DENSITY CURRENTS AND SHEAR-INDUCED FLOCCULATION IN SEDIMENTATION TANKS

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 geometrics. It solves the full two-dimensional elliptic governing flow equations with the k-epsilon 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.