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Water: Best Management Practices

Manufactured Products for Stormwater Inlets

Minimum Measure: Post-Construction Stormwater Management in New Development and Redevelopment

Subcategory: Other 

A variety of products called swirl separators or hydrodynamic structures have been widely applied to stormwater inlets in recent years. Swirl separators are modifications of traditional oil-grit separators.  They contain an internal component that creates a swirling motion as stormwater flows through a cylindrical chamber. The concept behind these designs is that sediments settle out as stormwater moves in this swirling path, and additional compartments or chambers are sometimes present to trap oil and other floatables. There are several different types of proprietary separators, each incorporating slightly different design variations, such as off-line application. Another common manufactured product is the catch basin insert. These products are discussed briefly in the Catch Basin Inserts fact sheet.


Swirl separators are best installed on highly impervious sites. Because little data are available on their performance (independently conducted studies suggest marginal pollutant removal), swirl separators should not be used as a stand-alone practice for new development. The best application for these products is as pretreatment to another stormwater device or, when space is limited, as a retrofit.


Limitations to swirl separators include:

  • Very little data are available on the performance of these practices, and independent studies suggest only moderate pollutant removal. In particular, these practices are ineffective at removing fine particles and soluble pollutants.
    • The practice has a high maintenance burden (i.e., frequent cleanout).
    • Swirl concentrators are restricted to small and highly impervious sites.
Siting and Design Considerations

The design of swirl concentrators is specified in the manufacturer's product literature. For the most part, swirl concentrators are a rate-based designs. That is, their size is based on the peak flow of a specific storm event. This design contrasts with most other stormwater management practices, which are sized based on the capture, storage or treatment of a specific volume. Sizing based on flow rate allows the practice to provide treatment within a much smaller area than other stormwater management practices.


Maintenance Considerations

Swirl concentrators require frequent, typically quarterly, maintenance. Maintenance is performed using a vactor truck, as is used for catch basins (see Catch Basin). In some regions, it may be difficult to find environmentally acceptable disposal methods. Due to hazardous waste, pretreatment, or groundwater regulations, sediments may sometimes be barred from landfills, from land applications, and from introduction into sanitary sewer systems. 


While manufacturers' literature typically reports removal rates for swirl separators, there is little independent data to evaluate the effectiveness of these products. Two studies investigated one of these products. Both studies reported moderate pollutant removal, but while the product outperforms oil/grit separators, which have virtually no pollutant removal (Schueler, 1997), the removal rates are not substantially different from the standard catch basin. One long-term advantage of these products over catch basins is that if they incorporate an off-line design, trapped sediment will not become resuspended. Data from the two studies are presented below. Both studies are summarized in a Claytor (1999).

Table 1. Effectiveness of manufactured products for stormwater inlets


Greb et al., 1998

Labatiuk et al., 1997


Investigated 45 precipitation events over a 9-month period. Percent removal rates reflect overall efficiency, accounting for pollutants in bypassed flows.

Data represent the mean percent removal rate for four storm events.




























a TSS=total suspended solids; TDS=total dissolved solids; TP=total phosphorus; DP=dissolved phosphorus; Pb=lead; Zn=zinc; Cu=copper; PAH=polynuclear aromatic hydrocarbons; NO2+NO3=nitrite+nitrate-nitrogen

Cost Considerations

A typical swirl separator costs between $5,000 and $35,000, or between $5,000 and $10,000 per impervious acre. This cost is within the range of some sand filters, which also treat highly urbanized runoff (see Sand Filters). Swirl separators consume very little land, making them attractive in highly urbanized areas.

The maintenance of these practices is relatively expensive. Swirl concentrators typically require quarterly maintenance. The most common method of cleaning these practices is a vactor truck, which costs between $125,000 and $150,000. This initial cost may be high for smaller Phase II communities. However, it may be possible to share a vactor truck with another community. Depending on the rules within a community, disposal costs of the sediment captured in swirl separators may be significant.


Claytor, R. 1999. Performance of a proprietary stormwater treatment device: The Stormceptor®. Watershed Protection Techniques 3(1):605-608.

Greb, S., S. Corsi, and R. Waschbusch. 1998. Evaluation of Stormceptor® and multi-chamber treatment train as urban retrofit strategies. In Proceedings:  National Conference on Retrofit Opportunities for Water Resource Protection in Urban Environments, Chicago, IL, February 9-12, 1998. U.S. Environmental Protection Agency, Washington, DC. 

Labatiuk, C., V. Natal, and V. Bhardwaj. 1997. Field evaluation of a pollution abatement device for stormwater quality improvement. In Proceedings of the 1997 CSCE-ASCE Environmental Engineering Conference, Edmonton, Alberta. Canadian Society for Civil Engineering, Montréal, Québec, and American Society of Civil Engineers, Reston, VA. 

Schueler, T. 1997. Performance of oil-grit separator at removing pollutants at small sites. Watershed Protection Techniques 2(4): 539-542.

King County, WA.  2000.  King County Surface Water Design Manual. [www.kingcounty.gov/environment/waterandland/stormwater/documents/surface-water-design-manual.aspx Exit EPA Site].  Accessed November 10, 2005.

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