Hindered and compression settling: parameter measurement and modelling.

The Vesilind settling velocity function forms the basis of flux theory used both in state point analysis (for design and capacity rating) and one-dimensional dynamic models (for dynamic process modelling). This paper proposes new methods to address known shortcomings of these methods, based on an extensive set of batch settling tests conducted at different scales. The experimental method to determine the Vesilind parameters from a series of bench scale settling tests is reviewed. It is confirmed that settling cylinders must be slowly stirred in order to represent settling performance of full scale plants for the whole range of solids concentrations. Two new methods to extract the Vesilind parameters from settling test series are proposed and tested against the traditional manual method. Finally, the same data set is used to propose an extension to one-dimensional (1-D) dynamic settler models to account for compression settling. Using the modified empirical function, the model is able to describe the batch settling interface independently of the number of layers.     

Computational fluid dynamics modelling of hydraulics and sedimentation in process reactors during aeration tank settling.

Aeration tank settling is a control method allowing settling in the process tank during high hydraulic load. The control method is patented. Aeration tank settling has been applied in several waste water treatment plants using the present design of the process tanks. Some process tank designs have shown to be more effective than others. To improve the design of less effective plants, computational fluid dynamics (CFD) modelling of hydraulics and sedimentation has been applied. This paper discusses the results at one particular plant experiencing problems with partly short-circuiting of the inlet and outlet causing a disruption of the sludge blanket at the outlet and thereby reducing the retention of sludge in the process tank. The model has allowed us to establish a clear picture of the problems arising at the plant during aeration tank settling. Secondly, several process tank design changes have been suggested and tested by means of computational fluid dynamics modelling. The most promising design changes have been found and reported.     

Turbidity current in a circular settling tank.

This paper deals with turbidity currents in a circular settling tank. A mathematical model with a k-epsilon turbulence model has been developed. Using this mathematical model, the following unique properties of turbidity currents in a circular settling tank are demonstrated: turbulence induced by the turbidity currents remains after most sediment particles have settled down. This residual turbulent diffusivity has a serious effect on the settling of finer particles. This phenomenon is a very important result in this study. Especially, in the case of a smaller densimetric Froude number, which is a stronger density effect, this residual turbulence effect increases, and also decreases the removal ratio in the downstream with low concentration. Generally, the bottom density current enhances the sediment transport near the tank bottom, while the bottom shear gives reversal influence. When the settling velocity is high, the settling ends under the developing stage both of the turbidity current and of the bottom boundary layer. On the contrary, if the settling velocity is low, the sediment travels a long distance, where the boundary layer is built up, resulting in the reduction of sediment transport near the tank bottom. The overall properties of the density-affected settling tank are also investigated in terms of the removal ratio.     

Evaluation of uncertainties in settling velocities of particles in urban stormwater runoff

Field experiments were carried out to contribute to the assessment of the VICAS protocol aiming to measure settling velocities of particles. Samples of deposited sediments have been taken in the Django Reinhardt stormwater detention and settling tank in Chassieu, France, using sediment traps located on the tank bottom. The first set of experiments was designed to assess the VICAS protocol in terms of mass balance and repeatability. A bias in the measurement of settling velocities distributions of deposited sediments (i.e. particles with high settling velocities) was suspected and confirmed by specific tests. Uncertainties in the final distribution curves have been evaluated by using Monte Carlo simulations and the law of propagation of uncertainties. All uncertainty calculations were implemented in a MatLab code named UVICAS used for each experiment. This code allows analysing the main sources of uncertainties and their evolution during experiments. Uncertainties in the final distribution curves decrease with increasing values of settling velocities and are lower than 1%.     

基于Acitiflo^（R）加砂高效沉淀池试验分析

Acitiflo^（R）加砂高效沉淀池工艺在絮凝池中投加微砂作为絮体的核心,增强混凝絮凝沉淀的处理性能,试验进一步验证了Acitiflo^（R）加砂高效沉淀池用于自来水厂初沉池的处理效果和运行数据的初步确定,分析Acitiflo^（R）加砂高效沉淀池工艺对浊度和藻类去除的效果,预加氯不会影响Acitiflo^（R）加砂沉淀池的浊度去除效果,能增加藻类去除率。     

The influence of settling time on the formation of aerobic granules.

