Pollutant Transport and Mixing Zone Simulation of Sediment Density Currents

Prediction of water column concentrations of suspended sediment is often necessary for environmental impact assessment of point source industrial discharges. For example, in “flow lane” or “open water” disposal, suction dredges discharge large volumes of suspended sediment into shallow water disposal locations. A sediment density current mixing model is presented here as part of the D-CORMIX expert system for hydrodynamic simulation of mixing zone behavior. This density current model extends the CORMIX decision support system to simulate continuous negatively buoyant discharges with or without suspended sediment loads on a sloping bottom with loss of suspended particles by sedimentation. Sedimentation is modeled using Stokes settling for five particle size classes. Density current width and depth, trajectory, total solids, tracer concentration, dilution, and particle size concentration are predicted. In addition, location and widths of sediment deposits, accretion rates, including particle size fractions within the spoils deposit, are predicted. The model results are in good overall agreement with available field and laboratory data.     

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.     

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。      

On-Line Control of Polymer Addition to Prevent Massive Sludge Washout

An experimental method that quantifies the effect of polymer dosing on sludge settling characteristics is proposed. This method consists of recording batch settling curves at a grid of sludge and polymer concentrations. The effect of the polymer was found to depend on the mixing time between the dosing of the polymer and the start of the batch sedimentation. The recorded effects could be successfully implemented in a 1D dynamic settler model. From the literature it was concluded that keeping the sludge blanket below a certain critical height is an effective way of controlling the effluent suspended solids. From a model-based analysis this strategy appeared to be sound. Different control strategies using, respectively, the sludge blanket height [feedback (FB)], the hydraulic loading [feedforward (FF)], the solids loading (FF), or the sludge volume loading (FF) were tested for their ability to keep the sludge blanket below the critical height. The control strategy based on the hydraulic loading was the least efficient with respect to minimizing polymer dosage. The others appeared equally effective provided that they were properly tuned. Using the excess of the critical sludge blanket height as a measure of effectiveness, strategies based on more than one measured variable appeared to be less sensitive to suboptimal tuning.     

全尾砂无耙深锥稳态浓密性能分析

结合沉降和压滤实验, 对脱水性能数据进行曲线拟合获得连续网状结构形成浓度、压缩屈服应力和干涉沉降系数, 引入Usher提出的稳态浓密性能预测算法, 建立了无耙深锥浓密模型, 分析了絮凝剂单耗、底流中固相的体积分数、泥层高度等对固体通量和固体处理能力的影响规律.研究结果表明: 絮凝剂添加量对沉降区域影响大于压密区域, 20 g·t-1时浓密性能较好, 底流中固相的体积分数越大固体通量越小; 在沉降区域, 固体通量仅与浓度有关, 不受泥层高度影响; 在压密区域, 固体通量为浓度与泥层高度的方程; 模型参数范围内, 当泥层高度 < 3.5 m时, 固体处理能力为浓度与泥层高度的方程, 当泥层高度>3.5 m时, 固体处理能力与固体通量随底流中固相的体积分数变化规律一致.      

赣南某钨矿山废水悬浮物处理试验研究

采用混凝法处理钨矿山废水中的悬浮物,研究不同混凝剂、混凝剂用量、搅拌强度、pH值、沉降时间等对废水处理效果的影响,得出优化操作条件。试验表明:采用石灰乳+FeCl3工艺处理该尾矿库外排水效果好,最佳工艺条件为:废水pH值调节至10左右,FeCl3的投加量为15mg/L左右,搅拌强度为250r/min,快速搅拌0.5min,然后50r/min慢速搅拌5min,沉降时间为60min。在此工艺条件下,悬浮物去除率可达到98.4%,上清液悬浮物含量可降至5.4mg/L。     

Integrated Methodology of Design for Construction Site Sedimentation Basins

The performance of four sedimentation basins (SBs) at a Pennsylvania Department of Transportation I-99 highway construction site was evaluated based on their particle removal ability. Suspended solids data from the basins indicated poor particle removal, peaks in suspended solid concentration that correlated with rainfall peaks and possibility of sediment resuspension. The current Pennsylvania Department of Environmental Protection basis for basin design is the allocation of 28?m3 (1,000?ft3) of sediment storage zone and 140?m3 (5,000?ft3) of drainage zone per disturbed acre of the drainage basin. Overflow rate, which is a scientific basis for particle removal, is currently not considered in the design of basins. This paper presents a methodology for developing an integrated design for SBs, applying rainfall probability plots to determine an appropriate basin settling volume. The revised universal soil loss equation is used to identify sediment zone volume, and an overflow rate is determined to design basin surface area. The method presented is a comprehensive procedure for designing SBs with flexibility to choose the extent of particle removal and runoff capture, and to vary construction costs.     

