Modeling of Class I Settling Tanks

Sedimentation is one of the earliest and most important unit operations in water and wastewater treatment. Conventional approaches for studying sedimentation of Class I settling tanks did not present enough information on suspended particle size distribution in the effluent. This information is very important for further treatment units such as filtration. In this research, a relatively simple and practical mathematical model is introduced to study sedimentation of non-uniform particle size in Class I settling tanks. The model is capable of providing such information as removal efficiency, size distributions in sludge and in effluent suspension, and thickness of bottom sludge. If desired removal efficiency is provided, the length of the tank can also be determined. Through numerical experiments, a sensitivity analysis was performed to examine the effects of tank dimensions, overflow rate, and detention time on the removal efficiency. Comparison with other models and a set of experimental data indicates a good agreement.     

Computational Fluid Dynamics Modeling for the Design of Large Primary Settling Tanks

Settling tanks are used to remove solids at wastewater treatment plants. Many numerical models have been proposed to simulate the settling process and to improve tank efficiency. In this research, a three-dimensional (3D) numerical model is developed to simulate large primary settling tanks. In the proposed model, the non-Newtonian properties of the sludge flow in the settling tank are described by a Bingham plastic rheological model. To eliminate the singularity inherited in the rheological model, a modified constitutive relation is used in both the yielded and unyielded regions. Hindered settling of particles in the settling tank is also modeled. Tracer study, where a massless scalar is injected and transported, is done to investigate the tank’s residence time. This numerical model is used to improve the design of the primary settling tanks, which will be built in Chicago. The Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) is in the process of building new preliminary treatment facilities at their Calumet Water Reclamation Plant (CWRP), including twelve 155-ft-diameter primary settling tanks (PSTs) designed to treat flows up to (480?million?gal./day (MGD). The computational fluid dynamics (CFD) model simulated solids removal efficiencies based on a particle size distribution similar to the one observed in the CWRP influent. The results were used to establish the design basis for tank side-water depth, inlet feedwell dimensions, etc., resulting in improved performance and substantial reduction in construction costs.     

Numerical Modeling of Type I Circular Sedimentation Tank

A numerical model for Type I circular sedimentation tanks (center feed) has been developed to provide useful information for circular tank operation. The unsteady flow process in circular sedimentation tanks is divided by numerous time intervals in which flow and sediment transport are considered to be steady. The other feature for this model is that particle size distribution of raw water is nonuniform. A numerical experiment based on the proposed model is carried out, and the results were compared with results from other models. The comparison shows that this model can provide more information, such as variations of water elevation, overall removal efficiency, bottom sludge thickness, and particle size distribution at outlet. In addition, this model had the advantage of optimizing the tank dimensions based on the sludge raking frequency and preferred removal efficiency. After simplification of the model, this model is practical in determining the overall removal efficiency and dimension of the tank based on different inflow situations for tank design.     

Design of Secondary Settling Tanks Using a CFD Model

A computational fluid dynamics (CFD) model is presented and applied in the design of the secondary settling tanks of Psyttalia Wastewater Treatment Plant in Athens, Europe’s largest sewage treatment facility. The tanks are of the Gould Type II consisting of the following regions: an inlet-flocculation chamber with an inlet baffle, two zones of settling separated by an intermediate baffle, an outlet region, and a sludge collection region. The number of tanks and their dimensions were determined with an empirical design procedure. Then, theoretical considerations, information from similar existing tanks, and preliminary CFD calculations were combined to determine the dimensions of the main regions and the positions of the baffles. Finally, detailed CFD calculations were performed to examine the performance of the tanks for various design conditions. Computations showed that the flow in the inlet-flocculation region was completely mixed; while in the settling regions a “three-layer” structure with relatively constant layer heights was observed. CFD results were processed to determine parameters of practical interest, including the heights of the sludge blankets and the effluent suspended solids concentrations; these parameters were correlated satisfactorily with the Hazen number, which is used as a scaling parameter in primary settling tanks.     

