The Effect of Ozone on the Reversibility of Floc Breakage: Suspensions with High Humic Acid Content

The effects of pre-ozonation and humic acid concentration on the formation, breakup and reformation of flocs generated using aluminium sulphate and polyaluminium chloride was investigated using conventional jar test procedure and by continuous optical monitoring. Using a range of different humic acid and ozone concentrations, it was found that increasing either concentration was detrimental to floc formation, leading to reduced turbidity removal. Once ruptured, only limited regrowth of flocs occurred, indicating irreversible floc breakage. The findings showed that the use of pre-ozonation did not provide beneficial results to improve turbidity removal. Nevertheless, improved dissolved organic carbon removal was achievable.     

Assessing floc strength using CFD to improve organics removal

Floc characteristics play a major role in the removal of contaminants from water in physico-chemical treatment processes. The efficiency of the main removal processes is a function of floc size, strength and density. Changes in these parameters affect floc removal and hence the removal of adsorbed organic matter. Coagulation and flocculation efficiency and floc strength are often assessed using a jar tester. Here, CFD was used to model the flow field within a standard jar test apparatus and, using a Lagrangian particle trajectory model, to study the effects of turbulence on individual flocs. Combining numerical and experimental data, velocity gradient values at which floc breakage occurs are postulated for three different floc suspensions. Although the threshold values are determined using jar test and CFD data in combination, they are based on the flocs’ resistance to induced velocity gradients. This is a significant result, as previous breakage thresholds have been expressed in terms of mixing speed and cannot be applied at full scale. The results shown here can be adopted for use in other situations and can be used to assess the performance of existing flocculators or to design new installations.     

Evaluation of mixing conditions using an on-line monitoring technique

Effects of mixing conditions on flocculation were investigated by using a photometric dispersion analyzer (PDA) as an on-line monitoring technique in this study. Both river water and synthetic waters of humic acid (HA) and kaolin solutions were used and polyaluminum chloride (PACl) were used as a coagulant in this study. A clear relationship between F-index and residual turbidity was observed. Residual turbidity was low at high F-index. The mixing effects were also found closely related to the floc formation. When the floc formation was governed by a combination of charge neutralization and sweep floc, rapid mixing was important, but it was not important when the floc formation was governed by the sweep floc mechanism. The coagulant dosage governed the floc size and strength in the sweep floc region. The higher the coagulant dosage was, the larger but the weaker the floc was. Rapid mixing effects were different, depending on raw water characteristics. Fast and large floc formation was observed in flocculation of the kaolin solution, compared to that of the HA solution. Small HA would be mostly adsorbed onto the hydroxide precipitate after the precipitate formed. The adsorption could retard further floc growth. The resulting floc was small, and the floc formation was slow. However, kaolin helped flocculation by bridging the hydroxide precipitates, leading to fast and large floc formation. Temperature affected the flocculation kinetics as well as the floc size. A large floc formed at high temperature. The flocculation kinetics became fast with increasing temperature.     

Coagulation of Particles Through Rapid Mixing

As is often assumed, rapid mix effectively distributes the dosed coagulant in suspension to promote subsequent flocculation of suspended particles and coagulant molecules. This study determined that the role of rapid mix is considerably more complex than conventionally assumed. Particles in the raw water samples from the Banxing Water Works of Taipei City were coagulated at high shear environment, during which flocs of size 40-70 μm were produced. The produced floc interior was compacted, and residual turbidity and the amount of dissolved natural organic matter (NOM) in suspension were substantially reduced. The subsequent stage of rapid mix slightly reduces floc size, further compacts the floc interior, and expels fine particles and some adsorbed NOM from the flocs. A sudden reduction in shearing rate produced large sized flocs with not so compact structures. Moreover, this action released fine particles from flocs into the suspension and enhanced adsorption of NOM onto solid surfaces.     

Aggregation and breakage kinetics of fresh cement paste

The influence of shear-induced forces on the microstructure of fresh cement pastes was studied. Aggregation and breakage kinetics of the paste matrix are highly influenced by the shear history. It was found that the kinetics of re-aggregation is relatively slow, and time scale for recovery is longer than the time needed for breakdown. When the aggregation kinetics dominates, network interactions among particles develop and the average floc size increases. When the breakage kinetics dominates, network interactions among particles are broken and are accompanied by a decrease in the average floc size. The results suggest that there is a limiting size to floc growth. Minor additions of clays can significantly impact the structural network development and result in a more flocculated structure. The flocs produced by the clays were highly stable flocs with strong interparticle bonds that were able to oppose floc breakage and floc erosion.     

