There’s something new on the farm. In the small town of Elk Mound in western Wisconsin, Five Star Dairy has welcomed the addition of an anaerobic digester built by Microgy Inc. The 850-head dairy is one of four dairies across western Wisconsin employing the latest technology for manure recycling and energy generation with its installation of a 750,000-gallon, high-rate manure digester to produce methane—enough to supply the electrical needs of 600 homes in the area by powering a generator. This power will amount to roughly 775 kW of renewable electric energy. Dairyland Power Cooperative will distribute the power locally through Dunn Energy Cooperative’s distribution system.

Ribbon-cutting banner at Five Star Dairy

Five Star Dairy has been a productive farm as well as a good land steward since the 1930s. The dairy has worked to protect the land on which it is located from gully erosion and soil-nutrient depletion through crop rotation as well as careful use of manure. Fulfilling various cleanliness standards for milk and milking equipment, ensuring ease of handling and cleanliness of animals, supplying plenty of fresh air, and implementing steps to curb water usage, all contributed to Five Star being awarded the Dairy Quality Assurance Stewards of the Land designation in 2004 by the Milk and Dairy Beef Quality Assurance Center Inc.

Since Five Star Dairy applied for its Wisconsin Pollution Discharge Elimination System permit from the Wisconsin Department of Natural Resources, it has been violation-free. The dairy is also something of a perpetual motion machine in that it produces nearly all its own food for the dairy on the land it owns or rents. As specified in their nutrient management plan, all the manure produced at the dairy is used as natural fertilizer and soil amendment. Crops, such as corn and beans, are rotated with alfalfa, and plowing is done with a chisel plow to cut down on erosion. The new anaerobic digester will also produce a fertilizer stream that can be spread on the land continuously throughout the growing season.

Microgy Installs Anaerobic Digester
Microgy Inc., headquartered in Golden, CO, is the primary operating subsidiary of Environmental Power Corp., which was founded in 1983, primarily for the purpose of developing renewable energy projects.

Lee Jensen, owner of Five Star Dairy in Wisconsin

Five Star Dairy is the first of Microgy’s projects to generate electricity from the biogas that is produced from the digester. Microgy currently has two other projects in commissioning or in construction in Wisconsin involving their anaerobic digester technology. One of the projects, near Lafarge, WI, is up and running. It has been producing biogas and is in the process of commissioning its electric generating equipment. The other is in the final stages of construction and will start up in the fall of 2005. Dairyland Power Cooperative will buy the gas produced from all three units. Microgy also has plans to start many more projects across the country.

Randy Hull, Microgy’s new president, comes to the company with more than 20 years of experience in power generation with GE, as well as executive roles in the chemical, petroleum, and food industries. “I simply could not pass up the opportunity to join the team in developing the emerging merchant and onsite renewable energy business, where we generate cost-effective energy (without the requirement of government subsidies), and where we simultaneously have a very positive impact in terms of reducing damaging nutrient runoff from agricultural waste to our streams and waterways,” Hull says.

Anaerobic digestion has been around for many years and, in a sense, in the animal world it’s a continuation of the digestion process. When agricultural animals—dairy, swine, and poultry to some degree—leave behind their excrement, there is a fairly large portion remaining that is undigested. This leftover material is known as volatile solids. Typically, what takes place in the farm environment is that all the excrement gets washed into a lagoon and sits for four to six months while the digestion process continues to proceed—very slowly. Some methane is naturally produced from the lagoon.

“Methane produced in the lagoon and released to the atmosphere is actually 20 times more potent as a greenhouse gas than CO2,” says Hull. “Over the five to six months that this natural process occurs, the lagoon produces methane and other odors. In the lagoon, the manure makes a slow transition from a largely organic state, with pathogens and harmful bacteria, into a largely inorganic state, where the nutrients, like nitrogen and phosphates, are more readily absorbed by plants and thus the material can be used as natural fertilizer.” The lagoon process is slow and inefficient, and the transition of the nutrients from organic to inorganic is incomplete. The digester is a controlled and predictable environment and the conversion of wastes is more efficient and complete.

The speed and conversion efficiency of pumping fresh manure from the barn into Microgy’s digester means far less methane escapes into the atmosphere. The benefits from the reduced ozone-depleting emissions from fewer storage lagoons would mean a reduction of nearly 2,800 metric tons per year of C02-equivalent emissions.

Once onsite, the anaerobic digester takes all the farmer’s manure, converts these compounds to readily usable nutrients, and cuts down—for the most part—the harmful runoff nutrients. The system actually has the ability to co-process other food processing wastes, such as fats, oils, and greases, and mix them with manure continuously at a constant temperature of 130°F. A small amount of the gas that is created is burned to create this constant heat or, if power is being produced, some waste heat is taken from a diesel engine to generate the heat for the digester.

