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

Abstract

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. • Key wordscomputational fluid dynamics; density-driven flow; secondary settling tank; wastewater treatment

Editorial

“…To select a single article from this informative collection was a particularly difficult challenge—kudos to all authors of these excellent contributions to Water Environment Research . However, I eventually elected to highlight Gao and Stenstrom's (2020) article Development and applications in computational fluid dynamics modeling for secondary settling tanks over the last three decades: A review as this issue's Editor's Choice article (as such, it will be freely available for 45 days). This review article stood out as it systematically merged “old” secondary settling tank design/performance and “new” advances in computational fluid dynamic (CFD) modeling.…”