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.     

Simplified Model for the Class-I Settling Tanks Design

The purpose of this paper is to simplify the original one-dimensional mathematical model that is used to simulate the performance of nonuniform particle sizes in Class-I settling tanks. This paper shows that the modified model is both efficient and economical. The model is capable of providing such information as removal efficiency, particle size distributions in sludge and effluent suspension, and the thickness of bottom sludge. If the desired removal efficiency is provided, a simple equation from the simplified model can be used to calculate the length of the tank. Moreover, the model is quite straightforward and does not require a computer program to obtain results. It deals with only one parameter, the sediment coefficient α, which shows great advantages. Here α is taken as 1.2 for the Class-I settling tanks, however calibration is still recommended. Effluent information, which is important for further treatment units, can be calculated directly from the simplified model. The simplified model is qualitatively reasonable in comparison with other models.     

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.     

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.     

热轧浊环系统高速过滤器反洗水处理工艺优化

阐述了热轧浊环水处理系统高速过滤器反洗水处理工艺,通过典型案例的分析发现热轧浊环水处理系统高速过滤器反洗水处理工艺运行过程中存在平流沉淀池、反洗水调节池和污泥浓缩池积泥,热轧浊环水水质超标和板框压滤机出泥率低等缺陷。通过采取工艺优化措施,热轧浊环水处理系统高速过滤器处理后的热轧浊环水水质明显提高,出泥量增加,平流沉淀池、反洗水调节池、污泥浓缩池积泥减少,污泥处理设施故障率低,从而降低了设备维护和备件费用,实现了设备自动化运行,系统运行效果良好。     

Effects of Bubble Size and Diffusing Area on Destratification Efficiency in Bubble Plumes of Two-Layer Stratification

Effects of bubble size, bubble diffusing area, and other parameters in air diffusers on destratification are studied using laboratory- and pilot-scale tanks with two layers. Dimensionless group involving such variable as bubble size, bubble diffusing area, and tank area are used to quantify these effects. Based on the results of experiments, a model is developed to predict destratification efficiency. Bubble diameter and overall tank area are found inversely related to destratification efficiency while bubble diffusing area is directly related to destratification efficiency.     

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。      

Bed Shear Stress Boundary Condition for Storage Tank Sedimentation

Computational fluid dynamics-based (CFD) software tools enable engineers to simulate flow patterns and sediment transport in ancillary structures of sewer systems. Lagrangian particle tracking represents a computationally efficient technique for modeling sediment transport. In order to represent the process of sedimentation in storage tanks, careful consideration must be given to the boundary condition at the bottom of the tanks. None of the boundary conditions currently available in the FLUENT CFD software appears to represent the observed behavior of sediment particles, which may become resuspended after first contact with the bed if the local flow velocity is sufficiently high. In this study, a boundary condition based on bed shear stress has been implemented in FLUENT and evaluated against laboratory data. A particle is trapped if the local bed shear stress is below the critical bed shear stress; otherwise, the particle is resuspended. The approach gives satisfactory agreement with measured sedimentation efficiency data, and the simulated spatial distribution is very similar to the sediment distribution observed in a laboratory tank.     

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.     

2,4,6 Tri-Chlorophenol Containing Wastewater Treatment Using a Hybrid-Loop Bioreactor System

A hybrid-loop bioreactor system consisting of a packed column biofilm and an aerated tank bioreactor with an effluent recycle was used for biological treatment of 2,4,6 tri-chlorophenol (TCP) containing synthetic wastewater. The effects of sludge age (solids retention time) on chemical oxygen demand (COD), TCP, and toxicity removal performance of the system were investigated for sludge ages between 5 and 30?days, while the feed COD (2600±100?mg?L?1), TCP (370±10?mg?L?1), and the hydraulic residence time (25?h) were constant. Percent TCP, COD, and toxicity removals increased with increasing sludge age resulting in nearly complete COD, TCP, and toxicity removal at sludge ages above 20?days. Biomass concentrations in the packed column and in the aeration tank increased with increasing sludge age resulting in low reactor TCP concentrations, and therefore, high TCP, COD, and toxicity removals. More than 95% of COD, TCP, and toxicity removal took place in the packed column reactor. Volumetric rates of TCP and COD removal increased due to increasing biomass and decreasing effluent TCP and COD concentrations with increasing sludge age. The specific rate of TCP removal was maximum (120?mg?TCP?gX?1?day?1) at a sludge age of 20?days. TCP inhibition was eliminated by operation of the system at sludge age above 20?days to obtain nearly complete COD, TCP, and toxicity removal.     

