Renewable energy technology is powering farms and small communities while controlling organic waste volume and odor.

What Is Anaerobic Digestion?

Focus on Energy | Focus on Energy

Renewable energy technology is p

Renewable energy technology is powering farms and small communities while controlling organic waste volume and odor.

submitted by Focus on Energy  

Farmers searching for answers to increasing energy costs as well as environmental pressures surrounding livestock and milk production are looking in the direction of anaerobic digester systems. While the costs of designing, implementing, and maintaining these systems - sometimes referred to as biogas recovery systems --may seem daunting, the benefits are meriting serious consideration. Originally looked upon as providing mainly energy benefits, these systems account for manure treatment cost savings, nutrient conversion, odor and pathogen control, and byproduct recovery. A deeper look into anaerobic digester systems can reveal whether one is right for your operation.

What is an anaerobic digester?

Methane is the main component of natural gas as well as biogas. An anaerobic digester system is an enclosed tank that excludes oxygen and through which manure is passed and broken down by naturally occurring bacteria, producing biogas. The digester retains the decomposing manure for sufficient time at a designed temperature to allow the growth of methanogenic bacteria in a "steady-state". One of the byproducts of the decay of this organic waste material is biogas. This biogas is composed of approximately 55 to 70 percent methane, which a farm can then use as a fuel for heating or to produce electricity and heat. If biogas is not used for some application, it must be flared off to prevent the methane from damaging the atmosphere.

Three main kinds of digesters work well in Wisconsin, each offering unique characteristics. The first is a plug flow digester. This system is made up of a tank, or a long covered trough that gets filled daily with manure. New manure is constantly pushing old waste towards the discharge end, while coarse solids in the manure form a sticky material as they are digested, called a "plug". This type of system is not recommended for swine manure because that type of waste has too low a percentage of solids.

The second type of digester is the complete mix digester. Used for swine or dairy manure containing two to ten percent solids, it uses a flush system to collect manure and then processes it in a heated tank, which is either above-or below-ground. A mechanical-or gas- mixing system keeps the solids in suspension, which speeds the digestion process.

A third type, a temperature-phased digester combines two types of digestion technologies into a two-stage reactor, increasing methane yields. This process shows potential for great success but is still somewhat untested for use on farms.

Which particular system is right for you is unique to your operation and should be fully examined during the planning and design phase.

Environmental and economic benefits

Early dairy digester systems in the United States were installed to produce energy during the energy crisis of the 1970s, while the first swine manure digester systems were built simply to control odor problems. In subsequent years though, the benefits of these systems have painted a promising picture. Here in Wisconsin there are eight systems in place - more than any state in the Midwest, with at least eight more to be constructed in the next year. In Wisconsin, farmers also have an opportunity to work with Focus on Energy, the state's renewable energy and energy efficiency initiative, to receive technical assistance and secure additional funding for the implementation of anaerobic digesters.

As farms become larger, anaerobic digester systems are becoming a viable means of generating on-farm energy while managing animal wastes. "Focus on Energy can assist farmers with questions about farm digester technology, project implementation and funding" according to Larry Krom of the Focus on Energy Renewable Energy program.

Farms introducing anaerobic digesters have realized numerous environmental benefits. Anaerobic digesters consume noxious compounds in manure, thus reducing odor emissions by up to 97 percent over fresh manure. The effluent of a digester has an earthy smell with some ammonia present. Flies are also controlled due to the reduced volatile organic content of the manure.

The biosolids produced from anaerobic digesters offer an excellent soil amendment as well, either in direct-use or when blended with other materials. The biosolids contain higher concentrations of nitrogen, phosphorus, and potassium than raw manure. And because the manure digestion is anaerobic, most weed seeds and pathogens are killed during the process. Other benefits include greenhouse gas reduction, ammonia control, water quality protection, reduction in methane emissions, and surface water protection from phosphorous.

Is an anaerobic digester system an economically sound investment? The methane produced in the process can be used in an engine-generator to produce electricity --up to two or three times the electricity a farm normally uses. A typical dairy farm with 600 cows can produce up to 54,000 cubic feet of biogas a day. With this volume, a 120 kW (kilowatts) generator can produce about 2,650 kWh (kilowatt hours) of electricity per day as well as significant heat recovery. This recovered heat is in turn used to heat the digester, and can be used for water or space heating.

How much gas is produced ultimately will determine how much money a farm can save using methane generation. The feed ration, solids content of the manure, length of time the manure is in the digester, and the bedding material all factor in the bottom line economics.

While the capital costs of these systems vary case-by-case, there is an excellent opportunity for return on equity. Assuming a qualified company has properly designed and installed the system and it is properly maintained, system costs can be recouped in as few as six to eight years.

