The Efficiency of Storm Water Sedimentation Tanks for Fine Particles in Urban Run-off

The sedimentation process seemed to be a well-known process to treat the urban runoff before discharging it into a water body. The flow mechanisms in real-life sedimentation tanks have now been investigated closer to optimize them for finer particle fractions and interesting findings occurred. The aim of the study was to investigate the sedimentation efficiency of finer mineral particle fractions, which are suspected carriers of heavy metal loads, with the help of an induced flow characteristic. Preparatory studies showed that recirculation zones could have a positive effect on the settlement of finer particles, especially if the axis of the recirculation is horizontal and perpendicular to the flow direction. Therefore the ratio of inlet-height (H_Inlet) to the height of the sedimentation tank (H_Tank) was varied systematically to maximize the sedimentation efficiency (η (-)) of fine mineral particles. It was found that the particle behaviour is influenced by the shear-layer between the main flow and the various sizes of the circulation zone, which depends on the height-ratio at the inlet.