Coagulation of Particles Through Rapid Mixing

As is often assumed, rapid mix effectively distributes the dosed coagulant in suspension to promote subsequent flocculation of suspended particles and coagulant molecules. This study determined that the role of rapid mix is considerably more complex than conventionally assumed. Particles in the raw water samples from the Banxing Water Works of Taipei City were coagulated at high shear environment, during which flocs of size 40–70 μm were produced. The produced floc interior was compacted, and residual turbidity and the amount of dissolved natural organic matter (NOM) in suspension were substantially reduced. The subsequent stage of rapid mix slightly reduces floc size, further compacts the floc interior, and expels fine particles and some adsorbed NOM from the flocs. A sudden reduction in shearing rate produced large sized flocs with not so compact structures. Moreover, this action released fine particles from flocs into the suspension and enhanced adsorption of NOM onto solid surfaces.     

交替水流在絮凝过程中的作用:理论分析

通过分析交替旋转水流对絮凝过程的影响,揭示了这一特殊的水流运动方式对絮团的形成具有促进作用,同时对形成的絮团具有特别的剪切与修饰作用。与常规的旋转水流相比,交替旋转水流具有下列特点:(1)有利于颗粒的剪切絮凝(松弛效应);(2)有利于原始颗粒或小絮团的钻隙作用(投掷效应);(3)有利于絮团在连接薄弱处通过断裂或滑动而重组(揉搓效应)。这些作用的结果,是有利于形成更为密实的絮团结构,降低固液分离后絮团的含水率。     

Comparative Coagulant Demand of Polyferric Chloride and Ferric Chloride for the Removal of Humic Acid

Abstract

This paper is concerned with the potential advantages of the use of polyferric chloride as a coagulant for the removal of natural organic matter (e.g., humic acid) in water treatment. In particular, this paper assesses the effects of the basic nature of polyferric chloride, the type of water, and the coagulation pH on the humic acid removal performance. The comparative dose demand of polyferric chloride and ferric chloride was assessed in terms of a good humic acid removal efficiency (> 50%) achieved. It was found that a polyferric chloride with a basic ratio of 0.3 can achieve the best humic acid removal. The water having both humic acid and colloidal particles was favorable to the floc development and a better humic acid removal. For the same coagulation conditions, the dose demand of polyferric chloride was 50% less than that of FeCl₃ for a > 50% humic acid removal.     

Strength and breakage of activated sludge flocs

The breakage of activated sludge flocs under turbulent shear conditions was investigated as a function of floc size. Municipal activated sludge flocs were fractionated by sieving to narrow size fractions. Shear stress distribution functions for the breakage of activated sludge floc samples were obtained. It was found that by increasing the floc size, this distribution was skewed towards smaller shear stress values and became broader. Results of experiments showed that the median shear stress, τ₅₀, required for floc breakage reduced by about 23% from 3.9 Pa for 45-63 μm sieve fraction to 3 Pa for the 150-180 μm sieve fraction. Under steady conditions, the median shear stress for the breakage of fragments that formed due to the breakage of larger flocs was as much as three times larger than that of the original flocs.     

聚合氯化铁混凝—超滤—氯消毒处理原水的研究

采用聚合氯化铁(PFC)作为混凝剂对腐殖酸-高岭土模拟水样进行混凝—超滤—氯消毒处理,分析混凝絮体特性和超滤处理过程对消毒副产物前驱物的生成趋势的影响。结果表明,PFC混凝效果随着投加量的增大而增强;投加量为12 mg/L时,酸性条件有利于增强混凝效果;超滤过程可以去除混凝出水中的大分子有机物,但对快速反应物的去除效果却不明显;在絮体粒径500μm的范围内,絮体粒径的增大有利于快速反应物的去除,从而降低了消毒副产物的生成潜能。     

Flocculation of Clay Suspensions in the Presence of Humic and Fulvic Acids

Surface waters which contain suspended clay and organic matter, differ strongly in response to flocculation from suspensions of “pure” reference clays in controlled systems. The mechanism of interference was investigated by means of a synthetic system, consisting of suspensions of reference clay with humic and fulvic acid added. These organic soil acids increase the colloidal stability of the suspension. Due to chemical reaction taking place between the organic acids and the cationic flocculant, higher flocculant doses were required in systems where interaction of clay and organic matter occurred.     

