Turbulence and interphase mass diffusion assumptions on the performance of 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

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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“…Although CFD models have been applied to study SSTs in the last three decades, there are some uncertainties of model parameters and only three recent papers (Gao & Stenstrom, , , ) studied the uncertainty of these parameters on the performance of SSTs. These model parameters are (a) generation of turbulent kinetic energy (TKE) due to buoyancy ( G b ) in the TKE equation of the k‐ε family turbulence models; (b) buoyancy coefficient ( C 3 ) in the turbulent dissipation rate (TDR) equation of the k‐ε family turbulence models; (c) turbulent Schmidt number (

σ_{c}

) in the sludge transport equation; (d) the coefficient in the second exponential term of the Takács settling equation ( r p ); and (e) dry solids density (

ρ_{p}

).…”

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 99%

“…However, none has simulated the same cases with and without G b coupling. Therefore, Gao and Stenstrom () studied the need for coupling the G b by simulating same cases with and without coupling G b .…”

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 99%

“…Gao and Stenstrom () used a Fluent‐based 2D SST model to study the effects of G b at Darvill SST reported by McCorquodale et al (). The inlet baffling structure of the Darvill SST is very poor and has only a center well.…”

Section: Applications Of Cfd Modeling Research and Engineering Practicementioning

confidence: 99%

“…The application of C 3 in the TDR equation and

σ_{c}

in the sludge transport equation has varied widely among different studies. Gao and Stenstrom () showed that among the performance indicators (ESS, SBH, RAS concentration), none of them is sensitive to the change of C 3 and only SBH is sensitive to the variation of the

σ_{c}

. Therefore, C 3 can be removed from the TDR equation to improve the rate of convergence and reduce computing time and

σ_{c}

can be used to adjust SBH, when ESS and RAS are well calibrated to the field data.…”

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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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

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