Calibration Testing and Modelling of a Hydrodynamic Separator for Urban Drainage Particulate Matter Separation and Scour

A common unit operation (nominally a BMP) for separation of coarse particulate matter (PM) and urban detritus in the built environment and areas under development is a screened hydrodynamic separator (SHS). These devices are intended to separate such gross PM and associated pollutants without providing hydrologic restoration. This study examines the pilot- scale performance of a common screened (2400 µm screen) hydrodynamic separator using a computational fluid dynamics (CFD) that was calibrated and validated utilizing pilot-scale testing data. The design of pilot-scale testing experiments utilized flow rates ranges from less than 2% to 125% of published design flow, and followed a common North American regulatory protocol for testing. To ensure that the CFD model results were defensible, pilot- scale testing was required. The resulting calibrated CFD modeling results indicate that for the class of hydrodynamic separators examined, while coarse PM and detritus can be effectively separated, the SHS could not separate fine PM (< 75 µm) which was also prone to scour. It was determined that regular maintenance and cleaning is mandatory to reduce scour and improve effluent chemistry. The study illustrates that lack of maintenance can lead to significant scour of existing PM inventories in the SHS, potential re-partitioning, and changes in water chemistry for runoff stored between wet weather events in a SHS. Results suggest that unit operation systems must include hydrologic restoration, frequent maintenance, frequent sludge/solute management; and considered only after source control measures have been shown to be insufficient for urban drainage chemistry and load modification.