Uncertainty Analysis of Conventional Water Treatment Plant Design for Suspended Solids Removal

An approach that links a Monte-Carlo simulation based reliability program with the water treatment process behavior and performance model is presented for uncertainty analysis of the conventional water treatment plant (WTP) design. The expected mass concentration of suspended solids (SS) in the effluent water is employed as a measurement of system reliability. The approach for uncertainty analysis of a WTP is illustrated with a hypothetical case study. The parameters contributing significantly to the variability of SS concentration in the effluent water are identified. From an operational viewpoint, the variability in effluent water quality resulting from uncertainty in the system parameters is investigated. Also, improvement in the reliability of WTP using modified design parameters is investigated along with its cost implications. From the results, a method to calculate a set of safety factors corresponding to various performance reliability levels is proposed.     

SSPRSD Using a Filamentous Fungal Strain Penicillium expansum BS30 Isolated from Wastewater Sludge

A filamentous fungal strain (Penicillium expansum BS30) isolated from a municipal wastewater treatment plant was used in this study to simultaneously reduce sludge solids, pathogens, and improve the sludge settling and dewaterability [simultaneous solids and pathogens reduction, settling and dewatering (SSPRSD)] in shake flask and 10-L bioreactor experiments. The fungal strain role in the SSPRSD process was evaluated at different temperatures and inoculum (spores) concentrations. The best performance of the process was achieved at incubation temperature of 25°C and inoculum concentration of 106?spores/mL. At these optimal conditions, suspended solids (SS) and volatile SS were degraded >50 and >53%, respectively. The capillary suction time value recorded (<13?s) was lower than that required for sludge dewaterability (<20?s). The populations of total coliforms and Salmonella (pathogen indicators) were reduced by two and four log cycles, respectively. A study on molecular screening of penicillin biosynthesis gene cluster and toxic organic compounds degrading machinery of the fungal strain was also conducted. It was found that the fungal strain possessed the penicillin-producing gene and toxic organic compounds degrading genes, and therefore may be helpful in degrading these compounds.     

Dissolved Oxygen-Stat Titration Respirometry: Principle of Operation and Validation

A method is presented to assess oxygen uptake rates which operates at constant dissolved oxygen concentration (DO). This method involves DO-stat titrations by controlled addition of a diluted H2O2 solution to a batch volume of activated sludge. The titrant flow rate is proportional to the biomass respiration rate, while the mass of H2O2 added is proportional to the oxidized organics. Classical batch respirometries (with and without gas flow, i.e., “open” and “closed”) were compared to the DO-stat technique using first NaSO2 as reducing chemical and then respiring aerobic biomass. An excellent correlation was found among estimates of oxygen consumption rate and short term yield coefficient by the three techniques. Inhibition by H2O2 was found to be negligible for biomass concentration as volatile suspended solids of ≥ 1.5–2 g?/L.     

Energy Audit, Solids Reduction, and Pathogen Inactivation in Secondary Sludges during Batch Thermophilic Aerobic Digestion Process

A typical secondary wastewater treatment sludge was digested aerobically in a batch digester in the temperature range of 10–80°C. Reaction rate constants were determined by measuring the amounts of volatile suspended solids removed at different time intervals during the process. The maximum value of the reaction rate constant (0.45?day?1) occurred in the temperature range of 55–60°C. The specific heat of biological oxidation was determined by energy balance calculations in the thermophilic range. Removal of indicator organisms in the sludge during the batch digestion was also studied. Sludge digestion at 60°C resulted in an appreciable reduction of indicator organisms as compared to digestion carried out at 55°C. Sludge digestion at 60°C resulted in more economic energy consumption, better volatile solids reduction, better sludge dewaterability, and more effective pathogen inactivation as compared to digestion of the sludge at 40°C.     

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.     

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.     

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%).     

