Implementation of CFD modeling in the performance assessment and optimization of secondary clarifiers: the PVSC case study

Development and applications in computational fluid dynamics modeling for secondary settling tanks over the last three decades: A review

Secondary settling tanks (SSTs) are a crucial process that determines the performance of the activated sludge process. However, their performance is often far from satisfactory. In the last 30 years, computational fluid dynamics (CFD) has become a robust and cost-efficient tool for designing new SSTs, modifying the geometries of existing SSTs and improved control techniques in wastewater treatment plants. The first part of this review paper discusses the different approaches to model the motion of particles in SSTs. The applications of different multiphase approaches and the widely applied single-phase approach in different SST studies are reviewed. The second part reviews current CFD research and engineering practice, focusing on the formation and the effect of density currents, effects of different design variables, parameter uncertainties in modeling structures, and atmospheric conditions. Finally, challenges and future improvements of sub-models (sludge settling, rheology, turbulence, and flocculation) in the SST model framework are identified. PRACTITIONER POINTS: The first journal review for the CFD applications in SSTs over the last decade. The controversy over the relationship between SOR and SST performance can be largely explained by the prediction of the CFD model. Density decoupling in the turbulence model is possible for well-baffled SSTs. The relative importance of three modeling parameters is summarized. Recommendations for future data collection are provided.

Evaluation of three turbulence models in predicting the steady state hydrodynamics of a secondary sedimentation tank

The secondary sedimentation tank (SST) is a sensitive and complicated process in an activated sludge process. Due to the importance of its performance, computational fluid dynamics (CFD) methods have been employed to study the underflow hydrodynamics and solids distribution. Unlike most of the previous numerical studies, in the present investigation, the performance of three different types of turbulence models, standard k-ε, RNG k-ε and Realizable k-ε, are evaluated. Firstly, two-dimensional axisymmetric CFD models of two circular SSTs are validated with the field observations. Next, comprehensive comparisons are presented of the model predictions of the key physical quantities, such as the concentration of effluent suspended solids (ESS), and returned activated sludge (RAS), sludge blanket height (SBH), turbulent properties and flow and concentration patterns. A surprising result shows that the prediction of the ESS concentration is not sensitive to the change of turbulence models; while remarkable prediction difference can be observed in the inlet zone and near-field of sludge hopper and SBH. The results suggest that more observations inside the inlet zone are needed to achieve better model calibration and correct application of the turbulence model, which can be crucial to optimizing the geometry of inlet structure and sludge hopper as well as changing return solids concentration for the operation.