Rectangular Settling Research Articles

Investigation of Rectangular Settling Tanks Performance in Drinking Water Treatment Plants

Investigation of Rectangular Settling Tanks Performance in Drinking Water Treatment Plants

CFD simulations of single-phase flow in settling tanks: comparison of turbulence models

ABSTRACT In the present work, single-phase CFD simulations are performed to study flows in the rectangular and circular settling tank. The three-dimensional flow was studied for the predictions of the velocity profiles in the settling tank. The performance of seven different types of turbulence models, standard k-ε model, low Re k-ε models such as AKN model, CHC model and Abid Model, k-ω model, low Re k-ω model and RSM model has been evaluated for the prediction of flow fields. The CFD model is validated with previously published experimental data. The comparison of velocity profiles, turbulent kinetic energy, turbulent Reynolds number and vorticity are presented. The predictive ability of Abid model is better than other models for prediction of the flow field for both rectangular and circular settling tank. The absolute error for the velocity predictions in the case of the low Re k-ε model is around 30% less as compared to the other models. The better prediction using the low Re k-ε models is attributed to the damping functions used in the model.

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

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.

Introducing the theory of successful settling in order to evaluate and optimize the sedimentation tanks

A rectangular settling tank in full scale is investigated using the Fluent software to increase its efficiency. First, the pure water is simulated in the absence of particles. Then particles are injected into the flow field and tracked by means of the discrete phase model. Three methods are presented to optimize the settling tank: (1) adding a baffle which is mounted in the bottom and is extended up to the near of free surface, (2) adding a baffle which is mounted in the free surface and is extended up to the near of the tank bottom and (3) install a bi-directional baffle which is mounted in the free surface and is extended up to the near of the bottom. These three suggestions are checked using the short-circuiting phenomenon and the successful settling theory. The successful settling theory states that a particle can be accounted as a “trapped” particle if (1) settled particle stays static and stable in the tank bottom and (2) settled particle doesn’t return to purified sewage. Although the second method provided higher efficiency, the third method was selected as the most appropriate method in order to optimize the settling tank.

Measurement of flow velocity in different secondary settling tanks for analysis of the flow

Settling tanks are essential parts of wastewater treatment process and their correct design is therefore crucial for its operation. Modeling of flow in these tanks is difficult, therefore it is necessary to find right tool to perform as precise work as possible. In this article flow velocity measurement in two different secondary settling tanks in Slovakia is presented. Rectangular settling tank in Dolný Kubín and radial settling tank with horizontal flow in Nižná nad Oravou were chosen for this purpose. Data gained from these measurement events will be used for building a model and followed by simulation. Additional data from operation of wastewater treatment plants are also used during the work.

Effects of convective-diffusive vertical mixing on the conception of rectangular settling basins

Collection efficiency η of rectangular settling basins is studied in this note for two cases: (i) with no mixing and (ii) with perfect vertical mixing. At first, well known reference solutions corresponding to suspended solids with constant settling velocity are presented. The effects on deposition of suspended solids with varying settling velocities are then studied. Finally, original exact solutions for η are obtained for gamma distributed settling velocities. The reference solutions are perfectly compatible with these more general solutions.

Modeling the effect of wind in rectangular settling tanks for water supply

A 2-D CFD model is applied to investigate the effect of co-current and counter-current wind velocities of up to 7.5 m/s in the settling tanks of EYDAP Water Treatment Plant of Aharnes, in Greece, and the following conclusions are drawn: (1) without wind, the flow field is characterized by a large recirculation region with significant short-circuiting, while for windy conditions a two-layer flow is observed, in which the surface layer follows the wind direction. The suspended solids’ concentration fields strongly depend on the corresponding flow fields. (2) Calculated local removal efficiencies, without wind, near water surface show a satisfactory agreement with measurements. When the sludge removal mechanism is operating, the calculated efficiency without wind is equal to 83.1%; this value is lower than the experimental value (86.0 ± 1.0%) and the predicted value by a 3-D model (85.7%). When co-current wind velocity increases, short-circuiting increases and the efficiency decreases; however, not noticeably (<0.3%). When counter-current wind velocity increases, short-circuiting is reduced and the efficiency increases by up to approximately 1.0%. (3) When the sludge removal mechanism is out of operation, the calculated efficiency without wind decreases to 68.1%, a value that is again lower than the experimental value (70.8 ± 1.0%) and the predicted value by a 3-D model (70.9%); the wind effect is similar to the case of operating sludge removal mechanism, with a somehow larger difference of up to 1.3%.

