Application of fractal dimensions to study the structure of flocs formed in lime softening process

The use of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. Fractal dimensions of flocs were measured directly on floc images and indirectly from their settling velocity. An optical microscope with a motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The directly determined fractal dimensions of the lime softening flocs were 1.11-1.25 for floc boundary, 1.82-1.99 for cross-sectional area and 2.6-2.99 for floc volume. The fractal dimension determined indirectly from the flocs settling rates was 1.87 that was different from the 3D fractal dimension determined directly on floc images. This discrepancy is due to the following incorrect assumptions used for fractal dimensions determined from floc settling rates: linear relationship between square settling velocity and floc size (Stokes’ Law), Euclidean relationship between floc size and volume, constant fractal dimensions and one primary particle size describing entire population of flocs. Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>50 μm) lime softening flocs better than Stokes’ Law. Settling velocities of smaller flocs (<50 μm) could still be quite well predicted by Stokes’ Law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster-cluster aggregation for small flocs (<50 μm) and diffusion-limited aggregation for large flocs (>50 μm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc formation mechanisms.     

Evolution of size distribution and transfer of mineral particles between flocs in activated sludges: an insight into floc exchange dynamics

The aggregation behavior of activated sludge flocs was investigated by monitoring the size distribution of flocs and transfer of mineral particles between flocs, under various conditions of agitation and dilution. The results showed that (i) the shape of the floc size distribution can be fitted with a gamma function, (ii) a steady-state mean floc size is reached for a given stirring rate, (iii) this stable floc size is shifted towards floc growth as sludge concentration is increased, (iv) under cycled-shear conditions, microbial aggregates break up and re-form in an almost reversible manner, (v) blending of raw sludge and sludge spiked with Aquatal mineral particles results in particle exchange between flocs and (vi) the detailed study of exchange kinetics indicates that some flocs do not participate to the aggregation dynamics. These experimental results suggest that the activated sludge floc size is governed by a flocculation/deflocculation balance, implying an exchange of floc constituents between microbial aggregates.     

Ageing of aluminium hydroxide flocs

The ageing processes of kaolinite suspensions flocculated with Al₂(SO₄)₃ under a variety of different but well-controlled flocculation conditions were studied by measuring their average diameter, diameter distribution, strength and electrophoretic mobility.Three periods could be distinguished in the ageing process. Initially shrinkage of the flocs is mainly due to cementation-aggregation. In the first 6–8 h, dependent on the flocculation conditions, the average diameter reduces from 500 to 220 μm. During the following 4 days a shrinkage to 180 μm occurs, mainly due to condensation-polymerization and crystallization. In the third period, the average diameter increases because of Oswald ripening of the crystals.During each period the change of the diameter as a function of age can mathematically be described by: d_f,i = d∞ + d′· t_i^+i−ω. With d_∞, d′ and ω constants dependent on the flocculation conditions.During ageing, the floc diameter distribution narrows. The shrinkage of the flocs during ageing is definitely not due to erosion of particles from the floc surface.The change of the electrophoretic mobility during ageing provides information on the thickness and homogeneity of the layer of insoluble hydroxides around the destabilized clay particles.The influence of a wide range of different flocculation process parameters on the floc ageing process is summarized, as is the influence of the ageing process on floc strength and floc density. The evolution of the effective floc density can be described by: .The constants K_p and a are functions of the ratio quantity of suspended solids vs coagulant dose.     

Stroboscopic determination of settling velocity, size and porosity of activated sludge flocs

A multi-exposure photographic method was developed for the combined measurement of the settling velocity and size of activated sludge flocs. The proposed method mainly differs from the previous stroboscopic tests by introducing a new experimental arrangement which ensures sharp images of flocs on photographs.The relationships between individual floc-settling velocity and the floc size were found to be linear or fractional power functions. All these relationships were well correlated.Based on a simplified floc structure assumption and results of experimental measurements of floc-settling velocity and size, the floc porosity was determined. The porosity increased at two distinct rates as the floc size increased.     

