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

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

Section: Methodsmentioning

confidence: 89%

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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“…As the current research focuses on the parameters in the turbulence model and the sludge transport model, only these two models are shown below. A more detailed description of CFD model non‐Newtonian flow behavior of the activated sludge and buoyancy effects is shown in Gao and Stenstrom ().…”

Section: Methodsmentioning

confidence: 99%

“…The simulation results are compared with the field data from Griborio () and shown in another paper (Gao & Stenstrom, ). ESS and RAS concentration are fitting parameters.…”

Section: Methodsmentioning

confidence: 99%

Turbulence and interphase mass diffusion assumptions on the performance of secondary settling tanks

2019

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Secondary settling tanks (SSTs), also known as secondary sedimentation tanks or secondary clarifiers, are a basic yet complicated process in a biological water resource recovery facility. In order to understand and improve SST performance, computational fluid dynamics methods have been employed over the last 30 years. In the present investigation, a Fluent-based two-dimensional axisymmetric numerical model is applied to understand the effects of the buoyancy term (G b ) in the turbulent kinetic energy (TKE) equation and two model parameters (the coefficient of buoyancy term (C 3 ) in the turbulent dissipation rate equation and the turbulent Schmidt number (σ c ) in the sludge transport equation) on the performance of an SST. The results show that the hydrodynamics can only be correctly predicted by buoyancy-coupled TKE equation, unless the mixed liquor suspended solids is low and sludge settling velocity is extremely high. When the field observations show the SST is operating well, the buoyancy-decoupled TKE equation predicts the correct result, but the buoyancy-decoupled TKE equation may predict failure. Care is required in selecting the correct modeling technique for various conditions. This study provides guidance on how to avoid modeling problems and increase rates of convergence. © 2018 Water Environment Federation • Practitioner points• C3 can be set to zero to improve rate of convergence and reduce computing time.• σc can be used to adjust SBH, when ESS and RAS concentrations are well calibrated to the field data, but the SBH does not fit field observation.• Key words buoyancy effects; computational fluid dynamics modeling; Realizable k-ε model; secondary settling tank; turbulent Schmidt number *WEF Member

“…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