Fall 1999
Methane recovery from manure: Control odor and produce energy
by Paul Miller, USDA-Natural Resources Conservation Service, Des Moines
It might surprise a lot of people, but manure can be nearly odor free. And it could make you money! New advances in methane digestion technology are finding success in Iowa. Several different types of anaerobic digesters have recently been installed in Iowa with assistance from the USDA-Natural Resources Conservation Service (NRCS) and AgSTAR. AgSTAR is a voluntary program of the U.S. Environmental Protection Agency that is designed to encourage the widespread use of livestock manure as an energy source.
Methane Process. According to Shihwu Sung, assistant professor in environmental engineering, and director of Anaerobic Systems Engineering at Iowa State University, anaerobic digestion occurs when bacteria produce biogas by decomposing organic matter, such as manure, in an environment without air. The process involves the following three steps:
- Hydrolytic bacteria convert complex particulate matter into dissolved compounds with low molecular weight.
- Acidongenic/acetogenic bacteria convert the dissolved compounds into organic acids and hydrogen.
- Methanogenic bacteria finally consume these acids or hydrogen to generate methane and carbon dioxide.
Anaerobic
digesters are sealed with covers that trap the biogas produced in the
digester. The biogas is then pulled from the digester by providing a slight
vacuum on a pipe with a gas pump or blower. Biogas, which contains 60–80
percent methane and has a heating value of approximately 600–800
Btu/ft3, is then used to produce energy. Methane can power
an engine generator to produce electricity and can be used to operate
a boiler or space heater, as well as chilling and refrigeration equipment.
Gas that is not used for energy production is ignited and flared to reduce
methane emissions and odor.
Success Stories. Gary Boland of Williamsburg, Iowa, wanted to reduce the odor coming from his earthen manure basin that served his pig nurseries. A floating cover, placed over the basin and kept afloat on top of the manure with 10-foot-long foam board logs, captures the biogas (approximately 60 percent methane). Methane produced by the stored manure is burned in a solar-operated flare, reducing or eliminating odor. The manure can still be used as fertilizer because none of the nutrients are lost or destroyed. Gary’s extra cost for the cover was about $7.50 per head for his 2,700 head nursery (about $1.00 per square foot of basin surface) (see article on pages 2–3).
Steve Crawford, a Story County hog producer, is testing new anaerobic digestion technology. The anaerobic sequencing batch reactor (ASBR), developed at Iowa State University, has been highly successful at converting swine manure to biogas. The ASBR is currently producing biogas with more than 70 percent methane and is using the biogas to operate a boiler to produce heat. This on-farm system treats the manure from 2,800 hogs and is being used for a demonstration site. The per-head cost was approximately $65 based on 5,000 head the unit was designed to handle.
SWIneUSA, located in Union County, is currently operating a complete mix anaerobic digester on a 5,000-head farrow-to-wean swine operation. The biogas from the $100-per-sow digester is operating an engine generator that currently produces 60 kilowatts of electricity for use at the operation. The unit provides most of the energy needed by the farm. Waste heat from the engine generator is captured and used to heat the digester.
Pollution Concerns. Growth of the livestock industry has generated the need for improved methods of manure management that are cost-effective and reliable. Pollutants from decomposing livestock manure can cause major problems, including surface and groundwater contamination as well as surface air pollution caused by odors, dust, and ammonia. Then there is the additional concern over the contribution of methane emissions to global climate change. In response, researchers have developed advances in biogas technology. The technology promotes the recovery and use of biogas to generate electricity or for heating and cooling needs.
The First Farm-Based Digester. In 1972, a farm near the town of Mt. Pleasant, Iowa, became the site of the first farm-based methane digester in the United States. The McCabe farm, a hog production facility, was near a town that was expanding to the farm’s border and the McCabe’s had to find an odor-free system of managing swine manure. It took several years of development, but a successful digester was created by adapting technology from a municipal wastewater treatment facility. The system has experienced very few problems and is used solely for odor control.
Major Improvements. The recovery of methane from animal manure is not new technology. More than 2 decades of research has gone into biogas systems that were developed in the 1970s when oil prices began to escalate. These early systems often failed. Biogas systems such as anaerobic digesters have a much greater success level now because of the improved technical support and increased profitability through the sale of manure by-products. Some dairy facilities report that they generate more revenue from the sale of electricity and other by-products than from the sale of milk. Aside from the moneymaking factors, digesters do help reduce odors. And that is a major concern for many livestock producers in Iowa.
For More Information. The USDA–NRCS can help landowners decide if a biogas system is appropriate for their operation. AgSTAR estimates that more than 2,000 livestock facilities across the United States could benefit from biogas recovery systems. For more information, please contact me at 515-284-4370 or the AgSTAR Program at 1-800-952-4782. For Internet resources on manure management and the AgSTAR program, visit http://extension.agron.iastate.edu/immag under the Publications link.
