Effects of current density and electrode material on the dewaterability of the thickened activated sludge by electro‐flotation

BACKGROUND: Electro‐flotation (EF) could improve thickening and subsequent dewatering characteristics of activated sludge although the settling characteristics of the sludge were very poor. The objectives of this study were to evaluate the performance of EF in the thickening and dewatering of activated sludge and the effects of current density and electrode material on the dewaterability of the sludge. RESULTS: The specific resistance to filtration (SRF) of the sludges thickened by EF was much lower than that by gravity sedimentation. The SRF of the thickened sludge decreased exponentially with increase of gas generation rate of the EF system. Gas generation rate could be controlled by varying the current density of the electrode. Degassing of the microbubbles by moderate mixing of the thickened sludge layer deteriorated the dewaterability of the sludge. Therefore, it is obvious that the gas bubbles entrapped in the thickened sludge play a key role in the observed dewaterability improvement. The effect of electrode material on the thickening and dewatering efficiency was trivial compared with the effect of current density of the electrode. CONCLUSION: The EF system was quite promising as an alternative to gravity sedimentation for thickening of activated sludge and the dewaterability was also improved by microbubbles generated in the system. Copyright © 2009 Society of Chemical Industry     

Local solidosity of microcrystalline cellulose during dead-end filtration and sedimentation

Solid–liquid separation by filtration and sedimentation are important operations used in a wide range of industries. One important characteristic of both the filtration and sedimentation processes is the solidosity of the filter cake/sediment that is formed, affecting the efficiency and design of the separation. In this study local solidosity was investigated using a γ-attenuation method during both filtration and sedimentation experiments for microcrystalline cellulose, a highly crystalline cellulose with particles of about 2–80 μm in diameter. Constitutive relationships for the solidosity were investigated using both filtration (i.e. cake build-up and expression) and sedimentation data for experiments at different pH and suspension concentrations. The sedimentation behaviour under these different conditions was also investigated. It was found that a three parameter empirical model could be used to describe the constitutive relationship between local solidosity and local solid compressible pressure for the sediment formed and the filter cake after both cake build-up and expression. This correlation worked well for the material investigated even at low solid compressible pressures.     

Numerical modelling of filtration with and without sedimentation runs

From experimental batch runs of filtration with sedimentation above the cake formed, a procedure is proposed and applied to experimental data for obtaining the relationships between the settling solids velocity in the hindered settling zone, the local specific resistance and the effective pressure and the solids concentration. Considering the material and momentum balances in the hindered settling zone above the cake, in the cake and in the filter membrane, a simulation program was obtained that satisfactorily reproduces the experimental runs of filtration with sedimentation runs. This simulation work confirms the hypothesis considered in the theory of filtration with sedimentation runs, e.g. the fact that the characteristic lines arise tangently from the cake surface. In addition, the method developed for obtaining the different relationships considered has been demonstrated as correct. The simulation of filtration without sedimentation runs (stirring the suspension above the cake) has also been studied.     

Flotation efficiency of activated sludge flocs using population balance model in dissolved air flotation

The activated sludge process is one of the most frequently used processes for biological wastewater treatment. Conventional gravity sedimentation (CGS), which is widely used as a secondary clarifier in activated sludge processes, has a routine problem due to floating tendency, called bulking, caused by filamentous microorganisms. Dissolved air flotation (DAF) has been applied as potential alternative to CGS as a secondary clarifier. A series of experiments were performed to measure physico-chemical characteristics and removal efficiency of activated sludge flocs. The removal efficiency of flocs corresponding in lag and exponential growth phases was lower, while that of flocs both in stationary and endogenous phases considerably increased. The rise velocity of floc/bubble agglomerates was calculated by using a population balance (PB) model explaining the distribution of floc/bubble agglomerates. The experimental results of flotation efficiency showed a similar tendency with the results predicted by PB model for the rise velocity and distribution of floc/bubble agglomerates. It was found from our study that the DAF process was very effective as a secondary clarifier in the activated sludge process.     

The Effect of Particle Sedimentation on Gravity Filtration

ABSTRACT

Simulation of cake formation of mono-sized and dual-sized particles under gravitational sedimentation and filtration is presented. The dynamic analysis proposed by Lu and Hwang in 1993 is applied to examine the local cake properties formed under a falling head by considering the hindered settling effect of particles in the slurry and the variation of the pressure drop across the filter septum. Results of this study show that, at a given position in a cake, the solid compressive pressure reaches a maximum value and then decreases for a gravity filtration due to the decrease in the driving head. A cake constructed with dual-sized particles has a more compact structure than does one with mono-sized particles, and larger particles will form looser packing than will smaller ones for mono-sized particles. A dual-dispersed suspension with a lower fraction of large particles will result in the lowest cake porosity and the highest specific filtration resistance of cake. Comparison of the porosity distribution in filter cake formed by means of gravity filtration and constant head filtration shows that the porosity near the filter septum of gravity filtration has a convex behavior while that of constant head filtration has a tendency toward concavity. This discrepancy is mainly due to the change in the driving head during the filtration process. Both theoretical and experimental results show that the uniformity of particle size distributions in the filter cake will be much better when the relative settling velocity between large and fine particles is reduced.     