Development of antifouling properties and performance of nanofiltration membranes modified by interfacial polymerisation

Two types of bisphenol monomers, Bisphenol A (BPA) and Tetramethyl Bisphenol A (TMBPA), with different concentrations of bisphenol aqueous solution (0.5% to 2.%w/v) and various interfacial polymerisation times (10 s, 30 s and 60 s) in the fixed 0.15%w/v organic solution of trimesoyl chloride (TMC)-hexane were studied. Irreversible fouling of both unmodified polyethersulfone NFPES10 and modified polyester thin-film composite polyethersulfone membranes were studied using humic acid model solutions at two different pH values, pH 7 and pH 3. It was observed that polyester thin-film composite membranes prepared by BPA exhibited fewer tendencies for irreversible fouling by humic acid molecules at neutral environment compared to unmodified NFPES10 and TMBPA-polyester series. This is most probably due to high electrostatic repulsion force between negatively charged of BPA-polyester layer and highly negative charged of humic acid at pH7. However, some modified membranes with rougher surfaces were severely fouled by humic acid molecules at acidic environment, pH 3. Under this acidic environment, carboxylic acid groups of humic acid lost their charge and the macromolecules of humic acid have smaller macromolecular configuration due to the increased hydrophobicity and reduced inter-chain electrostatic repulsion. Thus the molecules of humic acid may be preferentially accumulated at the valleys of the rougher membrane surface blocking them and resulting in a more severe fouling. In addition, the modification also affected membrane pore size and pore size distribution as shown by AFM images. It was also observed that the smaller pore size generated after modification does not have significant effect on humic acid removal due to the larger size of humic acid molecules. All the modified membranes posses smaller pore size than the unmodified NFPES10 (1.47 nm) in the range of 0.8–1.34 nm.     

Breakage–reflocculation implemented by two-stage shear for enhancing waste-activated sludge dewaterability: Effects of shear condition and extracellular polymeric substances

The conditioning of waste-activated sludge (WAS) before dewatering is crucial for enhancing sludge dewaterability. The breakage–reflocculation that was implemented by two-stage shear (drastic first-stage shear for breakage and moderate second-stage shear for reflocculation utilizing the bioflocculation function) which was proposed as a novel WAS conditioning method with several advantages (simple operation, lower cost, and none added reagent) compared to traditional methods. Effects of the shear condition and extracellular polymeric substances (EPS) on breakage–reflocculation were orderly investigated. Two equations were developed by response surface methodology for predicting breakage–reflocculation conditioning performance. Analysis of variance (ANOVA) indicated that individual effects of first-stage shear rate (G₁), second-stage shear rate (G₂), second-stage shear time (t₂), and interactive effect of G₁G₂ were significant. More compact WAS flocs with better dewaterability and larger floc size formed through breakage–reflocculation. This was reflected in that the capillary suction time decreased by 16.9% and mean floc size increased by 24% under the optimum shear condition. In addition, the loosely bound EPS was revealed to be closely negatively correlated with breakage–reflocculation conditioning performance, indicating its adverse role in breakage–reflocculation. The breakage–reflocculation could be used as an independent conditioning method with low cost or a part of combined method.     

Evaluation of humic acid photocatalytic degradation by UV–vis and fluorescence spectroscopy

Photodegradation of humic substances causes drastic changes in the UV–vis absorption and fluorescence properties of humic acids. In this study it is intended to fulfill the lack of knowledge about the spectral changes of humic acids during photocatalytic oxidation processes and elucidate the effects observed on the molecular size distribution of humic acid focusing on their analysis by UV–vis and fluorescence spectroscopy.

As confirmed by the spectroscopic evaluation of the molecular size distribution data, photocatalytic degradation of humic acid leads to the formation of lower molecular size (small fractions) and higher UV absorbing compounds. For fractions less than 10 kDa, UV₂₅₄ absorbing moieties in treated humic acid samples become higher than that of raw humic acid designating the generation of new species during photocatalysis. UV–vis spectroscopic changes were also evaluated by the parameters relating to the concomitant removal of the total organic carbon as well as by the ratios using absorption values at discrete wavelengths. Moreover, the fluorescence spectra of treated humic acid samples show decreasing intensity profiles with increasing photocatalytic irradiation time.     

Modelling of flocculation using a population balance equation

In this paper, a model based on a population balance equation (PBE) is developed. It aims at reproducing experimental floc size distributions obtained at steady state in a jar-test. The objective is to develop a simple model, based on physical phenomena, and that does not contain any adjustable parameters. Floc size distributions obtained using a part of a particle image velocimetry (PIV) device and image analysis are used to develop mathematical expressions for the aggregation and breakage kernels. A critical volume beyond which breakage is of significant importance is identified and related to the hydrodynamics. Hydrodynamic sequencing allows the distribution of the daughter particles resulting from a breakage event to be established. The model is finally successfully validated against experimental results.     