The operation of the digester takes place at 130°F, 365 days a year. Because it operates at this constant high temperature, thermophilic bacteria are used and the system creates a more efficient and rapid digestion. According to Hull, at present there are some 30–40 digesters out on farms in which the farmer has done some engineering himself. In some cases it is simply a cover on a lagoon to capture some of that escaping methane gas—perhaps only 10% of the gas that is ultimately recoverable. “The problem with that system though, is that the mesophilic bacteria operate well at a temperature of 70 to 90 degrees Fahrenheit,” says Hull. “But if you are in such places as Wisconsin, Minnesota, Pennsylvania, or Vermont, those bacteria are going to be asleep four to five months out of the year. At that time you will be getting very little anaerobic digestion going on. Our system is producing at both a much higher rate and continuously seven times 24, 365 days of the year.”

There are other anaerobic digesters in use on farms called “plug-flow digesters.” The farmer brings his manure to a large chamber where he puts in the animal waste. But the farmer is not mixing it, adding any other materials or waste products, and he is not heating that vessel. “Though they do recover methane from that process, we are able to recover typically three to four times the methane,” says Hull.

Escaping with the methane from the plug-flow digesters are sulfur compounds and organic vapors that create a noxious odor. Many farmers use their systems primarily to eliminate an odor problem in addition to receiving a little power or gas. “Typically, these systems have not been very economical,” says Hull. “By the time you consider all the controls required, the feed and collection equipment, and the gas recovery system required, and then ask the farmer to come up with a couple hundred thousand dollars, and then operate and maintain this system, it’s challenging. And you know what? They have enough challenges these days just running an efficient, healthy dairy or pig farm in a very competitive market.”

What Microgy is doing differently is putting up the funding, developing the project with the off-taker—such as the utility or the natural gas company—and essentially offering a much lower risk and lower cost option to the farmer for processing his manure, without any financial investment by the farmer. Because these systems can stand on their own financially, without any need for subsidies, Microgy can either do the financing on its own or in conjunction with the off-taker. The farmer does not need to make an investment and gets a good deal out of this through reductions in bedding costs and less load on his soil from a nutrient standpoint. In addition, the material can be applied over a lower acreage on a continuous basis. Most importantly, Microgy operates and maintains these systems and allows the farmer to do what he does best. Microgy in turn makes a profit through all the gas and power sales associated with the systems.

The digested manure leaving the Microgy anaerobic digester has been rapidly converted to a predominantly inorganic state over a period of 20 days. The solids are then separated from the digested liquid, called the digestate. All through the process, the digester is continuously fed manure, and digestate is continuously removed from the digester to maintain the liquid level in the digester.

In the digestate, a higher portion of the phosphorous is converted to an inorganic form and exists as a solid. In fresh manure the phosphorus is bound chemically in an organic form. The digestate can be separated in simple separators to produce digested biosolids—which includes roughly 40% of the phosphorus—and liquid. Farmers like these solids because they are very soft, light, and homogenous, and can be used very easily for bedding for the dairy cows. So, instead of spending $80,000 a year for bedding materials for a 1,000-cow farm, the farmer is now getting separated solids for the cost of operation of the separator. This material has proven to be better than sawdust, which seems a bit more prone to carry disease and germs, and is more of an irritant.

Phosphorous removal in a separator is more complete since a large portion of the phosphorus is converted to inorganic forms with the anaerobic digester. In many parts of the Great Lakes and in other areas of the country, phosphorous levels represent the big environmental hazard. The remaining liquid with most of the solids removed includes the remaining nutrients, which are primarily in an inorganic state. The liquid can be land-applied continuously and immediately throughout the growing season.

Since the nutrient load has been reduced and the remaining nutrients have greater crop availability, dairy farmers and hog growers do not need as many acres over which to apply the manure. This is a factor that is very closely regulated by environmental agencies. The liquid material can now be spread over a smaller footprint, with less stress on the local environment, and the farmer can potentially add stock to his herd, at the same time freeing up more acreage for use as cropland. Another benefit is that, in a thermophilic digester, pathogens and weed seeds are killed, which in turn lowers the need for herbicides.

The anaerobic digester removes approximately 50% of the volatile solids of the manure, and the separator removes another 40% of the post-digestion solids. In addition, odor is greatly reduced. An added benefit of digestion is a reduction of flies due to reduced organic wastes. Fewer flies result in less use of insecticides and less stress on the environment from that source.

