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 and cost to construct a slanted bed biofilter for a pit fan on the new South Dakota State University Swine Education and Research Facility wean-finish unit. 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 2610 cfm pit ventilation fan
- Restrictions in space: Gravel driveway, so we needed to minimize the space used downwind from 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 14 ft by 4 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.
To minimize the footprint in front of and to the sides of the fan, the “horizontal” filter design was built with two seven foot sections slanting down from above the top of the fan. Each side of the filter is 7 feet by 4 feet with room for a total of 22 inches of media.
The base structure of this biofilter is made of treated plywood and 2x4s. The media bed base is made of pallets. On top of the pallets is wire mesh overlapped several times (Figure 2). Plastic sheeting was used as a flexible air transfer point between the fan and biofilter. There is a layer of plastic sheeting as well between the ground and filter walls. On the front panel of the biofilter is a square cutout. The cutout is removable to allow access to the filter plenum, but also to add a dust impact filter, if necessary.
Material Costs & Labor
This biofilter cost approximately $320 in materials, and 23 hours labor. The expenses can be broken down into three general categories.
- Treated Wood: $200
- Bed Materials (Wire Mesh): $70
- General Hardware (Fasteners, Plastic Wrap, Etc.): $50
- 23 Hours Labor (Wage Dependent): $230
There was some bowing of the plywood at the bottom ends of the media bed. Extra structural support is worth the investment, especially when the media bed is angled. Figure 2 shows the flexible connection is angled upward from the fan to the filter. We have noticed moisture exhausted from the fan collecting and pooling in the transfer point. The simple solution is to widen the opening point into the air plenum, so that the bottom of the flexible connection angles downward to the biofilter and water can drain to the biofilter base. Making depth marks on the inside of the bed prior to filling with material (in dry conditions) helps to easily monitor settling of media. 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 maintaining the media bed moisture content between 40% and 65% (wet basis) easier.
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.