Breakage and re-growth of flocs formed by alum and PACl

The efficiency of coagulation and flocculation can be significantly improved through flocs breakage and re-growth under appropriate conditions. Investigation was carried out to examine the formation, breakage and re-growth of flocs formed by kaolin or the mixture of kaolin and humic acid with aluminum sulfate (‘alum’) or polyaluminum chloride (‘PACl’). The reversibility and mechanism in the process of flocs breakage and re-growth were explored through conventional jar test procedure and continuous optical monitoring (PDA). There was full re-growth of flocs in kaolin water coagulated in charge neutrality (alum or PACl), which indicated a significant reversibility of flocs break-up process; the process displayed a distinct irreversibility in sweep flocculation. The addition of humic acid weakened the reversibility, and alum showed better shear resistance than PACl. The residual turbidity after breakage and re-growth was lower than before breakage when charge neutrality dominates the coagulate mechanism, but the result was reverse in sweep flocculation.     

腐植酸类物质化学研究的独特性及新进展

简述了国内外腐植酸类物质化学研究的独特性及较有特色的最新进展,包括腐植酸及黄腐酸等的特殊来源与分子结构,腐植酸或黄腐酸的某些化学反应,与有机毒物分子或金属离子的相互作用特征,腐植酸等作为反应催化剂及载体或化学助剂的性能特色,作为表面活性剂、染色助剂以及用于检测微量有机物的传感器等的合成和性能研究,最后对如何检测微量腐植酸及如何从环境体系中降解或去除腐植酸等进行了初步探讨。     

Floc breakage in agitated suspensions: Effect of agitation rate

A population balance equation that governs the floc size distribution in turbulent flow, incorporating both the splitting and erosion mechanisms of floc breakage has been presented in previous studies[6,7]. Experiments have been conducted in which transient floc size distributions of dilute suspensions of Kaolin-hydrous ferric-oxide flocs undergoing breakage in agitated batch system are obtained over turbulent microscale shear rates ranging from 60 to 400 sec^?1. Floc size distributions over a range of floc diameters from about 1 to 100 μ m are measured by overlapping techniques, including computerized optical scanning of photographs and rapid electronic sizing of individual flocs using a light blockage transducer. The experimentally determined size distributions are then fit to those computed from the population balance equation, using constrained nonlinear least squares. The splitting frequency of parent flocs is found to vary as the 0.33 power of the parent floc volume and 0.71 power of the shear rate. The average number of daughter fragments produced upon splitting of individual flocs is found to be about 2.5. The mean and standard deviation of the daughter fragments produced upon splitting are found to be in fixed ratio to one another, independent of the parent floc size and shear rate. An appropriately nondimensionalized coefficient characterizing floc erosion has been found to be independent of shear rate.     

The effect of rapid mixing on the break‐up and re‐formation of flocs

The effect of rapid mixing on floc formation, breakage and re‐formation using aluminium sulfate (‘alum’), polyaluminium chloride and a cationic polyelectrolyte were investigated using a continuous optical monitoring technique. For the aluminium‐based coagulants it was found that, with shorter times of rapid mix, larger flocs were formed, but only limited re‐growth occurred in all cases, indicating a significant irreversibility of the floc break‐up process. For cationic polyelectrolytes, the re‐growth of flocs occurred to a much greater extent and with longer rapid mix times floc breakage was almost fully reversible. Residual turbidity values before floc breakage and after re‐formation were consistent with the dynamic monitoring results. Copyright © 2004 Society of Chemical Industry     

高剪切条件下纳米腐殖酸的制备与表征

以风化煤为原料,采用碱溶酸析沉淀法配加高剪切技术制备了平均粒径为60 nm的腐殖酸。通过正交实验得出制备纳米腐殖酸最优化工艺条件为:反应温度60℃,反应时间150 min,氨水质量分数25%,固液比1∶10,晶粒调整剂加入量为反应溶液质量的0.15%,并用激光粒度分析仪、红外光谱仪(FTIR)、凝胶色谱分析仪(GPC)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)及比表面积分析仪(BET)对产物纳米腐殖酸的粒径分布、化学结构、结晶性能、分子量大小、形貌、比表面积等进行表征。结果表明:粒度分布均匀,分散性良好,平均粒径为60 nm,比表面积为110.31 m2·g-1,数均相对分子量为119,且双键结构和官能团含量明显增加,形状呈球形,比表面积增大,分子质量减小。