Numerical simulation of a secondary clarifier in a sewage treatment plant using modified Bingham model

A numerical scheme for simulation of activated sludge sediment flow in a circular secondary clarifier is developed. The flow and settling processes are simulated, using the k- ε turbulence model on a two-dimensional axisymmetric and orthogonal grid. To incorporate the sedimentation of the activated sludge in the field of gravity, a convection-dispersion equation governing the mass transfer in the clarifier is extended. The computational domain includes the sludge blanket where the viscosity is affected by the rheological behavior of the sludge. The double-exponential equation is used to describe the dependence of the settling velocity on the solids concentration. The compression and consolidation process of the activated sludge is simulated by this equation. The experimental data provided by Weiss et al. show that the rheograms follow the Bingham law at low-shear rates. The modified Bingham model was introduced to overcome the blanket height overestimation problem with the results showing that the local sludge distribution in the clarifier has an excellent agreement with the concentration profile measurements by Weiss et al. (2007) and the sludge viscosity dominates the flow and sedimentation of activated sludge within the sludge blanket.