Producer-agricultural engineer develops simple system that effectively filters out manure odors and reduces neighborhood complaints.
Neighbors driving by NPPI's 1,500-sow, farrow-to-finish operation near Hector, MN, don't complain about hog odors. Biofilters, operating since the complex was built in 1997, have eliminated the majority of the odors as air is exhausted from the barns.
Only when pits are pumped out twice a year for application to nearby fields, do neighbors even notice the gases given off by the manure, says Dick Nicolai.
He raised hogs for 30 years and still owns part of the operation, but now works as an agricultural engineer at South Dakota State University (SDSU) in Brookings.
Biofiltration Not New
Biofiltration occurs when microorganisms mix with oxygen, resulting in a dilution of offensive waste compounds, says Nicolai.
Biofiltration is not new to agriculture, he explains. The drain field for a septic tank system offers similar benefits as the soil serves as a biofilter to break down and absorb the gases from the waste.
In the same way, hog barn gases exhausted through a biofilter's media, usually comprised of compost and woodchips, absorb and convert the odorous compounds, thereby greatly reducing the release of those gases, says Nicolai.
“Volatile organic compounds are broken down, producing carbon dioxide and water, which results in the elimination of about 90% of the odors,” he explains.
The process is fairly simple: air from a hog building, drawn through a manure pit, exits out the exhaust fans. Instead of exhausting gases directly into the atmosphere, the air from the exhaust fans is directed down through an air plenum (shipping pallets) and then through the biofilter (Figure 1).
“It's the media or the compost where the action takes place,” says Nicolai. The exhausted air is absorbed into the biofilter media and comes into contact with microorganisms that break down the offending gases.
For the best results, Nicolai suggests using a media mixture of 20-30% compost and 70-80% woodchips to maintain adequate porosity — vital to proper airflow without backflow. The media mixture can consist of yard waste, municipal waste, etc.
When working properly, biofilters will regularly eliminate 90% of odors from hydrogen sulfide. A fair amount of ammonia odors can be prevented, provided the biofilter is kept wet, he says.
There are three things to remember when it comes to maintenance and operating the biofilter efficiently, says Nicolai.
First, producers must do a good job with rodent control. “This is the number one issue that pork producers are afraid of when it comes to biofilters because they say ‘well, this is just going to attract them and make the rodent issue worse,’” he explains. “My contention is, you've got a rodent problem anyway without a biofilter, and if you do a fairly good job of rodent control, that will be good enough for a biofilter.
“Most producers put out rodent bait stations and think, ‘okay, they are out there, I don't need to look at them now,’” says Nicolai. But bait stations need to be serviced every month or two to keep the bait fresh. Arrange bait stations systematically around the perimeter of your buildings, he suggests. Be sure to place bait stations in the small area between the building and the biofilter.
Second, “You need to keep the compost wet so you create a biofilm on the surface of that media to provide a place for the microorgranisms to live,” he adds. The goal is to keep the content of the biofilter media at a 30-60% moisture level to break down the odorous compounds.
One of the biggest challenges that Nicolai is still dealing with after eight years of working on biofilters for hog buildings is finding a sensor to monitor moisture content. In the meantime, he suggests installing a lawn sprinkler to sit on the biofilter media. Water the compost area for an hour every day during the summer and in mild weather. In winter, in the northern states, snowfall and other precipitation are usually enough to keep the biofilter plenty wet.
Third, use a herbicide to keep weeds knocked down.
Also, don't let people or cattle walk on biofilters, because the media will become compacted and lose porosity.
Occasionally repair air leaks around the perimeter of the biofilter to avoid short-circuiting the system, says Nicolai.
Biofilters may not be for everyone. But biofilters can be a tool to keep neighbors satisfied, or a community less concerned about hog odors.