A review of floc strength and breakage

The main focus of the paper is to review current understanding of floc structure and strength. This has been done by reviewing current theoretical understanding of floc growth and breakage and an analysis of different techniques used for measuring floc strength. An overview has also been made of the general trends seen in floc strength analysis. The rate of floc formation is a balance between breakage and aggregation with flocs eventually reaching a steady-state size for a given shear rate. The steady-state floc size for a particular shear rate can, therefore, be a good indicator of floc strength. This has resulted in the development of a range of techniques to measure floc size at different applied shear levels using a combination of one or more of the following tools: light scattering and transmission; microscopy; photography; video and image analysis software. Floc strength may be simply quantified using the initial floc size for a given shear rate and the floc strength factor. More complex techniques have used theoretical modelling to determine whether flocs break by large-scale fragmentation or smaller-scale surface erosion effects, although this interpretation is open to debate. Impeller-based mixing, ultrasound and vibrating columns have all been used to provide a uniform, accurate and controllable dissipation of energy onto a floc suspension to determine floc strength. Other more recent techniques have used sensitive micromanipulators to measure the force required to break or compress individual flocs, although these techniques have been limited to the measurement of only a few hundred flocs. General trends emerge showing that smaller flocs tend to have greater strength than larger flocs, whilst the use of polymer seems to give increased strength to only some types of floc. Finally, a comparison of the strength of different types of floc (activated sludge flocs, organic matter flocs, sweep flocs and charge neutralised flocs) has been made highlighting differences in relative floc strength.     

Stability of particle flocs upon addition of natural organic matter under quiescent conditions

In this research, the influence of two natural organic polymers (polysaccharide and humic acid) on the stability of colloidal aggregates was examined. The primary objective of this research was to determine whether addition of organic matter to floc suspensions results in the fragmentation or stabilization of aggregates. A second objective was to determine how the size of aggregates and the composition of organic matter influence the floc breakup or stabilization process. It was found that the stability of aggregates depended on the type of organic material present as well as floc size. For example, humic acid increased the stability of aggregates more effectively than polysaccharides of larger size. It was also found that the addition of humic acid or polysaccharide generally decreased the rate of coagulation of small aggregates but had less influence on large aggregates. In no case did the addition of polysaccharide or humic acid result in the fragmentation of particle aggregates. The existence of strong interparticle forces within flocs prevented aggregate breakup upon adsorption of natural organic polymers. The results presented here provide important new information regarding the influence of NOM on the behavior of particles in aquatic systems.     

Impacts of structural characteristics on activated sludge floc stability

Activated sludge samples from seven full-scale plants were investigated in order to determine the relationship between floc structure and floc stability. Floc stability was determined by shear sensitivity and floc strength. Floc structure was considered in terms of two size scales, the micro- and macrostructure. The microstructure refers to the organization of the floc components, such as the individual microorganisms. The macrostructure refers to the overall floc. The floc macrostructure was characterized by filament index, sludge volume index, size, and fractal dimension. It had a significant impact on floc stability. Large and open flocs with low fractal dimensions containing large number of filaments were more shear sensitive and had lower floc strength compared to small and dense flocs. Fluorescent in situ hybridization analysis indicated that the organization of the bacterial cells might also have an effect on the floc stability.     

Micro-profiles of activated sludge floc determined using microelectrodes

The microbial activity within activated sludge floc is a key factor in the performance of the activated sludge process. In this study, the microenvironment of activated sludge flocs from two wastewater treatment plants (Mill Creek Wastewater Treatment Plant and Muddy Creek Wastewater Treatment Plant, with aeration tank influent CODs of 60-120 and 15-35 mg/L, respectively) were studied by using microelectrodes. Due to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the surface layer (0.1-0.2mm) of the sludge aggregate at the Mill Creek plant. The presence of an anoxic zone inside the sludge floc under aerobic conditions was confirmed in this study. When the dissolved oxygen (DO) in the bulk liquid was higher than 4.0mg/L, the anoxic zone inside the activated sludge floc disappeared, which is helpful for biodegradation. At the Muddy Creek plant, with its lower wastewater pollutant concentrations, the redox potential and DO inside the sludge aggregates were higher than those at the Mill Creek plant. The contaminant concentration in the bulk wastewater correlates with the oxygen utilization rate, which directly influences the oxygen penetration inside the activated sludge floc, and results in redox potential changes within the floc. The measured microprofiles revealed the continuous decrease of nitrate concentration inside the activated sludge floc, even though significant nitrification was observed in the bulk wastewater. The oxygen consumption and nitrification rate analyses reveal that the increase of ammonia flux under aerobic conditions correlates with nitrification. Due to the metabolic mechanisms of the microorganisms in activated sludge floc, which varies from one treatment plant to another, the oxygen flux inside the sludge floc changes accordingly.     