Modeling a Retention Treatment Basin for CSO

Combined sewer overflows (CSOs) result in hazardous and unsightly contamination of receiving waters, particularly swimming areas. The removal of suspended solids and associated biological oxygen demand (BOD) can accelerate the recovery following a CSO event. This paper presents a numerical model to simulate the solids removal efficiency of a retention treatment basin (RTB) that utilizes polymers to improve the flocculation and settling rates for the suspended solids. The model includes settleable, nonsettleable, and floatable solids. The sludge is treated as a non-Newtonian fluid. Discrete, zone, and compression settling/floatation regimes are included. In-tank flocculation and a storage zone for sludge flushing are also included in the model. The model was calibrated and validated with data from a RTB pilot plant, and was applied to evaluate preliminary designs for a prototype RTB for the City of Windsor. The calibrated model showed that the optimum location of the target baffle was approximately 30% of the distance to the scum baffle. For design flows of 20?m/h and run durations of up to 2?h, it was found that the removal was insensitive to slopes from ?1 to ?3% and depths greater than 2.5?m (L/H = 10). The simulations indicate that 70 to 78% of solids removal can be achieved at surface overflow rates up to 25?m/h.     

基于固液两相流模拟的选矿循环水深度澄清装置优化

部分选矿循环水中含一定量的高分散性悬浮颗粒,仅依靠简单浓缩沉降难以澄清,无法达到回用要求。针对这一难题,提出了一种选矿循环水固体悬浮物澄清装置。为优化装置的结构参数与运行参数,建立了选矿循环水深度澄清装置的二维物理模型,基于计算流体力学（CFD）的方法,选用Mixture和RNG k?ε 模型对装置主要的结构参数与运行参数展开了数值模拟研究。研究发现适当降低水力循环区喷嘴长度,增加喉管与喷嘴管径比、颗粒沉降区开口尺寸、装置直径等结构,能够降低颗粒沉降区平均湍动能,由于湍动能为单位质量流体由于紊流脉动所具有的动能,故降低了颗粒沉降区流场的紊流程度,增加了水流的稳定性,提高了装置对悬浮颗粒的去除效果;同时发现降低入口流速、增加悬浮颗粒粒径有助于提高悬浮物的去除率,当进水流速为0.1 m·s?1、经过混凝的悬浮颗粒形成粒径大于100 μm时,装置对选矿循环水中的悬浮颗粒去除效果显著。      

Two-Dimensional Numerical Simulation of Primary Settling Tanks by Hybrid Finite Analytic Method

In order to investigate the turbulent flow and mass transfer in primary settling tanks, numerical simulations are conducted by using a modified k?ε two-layer model based Boussinesq’s approximation to model the Reynolds stress in primary settling tanks, and solving the governing equations using a hybrid finite analytic method (HFAM). The simulation results obtained using the mathematical model are compared with the experimental data and simulation results available in the literature, and the results of comparison indicate that the profiles of the primary velocity field are in line with the experimental results and the flow-through curve obtained using the mathematical model are in good agreement with the curves based on experimental data. It is therefore concluded that the HFAM approach can be used to simulate the turbulent flow and mass transfer in a primary settling tank, and the modified k?ε two-layer model can be used to establish the velocity field distribution at the bottom of a primary settling tank.     

Using the Kinetics of Biological Flocculation and the Limiting Flux Theory for the Preliminary Design of Activated Sludge Systems. I: Model Development

Current activated sludge models consider that the removal of biodegradable organics by suspended growth includes rapid enmeshment of the organic particles in the microbial floc, hydrolysis of the complex organic molecules into readily biodegradable organic substances, and oxidation of dissolved organic substances. All of the models assume hydrolysis is the rate-limiting step, but none consider the role that the kinetics of biological flocculation and the sludge settling characteristics may play in defining the activated sludge operating parameters. Several researchers have studied the kinetic of biological flocculation, and have analyzed its role on the removal of particulate COD in suspended growth reactors. It has been demonstrated that a large proportion of the organic matter present in sewage can be removed by biological flocculation using short hydraulic retention times and subsequent settling. This paper demonstrates that the one-dimensional limiting flux theory may be useful for coupling the sludge settling properties with the aeration tank behavior, and is a reasonable first approximation that can be used for activated sludge system design and operation.     

Computational Study of Particle-Eddy Interaction in Sedimentation Tanks

Sedimentation tanks are used in the process industry to separate the solid particles from the slurry to get the clarified liquid. A detailed study of the hydrodynamics of sedimentation tanks is presented here using an Eulerian-Lagrangian approach to study the motion of solids in the tank. The model, in its present form, is applicable only to nonflocculent discrete (Type I) settling. It is shown that a typical particle-eddy interaction can be characterized by a lower cut-off size below which the particles would be entrained by the eddy; and an upper cut-off size above which the particle would continuously settle through the sedimentation tank in spite of the recirculation. The effect of inlet configuration on the flow field as well as on the settling characteristics has been investigated. The simulations show that both the upper and the lower cut-off sizes for a sedimentation tank are considerably reduced by providing a tulip type of inlet with a conical deflector as compared to a straight inlet.     

