Incorporation of a compression term in a CFD model based on the mixture approach to simulate activated sludge sedimentation

Abstract

The modeling of Secondary Settling Tanks has become an important decision tool due to its use in assisting tank design, troubleshooting, optimization and control. The density currents created by the dispersed phase (sludge) and the turbulence generated at the inlet make this phenomenon three-dimensional. This research is focused on a CFD tool development including not only the hydrodynamics of the settling tank but also the recent advances in both hindered and compression settling modeling. The existing OpenFOAM® solver driftFluxFoam, which calculates the settling velocity of the dispersed phase as the drift velocity, was improved by adding in the model equation a power-function and a second order derivative term to describe hindered settling and compression respectively. The modified solver was validated against batch settling experimental data. Consequently, simulations were made using an axisymmetric circular clarifier geometry, to study the effects on the sludge blanket height and sludge concentration using the compression function.