The impact of pH on floc structure characteristic of polyferric chloride in a low DOC and high alkalinity surface water treatment

The adjustment of pH is an important way to enhance removal efficiency in coagulation units, and in this process, the floc size, strength and structure can be changed, influencing the subsequent solid/liquid separation effect. In this study, an inorganic polymer coagulant, polyferric chloride (PFC) was used in a low dissolved organic carbon (DOC) and high alkalinity surface water treatment. The influence of coagulation pH on removal efficiency, floc growth, strength, re-growth capability and fractal dimension was examined. The optimum dosage was predetermined as 0.150 mmol/L, and excellent particle and organic matter removal appeared in the pH range of 5.50-5.75. The structure characteristics of flocs formed under four pH conditions were investigated through the analysis of floc size, effect of shear and particle scattering properties by a laser scattering instrument. The results indicated that flocs formed at neutral pH condition gave the largest floc size and the highest growth rate. During the coagulation period, the fractal dimension of floc aggregates increased in the first minutes and then decreased and larger flocs generally had smaller fractal dimensions. The floc strength, which was assessed by the relationship of floc diameter and velocity gradient, decreased with the increase of coagulation pH. Flocs formed at pH 4.00 had better recovery capability when exposed to lower shear forces, while flocs formed at neutral and alkaline conditions had better performance under higher shear forces.     

The state of suspended sediment in the freshwater fluvial environment: a method of analysis

Primary particles in rivers are frequently and perhaps characteristically transported as larger flocculated particles. Typically floc size has been determined by destructive, indirect laboratory methods or by laborious microscopic measurements.A unique direct observation digitizing method of floc characteristic analysis was developed and applied to suspended solids from Sixteen-Mile Creek in Southern Ontario. Suspended sediment particles in the creek are strongly flocculated, and the flocs are relatively stable within the realms of the sampling and analytical method.The digitizing method allowed for direct observation of floc structures which is impossible with conventional methods of sediment sizing. The method provides reasonable results with good precision on floc equivalent spherical diameter, surface area, perimeter and circularity. It is limited, however, by its labour intensive nature, possibility of sediment overlap, the individual investigator's criteria and by the photographic and microscopic instruments and techniques used. Other sizing techniques, or the use of the digitizing method by other investigators, may produce different results.     

Breakage and re-growth of flocs: effect of additional doses of coagulant species

Several polyaluminum chloride (PACl) coagulants were prepared, with different OH/Al ratios (B values), and characterized by Ferron assay. These were used in studies of floc formation, breakage and re-growth with kaolin suspensions under controlled shear conditions, using a continuous optical monitoring method. Particular attention was paid to the effect of small additional coagulant dosages, added during the floc breakage period, on the re-growth of broken flocs. The results showed that the re-growth ability was greatly dependent on the nature of the PACl species added as second coagulant. The re-growth ability of broken flocs was greatest when the second coagulant was PACl₀ (i.e. AlCl₃, with B = 0) and least with PACl₂₅ (B = 2.5). In the latter case there was no effect on floc re-growth, irrespective of the initial coagulant used. PACls with intermediate B values gave some improvement in floc re-growth, but less than that with PACl₀. Additional dosage of PACl₀ gave re-grown flocs about the same size or even larger than those before breakage. The re-growth of broken flocs is significantly correlated with the species Al_a (monomeric) and Al_b (polymeric), as determined by Ferron assay. The amorphous hydroxide precipitate formed from PACl₀, (mainly Al_a) can greatly improve the adhesion between broken flocs and give complete re-growth. However, for PACl₂₅, mostly composed of Al_b, the nature of the precipitate is different and there is no effect on floc re-growth.     

Strength of natural soil flocs

To obtain the strength of flocs against breakup is crucial for controlling flocculation in water treatment and predicting transport of colloidal particles in aqueous environments. Recently, the author reported a method to obtain floc strength from a simple experiment of floc breakup subjected to a laminar converging flow. In this study, this method was applied to natural soil flocs. The flocs were formed by coagulation with 0.5 M NaCl (pH 5.4-5.5, pH 6.6) solutions, 0.1M CaCl2 (pH 6.4-6.9) solutions, or acidified distilled water with dilute HCl (pH 5.6). Obtained floc strengths were 0.3, 0.7 and 4 nN for Na-, Ca-, and H-coagulated flocs, respectively. Also, floc strength did not change with floc size. These values of floc strengths were 1-3 orders smaller than those of flocs formed with polymer flocculants and/or precipitated ferric or aluminum coagulants.     