Computing Shear Flow and Sludge Blanket in Secondary Clarifiers

Recent developments in computing turbulent and buoyant flow in sedimentation tanks are introduced. The test case is a circular, center-feed secondary clarifier with inclined bottom and central sludge withdrawal. Axisymmetry is assumed, and the flow and settling processes are modeled in a radial section by using the k-ε turbulence model on a two-dimensional, nonorthogonal grid. The computation domain includes the sludge blanket where the viscosity is affected by the rheological behavior of the sludge. The aim of the present study is to evaluate the sensitivity of the flow and concentration fields to parameters that characterize (1) the rheological properties of highly concentrated regions; (2) the settling of sludge; and (3) the effect of stratification on the turbulent diffusion. The overall appearance of the fields proves to be similar, whereas the regions of high velocities and high gradients are strongly affected by using different parameters or approaches on rheology, settling, and diffusive transport, resulting in different sludge blanket heights.     

Using the Kinetics of Biological Flocculation and the Limiting Flux Theory for the Preliminary Design of Activated Sludge Systems. II: Experimental Verification

Current activated sludge models consider that the removal of biodegradable organics by suspended growth includes: rapid enmeshment of the organic particles in the microbial floc, hydrolysis of the complex organic molecules into readily biodegradable organic substances, and oxidation of dissolved organic substances. All of the models assume that hydrolysis is the rate-limiting step, but none considers the role that the kinetics of biological flocculation and the sludge-settling characteristics may play in defining the activated sludge operating parameters. Several researchers have studied the kinetics of biological flocculation, and have analyzed its role on the removal of particulate chemical oxygen demand in suspended growth reactors. It has been demonstrated that a large proportion of the organic matter present in sewage can be removed by biological flocculation using short hydraulic retention times and subsequent settling. The first paper demonstrates that the one-dimensional limiting flux theory may be useful for coupling the sludge-settling properties with the aeration tank behavior, and the second paper presents experimental evidence that the proposed model is a reasonable first approximation that can be used for activated sludge system design and operation.     

Vortex Plate for Enhancing Particle Settling

The feasibility of enhancing suspended solids settling by using the newly proposed vortex plates in clarifiers, instead of conventional smooth lamellae, was studied using computational fluid dynamics (CFD) modeling and laboratory experiments in which suspended particles were mimicked by crushed walnut shells and glass beads. The vortex plate was formed by attaching perpendicular ribs to the plate, forming slots of 25×25?mm (depth×width) and placing the plate parallel to the longitudinal clarifier axis at an angle of 60° from the horizontal. Rib walls were placed either in vertical planes, perpendicular to the clarifier longitudinal axis, or were slightly sloping in the main flow direction (20° about the vertical). Three hydraulic concepts were explored with respect to enhancing suspended particle settling: (1) the use of flow energy to generate steady vortices inside the slots and thereby entrain particles into the slots, where they would be sheltered from the fast horizontal flow and could settle without much hindrance; (2) enhancing the particle settling by increasing the contact surface area and thereby reducing the length of travel of settling particles; the same principle is used in conventional lamellar settlers but the surface area of a vortex plate is three times that of a smooth lamella; and (3) increasing the particle collision frequency within the swirling flow inside slots to prompt particle flocculation. The CFD modeling and experimental observations confirmed the formation of strong vortices in the parallel slots of the vortex plate. Such vortices entrained the passing by particles and retained some of them in slots, which provided a quiescent settling zone. Both the simulation and measured results indicated that the vortex plate contributed to a slightly improved removal of suspended particles. A CFD particle tracking model was applied to clarifiers with two vortex plates or two smooth plates and indicated that the vortex plate removed about 8% more particles than the smooth plate. In laboratory tests with plate arrays, the vortex plate array also contributed to better particle removals, especially for slower settling particles and larger inflow rates (by up to 26%).     

Hydraulic Model for Sedimentation in Storm-Water Detention Basins

Treatment of storm-water runoff may be necessary before discharge to surface waters. In urban areas, space constraints limit selection of conventional treatment systems, and alternative systems are needed. This research program involves design and laboratory testing of a small footprint nonproprietary detention basin which consists of pipes and box culvert sections with a specialized inlet and outlet system. This system can be placed below grade near the roadway section as part of the conventional drainage system and does not require additional right-of-way. A mathematical model, based entirely on hydraulic principles, is developed to estimate particle removal efficiency of the rectangular detention basin for the treatment of storm-water runoff by extending ideal horizontal tank theory under the condition in which water level is varied. A physical model was built in 1/5 scale to measure particle removal performance and validates the conceptual model. Experiments were performed for steady inflow conditions with different inflow rates, durations, and suspended sediment concentrations. Measured time series outflow suspended sediment concentrations and particle removal efficiency compare well with calculated results from the conceptual model. The outflow particle-size distribution can also be estimated using the conceptual model.     