Using CFD Techniques in Teaching Rectangular Settling Tank Hydrodynamics

Abstract Sedimentation is one of the most important phenomena applied in wastewater treatment. The paper presents the CFD techniques used in teaching students the influence of the inlet baffle on the primary settling tank hydrodynamics. Different sizes and dimensions of the baffle are studied in order to highlight the optimum configuration that assures uniform flow in the settling area and prevents dead zones. By this mean the students have the possibility to evaluate multiple configurations in short time intervals and with low costs, when compared to hands-on experiments.

Near field vorticity distributions from a sharp-edged rectangular jet

Experimental results on the near field development of a free rectangular jet with aspect ratio 10 are presented. The jet issues from a sharp-edged orifice attached to a rectangular settling chamber at Reh∼23,000, based on slot width, h. Measurements on cross plane grids were obtained with a two-component hot wire anemometry probe, which provided information on the three dimensional characteristics of the flow field. Two key features of this type of jet are mean axial velocity profiles presenting two off axis peaks, commonly mentioned as saddleback profiles, and a predominant dumbbell shape as described by, for example, a contour of the axial mean velocity. The saddleback shape is found to be significantly influenced by the vorticity distribution in the transverse plane of the jet, while the dumbbell is traced to two terms in the axial mean vorticity transport equation that diffuse fluid from the centre of the jet towards its periphery. At the farthest location where measurements were taken, 30 slot widths from the jet exit, the flow field resembles that of an axisymmetric jet.

Critical modeling parameters identified for 3D CFD modeling of rectangular final settling tanks for New York City wastewater treatment plants

New York City Environmental Protection is in the process of incorporating biological nitrogen removal (BNR) in its wastewater treatment plants (WWTPs) which entails operating the aeration tanks with higher levels of mixed liquor suspended solids (MLSS) than a conventional activated sludge process. The objective of this paper is to discuss two of the important parameters introduced in the 3D CFD model that has been developed by the City College of New York (CCNY) group: (a) the development of the 'discrete particle' measurement technique to carry out the fractionation of the solids in the final settling tank (FST) which has critical implications in the prediction of the effluent quality; and (b) the modification of the floc aggregation (K(A)) and floc break-up (K(B)) coefficients that are found in Parker's flocculation equation (Parker et al. 1970, 1971) used in the CFD model. The dependence of these parameters on the predictions of the CFD model will be illustrated with simulation results on one of the FSTs at the 26th Ward WWTP in Brooklyn, NY.

Development of a flocculation sub-model for a 3-D CFD model based on rectangular settling tanks

To assess performance and evaluate alternatives to improve the efficiency of rectangular Gould II type final settling tanks (FSTs), New York City Department of Environmental Protection and City College of NY developed a 3D computer model depicting the actual structural configuration of the tanks and the current and proposed hydraulic and solids loading rates. Fluent 6.3.26™ was the base platform for the computational fluid dynamics (CFD) model, for which sub-models of the SS settling characteristics, turbulence, flocculation and rheology were incorporated. This was supplemented by field and bench scale experiments to quantify the coefficients integral to the sub-models. The 3D model developed can be used to consider different baffle arrangements, sludge withdrawal mechanisms and loading alternatives to the FSTs. Flocculation in the front half of the rectangular tank especially in the region before and after the inlet baffle is one of the vital parameters that influences the capture efficiency of SS. Flocculation could be further improved by capturing medium and small size particles by creating an additional zone with an in-tank baffle. This was one of the methods that was adopted in optimizing the performance of the tank where the CCNY 3D CFD model was used to locate the in-tank baffle position. This paper describes the development of the flocculation sub-model and the relationship of the flocculation coefficients in the known Parker equation to the initial mixed liquor suspended solids (MLSS) concentration X0. A new modified equation is proposed removing the dependency of the breakup coefficient to the initial value of X0 based on preliminary data using normal and low concentration mixed liquor suspended solids values in flocculation experiments performed.