Creating the System
The digester itself is an above-ground steel tank approximately 50 feet in diameter and 60 feet tall. The digester is constructed of carbon steel to specifications that meet the American Petroleum Institute (API) standards for oil storage systems. The digester is structurally designed to handle the stress imposed from the pressure of 750,000 gallons of manure with sufficient safety factors to provide a useful life of 30 years. Since the interior of the tank is oxygen-free due to the anaerobic processes encountered, no oxidation takes place. The exterior sides and the top of the tank are covered, insulated, and protected from the elements. The insulating panels incorporate exterior aluminum siding into individual panels that are attached to the sides and top of the digester, creating a weather-proof covering. At Five Star, the aluminum siding is painted to match the barn exteriors. The entire digester plant is designed to blend in with the existing facilities.

Mike Casper, Microgy’s Midwest development manager, says the technology for the anaerobic digester project was first developed in Denmark around 1988. “Though there are around 28 of the systems now operating in Denmark, I’m not aware of any operating thermophilic systems in the United States other than Microgy’s,” says Casper

The technology of Microgy’s anaerobic digester has already been shown to be effective in the facilities where it has been operating in Europe over several decades. Europe’s decreasing available acreage, coupled with increasing market demands has reflected the conditions in which American farmers currently find themselves. The US farmers who have successfully applied this technology to make their farms more economically and environmentally efficient may set a trend for other farms throughout the region and the country.

Casper visited five of the Danish project sites in March 2005. Microgy worked very closely with the Danes on the design, startup, and commissioning of the systems running in Wisconsin. Dan Eastman, Microgy’s senior vice president for development, worked with those who had developed the system in Denmark. He came up with the license agreement to be the exclusive licensee of this technology for North America. “The project went as well as it did in our working with the Danes because everything was coordinated so well,” says Casper.

New Applications
The anaerobic digester system provides farmers with additional alternatives for their nutrient management plan. Microgy believes their system is applicable to other industries, especially meatpacking operations, which are now showing great interest in Microgy’s system. In addition, cheese producers located close to major dairies have large natural gas and/or propane loads that can be reduced with biogas recovery from adjacent or nearby farms. “These processing operations may also have current onsite wastewater treatment plants, because the local municipal plant cannot handle the additional loads from these processors. However, our system can be a very beneficial add-on to such a facility and a means to reduce existing waste loading. This may be extremely beneficial if the processor is also considering an expansion.

“We see other benefits with the meat processors by reducing their wastewater loading in the range of 5 to 10 percent, processing some of their very low-value rendering sludge to higher value, clean biogas, and processing other plant wastes as well. Additionally, many of these facilities have covered anaerobic lagoons that will see their performance improved by the addition of higher temperature streams coming off the digester, with additional healthy bacteria going to their lagoons.”

Saving and Selling the Energy
By using the jacket-water waste heat from the engine, there will be a reduction in on-farm use of LP, up nearly 90%—about 10,000 gallons—annually. An additional 2 MM Btu/hr of exhaust heat from the engine is available and will displace the use of over 100,000 gallons of LP each year. This will be used for a planned back-end agricultural-feed drying and processing system.

“There are so many different alternatives for the gas output from the digester and there is so much of a perception from the past that the only option out there was to produce electricity, when in fact there are other options out there for it—especially now that gas prices and other energy prices have increased so much,” says Casper.

Ideally, from an energy standpoint, Microgy would like to take the biogas produced, scrub out the C02, and have a plant nearby that could use that gas. “Most farmers, however, don’t use a lot of natural gas,” says Hull. “They have some things that need power and occasionally use some propane. But they cannot use all the gas that we make. A cheese or meatpacking plant next door would be a perfect fit. We have recently announced a project in a western state where biogas will be sold to a large cheese-making plant. In another state in the west we will be doing a large project involving feedlot manure, where we’ll be taking the end-product of biogas, cleaning it up, and putting it into a pipeline nearby.”

Microgy’s second preference would be to take the gas and put it into a natural-gas pipeline grid. One of the company’s Texas facilities presently has a natural gas pipeline within three miles that they feed into from their project.

Creating power with a generator is another possibility and, with a power line always nearby, that is an option. In that case, the biogas is removed but the CO2 is not removed. The sulfur compounds are taken out to avoid the creation of SO2, and the 65% methane, 35% CO2 mixture is placed into a diesel generator set. The power created can then be sold either to the local company or to the independent power producer.

The future looks exciting to Microgy. “There certainly is plenty of manure out there,” says Hull. “There is also no shortage of other food wastes. We look forward to supplying our country with renewable energy that lessens our dependence on foreign sources, while simultaneously helping to clean up our environment.”

PETER HILDEBRANDT writes extensively on engineering and scientific subjects.

DE - January/February 2006