Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
3.4 Filtration Practices
3.4.1 Description and Purpose
Surface or underground filters that use sand, peat/sand, or compost filter media can be classified as filtration practices. Noted for their ability to consistently remove fine-grain suspended solids, filters are not usually employed alone. Typically, filters are combined with pretreatment measures that remove large-grain sediments and other constituents from stormwater prior to filtration.
Ultra-urban BMPs that use filters can be designed for placement above or below ground level and can operate as either off-line or on-line facilities. Off-line facilities can capture and treat the water quality volume (WQV) or the "first flush" of stormwater, which often contains the highest concentration of pollutants. Larger storm events are diverted around an off-line filtration facility, sometimes into another management system such as an extended detention pond. On-line facilities pass all of the stormwater through the system and, as a result, are generally larger structures.
Filtration practices are usually designed to provide only stormwater quality management due to the relatively high cost of both installation and subsequent maintenance, especially for underground filters. Filtration BMPs have, however, been installed and maintained at some sites for many years and have been found to provide consistent performance. Best of all, filtration practices provide turnkey performance that is independent of local conditions (e.g., soil infiltration, seasonal groundwater levels).
Due to a wide range of available designs, a practical filtration design can be found for roadside applications (e.g., a ground-level design set in a open space) and for congested ultra-urban applications (e.g., an underground design set below a parking area). In terms of its surface area requirements, the footprint of a filtration practice typically occupies between two and three percent of the drainage area it serves. Consequently, most applications of filtration practices are for small to medium drainage areas.
3.4.2 Design Alternatives
Often the commitment of land area for surface filters is too large for most ultra-urban applications. However, surface filters have been extensively employed by several urban municipalities, including Austin, Texas. Filter designs consisting of a settling area and a filter (most often with sand medium) have applicability to highway settings, particularly at cloverleaf interchanges. Pretreatment measures are not usually incorporated into their design since the settling area is designed with sufficient residence time to remove the large-diameter material that would accelerate clogging of the filter medium. The settling area is typically a sediment chamber with a permanent pool that provides for pretreatment by storing the WQV to allow settling of these larger diameter suspended solids.
An underground filter design is well adapted for applications with limited land area and can also be retrofitted into existing storm drain systems. Of all filter practices, the underground system is the most expensive, primarily due to the construction costs of subgrade vaults. However, it may be the only pragmatic option where multiple use of land area is required (i.e., where the committed land area must also be used for automobile parking or for public parks). There are a number of design configurations for underground filters, with operating installations in a number of locations including Washington, D.C., and Alexandria, Virginia (Claytor and Schueler, 1996).
A variety of media can be employed as filter media, including sand (only), peat/sand combinations, and compost. Each of these media has advantages and disadvantages. The longest performance record exists for the sand medium, which is used in the majority of existing facilities. However, more recent designs have employed peat/sand or compost in an effort to improve the removal of metals and oil and grease from stormwater. Some designs have resulted in proprietary systems that attempt to standardize and modularize the application of compost media.
Claytor, R.A., and T.R. Schueler. 1996. Design of Stormwater Filtering Systems. The Center for Watershed Protection, Silver Spring, MD.