Though removal efficiency is a good indicator of a devices’ effectiveness in terms of separating pollutants from stormwater, particularly under low flow conditions, it does not provide a complete description of device efficacy especially in terms of its ability to retain previously captured pollutants under moderate or high flows. The phenomenon of ‘Washout’ defined as the tendency to re-suspend (re-entrain) and discharge previously captured pollutants is of particular importance in practice as stormwater treatment devices are typically maintained on an infrequent basis.

The paper describes results of work undertaken at Liverpool (JM) University to determine sediment retention performance of different stormwater treatment chamber configurations under controlled laboratory conditions that provides a true comparative assessment using identical testing protocols and methodologies. General conclusions drawn from the results include the observation that the phenomenon of “pollutants washout” from stormwater treatment chambers is very real and must be taken into account in system selection for practical application. The rate of stored pollutants washout was found to be extremely sensitive to chamber design with re-suspension and washout rates ranging from close to zero to close to 100 percent depending on device configuration, nature and type of flow modifying components and whether or not sediments are stored in isolated storage zones.

Authors

Darrell A. Egarr, Mike Faram, Tim O’Doherty and Nick Syred

Abstract

Hydrodynamic vortex separators (HDVSs) are low energy solid-liquid separation systems. They have been applied for a number of duties including wastewater treatment, combined sewer overflow treatment and storm water treatment. Such systems operate whereby solids settle due to the force of gravity and sufficient residence time for this to take place is provided by the rotary nature of the path of the fluid through the separator. In this study, the HDVS is considered operating without an underflow component which may be the case in practice, for example, where collected solid material is removed on a batch basis.