Evaluation of the Effects of Inlet Conditions on Particle-Laden Flows in Secondary Sedimentation Tanks

Abstract

Sedimentation tanks are one of the most important and costly parts of the water treatment process in which particles remaining from previous stages settle down due to the force of gravity. In this research, particle-laden flows in secondary settling tanks are numerically investigated by considering the effect of the free surface using the volume of fluid (VOF) method. Comparison of the results of the current study with available experimental results shows the importance of size distribution and particle diameter deviation from the mean particle size D50. Moreover, examination of the effects of inlet position as well as inlet Froude number reveals that (1) a decrease in inlet Froude number exerts a positive effect by reducing the volume of the recirculation zone and a negative effect by increasing the maximum velocity. Therefore, an optimum Froude number can be obtained for each geometry that maximizes the tank efficiency. (2) The maximum efficiency occurs when the inlet position is close to the free surface; however, for this case, the recirculation zone is larger than that for cases in which the inlet is in the middle of the tank or close to the bed.