Engineered windbreak walls includes mosquito screen and treated lumber that covers the exhaust fans.
By Sanjay Shah, Richard Goforth, Jonas Asbill and Praveen Kolar, North Carolina State University Frp Industrial Exhaust Fan
Reducing air emissions, particularly odor, from swine farms is an important concern for the pork industry. Odor emissions occur from houses, lagoons and land application sites. Iowa State University’s Air Management Practices Assessment Tool provides information on practices for reducing emissions.
Here, we discuss a relatively new yet simple technology, the engineered windbreak wall for reducing emissions from tunnel-ventilated houses. The EWW was developed and tested with funding from the USDA National Resources Conservation Service and further testing was funded by North Carolina’s Department of Justice.
The EWW consists of a structure made of mosquito screen and treated lumber that covers the exhaust fans (Figure 1(a)). Computer modeling was used to balance cost with performance and limit backpressure to 0.05 in of water column. The screen slows the exhaust air causing dust to deposit on the screen and the ground.
Since a large fraction of odorous gases is transported on dust, trapping the dust reduces odor. When the front screen is clean, most of the exhaust air passes through it. However, as the front screen clogs with dust over time, more exhaust air leaves through the top and side screens, which are cleaner. Hence, the system traps dust and increases dispersion.
The two openings (Figure 1) prevent excessive fan backpressure due to screen clogging by the dust. The bottom opening is screened with two rows of Switchgrass to trap dust. In July 2016, a system was installed to treat 50,000 cfm of exhaust air from two 48-inch fans and one 36-inch fan from a tunnel-ventilated swine-finishing house (Figure 1(b)).
During August 2021 through June 2023, we measured odor concentrations with Nasal Rangers 10 times during spring, summer and fall, with one, two or all fans running. Odor concentration as dilution to threshold was measured 20 ft from one of the fans covered by the EWW (Test house) and at the same relative location in front of the uncovered fans in an adjacent house (Control house); both houses were, otherwise, identical.
Overall, the EWW reduced odor concentrations significantly (t-test p<0.01) by more than half (Figure 2). During most events, there was very little Switchgrass and its impact on odor reduction seems to be small. Further, since Switchgrass requires annual planting, it can be omitted to reduce cost. Hence, the EWW was effective in reducing odor emissions from the swine-finishing house.
We also measured ammonia deposition in front of the fans in the Test and Control houses. For this, petri dishes with boric acid solution were placed at 5, 10, 15 and 20 ft from the wall along the centerline of the fans for an hour. The petri dishes at 5 and 10 ft were inside the EWW. Ammonia deposition was calculated based on the volume and ammonia concentration in the petri dish. In all events, ammonia deposition was always higher at the 5 and 10 ft locations in front of the Test house (Figure 3), showing that more ammonia was deposited inside the EWW than in the same area in front of the uncovered fan. While the entire footprint of the EWW would have to be covered with acid-filled petri dishes and ammonia emissions from the fans would have to be quantified to calculate total deposition, ammonia emission was reduced substantially.
In addition to reducing odor and ammonia emissions, the EWW had average hourly backpressure less than our target backpressure of 0.05 in, indicating that dust on the screen had a small impact on airflow rate. One reason why European exhaust air treatment systems are not used in U.S. livestock houses is that they impose much higher backpressure on the exhaust fans. Further, rainfall readily cleans the screen. The system can be built with labor and tools available on the farm.
Very importantly, the EWW has been approved for Environmental Quality Improvement Program cost share by North Carolina NRCS beginning FY 2024. The EWW cannot be used in naturally ventilated houses and will be more expensive to install on sidewall-ventilated houses because the fans are dispersed along the sidewalls.
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