Floc structural characteristics using conventional coagulation for a high doc, low alkalinity surface water source

Removal of natural organic matter (NOM) is well established using metal salt coagulants. In addition, flocculant aids are also commonly used to improve solid removal. The objectives of this paper is to describe the impacts of both NOM and polymer on floc structure. The study offers a comparison of floc physical characteristics for coagulant precipitate flocs, organic-coagulant flocs and organic-coagulant-polymer flocs for optimum coagulant and polymer doses. A ferric sulphate-based coagulant was used as the primary coagulant and the polymer selected was a high molecular weight (MW) cationic polydiallyldimethylammonium chloride (polyDADMAC). Floc size, breakage, re-growth and settling characteristics were measured. Precipitate flocs were larger than organic flocs and had better settling characteristics when compared to NOM-coagulant flocs. When polymer was added, floc size and compaction was seen to further reduce. An explanation was offered in terms of the mode of flocculation involved. Floc breakage behaviour showed that polymer reduced the rate of floc degradation but did not greatly improve floc re-growth potential after breakage, which was generally poor for all of the suspensions.     

Coagulation dynamics of fractal flocs induced by enmeshment and electrostatic patch mechanisms

The size and structure of flocs during floc formation were monitored for various coagulation mechanisms. Two distinctive mechanisms, namely, enmeshment and electrostatic patch, govern the dynamics of kaolin particles coagulation by polyaluminum chloride (PACl). They were investigated by small angle static light scattering (SASLS) and solid-state ²⁷Al NMR. In addition, a novel wet SEM (WSEM) was used in-situ to image the morphology of the aggregate in aqueous solution. Synthetic suspended particles were coagulated by two PACl products, a commercial product (PACl) and one laboratory product (PACl-E). The PACl-E contained more than 60% Al₁₃ while the PACl contained only 7% Al₁₃, with large percentage of colloidal Al. For coagulation by PACl at neutral pH and high dosage where the strong repulsion between particles occurs, the enmeshment ruled by reaction-limited aggregation (RLA) results in larger sweep flocs as well as higher fractal dimensional structure. For coagulation by PACl-E at alkaline pH and low dosage, the flocs were coagulated predominately by electrostatic patch with Al₁₃ aggregates. At such condition, it is likely that diffusion-limited aggregation (DLA) predominately rule PACl-E coagulation. The fractal dimension (D_s) values of PACl and PACl-E flocs formed at enmeshment and electrostatic patch increased with dosage, respectively. When breakage of flocs occurs, the breakage rate of PACl-E flocs is slower than that of sweep flocs. By WSEM imaging, the adsorption of spherical Al precipitates onto the particles was observed to form sweep flocs with a rough and ragged contour, while the PACl-E flocs were formed with a smooth and glossy structure.     

吸附架桥机理主导下APAM的多级絮凝效能

对吸附架桥机理主导下阴离子聚丙烯酰胺(APAM)的絮凝过程进行了研究,通过改变絮凝剂投加工况,对比分析常规絮凝与多级絮凝在污染物去除效果、絮体性能、絮体生长动力学与污泥调理能耗等方面的差异。结果表明,相同投药量下,两级絮凝的出水浊度低于三级絮凝和常规絮凝,两级絮凝在最少的APAM投加量(2 mg/L)下达到最低的出水浊度(19.53 NTU);与常规絮凝相比,两级絮凝的絮体成长速率、平均粒径和沉降速率分别增加12.67%、30μm、36.74%。两级絮凝在投加间隔为240 s、投配比为1∶1条件下絮凝效能最优,出水浊度为15.34 NTU,絮体沉降速率为1.1 NTU/s,絮体密度达到1.123 4 g/cm³。絮体破碎再絮凝过程中,两级絮凝与常规絮凝破碎后均能恢复至破碎前水平,但破碎后均出现不可逆的絮体结构破损,粒径在0~100μm的絮体颗粒增多,粒径>400μm的絮体减少,破碎后两级絮凝的絮体强度因子(68.15%)高于常规絮凝(41.63%),两级絮凝的絮体强度和抗破碎剪切能力更高。在剩余污泥调理方面,两级絮凝产生的污泥只需要投加40mg/L的APAM就可以达到最低的滤饼含水率(75.5%)。因此,两级絮凝可以显著提升除浊效能与絮体性能,是强化絮凝的发展方向。     