下旋式重力旋流沉淀池的设计

在总结了多个旋流沉淀池的设计后,介绍了同心武和偏心武下旋武重力旋流沉淀池的设计和计算.下旋式重力旋流沉淀池在钢铁厂轧钢和连铸车闻的废水处理中被广泛应用.     

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 between 8 and 12 is optimal.     

Optimal Design of a Settling Basin for a Small-Scale Drainage Area

This paper develops a nonlinear programming model to optimally design a settling basin for a small-scale drainage area with a minimum total cost. It is assumed that the shape of the settling basin is rectangular parallelepiped, and it is connected to an open channel at both ends. Therefore, the decision variables include the scales of the settling basin (i.e., length, width, and height) and the scales of the channel (i.e., width and height). The design trap efficiency requirement, which must be greater than or equal to the required one of the considered watershed, makes up the main constraint. Other constraints consist of the upper and lower bounds of the decision variables, the equations for computing the trap efficiency, and the average flow velocity in the settling basin. The objective function is to minimize the total annual cost, which is the sum of the land, capital, and maintenance-operation cost. The developed model is solved by using a genetic algorithm. This model is applied to a subwatershed of the Wu-She Reservoir watershed in central Taiwan. The obtained results effectively demonstrate the applicability and practicability of the model.     

Three-Dimensional Simulation on the Water Flow Field and Suspended Solids Concentration in the Rectangular Sedimentation Tank

A 3D computational fluid dynamics model for describing the water flow and suspended solids (SS) concentration distribution in a rectangular sedimentation tank is presented. The interfacial momentum transfer, buoyant forces, and the effect of sediment-induced density currents are considered. A convection-diffusion equation, which is extended to incorporate the sedimentation of activated sludge in the field of gravity, governed the mass transfer in the clarifier. The double-exponential law is used to describe the dependence of the settling velocity on the concentration. The results show that during the dynamic settling process of the sludge, the mud surface rose slowly, and a period of time later, the mud surface kept stability and reached dynamic equilibrium in the tank. The distribution of velocity along the z axis in the rectangular tank is not uniform, and the surface return flow is found. The turbulent kinetic energy is larger and dropped drastically in the inlet zone, while in the settling zone the turbulent kinetic energy is relatively small. Density current is formed, and the clear water zone, flocculation zone, lamella zone, and compression zone are found. Furthermore, under certain operational conditions, the influence of inlet baffle length on SS settling in the rectangular sedimentation tank is discussed. The prediction by the present model for liquid flow and SS concentration is confirmed by the experimental measurement in a rectangular sedimentation tank in Sweden reported by Larsen in 1977.     

Tornado Vortices in Settling Tanks

Settling tanks are hydraulically designed on a settling velocity. Observations have indicated that out-of-the-basin sediment transport may be initiated by corner vortices, which are able to scour a sediment layer and significantly reduce the efficiency of sediment deposition. Using four different materials—with densities from close to water up to sand, and diameters from 0.55 to 3.0 mm—the various forms of vortex scour were experimentally determined. The densimetric particle Froude number could be identified as the scaling quantity that governs these processes. The scour process can be reduced by inserting so-called antivortex elements, by which the 3D corner flow characteristics are influenced to result in a more 2D flow over a wall. The location and length of these elements were found to be similar to elements recently recommended for standard gates.     

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.     

Hydrodynamics of Secondary Settling Tanks and Increasing Their Performance Using Baffles

Generally, the flow in settling tanks is stratified, but the effect of buoyancy force on the flow field depends on the inlet concentration of particles and flow bulk velocity. A common approach for increasing settling tanks performance is to use baffles which can reduce effects of the unfavorable phenomena such as short circuiting between inlet and outlet and density currents in primary and secondary settling tanks, respectively. The suitable position of the baffles is related to the importance of buoyancy force. As a result, effects of inlet Reynolds and Froude numbers on the strength of buoyancy force are studied for a secondary settling tank and the results show that neither Reynolds nor Froude numbers are sufficient to be considered alone. Effect of buoyancy force on the suitable baffle position is also investigated. Results show that in high Reynolds numbers, the flow field and baffle position are not affected by the inlet Froude number.