Aerobic granular sludge, without the addition of carrier material, has only been reported in one suspended growth system, the Sequencing Batch Reactor (SBR) operated with short fill and settling periods. Recent studies have demonstrated that extracellular polysaccharides increased with the formation of aerobic granules, and that the shear force may stimulate production of these polysaccharides. In the study described herein, two SBRs were operated with the same shear force (air flow rate 275 L h(-1)) and two different settling times (2 and 10 min). Only the reactor with 2 min settling formed completely granular sludge, although granules were present in both reactors. Community analysis using 16S rRNA PCR products and DGGE showed that the communities diverged quickly after reactor start-up. For samples taken at steady-state, the granular population was more stable and less diverse than the flocculent reactor. EPS extraction of samples using cation exchange resin yielded similar values for aerobic granular sludge and previously reported anaerobic granules. While differences in the protein and TOC content between the flocculent and granular reactors increased appreciably as the sludge became more granular, the protein to polysaccharide ratio was relatively constant. The experiment confirmed previous theories that short settling times in SBRs select for granular sludge. The settling time results in granular sludge having a higher EPS protein content and a less diverse but more stable population.     

带翼片侧向流斜板沉淀池内颗粒沉降过程的数值模拟

采用有限体积法研究了带翼片侧向流斜板沉淀池内水流流动的特性,并计算了带翼片与不带翼片斜板内水流的雷诺数,结果表明加入翼片可以明显降低水流的雷诺数,其数值为不带翼片斜板雷诺数的42%,并采用层流模型计算了两种情况斜板内水流的三维流场分布情况.在此基础上考察了加入稀疏颗粒后,颗粒物的运动轨迹,采用DPM模型进行计算,结果表明,加入翼片后颗粒物沉降的路径缩短,有利于固液分离;随着水流速度的增大,颗粒跟随性增强,颗粒物密度(采用1100kg/m3和1 400 kg/m3进行分析)增大,颗粒沉降性也增强.同时大颗粒粒径也有利于沉淀.     

Analysis of the methods for determining the settling characteristics of sewage and stormwater solids

In order to optimise the design and the management of sewer networks and settling tanks, there is a need to determine the settling velocity characteristics of sewage and stormwater solids. Devices and test methods for measuring settling velocities of these solids have been developed (Chebbo, 1992), (Michelbach and Wohrle, 1993) and (Tyack et al., 1993). In order to quantify the influence of the experimental procedure on the settling velocity grading curves, we have compared three selected methods (Cergrene, Aston, Camp) when tested with the same sample with the method developed by UFT (Michelbach and Wohrle, 1993). The results of the comparative tests indicate that the settling velocities are significantly higher with the UFT test procedure than the other methods. This is mainly explained by the fact that the UFT method takes only settleable solids into account. When the calculations of the four methods are made with the total mass of SS, the study has shown that the settling velocity curves of the same samples are different. The differences vary between the nature of the samples and the test procedures which were compared. The influence of the experimental procedure for separating a sample by settling velocities fractions is demonstrated. In conclusion, we explain the need to develop an international test procedure in order to be able to compare the results.     

Column studies to determine mercury settling in road runoff

A study was performed using a settling column to remove mercury (Hg) from road runoff. The emphasis was placed on the relationship between Hg removal and critical settling velocities, as well as the distribution of total suspended solids (TSS). The impact of rainfall characteristics and temperature on Hg removal was also discussed. Results indicated that Hg removal was significant within the initial 30 min of the settling process. The Hg removal rate increased with the decrease of critical settling velocity, and this trend became gradually significant, which could be used as an important reference for the optimization of sedimentation basin design. Hg removal at different settling times was significantly related to initial distribution of TSS. The impact of rainfall intensity on Hg removal was greater than other parameters of precipitation features, followed by rainfall. In addition, Hg removal was moderately related to temperature. The effect of antecedent dry periods on Hg removal was restricted by rainfall and rainfall intensity.     

Computational simulation of flocculent sedimentation based on experimental results

Computerised interpolation algorithms as well as the empirical model for analysing the flocculent settling data were developed. A mechanistic semi-empirical model developed from fundamental physical principles of a falling particle in a viscous fluid was tested against actual flocculation column data. The accuracy of the mechanistic model was evaluated using the sum of the squared errors between the interpolated values (real values) and the model predictions. Its fitting capabilities were compared with ?zer's model using nine flocculent data sets of which four were obtained from literature and the rest were actual data from the performed experiments. The developed model consistently simulated the flocculation behaviour of particles in settling columns better than ?zer's model in eight of the nine data sets considered. It is recommended that the model's performance be further compared with other models like the Rule based and San's model. The errors due to the use of interpolated values when determining the performance of the empirical models need to be investigated. Furthermore, a three-way rather than two-way interpolation should now be achievable using the interpolation algorithm developed in this study thereby reducing the effects of interpolation bias. The above work opens the way to full automation of design of flocculation sedimentation basins and other gravitational particle separation systems which at present are designed manually and are susceptible to a wide range of human and random errors.     

