Iowa State University, the University of Minnesota and South Dakota State University join ranks to identify strategies to cut barn odor levels.

Leading agricultural engineers agree that biofiltration supplies a proven means of lowering odor emissions from totally confined hog barns. Research suggests that biofilters reduce exhaust levels by 90% or more.

One of the drawbacks to biofilters, however, is the added energy demand that is placed on the ventilation system, according to Steve Hoff, professor of agricultural engineering at Iowa State University (ISU).

When a biofilter is added to the building equation, it produces backpressure on the fans, and in some situations that requires more power than the existing fans can provide to push the air through the biofilter, he points out.

As a result, when a biofiltration system is installed in a confinement building, new, higher-capacity fans must often be installed to deal with the increased backpressure exerted by the biofilters.

But Hoff strives to make the most of what biofiltration has to offer without ramping up costs. “One of the things I have been concerned about is to make sure we can use standard agricultural fans that are currently in our barns to push air through a biofilter,” he comments.

Dealing with Nighttime Odors

Because the atmosphere is very unstable and there is a lot of vertical mixing of the air during the daytime, especially in summer, hog odors do not travel as far.

In contrast, during the nighttime, air is emitted into a very stable environment and odors travel farther. That's why more odors are detected when people first get up in the morning, Hoff explains.

The goal, then, should be to focus on using the biofilter to lower the nighttime rate of dispersion of odors and gases into adjacent areas.

“Let's not try to biofilter 100% of the ventilated air if it is not needed,” states Hoff.

To that end, Hoff and his team, including agricultural engineer Jay Harmon of ISU, are studying the use of existing fans in a 600-head, two-room finishing barn with a deep manure pit. The pens were stocked at 7.75 sq. ft./pig. One side is operated with a biofilter comprised of wood chips, which, when kept properly wet, offer enhanced porosity and reduce the total demands on the ventilation system. The other side of the finishing barn, with no biofilter, serves as the control model.

The fan system has been set to produce about 42 cu. ft./min. (cfm)/pig, within the range of the critical minimum ventilation rate (40-50 cfm/pig) needed to adequately treat nighttime exhaust air during summer days. The setting represents about a third to half of the maximum ventilation capacity of 120 cfm/finishing pig.

Then, in order to capture potential energy savings when biofiltration is not needed, Hoff built a bypass system that in effect “bypasses” the biofilter and allows the fans to operate normally (Figure 1).

Using an on-site weather station, researchers keep track of atmospheric conditions. When the atmosphere is unstable, and there is good, natural mixing of the air (daytime), the biofilter is bypassed. When the atmosphere is stable, and exhausted air can travel greater distances and cause potential odor disturbances, the biofilter is operated. This occurs mainly at night, lasting approximately 8-10 hours.

The bypass consists of a series of louvers located on the front of the biofilter (on the exhaust side of the pit fans). “When you do not need to operate the biofilter, you simply open the bank of louvers and bypass the biofilter system,” Hoff explains.

In one year of testing, the biofilter has produced a 60% reduction in odors, and close to a 60% reduction in ammonia with a wetted, wood chip-based biofilter, he reports.

Hoff concludes: “What we are saying is, let's come up with a biofilter that may not give us those real high reduction efficiencies, but gives us ample reduction in gases, and allows us to use standard agricultural fans that we have in our hog barns today.”