Performance Assessment of Secondary Settling Tanks Using CFD Modeling

Amongst the principal separation processes used to treat effluents in the water industry is the sedimentation of solid particulates from the carrier fluid known as mixed liquor by the force of gravity. New York City DEP is in the process of upgrading its Upper East River water pollution control plants (WPCP’s) to incorporate biological nitrogen removal (BNR). The rectangular final settling tanks (FSTs) are a central link in the treatment process and often times a limiting factor in terms of the solids handling capacity especially when high throughput requirements need to be met. The objective of this study was to develop a 3D Computational Fluid Dynamics (CFD) model based on the exact geometry of the existing Gould II type FSTs in Battery “E” at the Wards Island WPCP and calibrate and validate it with in-situ data collected at the site. The goal of this study is to illustrate how the different modeling approaches applied on the physical phenomena that take place in the tank affect the outcome of a CFD model and its predictions. It is common for internal baffles to be added at the inlet and within the tank to handle high flow requirements such as wet weather storms. This model has been used as a tool to assess the internal behavior of such baffles and assess the clarifier’s performance based on different inlet baffle configurations.

Two-Dimensional LDV Measurement, Modeling, and Optimal Design of Rectangular Primary Settling Tanks

To optimize the design parameters of rectangular primary settling tanks, we used two-dimensional laser Doppler velocimetry (2D LDV) to conduct flow field measurements in five cases and used a previous model to simulate the flow field. The relative baffle submergence height and the ratio of tank length to height were optimized in a low suspended solid (LSS) concentration ( LSS<150–200 mg/L ) . The experimental and simulation results show that a large recirculation zone exists behind the reaction baffle and the flow magnitude is small in the recirculation region; the length of recirculation increases with an increasing flow rate; the length of recirculation increases as the depth of the submerged reaction baffle increases; and the variation of the reaction baffle height can affect the flow field more significantly than does the variation of the flow rate. We also determined that to reach a higher removal rate and to optimize the area dimensions of a sedimentation tank, a length-to-height design ratio betwee...

Sludge Floc Behavior in an Elongated Rectangular Settling Tank

The different sludge floc distributions along the rectangular secondary settling tank were confirmed experimentally. Along the settling tank, three different regions can be formed: the fast settling zone near the inlet, the compaction zone in the middle, and the slowly settling zone near the outlet. Further investigation of morphological change of sludge flocs also showed corresponding floc characteristics: the bigger size of particles in the front, the relatively smaller particles with high density in the middle, and the small loose flocs in the rear. These were determined by the hydraulics and floc construction. The preceding results can be used as a guide to design systems to collect loose sludge solids.

Development and Validation of a 3-Dimensional Computational Fluid Dynamics (CFD) Model for Rectangular Settling Tanks in New York City Water Pollution Control Plants

New York City provides secondary treatment to approximately 78.6 m3/s among its 14 water pollution control plants (WPCPs). The process of choice has been step-feed activated sludge. Changes to the permit limits require nitrogen removal in WPCPs discharging into the Long Island Sound. The City has selected step feed biological nitrogen removal (BNR) process to upgrade the affected plants. Step feed BNR requires increasing the concentration of mixed liquors, (MLSS), which stresses the Gould II type rectangular final settling tanks (FSTs). To assess performance and evaluate alternatives to improve efficiency of the FSTs at the higher loads, New York City Department of Environmental Protection (NYCDEP) and City College of New York (CCNY) have developed a three-dimensional computer model depicting the actual structural configuration of the tanks and the current and proposed hydraulic and solids loading rates. Using Computational Fluid Dynamics (CFD) Model, Fluent 6.3.26TM as the base platform, sub-models of the SS settling characteristics as well as turbulence, flocculation, etc. were incorporated. This was supplemented by field and bench scale experiments to quantify the co-efficients integral to the sub-models. As a result, a three-dimensional model has been developed that is being used to consider different baffle arrangements, sludge withdrawal mechanisms and loading alternatives to the FSTs.

