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




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System Performance

Water Quality Treatment
Water quality performance was moderate to poor across the range of pollutants commonly associated with stormwater treatment performance assessment. The following observations are based on median values that reflect the performance of the four systems evaluated at UNHSC: the VortSentry, the CDS, the V2B1, and the Aqua-Swirl.

The median annual average for removal of total suspended solids in these systems was well below the EPA’s recommended level for removal—they performed in the 30 percent range during the warmer months and 20 percent range in the winter. Likewise, they did not meet regional ambient water quality criteria for removal of petroleum hydrocarbons and zinc. No removal was recorded for nutrients, dissolved inorganic nitrogen, or total phosphorus.

The chart at top right is based on median values for the class of HDS systems evaluated; it reflects their performance in removing total suspended solids, total petroleum hydrocarbons, dissolved inorganic nitrogen, total phosphorus, and zinc. Values represent results recorded over 18 storms, with the data further divided into summer and winter components.

Water Quantity Control
Typically, HDS devices are flow-through systems. Therefore, they exhibit no peak flow reduction, volume detention, or lag time, as demonstrated by the chart at bottom right.

Maintenance
Maintenance of HDS devices includes the periodic inspection for floating debris, oil, and grease and the removal of solids by a vacuum truck. Systems in which the catchbasin is designed to be open and accessible allow for more thorough removal of sediment and are less costly to maintain. These devices did not requiure cleaning during their evaluation at the field site.

Cold Climate
As a class, the ability of HDS devices to remove sediments was significantly impacted during cold winter months. This is due to the increased viscosity of stormwater runoff and high concentrations of chloride, both of which combine to reduce particle settling velocity. Calculations of particle settling velocities at temperatures and chloride concentrations typically found in winter runoff demonstrated that HDS devices need about twice the time necessary to settle the same size particles in cold weather. When designed for installation in prolonged cold climate conditions, HDS devices that rely on particle settling for sediment removal need to be oversized to account for these changes in system performance.