Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring
Fact Sheet - Catch Basin Inserts
Catch basin inserts (CBIs) are designed either to hang from a drain-inlet frame or to be inserted well below the drain inlet in the sump area, taking advantage of additional space in the lower part of the catch basin. The information provided here refers to drain-inlet inserts that are mounted directly beneath the frame. Figure 22 shows a typical frame-mounted CBI. CBIs work by gravitational filtering to remove debris and large (gravel-sized) sediment particles entering the catch basin. Some of the insert models also are designed with an inner component that contains an oil-absorbent material to facilitate in the removal process.
CBI devices are designed to be suspended from the storm drain inlet structure. Hydraulically, they are designed with a high-flow bypass to prevent resuspension and washout. Only the designed flow rate should pass through treatment surfaces. The insert can contain one or more treatment mechanisms, which include filtration, sedimentation, or gravitational absorption of oils. Two outlets also are designed into the devices. The first outlet is for treated stormwater, and the second is for stormwater that exceeds the capacity of the device. In some manufactured CBIs, the overflow outlet is not a true bypass because excess water still contacts the treatment area prior to overflow. For such CBIs, due to the very short contact time and potential for flushing previously trapped materials, treatment may be compromised at higher flow rates (King County, 1996).
CBIs are not suitable for removal of fine particulate stormwater pollutants such as metals, nutrients, silts, or clays; however, inserts can be used in unpaved areas where the sediment concentration in the stormwater is expected to contain coarse material. In addition, CBIs are suited for sites where a substantial amount of debris is found in stormwater runoff. Areas where CBIs would be appropriate include unpaved roads or parking areas, construction sites, or unpaved industrial sites and lumber yards. Because oil/grit separators are not recommended for unpaved areas, CBIs could be used in lieu of them.
In a recent study by King County, Washington, and others (King County et al., 1995), six different CBIs were evaluated. The inserts tested did not remove significant amounts of pollutants associated with silt- or clay-sized particles; however, the inserts were capable of trapping and removing the coarser materials and debris that are typically found in unpaved areas. New inserts that were designed to remove petroleum hydrocarbons were found to reduce oil and grease concentrations by 30 to 90 percent; after some use, the sustained removal rates were reduced to 30 percent or less. While the inserts varied in their ability to remove oil and grease, most units exhibited some level of treatment if maintained on a regular basis. Inserts did not exhibit any ability to remove metals such as total copper, lead, or zinc. Tests on new and used insert units showed that the CBIs were not effective at removing total phosphorus associated with very fine sediment.
Siting and Design Considerations
Because of their limited ability to remove stormwater runoff pollutants, CBIs should not be used as a stand-alone BMP, but rather installed in conjunction with other BMPs. CBIs are best suited for installation as pretreatment for other BMPs to remove large sediment or debris from unpaved or pervious areas. It should be noted that there are different types of CBI designs and media and one type might not cover all possible pollutants. It is important, therefore, to specify which pollutant is of primary importance because systems optimized for large sediment or debris might not provide acceptable long-term removal of oils and grease, and vice versa. Because catch basin inserts are commercially available, design and installation information can be obtained from their manufacturers or distributors. Catch basin inserts developed by three vendors were evaluated by the Interagency Catch Basin Insert Committee in the Seattle, Washington, area (King County et al., 1995). General design criteria and siting recommendations can be found in the King County, Washington, Surface Water Design Manual (King County, 1996).
CBIs should be designed to perform acceptably for a reasonable design storm (e.g., 2-yr rainfall event) based on hydrologic characteristics and the percent of imperviousness of the site. At the same time, they should not interfere with the drainage for larger rainfall events (e.g., the 10-year rainfall event).
One of the major concerns with CBIs is the need to regularly clean the filter system or medium. Units designed for coarse sediment or debris removal tend to have more holding capacity and, depending on their location, will operate correctly if cleaned after every two or three major storms. Maintenance for CBIs configured for oil and grease removal is also a function of specific site conditions but in general is more intensive. In the majority of the cases, this maintenance focuses on removing accumulated fine-grain sediment from the filter surface or screens. The filter or medium has to be replaced less frequently because of saturation by oil and grease. Streetsweeping could potentially reduce the maintenance frequency for inserts that have this problem.
There is currently an effort to improve the design of CBIs to manage oil and grease and sediment. CBIs currently under development would separate sediment holding areas from the filter media. Captured sediment collected from several storm events would be stored in a dead-storage area at the base of the catch basin, thereby, preventing clogging of the filter media.
Most of the inserts are made of lightweight material and can be removed by one person; however, filter inserts allowed to fill up with sediment or debris may require two-person crews to lift.
Depending on the complexity of the unit, the CBI grate-mounted units can range in cost from as little as $100 up to $1,500. Variables affecting cost include the size of the insert, the type of filter medium, the filtering system, and the material used to construct the insert. Another consideration is the clean-out and maintenance requirements of a sump with an insert versus a sump without the insert. Costs for maintaining CBIs range from $10 to $100 per unit per month, assuming monthly replacement of filter media (King County, 1996). In a study conducted by the Port of Seattle, it took one person 90 minutes to clean 18 inserts. In contrast, it took two vacuum truck operators about three hours to clean 18 sumps (King County et al., 1995).
King County Surface Water Management Division, King County Department of Metropolitan Services, Snohomish County Surface Water Management Division, Seattle Drainage and Wastewater Utility, and Port of Seattle. 1995. Evaluation of Commercially-Available Catch Basin Inserts for the Treatment of Stormwater Runoff from Developed Sites. Interagency Catch Basin Insert Committee.
King County. 1996. Surface Water Design Manual. Draft. King County Surface Water Management Division, King County, WA.