A Tool for the Performance Assessment of Hydrodynamic Separators

Hydrodynamic separators are stormwater treatment devices with small footprints used for removing suspended sediments from stormwater runoff in urban areas. These devices have been tested by different laboratories under a set of specific conditions, e.g. a limited number of flow rates, under a specific water temperature (fluid viscosity), a specific particle size distribution and particle specific gravity. However, the question ahead of the primary users of these devices, i.e. cities, counties and other local government agencies and large retailers is: "How well do these devices remove suspended sediment from our stormwater runoff?" To address this question a tool has been developed to simulate continuous loading from small drainage basins in urban areas and to simulate the response of these devices in removing the suspended load from stormwater runoff. The tool is comprised of a hydrologic model and a generic device response model. The hydrologic model utilizes the SCS curve number method and the SCS unit hydrograph to simulate runoff. The input data into the hydrologic model are the drainage basin characteristics and 10 years of 15-minute precipitation data. The generic device response model utilizes the removal efficiency performance functions developed from the laboratory testing conducted on hydrodynamic separators. The input data for the device response model are 10 years of daily air temperature data, the particle size distribution, specific gravity, and influent concentration of suspended sediments in stormwater runoff. The tool has been applied to four small urban drainage basins in Minnetonka, MN using four hydrodynamic separators as stormwater treatment devices. The results of continuous simulation were compared to the performance assessment of the devices under 1-year and 2-year frequency storm events to determine if an event-based simulation is adequate to assess the performance of these devices.