A dry filter box is a pretreatment structure that is installed at grade at a curb cut or inlet to prevent sediment and other debris from entering a bioretention basin or other storm-water primary treatment BMP. The box provides a stable inlet, slows runoff velocities, and captures gross solids. It can also remove finer particles if a permeable filter panel is installed. The box concentrates sediment and debris into one contained collection area and thus simplifies the recurring sediment removal needed to insure the long-term functioning of the treatment BMP.
Capturing sediment within the box helps extend the life of a downstream primary treatment BMP by reducing the likelihood of plugging. The box is easy to maintain because it contains an easily accessible shallow sump that can be cleaned out with hand tools. Semiannual cleaning is typically required to maintain functionality.
The dry filter box is most effective when used in conjunction with a curb cut inlet into a bioretention basin with a contributing impervious drainage area of 1/2 to 1 acre. The box should be designed to allow overflow in the event the box becomes full.
Dry filter boxes can be installed in both new and existing BMP projects where there are concerns about the inlet and/or maintenance issues. Dry filter boxes installed to retrofit existing BMPs require replacement of the curb cut and inlet.
Cold Climate Suitability
During winter, dry filter boxes will likely become buried in snow and ice, no different from any other inlet type. Runoff will likely continue to enter the box under the snow or when a path is formed during snowmelt. When properly designed and installed, dry filter boxes will not shift or separate from the inlet as the ground freezes and thaws.
Water Quantity and Water Quality Benefits
The dry filter box is not a primary means of controlling water quantity. The dry filter box is a pretreatment BMP and maintenance tool; the primary treatment BMP provides the water quantity and quality benefits.
Dry filter boxes are positioned near the inlet of the primary treatment BMP to receive initial storm water inflow. When properly designed and installed, the dry filter box will slow runoff velocities and allow solids in runoff to settle and be filtered before entering the primary treatment BMP. BMPs lacking such pretreatment have little protection against the accumulation of sediment and debris transported in the incoming storm water. Sediment buildup in the primary treatment BMP is difficult to remove and severely compromises the treatment BMP by reducing infiltration rates and storage volumes and stressing the plant community, thereby reducing the water quality benefit of the BMP.
Critical components for an effective dry filter box design:
- Size of contributing drainage area: ideally less than 1 acre
- Elevation difference between curb inlet and dry filter box: 2 to 3 inches to avoid plugging of the curb inlet
- Placement of the bottom of the dry filter box: above the bottom of the BMP treatment basin to ensure no standing water in the box
A dry filter box can be constructed of any material that is able to withstand frequent inundation and temperature extremes (e.g., composite or concrete). The dry filter box should rest on a solid base to prevent settling. Concrete should be poured, if necessary, between the curb inlet and the dry filter box.
Depending on the characteristics of the contributing watershed and seasonal variation, common maintenance needs include periodic removal of accumulated sediment and debris from the dry filter box. Contributing watersheds with high sediment concentrations will require more frequent removal of buildup of sediments and solids and cleaning of any filter screen. Contributing drainage areas with high sediment concentrations during wet periods will require the most frequent maintenance of the dry filter box and filter.
*Note: Rain Guardian Turrent pretreatment chamber (Patent No. US 8,501,016 B2) available through Anoka Conservation District, Minnesota.