Density Currents and Shear‐Induced Flocculation in Sedimentation Tanks

Abstract

A numerical model for the computation of turbulent flows in sedimentation tanks is presented. Based on a nonorthogonal grid formulation, it can handle fairly arbitrary two‐dimensional geometries. It solves the full two‐dimensional elliptic governing flow equations with the k‐ε turbulence model. Effects on momentum, as well as turbulent kinetic energy balances, due to sediment‐induced density currents are taken into account. Solids with a broad settling velocity distribution are treated by considering different settling velocity classes. A simple flocculation model assuming only turbulent shear‐induced flocculation is formulated, and the importance of this cause of flocculation is examined. Predicted velocity and concentration profiles and removal efficiencies are compared with observations in a tertiary settling basin in Sweden. Results suggest that the flow field is significantly changed by seemingly small density differences, that shear‐induced flocculation is relatively unimportant, and that the influent settling velocity distribution is critical for accurately predicting of removal efficiencies.