A biofilter is an air filtration system that uses a layer of microbes on a support media to filter gases from the air exiting a livestock facility. The support media can be a wide variety of material but is most often woodchips. This article outlines the design considerations, experiences and cost to construct a horizontal bed biofilter for a pit fan on the new South Dakota State University Swine Education and Research Facility gestation room. For general background and design information on biofilters, please see the Biofilters Factsheet.
There are many considerations when designing and constructing biofilters. For this site and barn, our design process started with the following specifications and goals in mind:
- Airflow: Exhaust air is from a 6500 cfm pit ventilation fan
- Restrictions in space: Gravel lot, but need to provide ample space for manure removal equipment and feed delivery approximately 30 feet away on either side of the fan
- Empty Bed Contact Time (EBCT, the time it takes the air to travel through the filter): Provide enough capacity for 2 to 3 seconds EBCT
- Adjustable: Allow for variable media depth, and provide flexibility in the air transfer point between the fan and filter to keep pressure off the fan as the ground can rise and fall due to freezing and thawing.
- Accessible: Enable future inspections of the air plenum below the filter for maintenance and research purposes
- Cost: Minimize cost
We used the design equations in the Biofilters Factsheet to determine that a 16 ft by 8 ft horizontal biofilter with 22 inches of woodchip media depth would provide an EBCT greater than 2 seconds. The anticipated pressure drop for this type of design was 0.2 inches of water.
The finished filter has a footprint of 16 feet by 8 feet, with 4 feet of air transfer from fan to the media bed. The air plenum (or space) below the media is 16 inches from ground level. The woodchip media bed is currently 12 inches, but the walls allow us to add additional media, up to 22 total inches (Figure 1). To make the biofilter adjustable and expandable, plastic sheeting was used as the flexible connection point between the fan and the filter (Figure 2). The end sheet of plywood on this filter can be easily removed for access to the air plenum, or configured for expansion if need be.
The filter has a layer of plastic sheeting along the ground and extending approximately 10 inches up the interior walls, creating a barrier to stop any excess water in the air plenum from seeping into the soil. There are cinder blocks with cattle panel resting on top to form the air plenum. On top of the paneling there is plastic ½” mesh overlapped twice to serve as the final layer before the wood chips were added (Figure 3). Based on this design and construction and a current media depth of 12 in, the estimated EBCT is 1.2 seconds. The pressure drop has been checked once with a manometer, and there is an additional 0.08 inches of water pressure against the exhaust fan. Changes in media depth and EBCT will be assessed against the pressure against the fan over time.
Material Costs & Labor
This biofilter cost approximately $320 dollars in materials, plus 23 hours labor. The expenses can be broken down into three general categories.
- Treated Wood: $200
- Bed Materials (Cinder Blocks, Cattle Paneling, Mesh, Etc.): $70
- General Hardware (Fasteners, Plastic Wrap, Etc.): $50
- Woodchips: Free from local landfill
We encountered some natural bowing of the wood, especially in large sheets of plywood. To improve the aesthetics one may consider using more 2x4s as a structural support to tie the plywood together and prevent bowing. The surface under this biofilter is gravel, however, some washout areas in the gravel resulted in gaps between one of the walls and the ground. The plastic liner helps block air from leaving through this gap, but there is a risk of rodent entry. Filling in the gravel after biofilter construction is an easy solution. Finally, while there are water spigots available on the exterior of the barn, including additional spigots on the barn exterior when and where biofilters are planned would make future management of any biofilter easier since the media should be 40% to 65% (wet basis) moisture for optimum efficiency.
Now that design and construction have been completed, the real work begins. Regular monitoring of the biofilter will allow us to observe the effectiveness of the unit. Quantitative (pressure drop, air quality, media settling) and qualitative (observations, odor sensations) measurements will be used to monitor the effectiveness and characteristics of this filter. To keep the biofilter working as designed, it will be imperative to keep the media moist and rodent free.