Comparison of the 1D flux theory with a 2D hydrodynamic secondary settling tank model.

The applicability of the 1D idealized flux theory (1DFT) for design of secondary settling tanks (SSTs) is evaluated by comparing its predicted maximum surface overflow (SOR) and solids loading (SLR) rates with that calculated from the 2D hydrodynamic model SettlerCAD using as a basis 35 full scale SST stress tests conducted on different SSTs with diameters from 30 to 45m and 2.25 to 4.1 m side water depth, with and without Stamford baffles. From the simulations, a relatively consistent pattern appeared, i.e. that the 1DFT can be used for design but its predicted maximum SLR needs to be reduced by an appropriate flux rating, the magnitude of which depends mainly on SST depth and hydraulic loading rate (HLR). Simulations of the sloping bottom shallow (1.5-2.5 m SWD) Dutch SSTs tested by STOWa and the Watts et al. SST, all with doubled SWDs, and the Darvill new (4.1 m) and old (2.5 m) SSTs with interchanged depths, were run to confirm the sensitivity of the flux rating to depth and HLR. Simulations with and without a Stamford baffle were also done. While the design of the internal features of the SST, such as baffling, have a marked influence on the effluent SS concentration for underloaded SSTs, these features appeared to have only a small influence on the flux rating, i.e. capacity, of the SST, In the meantime until more information is obtained, it would appear that from the simulations so far that the flux rating of 0.80 of the 1DFT maximum SLR recommended by Ekama and Marais remains a reasonable value to apply in the design of full scale SSTs--for deep SSTs (4 m SWD) the flux rating could be increased to 0.85 and for shallow SSTs (2.5 m SWD) decreased to 0.75. It is recommended that (i) while the apparent interrelationship between SST flux rating and depth suggests some optimization of the volume of the SST, that this be avoided and that (ii) the depth of the SST be designed independently of the surface area as is usually the practice and once selected, the appropriate flux rating is applied to the 1DFT estimate of the surface area.     

Effect of upflow velocity on the performance of an inclined plate membrane bioreactor treating municipal wastewater

An inclined plate membrane bioreactor (iPMBR) was introduced to meet the challenge of handling high mixed liquor suspended solids when operating at long sludge retention times. During the first 407 days of operation, the iPMBR was able to rezone more sludge (1.5-10.5 times greater) in its upstream, anoxic tank compared to its downstream, aerobic tank. This could extend membrane filtration by diverting most of the sludge from the aerobic zone. During this period, the upflow velocities through the inclined plates of the anoxic tank ranged from 2.3 x 10(-4) to 7.7 x 10(-4) m/s. After Day 407, the operating conditions were changed to determine whether the iPMBR would fail to create a sludge concentration difference between its two tanks. When the upflow velocity was increased to 1.8 x 10(-3) m/s, the sludge concentration difference between the two zones was removed. This indicated that the upflow velocity had increased sufficiently to overcome the settling velocities of most flocs, resulting in more solids being carried from the anoxic to the aerobic tank. For the configuration of this iPMBR, operating at flow rates where the upflow velocity through the inclined plates was less than 1.0 x 10(-3) m/s would be necessary to keep a significant sludge concentration difference between its two zones.     

新型平流式沉淀池沉淀效果试验研究

在进水口加设调流板、在溢流堰上沿水流方向设置溢流槽,从两个方面对传统平流式沉淀池进行了改进,设计出一种新型平流式沉淀池。通过试验研究分析了调流板和溢流槽对泥沙沉淀效果的影响,得出新型平流式沉淀池可以大大提高泥沙的沉淀效率,出池水流平均含沙量最大降幅可达94.8%,特别是对细颗粒泥沙的沉淀效果尤为明显。     

Theoretical analysis on non-uniformity of water distribution and influence of construction parameters on settling efficiency

