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.     

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.     

AGGREGATE CHARACTERISTICS AND TURBULENCE IN FLOCCULATION BASINS

An investigation has been undertaken using laser-Doppler velocimetry and flow visualization to better understand the interactions of turbulent eddies with the component processes of coagulation. Coagulant-aid activation, floc structure and density, and aggregate integrity are of particular interest. A 20 liter flocculation basin with two different paddle assemblies was constructed and examined extensively. The perforated blades produced a far mote energetic gross circulation at modest rates of rotation than obtainable with the hardware cloth paddles, and a larger initial aggregate growth rate. The hardware cloth paddle assembly appeared to produce greater floc volume fraction at large aggregate diameters, however. The objective of this study was to determine the extent to which the turbulent energy distribution can be manipulated to produce desired floc properties or floc growth patterns.     

Improvement in the coagulation performance by combining Al and Fe coagulants in water purification

This study evaluated the beneficial effects on the coagulation process of combining coagulants with Fe and Al in the removal of turbidity and DOC (dissolved organic carbon), and proposed conditions for proper operation of a coagulation process. In addition, the floc characteristics and sludge dewater ability were evaluated. Blended coagulants were more effective than single coagulants at lower concentrations in water purification. The optimal blended ratio for the removal of DOC was 1.45 mM as Al/mM as Fe. Also, blended coagulants were less affected than single coagulants by pH, temperature, and rapid mixing intensity. Based on the results of the change in the apparent molecular weight distribution (AMWD) of DOC in raw and treated water by coagulation, low molecular weight organic matters were removed by 40.7% for the blended coagulant (1.45 mM as Al/mM as Fe). Blended coagulants formed larger flocs than single coagulants did. At pH 6.0, floc strength and sludge dewaterablity were both improved.     

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 structure and strength of depletion force induced particle aggregates

Aggregating fine particulate matter is common practice in many industrial solid-liquid separation processes. Data obtained in this work on dilute aqueous dispersions of model colloidal polystyrene latex spheres indicate that depletion flocculation, which uses non-adsorbing polymer, can yield very compact aggregates. Flocculation of the negatively charged latex particles was induced by the addition of a poly(acrylic acid) at pH 10. The structural compactness of the latex flocs formed in the dilute dispersions was characterised using small-angle static light scattering in terms of mass fractal dimensions. Rheological measurements on the concentrated latex dispersions in the presence of the non-adsorbing polyacid showed Bingham yield stress behaviour. Both the compactness and strength of the latex flocs were found to be significantly dependent upon the level of the polyacid, as well as the concentration of the initial particles. In particular, as the level of the polyacid was raised the floc compactness decreased, whereas its strength increased. They were both seen to level off at high polymer concentrations. Atomic force microscopy measurements were made at varying concentrations of the polyacid to provide a qualitative explanation of the observed floc structural behaviour of the dilute dispersions. By combining the fractal dimension and the Bingham yield stress we were also able to estimate the energy required to separate the flocs into single units in the concentrated dispersions. It was concluded that the interparticle interaction energy is the key to understanding the dependence of both the floc structure and strength on the polymer concentration.     

Comparison of two cationic polymeric flocculant architectures on the destabilization of negatively charged latex suspensions

Flocculation studies between cationic polymers and oppositely charged colloidal particles are reported in which both flocculation kinetics and floc structures are systematically investigated. The flocculation rate constant, stability ratio and kinetics laws are experimentally determined using particle counting for two polymer architectures; a cationic linear polymer and a two-branched polymer. Comparisons are also made using NaCl at different ionic concentrations for the destabilization of the colloidal particles. Detailed measurements of electrophoretic mobility and kinetics rate constants on varying the polymer dosage are reported. Results suggest that the polymer architecture plays important roles on the polymer dosage for the rapid destabilization of the colloidal suspension. The branched polymer at optimal dosage exhibits the highest flocculation rate constant, whereas on the other hand, the linear polymer concentration range of flocculation is larger. In both cases, polymer flocculation is more efficient by a factor of 5-6 than charge screening effects due to the presence of salt. Analysis of the stability ratio indicates that tele-bridging flocculation and electrostatic forces dictate the stability of the charged latex particle suspension. It is shown that the fractal concepts which are valid for aggregation processes are also applicable here and branched polymers as well as linear polymers yield to the formation of compact flocs in comparison to those obtained with salt.     