Measurements of drop size in liquid/liquid systems by sedimentation

Sedimentation velocities and drop sizes have been observed in batch dispersions of two immiscible liquids during collapse of the dispersion created by agitation in a tank. It is shown that existing relationships between particle size, sedimentation velocity and fractional hold-up of dispersed phase can be used to obtain characteristic drop sizes from data on sedimentation velocity and hold-up in batch tests. This sedimentation technique is simple and rapid but approximate. It may be used to estimate drop sizes in stirred tanks when other methods usually fail, for example when the liquids are strongly coloured or hold-up of the dispersed phase is near 50%. In industrial laboratories interpretation of the characteristics of a particular system, particularly sensitivity of drop size to stirring conditions, will improve mixer-settler design procedures for example. It is unlikely that sophisticated photographic techniques will be widely used in this role.     

Optimization of Internal Bypass Ratio for Complete Ammonium and Phosphate Removal in a Dephanox-Type Two-Sludge Denitrification System

The capacity of complete simultaneous ammonium and phosphate removal was studied in a laboratory scale Dephanox system in relation to its internal bypass ratio (BPR). In this configuration, most of the ammonium detected in the effluent is ammonium bypassed by the system’s internal settler. Therefore, this research studies the possibility of complete simultaneous ammonium and phosphate removal by means of the balance of bypassed ammonium with ammonium requirement for growth of denitrifying phosphorus accumulating organisms in the anoxic tank. During these experiments, ammonium removal was governed by internal BPR and limited by sludge settleability. The predominant anaerobic-anoxic sludge developed a high settleability, allowing the application of drastic low BPRs. The system studied under many BPRs proved to achieve almost complete simultaneous ammonium and phosphate removal for BPRs ranging from 0.08 to 0.13 of the influent. A BPR lower than the inferior limit produced extreme accumulation of sludge into the internal settler, interfering in the distribution of sludge and consequently in removal efficiency. A positive effect of the internal settler was the extension of anaerobic contact time and anaerobic solids retention time. The increased phosphorus release suggests that a higher volatile fatty acids production might have occurred when raw wastewater was used as influent.     

Particle Destabilization in Highway Runoff to Optimize Pollutant Removal

Sedimentation column studies and simulations using particle size distribution suggest that low removal efficiencies of smaller particles in highway runoff would be obtained using sedimentation if coagulation-flocculation is not performed. Coagulation-flocculation studies, using metal salts (alum and ferric chloride) and one organic polymer in three molecular weights, were evaluated over the 2004–2005 storm seasons. Only the first flush or approximately the first hour of runoff was coagulated. Efficiencies were quantified with particle size distribution measurements and turbidity. Results with low dosages of metal salts were ineffective and did not improve water quality. High dosages of metal salts using a sweep floc mechanism were effective in dramatically lowering runoff turbidity, but resulted in large quantities of sludge production and required pH control. A cationic organic polymer at low dosages (<10?mg/L) was effective in coagulating highway runoff and reducing particle charge. Extended mixing time was required to achieve low turbidities ( ～ 5 NTU). A combination of organic polymer, followed by small doses of alum (<10?mg/L), reduced mixing time and produced high quality effluent.     

