Hydrodynamic Separators
About Hydrodynamic Separators
General Information Performance Data Water Quality Treatment Process Design

Many states allow hydrodynamic separators for primary stormwater treatment; however, there has been a trend toward limiting their use to pretreatment. The systems evaluated at UNHSC all demonstrated water quality treatment performances appropriate for pretreatment usage.

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About Hydrodynamic Separators

Hydrodynamic separators (HDS) are small, flow-through devices that remove sediment, trap debris, and separate floating oils from runoff. UNHSC evaluated four HDS designs from 2004 through 2006: the VortSentry, the Continuous Deflection Separator (CDS), the V2B1, and the Aqua-Swirl. While their proprietary designs vary, they all primarily rely on swirl action and particle settling to remove pollutants. The 2005 UNHSC Data Report presents individual results for these systems. In this report, performance data is presented as median values reflecting the class of systems. Their ability to address water quality was marginal. They appear to be most effective when used for pretreatment in areas where runoff is expected to contain sediment particles greater than 100 microns in diameter.

Where to Use It

Manufactured HDS devices are widely used throughout the United States, and there are many options on the market. Their small footprint makes them particularly suitable for urban areas, or as retrofits to existing stormdrain networks. They are relatively simple to maintain, making them ideal for use as pretreatment components in treatment trains that also include filtration or infiltration systems.


The approved use of HDS devices varies from state to state. This variability is due, in part, to the discrepancies that exist between laboratory-based and field-based performance data.

Many states approve the use of HDS devices for primary stormwater treatment, however, there is a trend toward limiting their use to pretreatment. Currently, some states require field-performance certification before HDS systems can be used for primary treatment. Other states restrict their use to pretreatment, or require that they are used in combination with other stormwater systems as part of a treatment train. This trend, combined with the widespread adoption of HDS devices, reflects the need for programs that provide independent, certified field-testing of system performance.

The installation cost of HDS devices ranges between $18,000 to $20,000 per acre of runoff treated, and this does not include system maintenance.

Designs for HDS devices are available from the manufacturers.