Experimentelle und theoretische Untersuchungen zur Filtration mit überlagerter Sedimentation in Drucknutschen

Sedimentation superimposed on industrial cake filtration leads to longer filtration times and often has a detrimental effect on subsequent process steps such as washing and demoisturing. The influence of sedimentation is seldom recognised in laboratory filtration experiments. Methods are presented for evaluation of pressure filter experiments with superimposed sedimentation which avoid the error made in the usual evaluation methods. For the case of zone sedimentation the article presents a graphical evaluation and a numerical method of modelling permitting scale up to any desired cake height. In superimposed classifying sedimentation simultaneous measurement of filtrate volume and cake height provide information about local variation in filter cake resistance. It is shown for a model system that, owing to sedimentation, the cake resistance shows a minimum at mean cake height and increases rapidly towards greater heights. The local cake resistance corelates with particle size distributions measured for layers of a horizontally cut filter cake. The method of evaluation presented permints determination of the flow resistance of the uppermost layers of a cake and hence estimation of the gas pressure necessary for demoisturing. The profiles of local filter cake resistance show that the relative cake layering is largely independent of the level of suspension filling. A scale-up model is presented for use in those cases where classifying sedimentation cannot be suppressed.     

Permeate flux decline in cross-flow microfiltration at constant pressure

Microfiltration processes are frequently used to separate solids from aqueous suspensions. The rejection of suspended matter is facilitated by means of a size exclusion mechanism and is affected by membrane properties, characteristics of the suspension and operating conditions. Therefore, the filtration performance of a single polymeric hollow-fibre membrane was investigated by monitoring the permeate flux decline for a filtration at constant transmembrane pressure (TMP). For these bench-scale experiments, a model suspension consisting of silica particles in xanthan gum solutions was used in order to represent the characteristics of biological suspensions such as activated sludge properly. In the framework of this study, it was confirmed that the permeate flux declines rapidly during the first stage of filtration until an equilibrium of particle deposition and entrainment is reached. The steady-state permeate flux was found to increase with an increase in cross-flow velocity, a decrease in solid concentration, a decrease in particle size (for this ratio of particle to pore diameter) and a decrease in apparent viscosity of the suspension. However, the equilibrium permeate flux was not affected by variations in TMP, which is in agreement with the limiting flux theory.     

Numerical simulation of secondary clarifier with activated sludge and suction lift removal system: Modified Casson model and sludge withdrawing sensitivity analysis

A computational fluid dynamics model that predicts the sedimentation of activated sludge in a circular secondary clarifier with activated sludge is developed. The axisymmetric single-phase flow is simulated by using a CFD code that has been written with Intel Visual Fortran. First, sludge withdrawing by suction-lift in the near-bottom region of the clarifier is simulated using suction at the bottom of clarifier. The flow and settling processes are simulated using k-e turbulence model on a two-dimensional and orthogonal grid. A convection-dispersion equation that is extended to incorporate the sedimentation of activated sludge in the field of gravity is used. The computational domain includes the sludge blanket where the viscosity is affected by the rheological behavior of the sludge. Experimental data provided by Weiss et al. show that the relationship between shear stress and shear rate follows the Casson law for the shear rates lower than 50 s-1. Plastic viscosity of activated sludge is not removed from the concentration diffusion, so using regular non-Newtonian models leads to overestimation of blanket height. Modified Casson model is introduced to overcome the blanket height overestimation problem. Results show that the local sludge distribution in the clarifier has excellent agreement with concentration profile measurements of Weiss et al., for different treatment plant loadings. Alternative sludge withdrawing methods include withdrawing from pipes position at the bottom of clarifier and withdrawing by using sink terms in governing equations are used. Results show that the first withdrawing method gives less error comparing to these withdrawing methods.     

