The technology behind swine manure digesters hasn’t changed much in the past decade, but new applications are redefining nutrient management nationwide.

“With odor, air-quality and water-quality issues facing the swine industry, we need to look at 21st-century solutions,” says John Norwood, a principal with Triple Bottom Consulting in West Des Moines, Iowa. “That means taking a new look at manure digesters.”

In northwest Iowa, pork producers, local communities and West Bend based Max Yield Cooperative are looking at converting manure into electricity and other value-added products through a centralized, anaerobic digester system.

Processing manure from area swine farms to produce methane gas can generate electricity for the power grid, explains Larry Arndt, MaxYield’s agronomy and technical services marketing director.  The residual byproduct of anaerobic digestion also contains manure nutrients without the odor. The byproduct can be land applied or can be dried and sold as fertilizer.

   “Anaerobic digestion provides not only odor abatement and pollution control, but also sustainable energy and nutrient-rich compost,” notes Arndt. “The compost is pasteurized and far more stable and transportable than liquid manure. Anaerobic digestion can convert a potential liability into a value-added product and profit center.”

The approach is on the right track, says Rodney Frazier, president of FBA Consulting in Memphis, Tenn. “We’ve got to find something that the public wants. People don’t want swine manure, but they do want electricity.”

Europe has had centralized manure digesters in place for decades, notes Shihwu Sung, associate professor of environmental engineering at IowaStateUniversity. “Denmark and the Netherlands have government-subsidized, centralized digesters that convert livestock manure to fertilizer and methane.”

The fact that digesters aren’t located on individual farms has been key to their success. “Most swine operations are too small to make on-farm digesters economically feasible,” notes Sung. “Most producers don’t have the time, interest or expertise to manage one.”

In late 2004, MaxYield Cooperative received a USDA grant to study the use of a community-based manure collection system. Last spring the Iowa Department of Natural Resources issued a second award to investigate composting options. MaxYield started identifying producers within a 20-mile radius of Whittemore, Iowa, who are interested in participating in a manure cooperative.

Equity interests in the business could come in the form of manure or cash. It also could offer returns to shareholders. “To do a value-added project, you need at least a 20 percent return on investment. We’ve had to make a lot of assumptions, but, the estimated 10-year average return for a system that processes 100,000 gallons a day is 27.8 percent,” says Ardnt.

Because of Iowa’s winters, the system would be in an insulated, enclosed building. “Smell is not an issue with this technology,” says Olaf Riedel, with Biopower Technologies, which supplies manure digester equipment.

The digester’s tank looks similar to a large Harvestore. It generates biogas in the tank’s temperature-controlled, oxygen-free atmosphere when bacteria break down the manure. The biogas is a mixture of 65 percent methane, 33 percent carbon dioxide and 2 percent other gases, including oxygen and hydrogen sulfide. Through a desulphurization process, the hydrogen sulfide can be removed.

As for the byproduct, the turf industry is a big potential market. The products also can be used for stream-bank stabilization, erosion control on slopes near roadways, and berm construction. “With stabilized organic matter content, these products can be used for turf topdressing, mulching and making mixes for golf courses,” says Ron Alexander, with the North Carolina-based R. Alexander Associates. Nutrients from the byproduct also are easy for plants to absorb. “The biggest challenge is building a market for the products and securing public acceptance,” notes Sung.

Other industries are interested in the digester’s byproducts as well. “We’ve been contacted by a large supermarket that’s interested in the thermal energy that the manure digestion process produces,” says Arndt. “They want to use it to heat a greenhouse.”

MaxYield hopes to move ahead on the project this next year.

Others are trying out similar processes. Premium Standard Farms completed a $9.5-million, manure-to-fertilizer plant near Green City, Mo. The project is located at the PSF Valley View farm, which averages 107,000 head of grow/finish pigs. The product of a five-year effort by PSF and Crystal Peak Farms, the plant operates without treatment lagoons and eliminates nutrient land application. It also captures methane, as a renewable fuel. 

“Manure from the unit can be processed in a digester and made into a dry, granular fertilizer with an approximate nitrogen, phosphorus and potassium analysis of 12-8-8,” says Dave Townsend, PSF’s vice president of environmental affairs. “The operation will be able to produce approximately 8,700 tons of fertilizer a year.”

The process starts when manure is thickened within internal recirculation-process-pump stations at each of the 14 sets of eight barns. The manure slurry is transferred to one of five digesters operated on a batch basis. Slurry from the digesters is pumped to a gravity thickening basin before going to a centrifuge building. Liquids are pumped to a lagoon, where they are processed through the freeze-thaw system during freezing weather to produce concentrated liquid fertilizer and treated water.

Meanwhile the thickened slurry is centrifuged to separate solids and produce a cake material. This is combined with the concentrated liquid fertilizer. The mixed material is fed to a rotary dryer and granulation system. “We use biogas from the digester to supply heat to dry and granulate the fertilizer product,” Townsend explains. Then the granulated fertilizer is processed through a second-stage rotary drum to char it, eliminate objectionable product odors and render the material pathogen-free. PSF has a contract with the J.R. Simplot Company to take the finished fertilizer pellets, which can be used on golf courses, lawns and more.

Methane produced from swine manure digestion also can be used in the biodiesel industry, says Norwood with Triple Bottom Consulting. Smithfield Foods has invested $20 million in a facility that converts swine manure into biodiesel. Smithfield is the major partner in Smithfield BioEnergy, which constructed the project at swine production facilities near Milford, Utah, at Circle Four Farms.

The project includes a collection system to pump manure to a central processing facility where it is concentrated. It then moves to a second facility that produces biogas. From there it’s piped to an enclosed plant where it’s converted into biomethanol. It can be transported to a plant outside of Utah and processed with a variety of vegetable- or animal-based oils to create biodiesel.

Centralized manure digesterss also could benefit the ethanol industry. Drying distillers grain is a major cost, with some plants spending $1 million a month, says Norwood. “Swine manure could be digested to generate methane that could be used to dry distillers grain.”

Fostering these innovative approaches to manure management will require a broad base of support. Sung believes green energy credits and carbon credits could help promote the technology and make it economically feasible.

“Manure digesters may not be the silver bullet for the swine industry, but they’re a step in the right direction,” says Frazier.

Darcy Maulsby is a freelance writer in Granger, Iowa

New Applications Redefine Manure Digesters

A manure digester’s tank looks similar to a large Harvestore. Biogas is generated in the digester’s tank when bacteria in the temperature-controlled, oxygen-free atmosphere break down manure. The biogas is a mixture of 65 percent methane, 33 percent carbon dioxide and 2 percent other gases, including oxygen and hydrogen sulfide. Through a desulphurization process, the hydrogen sulfide can be removed. Experts agree that most swine operations are too small to make on-farm digesters economically feasible. However, new projects are studying the prospect of community-based manure collection systems.

Composted livestock manure can be used in the turf industry, according to Larry Arndt (left) from MaxYield Cooperative, and Ron Alexander with R. Alexander Associates. MaxYield is gauging pork producers’ interest in an anaerobic digester system that can convert hog manure into fuel, provide odor abatement and produce a nutrient-rich compost.