Design Specification No 12: Filtering Practices
Stormwater filters are a useful practice to treat stormwater runoff from small, highly impervious sites. Stormwater filters capture, temporarily store, and treat stormwater runoff by passing it through an engineered filter media, collecting the filtered water in an underdrain, and then returning it back to the storm drainage system. The filter consists of two chambers: the first is devoted to settling, and the second serves as a filter bed consisting of a sand or organic filter media.
Stormwater filters are a versatile option because they consume very little surface land and have few site restrictions. They provide moderate pollutant removal performance at small sites where space is limited,. However, sand filters have limited or no runoff volume reduction capability, so designers should consider using up-gradient runoff reduction practices, which have the effect of decreasing the Treatment Volume (and size) of the filtering practices. Filtering practices are also suitable to provide special treatment at a designated stormwater hotspots. For a list of potential stormwater hotspots that merit treatment by filtering practices, consult the Stormwater Design Specification No. 8 (Infiltration).