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 \Ik\N–ϵ\N 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 removal efficiencies.
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