Destabilization of model silica dispersions by polyelectrolyte complex particles with different charge excess,hydrophobicity, and particle size

Nonstoichiometric polyelectrolyte complex dispersions (PECs) with different charge excess and hydrophobicity as well as different average hydrodynamic particle size were used to induce flocculation of oppositely charged silica dispersions. PECs were formed with poly(diallyldimethylammonium chloride) (PD) as polycation and poly(styrene‐p‐sodium sulfonate) (NaPSS) and poly(acrylamide‐co‐sodium acrylate) (PR2540) as polyanions. PD was used as single polymer flocculant too. Flocculation process was characterized by optical densities of supernatants OD₅₀₀, by UV/VIS spectrometry, and determination of average floc sizes D(v; 0.5) as well as volume distribution of floc sizes by laser diffraction and determination of sedimentation velocity s by means of LUMiFuge? 114. It was found that the reaction process between silica and the used flocculants could be divided into three intervals (destabilization, flocculation optimum, and restabilization) as it is known for all other polymer flocculants. For an effective flocculation of a charged substrate, both electrostatic as well as hydrophobic interactions play an important role. The interval up to the beginning of the flocculation optimum is mainly ascertained by electrostatic interactions (the charge density of the flocculant) but the broadness of flocculation optimum depends largely on hydrophobic interactions. Hydrophobic interactions also play an important role for shear stability of the formed flocs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3776–3784, 2007     

PAC-PDMDAAC复合絮凝剂净化海水的絮凝特性及其絮凝条件优化

以聚合氯化铝（PAC）和聚二甲基二烯丙基氯化铵（PDMDAAC）为原料制备了PAC-PDMDAAC复合絮凝剂,将其用于海水的净化处理。考察了PDMDAAC与Al的不同质量比对絮凝指数FI、形成絮体厚度及体系稳定动力学参数的影响,确定了PAC-PDMDAAC复合絮凝剂的PDMDAAC/Al最适宜配比。采用Box-Behnken响应面法分析了絮凝剂投加量、反应pH值之间的交互作用及其对浊度和COD_Mn去除效果的影响。实验结果表明：与聚合氯化铝（PAC）相比,PAC-PDMDAAC复合絮凝剂净化海水过程中形成絮体更大,沉降性能更好;随着PDMDAAC与Al质量比的增大,絮凝指数FI曲线上升越快速,絮体分层厚度越大,沉降速度越快,稳定动力学参数越大。PAC-PDMDAAC复合絮凝剂的最适宜混凝条件为：反应体系pH值为7.5,絮凝剂投加量为15.3 mg/L,在该条件下复合絮凝剂对海水COD_Mn和浊度的去除率均值分别为65.2%和87.4%,实验结果与响应曲面模型预测值基本相符。     

Stereological estimation of structural change in floc matrix following weak ultrasonication

Ultrasonication is an energy intensive process used to enhance organics’ solubilisation and used in promoting methane yield from wastewater sludge. Applying limited ultrasound without full disruption of floc structure or cell walls can significantly enhance methane yield from sludge at reduced energy cost. This work probed the spatial distribution of extracellular polymeric substances, cells in the original wastewater flocs, and the yielded fragments following weak ultrasonication using a process of multiple staining and a confocal laser scanning microscope technique. The stereological information of the three-dimensional architecture of the original wastewater floc and ultrasonicated fragments was extracted and compared. The local structural changes following weak ultrasonication were presented for the first time, which should also correlate with the enhanced methanogenesis thereby produced.     