During the reconstruction of horizontal flow tanks into inclined settling tanks in Chinese water plants, uniformity of water distribution has not been solved theoretically. Based on the concepts of hydraulics, a model of inclined tanks, including the ratio (L/B) of tank length (L) to width (B), diameter of inclined tubes (d) and height of the water distribution area (h1) and so on, was established to simulate and analyze the effects of these parameters on Non-Uniformity of Water Distribution (NUWD). The influences of NUWD on settling efficiency were also analyzed based on Yao's formula. Simulated results show that the ratio (L/B) has the greatest impact on NUWD, and the settling efficiency decreases with it. Under the conditions of q=10 or 20 m/h and L/B>or=5 or 3, the total forces imposed on down-sliding flocs tend to be zero, which reduces the separating efficiency. Moreover, critical settling velocity (CSV) of the first inclined tube will decrease with the increase of h1, and the optimal range of h1 will be 1.2-1.6 m. The difference of CSV between the first tube and the average value of the tank u0 (shown as Delta(uF0-u0)) will increase with d and surface load (q).     

二沉池一维动态模型的研究进展

对基于固体通量理论的二沉池一维动态模型研究进行较为全面的综述。二沉池一维动态模型大致可分为两类:考虑通量约束的模型和考虑弥散作用的模型,重点介绍了近年来有较大影响的模型。对模型中污泥重力沉降速率表达式、弥散系数、模拟过程中二沉池的分层数以及污泥混合液导致的密度流及短流进行了较详尽的讨论。最后,对二沉池一维动态模型的进一步研究工作提出展望。     

Ⅱ型和H型立柱对排沙漏斗水沙分离性能的影响大流量进水工况下主隧系统水气运动特性

为获取不同立柱体型对排沙漏斗水沙分离性能的影响规律,通过室内试验对不加设立柱、加设Ⅱ型和H型立柱支撑系统的排沙漏斗水沙分离性能进行研究。试验结果表明：无立柱时空气涡贯穿排沙底孔,涡径最大,清水区面积最大,漏斗室内的螺旋流强度最大;加设Ⅱ型立柱后空气涡产生偏移,涡径减小,清水区面积减小,漏斗室内的螺旋流强度减小;H型立柱下空气涡偏移距离最大,涡径最小,清水区面积最小,漏斗室内的螺旋流强度最小。相较于加设H型立柱,Ⅱ型立柱支撑系统下排沙漏斗的总截除率、泥沙排出率、漏斗室内淤积量、排沙耗水率与悬板下方不加设立柱时相近,为保证悬板安全及较高的截除率,可采用Ⅱ型立柱作为支撑体型。     

水厂沉淀池运行参数对沉淀效果影响的数值模拟

运用已建立的二维沉淀池数学模型以及k-ε紊流模型,采用非正交同位网格下SIMPLE算法求解水动力和悬浮物沉降的耦合模型,模拟平流沉淀池中悬浮物的沉降过程,在平流沉淀池不同进水浊度和产水量Q(或停留时间T)下进行计算,得出不同运行工况下沉淀池进水浊度与出水浊度之间的定量关系,分析水厂运行管理控制对沉淀池出水质量产生的影响,为水厂提高产水效率和控制沉淀池出水水质提供参考依据。     

牵拉抢险长管袋水下沉落的力学模拟

采用牵拉方法为充泥长管袋沉落定位,达到缓流堵口的目的,是黄河抢险堵口的新技术。本文对牵拉抢险长管袋这种新型的抢险柔性材料进行了抛掷沉降的模拟实验研究,得到了对管袋沉落过程的基本认识,获取了基本力学参数。在此基础上分析了牵拉长管袋沉降的力学特性,建立了牵拉长管袋沉落力学模型。通过不同工况的模拟计算,得到了牵拉绳长、大河流速、大河水深等与牵拉力之间的关系,发现了管袋碰撞着地时牵拉力的突跃现象。本文提出的计算方法及有关参数对完善牵拉长管袋设计,提高新材料抢险堵口技术具有重要的工程指导意义。     

Calculated substantiation of the settling velocity of sediments at the outlet of a settling basin

Conclusion On the basis of treating the author’s experiments and generalizing the results of earlier investigations a graph (Fig. 3) and dependence (5) were obtained for the first time for determining the average settling velocity of the flow at the outlet of a settling basin, which will made it possible to provide a suitable design substantiation of irrigation settling basins and canals of irrigation systems, to reduce the volume of construction and operating works, and will also enable the operating service to promptly maintain the balanced regimes of the liquid and solid phases in the settling basin and canal. Translated from Gidrotekhnicheskoe Stroitel’stvo, No. 3, pp. 53–56, March, 1997.