It is important to call your electric utility to inquire about the rate they will pay for renewably generated electricity. Electric utilities have a ready market for renewable electricity including fulfilling their renewable portfolio standard and supplying energy for their "green" power programs.

Is this system right for my farm?

The optimal time for adopting an anaerobic digester system is when the layout of the farm is being reconsidered. Managing these systems requires daily supervision (about one hour per day) to ensure they remain functioning as they were intended. To be sure it will work for your farm, several factors should be considered:

  • Start by looking at your manure handling capabilities. It is important that there be a year- round supply of fresh manure. Anaerobic digestion systems are available for wet or dry manures.
  • The size of the farm operation is key. While anaerobic digester systems can work with smaller farms, about 450 to 500 head --at a minimum --is the general rule of thumb for current technology.
  • The financing of a system should be factored in as well. While some financing barriers may exist, grants and tax credits are available to assist. All financing options should be explored including those through the U.S. Department of Agriculture, the U.S. Department of Energy, Focus on Energy and the State of Wisconsin.

Finally, installing a new system can take approximately eight months, and another two months will be needed to get it running at its optimal level. A digestion system should be engineered specifically for each individual farm operation, and then properly installed. The primary reasons for anaerobic digesters failing are poor design, improper installation, and poor management. Proper operation will provide a cost-effective energy solution with the opportunity for producing thousands of dollars worth of electricity every year. Doing your homework from the beginning can pay huge dividends down the road.


Gale Gordon's farm is a showcase for the renewable energy technology known as anaerobic digestion.

Gale Gordon is a third-generation dairy farmer. When asked the question of what his grandfather would think of what he has done with the homestead, he answered, "I think he'd have done the same, had the technology been available in his day."

The technology Gordon is referring to is anaerobic digestion - or turning manure into usable energy. Gordon and his son Kyle began looking at ways they could update their farm. With nearly 1,000 head of dairy cattle, the removal and disposal of the large amounts of manure was becoming nearly impossible - not to mention the odor and pests associated with it. To remedy the issue, Gordon and his sons began investigating anaerobic digesters - first reading up on it and then touring farms that had already implemented this still relatively new technology. They knew immediately it was what they needed to do, and more importantly what they wanted to do on their farm.

The timing for the installation of an anaerobic digester system couldn't have been better for the Gordon's. Alliant Energy was working with farms in the area to partner with and the Gordon farm was a perfect fit. Alliant currently owns the engine and generator that is powered by the methane gas extracted from the manure and from it the Gordon's state-of-the-art barn, office and milk house are powered and heated throughout the year. Because the 120 kW mixed plug flow system provides more than enough energy to handle these tasks, Gordon is able to sell back the additional energy to Alliant Energy.

In addition, Wisconsin's Focus on Energy Renewable Energy program was able to assist in the funding of the installation through a grant. Focus on Energy is the state's renewable energy and energy efficiency initiative from which farmers like Gordon can receive financial assistance for taking steps to improve their business while helping keep Wisconsin's energy and environmental needs in check.

"There are so many benefits to implementing an anaerobic digestion system," Gordon said. "If you have more than 500 head of cattle it makes sense for a number of reasons not the least of which are the environmental and energy impacts." Gordon also noted that the solids separated in the process makes excellent bedding for the herd and that the solids bind up some of the phosphorus. The bedding also cuts down on the e-coli virus as well as other pathogens that can be transferred to the animals by use of other bedding materials.

Odor and pest control are also very much a benefit to an anaerobic digester system as well. "Rarely will there be an odor of manure on the premises and flies are essentially nonexistent," Gordon said. "Because of the process in which the manure is broken down, the odors are trapped in the trough. It is also because of the process that all fly eggs and larvae are killed."

The end result of the process is a dry, acid and odor-free, nutrient-rich solid that Gordon uses, in addition to the bedding for the cows, to fertilize his 1,000 acres of fields and gardens. The dry material neutralizes the soil providing a richer soil for plants to grow in.

To learn more about Focus on Energy and its Renewable Energy program, or to find out if you qualify for incentives or a grant call 800.762.7077 or visit

About the Focus on Energy Renewable Energy Program

Focus on Energy is a public-private partnership that provides energy efficiency and renewable energy information and services to the state's energy utility customers. Focus on Energy's Renewable Energy program seeks to raise awareness, provide training and financing, enhance marketing, promote technical assistance, and support the installation of renewable energy technologies across Wisconsin. Focus on Energy provides applications, with full program details, at 800-762-7077 or at

The content & opinions in this article are the author’s and do not necessarily represent the views of AltEnergyMag

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