Evaluation of flocs resistance and reflocculation capacity using the LDS technique

In a previous paper we have shown the added value of using LDS to monitor flocculation. It can supply, simultaneously, information on flocs size and structure and enlighten flocculation kinetics and mechanisms. In this paper, LDS is applied to study deflocculation and reflocculation processes of precipitated calcium carbonate (PCC) induced by cationic polyacrylamides, when different types of shear forces are applied. LDS can detect the influence of polymer characteristics and concentration as well as of the type of shearing, on flocs resistance and reflocculation degree, which depend on flocs structure and on the type of bonds between particles. As expected, flocs formed by bridging mechanism reflocculate with difficulty while flocs formed by patching reflocculate to a higher degree. Flocs resulting from reflocculation are more compact than the original ones, as assessed by the mass fractal dimension. Reflocculation is also lower when the flocs are submitted to superficial shearing than when they are submitted to sonication. Shearing induced by sonication is sufficient to break down the flocs in many fragments while the increase of pump speed only detaches particles by erosion, at the flocs surface, where bonds are weaker. Results prove that LDS is useful to monitor deflocculation and reflocculation processes and to predict floc resistance under different conditions. Moreover, the whole study demonstrates the benefit of using LDS for a complete evaluation of flocculants performance in the different stages of flocculation: aggregation, stabilization, deflocculation and reflocculation.     

Characterization of floc size, strength and structure under various coagulation mechanisms

Flocs generated by various coagulation mechanisms exhibit different size, strength and structure. The properties and fractal dimensions of flocs formed under three common coagulation mechanisms, i.e. charge neutralization, sweep and bridging, were investigated at various hydraulic conditions. The results showed that the floc size decreased with the increasing average velocity gradient G and the stable floc size exponent γ was of the following hierarchy: charge neutralization (0.6107) > sweep (0.5618) > bridging (0.3674). Furthermore, fractal dimensions of flocs were the highest when formed by sweep and the lowest when generated by bridging flocculation. The mass fractal dimensions measured by light scattering were between 2.0 and 3.0 and the floc strength was between 0.01 and 0.58 N m^− 2. An intrinsic unity of the relationship among floc size, fractal dimensions, floc strength under the three coagulation mechanisms was demonstrated.     

Flocculation,precipitation, sedimentation and flotation for use as pretreatment stages for brackish water and seawater in desalination plants

The increased application of ion exchange and membrane processes for producing drinking water from qualitatively poor sources of raw water necessitates the use of advanced mechanical and chemical technologies as well as combinations of these. Our immediate objectives are as follows: (a) Use of precipitation and flocculation reactions for removing inorganic and organic matter; (b) separation of the suspension by using flotation and sedimentation; (c) application of these processes in compact units to minimize investment costs, and (d) inclusion of sludge treatment in water processing using mechanical techniques of dehydration.     

高效混凝沉淀池的运行管理

介绍了枣庄市污水处理厂高效混凝沉淀池的工艺流程、设计参数,重点分析了进水量、凝聚和絮凝搅拌强度、药剂投加点和投加量、絮凝区污泥浓度和污泥回流量、污泥停留时间和污泥位等参数对处理效果的影响。通过总结生产实践经验,建议应控制进水量均匀稳定,凝聚搅拌强度控制在80~120 r/min,絮凝搅拌强度控制在15~20 r/min,PAM投加量为0.6 mg/L,除磷时Fe Cl3与磷的物质的量比为1.4,絮凝反应区污泥质量浓度控制在120~180 mg/L,沉淀区污泥位控制在1.0~1.8 m。     

基于分形学的絮凝理论研究进展

絮凝过程中形成的絮体具有自相似性和标度不变性等特点,是一个典型的分形体。但传统的絮凝理论并未考虑絮体的分形结构,因而存在一定的缺陷。本文主要介绍了引入分形理论后,絮凝形态学和絮凝动力学的发展状况,重点阐述了絮体分形结构与其粒径、强度、密度、沉降速度等之间的关系,以及分形理论对Smoluchowski方程、传统絮体生长模式理论的改进。此外,本文同时指出分形学与絮凝理论的结合在很多方面尚未总结出系统的普适规律,尤其是非线性絮凝动力学仍需进一步完善。     

A model of the coagulation process with solid particles and flocs in a turbulent flow