Development of a Flocculation Sub-Model for a 3-D CFD Model Based on Rectangular Settling Tanks

To assess performance and evaluate alternatives to improve the efficiency of rectangular Gould II type final settling tanks (FSTs), New York City Department of Environmental Protection and City College of NY developed a 3D computer model depicting the actual structural configuration of the tanks and the current and proposed hydraulic and solids loading rates. Fluent 6.3.26 ™ was the base platform for the computational fluid dynamics (CFD) model, for which sub-models of the SS settling characteristics, turbulence, flocculation and rheology were incorporated. This was supplemented by field and bench scale experiments to quantify the coefficients integral to the sub-models. The 3D model developed can be used to consider different baffle arrangements, sludge withdrawal mechanisms and loading alternatives to the FSTs. Flocculation in the front half of the rectangular tank especially in the region before and after the inlet baffle is one of the vital parameters that influences the capture efficiency of SS. Flocculation could be further improved by capturing medium and small size particles by creating an additional zone with an in-tank baffle. This was one of the methods that was adopted in optimizing the performance of the tank where the CCNY 3D CFD model was used to locate the in-tank baffle position. This paper describes the development of the flocculation sub-model and the relationship of the flocculation coefficients in the known Parker equation to the initial mixed liquor suspended solids (MLSS) concentration X 0 . A new modified equation is proposed removing the dependency of the breakup coefficient to the initial value of X 0 based on preliminary data using normal and low concentration mixed liquor suspended solids values in flocculation experiments performed.

Development and Validation of a 3-Dimensional Computational Fluid Dynamics(CFD) Model for Rectangular Settling Tanks in New York City Water Pollution Control Plants

New York City provides secondary treatment to approximately 78.6 m3/s among its 14 water pollution control plants (WPCPs). The process of choice has been step-feed activated sludge. Changes to the permit limits require nitrogen removal in WPCPs discharging into the Long Island Sound. The City has selected step feed biological nitrogen removal (BNR) process to upgrade the affected plants. Step feed BNR requires increasing the concentration of mixed liquors, (MLSS), which stresses the Gould II type rectangular final settling tanks (FSTs). To assess performance and evaluate alternatives to improve efficiency of the FSTs at the higher loads, New York City Department of Environmental Protection (NYCDEP) and City College of New York (CCNY) have developed a three-dimensional computer model depicting the actual structural configuration of the tanks and the current and proposed hydraulic and solids loading rates. Using Computational Fluid Dynamics (CFD) Model, Fluent 6.3.26TM as the base platform, sub-models of the SS settling characteristics as well as turbulence, flocculation, etc. were incorporated. This was supplemented by field and bench scale experiments to quantify the co-efficients integral to the sub-models. As a result, a three-dimensional model has been developed that is being used to consider different baffle arrangements, sludge withdrawal mechanisms and loading alternatives to the FSTs.

Modeling and Implementation of Flocculation Zones in Rectangular Final Settling Tanks at the Newtown Creek Water Pollution Control Plant

Modeling and Implementation of Flocculation Zones in Rectangular Final Settling Tanks at the Newtown Creek Water Pollution Control Plant

Effect of inlet structure on flow in rectangular settling tank

A laboratory experiment and numerical simulations were executed to analyze the flow field in a rectangular settling tank. Special attention is paid to the influence of the inlet structure on the performance of the tank. Two cases were investigated by setting the different aperture height of the inlet wall D; H/D=3 and H/D=2, where H is the depth of the tank. In the case of H/D=3, that is relatively lower feed with higher velocity, the flow pattern looks like a shallow settling tank with high inlet velocity, near the inlet zone. While in the case of bottom half feed H/D=2, the densityaffected acceleration becomes predominant when densimetric Froude number is small. This density current enhances the turbulent diffusivity in a tank. It is also shown that the increased turbulent diffusivity decreases the treatment efficiency of finer sediments.

Behavior of density current in rectangular settling tank

Density current in a rectangular settling tank is investigated mathematically. The basic equation consists of equation of motion for horizontal and vertical direction together with the mass conservation equation of both fluid and sediment. As a turbulence model, the standard k-e model is employed. Flow velocity profiles and sediment concentration distribution are illustrated. When the temperature of inlet suspension is warmer than that of the tank water, the suspension flows downward with the positive buoyancy. This density current increases the turbulence in its accelerating stage, which prevents the settling of sediments. On the other hand, in the case of high sediment concentration, the turbidity current accelerates rather low region in a tank, which may bring the resuspension of bottom sediment. It is also shown that the flow field is very complicated one, because the density current brings up under the developing stage of bottom boundary layer. If the sediment particle has higher fall velocity, they settle down within a short distance. In such case, the density current gives its effect on the sediment motion only in the developing stage and this effect leads to the decrease of removal ratio.