Physical and chemical properties of activated sludge floc

Physical and chemical characteristics of activated sludge such as floc size, density, specific surface, carbohydrate content, dehydrogenase activity and settleability were investigated by seven parallel bench scale activated sludge units operated under different sludge ages (1.1–17.4 days). The analytical methods used included a dye adsorption technique for specific surface area determinations, the Coulter Counter method for floc size measurements and interference microscopy for floc density determinations. The typical floc sizes were found to be in the range 10–70 μm with floc densities in the range 1.015–1.034 g cm⁻³. A strong correlation between floc density and size was obtained. The specific surface areas measured (typically 100–200 m² g⁻¹ dry sludge) were found to be one to two orders of magnitude higher than the corresponding geometric floc surface areas, indicating a porous floc structure. Sludge settleability, for non-filamentous sludges, was well correlated to both floc size, density and specific surface area, but not to the sludge carbohydrate content, which was found to vary between 6 and 18%.     

Effect of shear on concentrated hydrous ferric floc rheology

The primary interest of this work is to determine the effect of continuous shear, variable shear rate, and shear history on concentrated hydrous ferric floc rheology. The floc suspensions are strongly shear degrading with the behaviour being adequately described by a power law model. This is supported by images of the floc particles, which indicate a breakdown in floc structure, with the release of water and formation of smaller particles, after the application of shear. A shear rate sweep of the sample produces a hysteresis loop due to the time dependency of the samples. Higher shear rates are shown to destroy more structure, resulting in an overall decrease in floc viscosity. This is irreversible, and occurs due to a breakdown in floc structure to release trapped water and produce smaller floc particles. Shear also causes an instantaneous reduction in viscosity. This effect is instantaneously recoverable, and is due to a decrease in floc particle aggregation. Floc behaviour within the shear rate range 20-250 s(-1) is also adequately modelled by the Sisko model.     

混凝剂加注量的自动控制新方法

报导了净水过程中使用计算机实时采集和定量分析絮凝池絮体图像,算出絮体沉淀“等效直径”作为控制混凝剂加注率的目标值,并用进水流量和沉淀水浊度信号自动修正目标设定植,加注量为进水流量与加注率之积。实验数据表明,该方法能适应水质和其他因素的变化自动调整加注量,达到了稳定沉淀水浊度的目的。     

混凝剂中的铝形态对藻类混凝过程的影响

为了考察混凝剂中的铝形态对藻类混凝过程的影响,使用3种具有不同铝形态分布的混凝剂对含藻水进行了混凝试验。结果表明,硫酸铝由于具有较低含量的Alb,电中和能力较差,故需要较大的投量才能去除藻类,形成絮体;含藻水体系中的有机物主要是腐殖酸及富里酸类物质,微生物代谢产物(SMP)在硫酸铝作混凝剂时得到较好的去除,而腐殖酸及富里酸的去除率较低可能是造成硫酸铝混凝效果较差的原因;Alc(Al(30))在混凝中的作用机理主要是吸附架桥作用,可有效去除水体中的有机物,Al₁₃的主要作用机理是电中和作用,可以有效去除水体中的颗粒物;Al₁₃与Al₃₀由于具有形态的稳定性,其混凝过程受pH值的影响较小。絮体强度因子随着pH值的升高先增大后减小,Al₁₃作混凝剂时絮体恢复因子随pH值的升高先增大后减小,而其他两种混凝剂所形成絮体的恢复因子随pH值的升高而增大。     

The shear sensitivity of activated sludge: an evaluation of the possibility for a standardised floc strength test

The presence of micron-sized primary particles is critical in many solid/liquid separation processes, and characterisation of stability with respect to the degree of dispersion is therefore relevant. Characterisation of floc strength so far has mainly dealt with the size of flocs and not addressed quantification with respect to primary particles. In the present work floc strength was quantified in terms of the shear sensitivity (kss) as a standardised parameter based on the recent adhesion-erosion model (AE-model). The shear sensitivity quantifies the degree of dispersion for low total solids contents and intermediate turbulent shear rates. It reflects the affinity of adhesion/erosion of primary particles to and from flocs and thus provides a means of characterising the interaction energy between the sludge colloids.