A mathematical model was developed on the basis of population balances to predict the floc size distribution in a coagulating suspension. The coagulation process is performed in a stirred tank reactor with a turbulent flow field. In the population model the influence of the different local energy charges inside the reactor is taken into account. Moreover two sorts of particles are distinguished, i.e. the originally present and completely dispersed primary particles and the flocs. Contrary to the primary particles the flocs can be disrupted due to pressure and shear forces as they are mechanically not very stable. This different behaviour requires separate population balances for the two sorts of particles. The model parameters that are necessary are adapted to one single experiment.For the steady state the results represent different floc size distributions dependent on the solid concentration and the energy charge. Moreover it is shown that the assumption of an ideally mixed reactor that is often used cannot be maintained to be always true for the prediction of the resulting floc size distribution. The calculation results achieved are validated by image processing measurements of coagulating quartz particles in an aqueous suspension.     

Effect of coagluant dosage and grain size on the performance of direct filtration

The performance of flocculation in direct depth-filtration was assessed in relation to the shear rate and the floc size in the filter pore. Quantitative measurements of the pressure drop of bed, as well as the turbidity and the floc size in the filtrate, and estimation of the shear rate in the filter during filtration were made to investigate the effects of chemical doses and grain sizes on the performance of direct filtration. Furthermore, a method was suggested to determine the porosity in the deposited layer. In this study, the product was estimated to be two to three times that in the initially clean bed after a 6 h operation of the filter using 50–100 ppm of alum. It was also found that the porosity of the deposited layer is very high up to a range of 0.93–0.99. The porosity decreased with the filtering time and filtration rate, and increased with the alum dosage. However, it was only slightly affected by the grain size used in the study.     

搅拌条件对氢氧化镁混凝性能及絮体特性的影响

通过氯化镁在碱性条件下经搅拌生成氢氧化镁处理活性橙染料模拟配水,研究了搅拌条件对氢氧化镁混凝性能和絮体特性的影响。以絮凝指数FI与zeta电位为考察指标,对絮体形成机理进行了探讨,并对絮体形貌进行观察,旨在揭示絮体形成过程与絮凝效果的关系。结果表明,延长快速搅拌时间有利于絮体形成,以搅拌45s为最佳,再延长则会导致絮体被打碎,脱色处理效果变差;慢速搅拌时间以3min为最佳,过长也会导致絮体破碎。在最佳搅拌时间基础上发现搅拌速度也是影响混凝性能和絮体特性的主要因素。快搅或慢搅搅速过低,则不利于形成絮体或絮体增长;若搅速过高,形成的絮体会被打碎,这些都使得脱色处理效果变差。实验条件下,在Mg²⁺计投加量为150mg/L时,以转速250r/min或速度梯度G值126.3,快速搅拌45s,之后转速60r/min或G值18.5,慢速搅拌3min,氢氧化镁混凝性能最佳,可充分发挥其吸附、电性中和、卷扫与网捕作用,对活性橙染料配水的色度去除率达到95%以上。     

水动力学条件对絮体形成的影响

利用颗粒运动方程,分析颗粒运动导致的碰撞絮凝,提出涡旋惯性离心力和剪切力是颗粒接触絮凝的主要动力因素,其中涡旋剪切力是主导动力;湍流过渡区控制絮体的成长尺度和密度;并讨论了提高絮凝效率的技术途径。     

Definition of a practical multi-criteria procedure for selecting the best coagulant in a chemically assisted primary sedimentation process for the treatment of urban wastewater

Chemically assisted primary sedimentation (CAPS) or chemically enhanced primary treatment (CEPT) consists of adding chemicals in order to increase the coagulation, flocculation and sedimentation of raw wastewater. Over the last twenty years, the use of CAPS has increased due to the development of better coagulants and flocculation enhancers, stricter standards as well as the need for low-energy treatment technologies, especially in developing countries. This paper defines a very simple multi-criteria procedure to be used in order to select the best combination of coagulant and dose when using jar tests. It is based on the adoption of the following 5 criteria: COD percentage removal, sludge volume after 2 h, coagulant dose, coagulant cost, pH percentage variation. The mathematical affordability of the procedure was tested by comparing it with the well known analytic hierarchy process.