Evaluating the Effects of Inlet Geometry on the Limiting Flux of Secondary Settling Tanks with CFD Model and 1D Flux Theory Model

2019

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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

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 93%

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 99%

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 99%

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 99%

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

2019

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“…Boundary conditions are shown in Table 2. Compared with our previous studies (Gao & Stenstrom, 2018a, 2018b, 2019a, 2019b, the only different boundary condition needed to simulate the windy conditions is the surface boundary condition, which is a constant horizontal velocity as the free surface along the wind direction (Gkesouli et al, 2016;Stamou & Gkesouli, 2015).…”

Section: Cfd Boundary Conditionsmentioning

confidence: 99%

The influence of wind in secondary settling tanks for wastewater treatment – A computational fluid dynamics study. Part II: Rectangular secondary settling tanks

2019

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Computational fluid dynamics (CFD) model is used to study the effect of wind on the performance of a rectangular secondary sedimentation tank (SST) in a wastewater treatment plant (WWTP). Unlike most of the previous CFD modeling studies which evaluated the wind effect on the sedimentation tank in only water treatment plants, this study evaluates a rectangular SST in a WWTP at different wind speeds and directions, and under different inflow loading conditions. The wind is qualitatively and quantitatively analyzed for a range of wind speeds and directions as well as loading rates. The net effect is to change the three‐dimensional hydrodynamics profiles, effluent suspended solids, and sludge blanket height. The simulation results show that the wind deteriorates overall clarification performance of the rectangular SST and has little effect on the sludge thickening under mild wind conditions until the speed increases an extreme windy condition. These CFD simulation results suggest that in strong windy climates, covering SSTs or protecting them from strong winds may be justified. Practitioner points This is the first comparison of wind effects on a rectangular secondary sedimentation tank The effects of varied flow conditions, wind directions, and velocities are compared Wind‐induced currents in the settling tank negatively affect removal efficiency Winds have strong negative impact on the performance of sedimentation tanks

The influence of wind in secondary settling tanks for wastewater treatment—A computational fluid dynamics study. Part I: Circular secondary settling tanks

2019

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Computational fluid dynamics model is used to understand the impact of wind on the performance of a secondary settling tank (SST) in a wastewater treatment plant (WWTP). Unlike most of the previous modeling studies which evaluated the wind effect on the settling tank in a water treatment plant, this study evaluates a circular SST in a WWTP at different current velocities and flow conditions. Performance indicators, such as effluent suspended solids and sludge blanket height, and three‐dimensional hydrodynamics profiles are compared among different windy conditions and the calm condition and under different wind directions and flow conditions. The simulation results show that the existence of wind has strong negative impacts on the overall performance of the circular SST. The prediction of ESS is doubled in the circular SST under the mild wind condition. Moreover, the circular SST is more sensitive to the wind along the inlet port direction. Practitioner points This is the first comparison of wind effects on a circular secondary settling tank Detailed computational fluid dynamics solution procedures to simulate a secondary settling tank Wind effects are investigated under multiple flow conditions, current velocities, and wind directions The performance of a circular secondary settling tank is very sensitive to the wind Wind along the inlet port direction has stronger negative impacts than it along 45° to the inlet direction