies of farm-based anaerobic digesters installed in Wisconsin. Interested readers should refer to the casebook itself for additional information on innovations, benefits, problems and other insights (web address provided at end of article).
A two-part article based on the 2008 Casebook provided details on each of the farm digester systems included in that edition (see “Farm Digester Progress In Wisconsin,” November and December 2008). The 2008 Casebook had a total of 17 farms with 22 digesters.
This article summarizes details from the 2009 Casebook by general categories, e.g., farm and digester characteristics, biogas treatment and use, codigestion. Information on systems was compiled over the period of August through November 2009. Information in the case studies was received directly from owners and operators of digester systems, and is generally not third-party verified. The casebook was produced to give those interested in digesters some insight into how these systems are working in Wisconsin. The task was made easier by the fact that these systems appear to be generally successful.

Farm Characteristics
All of the operational farm-based systems in Wisconsin are on dairy farms. These farms are spread throughout the state with some concentration in the east central region. While there are a total of 31 operating digesters on 22 farms, the digester statistics and descriptions in this article refer to installed systems on the 21 farms with 30 digesters that are profiled in the casebook.
Farms with installed digesters are predominantly bedding their cows with digested solids, although there are two that use sand. One farm in this group was using sawdust, but has recently switched to digested solids because sawdust has been increasingly costly and scarce.
In Wisconsin, AD systems have only made a commercial presence on larger farms. As illustrated in Figure 1, herd sizes for farms with operational digester systems in Wisconsin range from about 825 to 4,600 head.

DIGESTER CHARACTERISTICS
All of the farm digesters in Wisconsin fall into the classification of either mixed plug-flow or complete mix system types. They were provided by seven different digester designers. Table 1 lists some details on individual systems and design companies.
GHD, Inc. has the most systems by far; a total of 13 farms have 18 of their mixed plug-flow systems operating in Wisconsin. Microgy has the second highest number of installations with three farms using its thermophilic complete mix systems (operated at about 125°F). All remaining digesters are mesophilic systems, designed to operate in the 99° to 102°F range.
The growth in digesters in Wisconsin has been steady. Since 2002, an average of 3.75 new digesters per year became operational.

BIOGAS TREATMENT AND USE
Biogas pretreatment information was also gathered. Having excessive hydrogen sulfide (H2S) in the biogas can cause excessive engine wear – especially where water in the liquid phase is present. The primary benefit reported of reducing H2S and water levels is reduced frequency of oil changes, which translates directly into increased uptime for the engine and lower oil costs.
Virtually every system is using a device of some kind to reduce the moisture of the biogas. A number of farms now employ scrubbing systems or practices to reduce H2S levels in biogas prior to using it in their engines. Six farms have begun injecting air into the digester to drop out the H2S. Three are using a Biothane brand scrubber and two are using a GHD-designed H2S reduction system. One farm reported using an “iron sponge” type scrubber. (A detailed discussion of biogas conditioning systems is provided in the February and March 2010 issues of BioCycle.)
The most common use for biogas is fuel for an engine generator set to produce electricity for sale, and heat for on farm use. The most common brand of installed engine generator set is Guascor with ten installations in the state. There are eight Caterpillar sets, three Waukesha, two Jenbacher and one Deutz. Since the summer of 2008, installed generating capacity of farm-scale biogas systems in Wisconsin has grown by almost 60 percent from 7.3 to 11.6 megawatts. Some of this increase was from upgrading or addition of generation equipment at existing digester installations. One farm, Emerald Dairy, successfully demonstrated the feasibility of upgrading biogas to pipeline grade natural gas, and injecting it into the distribution system. However, low natural gas prices have made that option uneconomical at this time.

CODIGESTION
An estimated 1.3 million gallons/day of manure are being digested. In addition, nine digester owners are adding other substrates to the manure, bedding and wastewater in their influent. Three of these owners report adding other on-farm wastes such as chopped straw (to generate more bedding), waste corn silage and moldy or unused feed. Five farms regularly process off-farm wastes from food or beverage processing industries with their manure to boost biogas production. Some of these digesters add up to 20 percent food processing substrates.
Increases in biogas generated through addition of high energy feedstocks can only benefit the farm if it has the capacity to use the biogas. Otherwise, the biogas is flared. Some farms are using some limited on-farm biogas storage to maximize electricity production during energy demand peak hours when they can get the highest price.
Increasing interest in keeping compostable organics out of landfills will mean owners of digesters may have increased options for accepting off farm substrates. This could result in further increases in generating capacity at existing installations.
Energy Generation
Biogas production information was not available for many of the systems in this report due to the lack of gas metering on older systems. Because newer systems include biogas metering, this statistic is expected to be included in future editions of the Casebook once a suitable history has been established.
Nearly all the farms generating electricity have power purchase agreements with utilities. With the energy generation owners’ consent, the authors obtained purchase histories from the utilities for 16 of these farms. The combined generation for these systems is shown in Figure 2.
The generation equipment for these systems operated at an average capacity factor of 61 percent over this time period. It should be recognized that theoretical capacity as presented here cannot be practically achieved, given necessary downtime for oil changes and other maintenance. Numerous other conditions influence observed capacity factors. More detail on the generation histories of these systems can be found in the annotated generation histories and discussion in the individual case studies.

BUSINESS MODELS
The profiled farms are working under several different business models and contracts. These models can be categorized by ownership.
The most common type, adopted by 12 farms, has the farmer owning the digester and energy generation, and selling power to a utility. There is also one farm that generates electricity but uses it on the farm. Under another business model, a digester designer builds and operates a digester on the farm, the farm owns it, and sells biogas to a utility. Microgy has used this model and has three such systems in Wisconsin. Two farms have the digester and energy generation owned and operated by a third party.

DIGESTED SOLIDS
Markets for separated digested solids have been developing in Wisconsin. Digester owners are increasingly reporting that some solids are currently sold off farm, either to other farms for bedding or to gardeners or landscapers. These markets are still developing, and will likely be quite local. One digester owner (Clear Horizons on the Crave Brothers Farm) has bagged retail products sold as soil amendments.
Eight of the 21 farms are using all the solids they produce on their own farm, either as bedding, land applied as soil supplements, or both. Of the 12 farms selling some solids off-site, the average price is about $20/ton. Based on the owner’s estimates, the rate of solids generation is about 17 lbs/cow/day.
The practice of using anaerobic digesters at livestock operations has been increasingly fine-tuned. System providers continue to improve their designs, and owners and day-to-day operators discover innovative operational changes that improve overall performance. The accumulation of experience with the many Wisconsin farm-based digesters will continue to bode well for the success of future installations.

Joe Kramer is Senior Project Manager with the Energy Center of Wisconsin. Larry Krom is Project Manager, bioenergy and large windenergy, for Wisconsin’s Focus on Energy Renewable Energy Program. The casebook is available for download from: http://www.focusonenergy.com/InformationCenter/Renewables/Fact-Sheets-Case-Studies/Biogas.aspx.