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Harnessing manure's value
with methane gas digesters

Research and better system designs lead to renewed interest
in converting manure into energy and more useful fertilizer

By Karl Ohm

In the near future, agriculture appears poised to becoming a significant, if not major, energy producer that could usher in a more sustainable type economy, according to key researchers at the U.S. Department of Energy's National Renewable Energy Laboratory in Golden, Colorado.

New research with enzymes at Cornell University, for example, shows promise in making it much more economical to convert biomass, such as corn stover and other plant materials, into fuel.

For livestock farmers, advancements in molecular biology and bacteriology have also begun to rekindle interest in converting manure into methane ¾ or biogas ¾ that can power electric generators. Refinements are also being made in the design of the methane gas digesters and related equipment used to process and transform this valuable resource into value-added products.

Manure can be transformed into biogas through "anaerobic digestion," says Lut Raskin, University of Illinois civil and environmental engineer. This is nothing new. However, anaerobic digesters have come a long way since the 1970s, and U of I researchers have embarked on new research that aims to refine the process even further.

Producing biogas

In the anaerobic digestion process, bacteria break down organic matter very quickly in an oxygen-free environment. Unlike a lagoon, it is a closed system, so methane is contained and is, therefore, a usable fuel.

The system also seals in odorous gases or vapors. The ability to reduce odors is another reason why methane gas digesters are being seriously looked at today by farmers whose operations are near populated areas.

What's more, the fertilizer produced with an anaerobic digester is very stable and degrades slowly over time. And if Raskin and fellow researchers are successful, a digester will also be able to extract phosphorus from manure, allowing it to be used or sold separately.

The Anaerobic Sequencing Batch Reactor (ASBR), developed at Iowa State University, is a new digester that shows great promise for efficient and reliable processing of manure. Its biggest advantage over previous designs is its size, says Raskin. It is small, but can be used at the same loading rates as larger systems.

Hypothetically, if all the manure produced annually in Iowa from 20 million head of swine were processed through ASBR, the result would be a methane value of $40 million. At current market prices, the nitrogen in Iowa's swine manure would translate to about $60 million in fertilizer.

While these numbers are impressive, says Raskin, it is unlikely that pork producers or dairy farmers will invest in an anaerobic digester unless they are sure that it is reliable and affordable. An on-farm cost-benefit analysis of ASBR is on-going.

Raskin's team, partially funded by the Illinois Council on Food and Agricultural Research, aims to build on the promise of this technology by further reducing ASBR's operating costs and increasing the value of its products.

For example, separating out the phosphorus in manure provides environmental and economic advantages. Typically, farmers apply manure to ensure sufficient nitrogen for the crops. But in the process of applying enough manure to provide adequate nitrogen levels, they may over-apply phosphorus.

If nitrogen and phosphorus can be measured separately, this problem can be avoided.

Also, phosphorus extracted from manure can be sold to industry. Mining of phosphorus is becoming increasingly difficult and supply is limited. So industry and government efforts to recover phosphorus have recently begun to intensify, says Raskin. Animal manure is high in this nutrient and is a cheap resource.

One way to reduce the cost of a digester is to reduce its start-up time. It takes months for bacteria to multiply to the point where the anaerobic digester is up and running. If the start-up time is during colder seasons, a heater may be required.

By trying different sources of bacteria in the digester, Raskin says he hopes to find the fastest start-up route. With a quick start-up time, methane will be produced sooner, lowering operating costs in the first few months, and maybe even lowering the initial investment, if a heater is not necessary.

An early pioneer

Fairgrove Farms, a 650-plus cow dairy operation based near Sturgis, Michigan was one of the early pioneers in methane gas digestion. In 1981, John Pueschel and his brother, David, constructed a plug-flow digester system for about $150,000 that holds 120,000 gallons of manure.

John Pueschel (shown here), and his brother, David, dairy farmers from Sturgis, Michigan, were early pioneers in using a biogas digester system. They installed one back in 1981.
Photo: Karl Ohm

Manure is scraped twice daily from the free-stall barns into a below grade pit that transfers the manure into a underground V-shaped concrete tank. Designed by Perennial Energy, of Iowa, and modified by the Pueschel brothers, a plug-flow digester essentially allows fresh manure to come in one end while older digested manure is slowly pushed out the opposite end of the tank.

The methane is used to power an engine to produce electricity which has been saving, on the average, the farm $4,000 to $5,000 monthly in energy costs.  John says that the payback for the system was about four to five years. The big pluses of the system have been electricity cost savings, reduced odors and fly problems from manure and better fertilizer value, according to John.

Once fully digested, the manure effluent is transferred via a pipeline to a centrifuge where it is separated into solid and liquid fractions.

Solids reclaimed have a sawdust-like consistency and is used as an odor-free stall bedding material. John says the material has helped reduced mastitis when compared to conventional bedding materials. John also estimates that the dairy has reduced bedding expenses by as much as $3,000 monthly.

The liquid effluent flows by gravity into a storage lagoon and is spread directly on fields as fertilizer, using a boom system for better coverage. The liquid fertilizer is usually applied at rates up to 5,000 gallons per acre, which offsets the need for 100 lbs. of nitrogen, 25 lbs. of phosphate, and 100 lbs. of potassium. John estimates a savings of $25 to $30 per acre in fertilizer costs.

An integrated approach

Taking an integrated approach to the production of renewable energy has been the focus of PRIME Technologies, LLC, of South Dakota. It recently received $402,500 from the Value-Added Ag Subfund to study the feasibility of combining a beef feedlot, ethanol plant, anaerobic digesters and feed processing into one operation.

"Combining these four operations is a completely new concept," says Ron Wheeler, commissioner, Governor's Office of Economic Development. "People have combined feedlots with ethanol plants, but adding the other two elements gives it a whole new dimension."

The proposed facility would sit on 320 acres in southern Sully County. The integrated, environmentally sound, complex would include four distinct operations.

"This project is a good opportunity for our beef and corn producers in that is allows them to capture almost 100 percent of the value to their products," says Larry Gabriel, South Dakota Secretary of Agriculture. "A major goal of the Governor's Office of Economic Development is to enhance value-added agricultural opportunities in South Dakota. This project would certainly do that," Wheeler says.

According to Wheeler, capital investment on the project would be nearly $40 million. The project would use approximately 8 million bushels of corn and feed 65,000 - 75,000 head of local feeder cattle yearly. "Additionally, producers could see an increase of 10 to 20 cents a bushel in corn prices. Local tax revenues would increase and up to 50 new jobs could be created," Wheeler says.

The PRIME Technologies project is the largest project to be funded by the Agricultural Subfund. "As the awareness of this fund grows, we are seeing more and bigger projects being presented," Wheeler says. "We are pleased with the results to date and are optimistic this fund will continue to benefit South Dakota agricultural producers."

AgSTAR Program is resource
on methane gas digesters

In 1993, the U.S. Environmental Protection Agency (EPA), along with the U.S. Department of Agriculture(USDA)  and U.S. Department of Energy (DOE  launched the AgSTAR Program designed to promote cost-effective and efficient methods to reduce methane emissions from manure management.

Focusing largely on dairy and swine operations, one of the program's main goals is to assist farmers in partnering and allying with key firms well-versed in the areas of digester design, electrical interfacing, manure management and by-products.

To find out more about this unique program, please contact:

EPA AgSTAR Program
401 M Street, S.W. (6202J)
Washington, D.C. 20460

AgSTAR hotline number: 1-800-95-AGSTAR