Biofilters are low-cost, effective ways of reducing hog odors and gases from mechanically ventilated hog buildings, according to University of Minnesota research.

Biofiltration is not well understood, says Minnesota agricultural engineer Kevin Janni. We do know that the process uses microorganisms to break down odorous gases and expel harmless end products to the environment, he explains.

Janni observes, "We just know we take the media (filtration system), put some organic material in it, add water to it, blow air through it, keep it moist and it will do the job. It will remove a tremendous number of odorous compounds.

"Biofiltration works very well for treating odors because most odorous emissions are made up of numerous compounds at low concentrations that are readily oxidizable. The microorganisms use those exhausted gases for growth and what they actually convert them to we are not sure," continues Janni. The byproducts of the oxidation process are water, carbon dioxide, mineral salts, some volatile organic compounds and various microbes.

Biofilters were first used on livestock facilities in Germany in the late 1960s and in Sweden in 1984. Used in pig and calf shelters, they were reported to be 70% efficient in removing noxious odors and gases.

Air is exhausted from the hog building using ventilation or pit fans connected by a duct to the biofilter plenum, explains Janni. The plenum distributes the air evenly across the biofilter media which screens out odors and gases before the contents are exhausted into the atmosphere (see Figure 1).

Fellow Minnesota researcher Richard Nicolai first proved the value of the biofilter system that he designed in 1996. The 20 x 20 ft. biofilter treated the air exhausted from a pit fan on a farrowing barn.

Using an olfactometer, odor was reduced an average of 78% as it passed through the biofilter. Reduction ranged from 29% in April to 96% in August. The biofilter was less effective in April because it became too dried out.

It is important that a 45-50% moisture level be maintained in the biofilter, stresses Janni. Hydrogen sulfide was reduced 86%, ammonia was lowered an average of 50% (ranging from 28% in March to nearly 100% in July). The biofilter functioned well through the winter and only needed moisture added during mild and warm weather to prevent excessive drying, explains Janni. In a second Minnesota trial in 1997, four small biofilters (5 x 7 ft.) were built to treat exhausted air from a deep pit nursery. Odor was reduced an average of 89% with hydrogen sulfide concentrations lowered an average of 96% and ammonia reduced an average of 78%, Janni reports.

Last year, University of Minnesota scientists collected data from a full-size (6,900 sq. ft. in three sections) biofilter used to treat all of the ventilation air expelled from a 750-sow gestation/farrowing facility at a farm near Hector, MN, owned and operated by Nicolai. Average odor levels were cut 82% during 10 months of operation; hydrogen sulfide was dropped 80%, ammonia 50%. Biofilter Challenges

Because the biofiltration process involves bacteria and fungi that live on the media surface, it is not like a dust filter that needs periodic cleaning, stresses Nicolai. Rather, the living system must be supported with the correct nutrients.

"For a biofilter to operate efficiently, the media must provide a suitable environment in which microorganisms can live and reproduce, have good moisture-holding capacity and have a high porosity," says Nicolai.

An early biofilter used dark red kidney bean straw and compost. Later Minnesota studies used shredded wood and compost. Janni says wood maintains biofilter porosity longer than bean straw. Compost provides a vital source of microorganisms and nutrients. Table 1 lists the characteristics for various relatively low-cost biofilter media that are available in the upper Midwest, says Nicolai.

As mentioned, proper moisture levels must be maintained in biofilter media. Too much moisture will reduce the media porosity and too little will dry it out and it will not work. The challenge, says Janni, is how to develop a system that will periodically provide water to the filtration system so it isn't up to the producer to do it manually.

Another challenge is to keep the biofilter media from plugging due to dust buildup. Plugging is expected to occur in biofilters used for extended periods of time, but it is unknown when dust plugging will occur, says Janni.

Good rodent control is essential, too. Rodents burrowing through the media seeking warmth will cause channeling and poor filtration results. Rabbits, woodchucks and badgers are also suspected of burrowing through and nesting in biofilters, he points out.

Excessive vegetation on the biofilter surface can reduce its efficiency by causing channeling and limiting oxygen exchange. Roots can plug the system. Remove noxious weeds before they produce seed.

Ventilation Changes A biofilter system cannot be installed with existing ventilation. "Biofilters create a pressure drop the fans must overcome to blow the air through the media," says Janni "This pressure drop and its impact on the ventilating system must be taken into account when designing a biofilter. Adding a biofilter to a duct connected to the ventilating fans of a livestock building without accounting for this added pressure drop will cause significant ventilation problems." Most shallow (1 ft. deep) biofilters can treat between 7 and 17 cfm. of ventilation air per square foot of biofilter without creating an excessive pressure drop, he explains.

Naturally ventilated barns with pit fans offer a different challenge, says Janni. The biofiltration system will treat the pit fan exhaust, but the odor emissions from the rooms above the pits would not be covered. Janni says an option being considered at the University of Minnesota is to treat the rooms by sprinkling soybean oil to control the dust and keep down odor levels.

Cost Considerations Amortizing construction and operating costs over three years, the cost of the full-scale biofilter system discussed in this article came to 22 cents/pig produced/year. Rodent control costs came to $275/year. Other operating costs included $125/year to cover the cost of sprinkling and for the extra power to run the ventilation system, according to Nicolai.

Design Details The on-farm research trials led to development of a publication covering specific biofilter design elements and management recommendations. The Biosystems and Agricultural Engineering Update (BAEU-18) can be obtained from the University of Minnesota Department of Biosystems and Agricultural Engineering, 1390 Eckles Ave., St. Paul, MN 56078-6005 or by calling (612) 625-9733. Design information is also available on the Web at www.bae.umn.edu.

The publication will be updated as more information becomes available. Further research on biofilters is ongoing on three separate poultry, dairy and swine sites around the state, says Janni. That project should be wrapped up this summer.