厌氧反应器内颗粒污泥沉降速率的测定与研究

采用重量沉降法对上流式厌氧污泥床和厌氧颗粒污泥膨胀床生产装置中颗粒污泥的沉降速度进行了测定,将测定结果与沉降速度模型法计算的结果进行了对比,2种方法之间的误差不超过10%。因此认为重量沉降法与沉降速度模型法可作为生产实践中厌氧颗粒污泥沉降速度的确定方法。     

Optimization Study of Municipal Sludge Conditioning,Filtering, and Expressing Dewatering by Partial Least Squares Regression

The effect of different conditioner doses and different applied pressures on the dewaterability of municipal sludge during the filtration and expression stage was assessed using a series of experiments referred to as a uniform design. The relative importance of conditioner dose and applied pressure on the dewatering characteristics of municipal sludge was evaluated with a quadratic model using partial least squares (PLS) regression analysis. It was shown that the developed quadratic model was able to accurately predict the dewatering characteristics during the whole process. It was further shown that lime as a physical conditioner had the most significant impact on filtration rate, expression rate, cake dry solids content, net sludge solids yield, and total cycle time. Aluminum polychloride as the inorganic flocculant had a quite complex effect although not as significant as lime. Filtration pressure had only a slight effect on dewaterability of the municipal sludge, though higher feeding pressure was needed in order to enhance the total feeding amount. Furthermore, expression pressure was only of benefit to cake dry solids content and expression rate and failed to improve other dewatering characteristics of the municipal sludge.     

Drying of water treatment process sludge in a fluidized bed dryer

The drying characteristics of water treatment process (WTP) sludge were investigated with a fluidized bed. The equilibrium moisture ratio of WTP sludge increased with relative humidity and decreased with temperature of drying air. However, equilibrium moisture ratio of WTP sludge was more sensitively dependent on relative humidity than temperature of drying air. When the sludge was dried in a batch fluidized bed, the drying rate of sludge decreased as the moisture ratio of sludge in the bed decreased. The periods of constant drying rates were apparently not observed on the drying rate curves. In addition, the maximum drying rates were increased with bed temperature and superficial air velocity. As the fluidized bed was operated continuously, the degree of drying of WTP sludge increased with bed temperature but was weakly dependent on superficial air velocity. However, the drying efficiency was decreased with bed temperature and relatively insensitive to superficial air velocity and increased with feed rate of sludge.     

Combined gravity and mean flow effects on agitated slurries in a dead-end filtration set-up

In previous work on dense, agitated particle-fluid mixtures in a dead-end filtration set-up the gravity effects that arise from a mass density contrast between particulate and fluid phase have been neglected. As a consequence the granular temperature theory that leads to a solidosity profile can be scaled by one parameter: the ratio of the mean fluid velocity in the filter to the oscillation velocity amplitude. In the present paper the effects of gravity are included, through a term that contains the mass density contrast. The effective gravity force may be net-buoyant or net-sinking. An extra scaling parameter must be introduced, which represents the ratio of the sedimentation velocity to the dead-end filter velocity. The results of the study are reported in a parameter range (fluid velocity, amplitude of agitation, mass density contrast and particle size) that highlights the relevant physics by looking at limiting cases and is also informed by a practical work environment. The key outcome is the solids volume fraction profile in the filter.     

Comparison of various settling velocity functions and non-Newtonian fluid models in circular secondary clarifiers

An axisymmetric single-phase model that predicts the sedimentation of activated sludge in a circular secondaryclarifier is developed. The k-ɛ turbulence model is used on a two-dimensional, orthogonal and stepwise grid. The concentration equation, which is extended to incorporate the sedimentation of activated sludge in the field of gravity, governs the mass transfer in the clarifier. The computational domain includes the sludge blanket where the viscosity is affected by the rheological behavior of the sludge. Results in case of non-Newtonian fluid model are compared with another numerical approach provided by Lakehal et al. Non-Newtonian fluid models—Bingham, Casson, and Herschel-Bulkley—are used. The influence of settling velocity functions and non-Newtonian models on the flow behavior isinvestigated. Finally, the best models are introduced and the ways that the non-Newtonian model introduces the plastic viscosity are discussed.     

Ultrafilter Conditions for High‐Level Waste Sludge Processing

Abstract

Optimal filtration conditions were evaluated for the ultrafiltration process planned for pretreating high‐level waste (HLW) sludge in the Hanford Waste Treatment Plant. This sludge must be filtered in the pretreatment process to remove sodium and, consequently, reduce the number of canisters for storage. The evaluation, which was based on Hanford HLW slurry test data, was performed to identify the optimal pressure drop and crossflow velocity for filtration at both high and low solids loading. Results from this analysis indicate that the actual filtration rate achieved is relatively insensitive to these conditions under anticipated operating conditions. The maximum filter flux was obtained by adjusting the system control valve pressure to between 400 kPa and 650 kPa while the filter feed concentration increased from 5 wt% to 20 wt%. However, operating the system with a constant control‐valve pressure drop of 500 kPa resulted in a reduction of less than 1% in the average filter flux. Also, allowing the control valve pressure to swing as much as ±20% resulted in less than a 5% decrease in filter flux. This analysis indicates that a back pressure setting of 500 kPa±100 kPa will give effectively optimal results for the system of interest.     