One-Dimensional Modeling of Dam-Break Flow over Movable Beds

A one-dimensional model has been established to simulate the fluvial processes under dam-break flow over movable beds. The hydrodynamic model adopts the generalized shallow water equations, which consider the effects of sediment transport and bed change on the flow. The sediment model computes the nonequilibrium transport of bed load and suspended load. The effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity. In particular, a correction factor is proposed to modify the Van Rijn formulas of equilibrium bed-load transport rate and near-bed suspended-load concentration for the simulation of sediment transport under high-shear flow conditions. The governing equations are solved by an explicit finite-volume method with the first-order upwind scheme for intercell fluxes. The model has been tested in two experimental cases, with fairly good agreement between simulations and measurements. The sensitivities of the model results to parameters such as the sediment nonequilibrium adaptation length, Manning’s roughness coefficient and the proposed correction factor have been verified. The proposed model has also been compared to an existing model and the results indicate the new model is more reliable.     

Custom 450钢异形方管热挤压成形的数值模拟及试验研究

摘要：为了确定Custom 450钢最佳的热变形区间以指导实际热挤压过程的工艺参数设计,采用Gleeble 3800热模拟试验机在真应变为0.2~1.0,应变速率为0.01～10s-1,变形温度为900~1200℃的条件下开展了热压缩试验,确定了该钢发生完全动态再结晶的工艺参数区间;基于热压缩试验结果,采用热力耦合有限元方法对Custom 450钢异形方管在不同挤压工艺参数下的成形过程进行了数值模拟,并依据模拟结果进行实际挤压试验验证,确定了最佳的热挤压工艺。研究结果表明,Custom 450钢异形方管的最佳挤压速度为180mm/s,最佳坯料预热温度为1200℃,依据模拟结果制备的异形方管形状完整,平直度好,具有优良的显微组织与力学性能均匀性。     

High-Rate Biodegradation of Phenol by Aerobically Grown Microbial Granules

This study demonstrates that aerobic granules can be developed to achieve high phenol loading rates in a sequencing batch reactor. The reactor was started at a loading rate of 1.5 kg?phenol?m?3?d?1 with phenol-enriched activated sludge as inoculum. Granules first appeared on Day 9 after startup and quickly grew to become the dominant biomass in the reactor. The phenol loading was then adjusted stepwise to a final value of 2.5 kg?phenol?m?3?d?1. At this high loading, phenol was completely degraded and high biomass concentration was maintained in the reactor. The biomass continued to possess a good settling ability, with a sludge volume index of 60.5 mL?g?SS?1 (SS stands for suspended solids). Granules remained stable, without significant deterioration in granule structure and physiology, even at the maximum phenol loading rate tested. The applied selection pressure enabled the micro-organisms to aggregate into granules, and the compact structure of the aerobic granules served both to retain biomass and protect the microbial cells against the phenol toxicity. High specific phenol degradation rates exceeding 1 g?phenol?g?VSS?1?d?1 (VSS stands for volatile suspended solids) were sustained up to phenol concentrations of 500 mg?l?1, and significant rates continued to be achieved up to a phenol concentration of 1,900 mg?L?1. The phenol-degrading aerobic granules can be exploited to design compact high-rate aerobic granulation systems for the treatment of industrial wastewaters containing high concentrations of phenol and other inhibitory chemicals.     

Electrochemical Reduction of 2,4-Dinitrotoluene in a Continuous Flow Laboratory Scale Reactor

An electrochemical laboratory scale reactor was used to treat 2,4-dinitrotoluene (DNT). Experiments were conducted by using a graphite carbon cylinder impregnated with glassy carbon (zero porosity) as the cathode and a platinum wire as the anode. All experiments were conducted under anoxic conditions. Initially, experiments simulating batch conditions were conducted to obtain the optimum operating conditions for the reactor. During this batch-mode study, the effect of various parameters such as applied current, electrolyte concentration, and type of electrolyte on the reduction of DNT were evaluated. Results showed that the rates of DNT reduction increased with an increase in current or concentration of electrolyte. Based on the results obtained from the batch simulation experiments, continuous flow experiments were conducted at three different currents and one electrolyte concentration. The ionic strength of the feed solution was maintained at 0.027 M. A current of 200 mA (current density 0.088 mA/cm2) provided a stable reduction of DNT at the 80% level for a period of 14 days after which reactor cleaning was necessary for removal of suspended solids that were formed within the reactor. End products determined for the experiments showed 80–100% molar balance closure.