Author(s): N. Vosswinkel; B. Micke; M. Wietbuscher; S. Gordon; R. Mohn

Linked Author(s): Rainer Mohn

Keywords: Sedimentation tanks; Fine particle separation; Recirculation zones; Storm water treatment; Numerical modeling

Abstract: Insufficient hydraulic conditions are likely to cause suboptimal conditions for sedimentation in rectangular storm water treatment tanks. This study focuses on the presence of recirculation zones and their effect on separation efficiency of fine particle fractions. Sedimentation tanks are commonly used as storm water treatment facilities to reduce the emission of particulate matter. Particles in urban runoff and substances bound to these particles contribute substantially to the pollution of surface waters. The overflow of separate sewer system has been identified as being largely responsible for the presence of heavy metals in the environment. More than 70 % by mass of the total heavy metal load is bound to fine particles with a diameter of less than 44 μm. Therefore, the target parameter AFS63 (name according to German Standards; it includes all fractions with a diameter between 0.45 and 63 μm of Suspended Solids [SS]) was defined in the German regulatory of DWA-A 102. Previous investigations have revealed that the separation efficiency for AFS63 can be significantly improved by straightening of streamlines within the sedimentation tank to avoid flow separation and recirculation zones. The profile of horizontal velocity components shows then a kind of plug form, that means a uniform velocity distribution near to the mean horizontal velocity. In contrast, our investigations show that straightening the flow does not improve but deteriorate the sedimentation of the finer fractions of AFS63. That may damage the separation of heavy metals. Our numerical model tests indicate that the separation efficiency for finer fractions could be improved significantly by a prolonged residence time in recirculation zones. As these recirculation zones occupy space, the straightened flow areas are impacted by higher acceleration and shorter residence time. Therefore, there is a need to optimize a sedimentation tank by selectively partitioning into areas with straightened flow and with controlled recirculation zones.

DOI: https://doi.org/10.3850/978-981-11-2731-1_110-cd

Year: 2018