悬浮污泥过滤技术在海外油田采出水回注处理中的应用研究初探

悬浮污泥过滤技术集混凝沉降和过滤于一体。分析了大庆油田和胜利油田采出水处理站悬浮污泥过滤技术的应用效果和存在的问题。介绍了悬浮污泥过滤技术在伊拉克油田采出水处理中的应用现状,分析了影响净化效果的主要因素,并与传统油田采出水处理工艺进行了比较,     

动态膜在错流微滤系统中的应用

利用6000目的高岭土作为动态膜涂膜材料,在相同的条件下过滤活性污泥和二级出水.实验结果显示,动态膜过滤在出水水质和通量方面明显优于直接过滤,而处理二级出水在膜通量方面没有优势.清洗实验结果表明动态膜能有效降低膜内部污染,并能恢复到新膜通量的90%.污泥浓度越大,通量越小.提高膜面流速和增加压力都能有效的提高膜通量,但提高膜面流速更经济.     

乳饮料废水A/O处理工艺设计探讨

针对乳饮料废水水量大、水质不均匀、pH值波动大等特点,介绍了物化-A/O处理工艺的设计,对各处理构筑物的设置要求、参数选用、设备选型、附件安装等方面提出了一些建议。提出要注重事故池、中和池的设计;建议气浮采用平流式加压溶气形式;二沉池不建议选用斜管沉淀池;对填料支架的做法应建立相应标准和规范;对物化污泥和剩余污泥,可通过设置框式搅拌机先行混合,再进行压滤。     

Electrokinetic effects on fluid flow and particle transport

The effect of counter-electroosmotic flow on the particle trajectories, the particle equilibrium position, and the critical flux was for the first time evaluated in normal flow filtration using numerical solution of the two-dimensional coupled Navier-Stokes, Nernst-Plank, and Poisson equations for a slit pore having a converging entrance. It was shown that the numerical results for the velocity profiles, ion concentrations, and induced streaming potential were in good agreement with analytical expressions obtained for a simple slit shaped. Numerical simulations for particle transport were performed at both constant pressure and constant filtration velocity in the presence of counter-electroosmosis. A significant shift in the particle trajectory and final equilibrium location were shown at constant pressure due to the reduction in the filtrate flux associated with the counter-electroosmotic flow arising from the induced streaming potential. However, simulations conducted at a constant filtration velocity showed only a very small effect of counter-electroosmosis, with the equilibrium position varying by less than 5% for calculations performed in the presence/absence of counter-electroosmosis. This result stems from a very small distortion in the velocity profile in the region above the pore due to the greater contribution from counter-electroosmosis in the region immediately adjacent to the pore wall. This paper will provide a useful framework to evaluate particle transport in the presence of electrokinetic phenomena.     

浆态床FT合成反应器的固液分离现状和进展

本文就浆态床ＦＴ合成的固液分离体系的性质，重力沉降、加压过滤、离心分离、磁分离技术，超临界流体萃取分离各种固液分离工艺在浆态床ＦＴ合成中试验应用情况，以及工艺开发的进展作一概述。     

基于微观结构的褶式滤芯拟态化模型及其过滤性能的数值模拟

基于随机多层纤维过滤介质算法建立褶式滤芯三维拟态化结构模型,对褶式滤芯内部气-固两相流动进行数值模拟,计算不同运行参数及结构参数下滤芯的压力损失及过滤效率,并与文献计算值进行比较. 结果表明,压力损失随过滤风速增大呈线性增加;随褶尖角增大,压力损失呈先减小后增加,压力损失计算值与文献计算值吻合较好. 褶尖角和过滤风速一定时,过滤效率随粒径增加先减小后增大,在给出的颗粒直径范围内存在最易穿透颗粒直径(MPPS). 不同过滤风速下,当颗粒粒径小于0.5 mm时,扩散作用使过滤效率随过滤风速增加而减小;大于0.5 mm时,惯性作用使其随过滤风速增加而增加;MPPS随风速增加而减小;本计算值与文献计算值趋势一致. 不同褶尖角下,当颗粒粒径小于1 mm时,扩散作用使过滤效率随褶尖角增大而减小;大于1 mm时,惯性作用使其随褶尖角增大而增加.