Particle suspension in (air-agitated) pachuca tanks

Particle suspension is an important parameter in the design of an energy-efficient Pachuca tank. Unfortunately, very little attention has been focused on the suspension behavior of air-agitated Pachucas. In the present investigation, therefore, extensive experiments have been carried out in three laboratory-scale Pachuca tanks to examine the effect of design and operating parameters, as well as scale-up, on particle suspension. A mathematical model that combines the Bernoulli’s equation and the theory of transport of particles in the horizontal flow of a liquid has been developed to predict the critical gas velocity for particle suspension in Pachuca tanks. Some important results, crucial to the design and scale-up of Pachuca tanks, have emerged. Full-center-column (FCC) Pachuca tanks with a draft tube-to-tank diameter ratio (D _d/D_t) on the order of 0.1 are found to be energetically more efficient in suspending particles than free-air-lift (FAL) and stub-column (SC) Pachuca tanks. It is also observed that taller tanks require lower air flow rates for particle suspension than shallower tanks. Finally, it is explained why industrial Pachuca tanks operate at lower air velocities than laboratory-scale tanks.     

福建省某工业园区污水处理厂的工程设计

结合福建省某工业园区的工程概况和入驻企业的特点,园区污水处理厂工程采用预处理+水解酸化+多级AO+高效沉淀池+臭氧BAF处理工艺,并详细介绍了设计进出水水质、工艺流程、工程设计等。工程投运后,出水水质满足环评要求,社会效益和环境效益显著。     

Model to Quantify Removal and Inactivation of Microorganisms Occluded in Effluent Wastewater Particles Using Filtration and Disinfection Systems

A model is presented to quantify microbial occlusion in effluent particles and improve understanding of the removal and inactivation of occluded microorganisms in filtration and disinfection systems. Microbial occlusion in particles is described in the model as a function of the particle size distribution, the microbial density (N1), which is the average quantity of target microorganisms within the subset of particles that contain microorganisms, and the frequency of association (N2), which is the ratio of particles that contain at least one target microorganism. To demonstrate the model, undisinfected secondary effluent samples were collected from an extended aeration treatment facility and analyzed to determine values for the model variables, N1 and N2 for heterotrophic bacteria and aerobic endospores and N2 for total coliform bacteria. Heterotrophic bacteria were present in most effluent particles (73–100%) at high densities (7–93 per particle), whereas aerobic endospores and total coliform bacteria were only present in a small percentage of effluent particles (0.1–6%) and the density of aerobic endospores in effluent particles was not appreciably higher than one per particle.     

Postdigestion Struvite Precipitation Using a Fluidized Bed Reactor

Controlled precipitation of struvite from postdigestion, sludge lagoon supernatant was studied using a pilot scale fluidized bed reactor (FBR). The process effectively and rapidly removed struvite constituents from the supernatant and produced effluent significantly undersaturated for struvite. Recently published information on struvite thermodynamic and kinetic properties was incorporated into the FBR system design and selection of components. Struvite crystals were selected as seed media, which enabled precipitation of nearly pure crystals and allowed nucleation, with its associated lag time, to be bypassed. Mixing energy in the FBR was demonstrated to be sufficient to overcome transport limitations to struvite growth, allowing growth kinetics to be governed by the surface integration rate of constituent ions into the growing crystal lattice structure. Struvite removal was optimized by incorporating pH elevation into the FBR system. Process fluid was circulated in the pH adjustment tank∕FBR system, which was configured as a continuous feed stirred-tank reactor. Struvite removal efficiency exceeded 80% in continuous feed experiments where the hydraulic detention time exceeded 1 h.     

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.     

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.     

Fluid flow in pachuca (Air-Agitated) tanks: Part II. Mathematical modeling of flow in pachuca tanks

A generalized mathematical model that combines Bernoulli’s equation and thek-ε model of turbulence, using only the gas flow rate and tank geometry as inputs, has been formulated to predict the fluid flow pattern in industrial-scale full-center-column Pachuca tanks. Predictions from this model reproduce reasonably well the trends that were observed in experiments when design and operating parameters, such as superficial air velocity and tank height/tank diameter ratio, were varied. Results from these calculations indicate that design parameters, such as tank height/tank diameter and draft tube/tank diameter ratios, have a significant effect on the flow pattern in full-center-column Pachuca tanks at large tank diameters or small tank height/tank diameter ratios.