Influence of Model Parameters and Inlet Turbulence Boundary Specification Methods in Secondary Settling Tanks: Computational Fluid Dynamics Study

Abstract

Computational fluid dynamics (CFD) has been applied in secondary settling tank (SST) analysis for more than 30 years. Although great progresses has been achieved, there are still some uncertainties in CFD SST modeling. In this study, a numerical model is used to understand the effects of two model parameters: rp, accounting for particles with poor settling properties in the Takács settling model; and ρp, the dry solids density. Both parameters are used in the user-defined functions to couple solids transport on settling velocity. Also, the differences in turbulence specification methods in prediction of SST performance are evaluated. The results show that the prediction of effluent suspended solids (ESS) is very sensitive to rp. The incorrect specification of rp may mask real improvements in SST geometry, and may even cause clarification failure prediction under the normal inflow conditions. The value of ρp has less impact on ESS prediction, and its specification is less critical to maintain ESS prediction consistently with field data. Neither return activated sludge concentration (RAS) nor sludge blanket height (SBH) is sensitive to the effects of rp and ρp. Additionally, the initial turbulent kinetic energy and turbulent dissipation rate specification methods produce only local effects on the hydrodynamics near the inlet boundary, and none of the performance indicators (ESS, RAS, or SBH) are sensitive to inlet boundary turbulence specification methods.