Impact of Aggregate Size and Structure on Biosolids Settleability

Settling is an important solid-liquid separation process in wastewater treatment. While solids concentration is an important factor influencing settling velocity of particles, other solids characteristics including particle size, shape, and structure also play an important role in settling rate. The “compactness” of bacterial aggregates in particular is recognized to exert a great influence on solid phase dynamic behavior since it has a substantial effect on fluid flow through the aggregate, which, in turn, affects the particles buoyancy. With the recognition that biosolids structure can be described by fractal methods, we now have a convenient means of parameterizing aggregate “compactness.” In this article, we examine the fractal nature of bacterial aggregates (which are the main component of the solids in wastewater treatment processes) using small angle light-scattering methods and assess the impact of aggregate compactness (as described by fractal dimension) on settling velocity of both single aggregates and large aggregate clusters. The results indicate that settling velocity is strongly dependent on both size and aggregate structure, with the larger and less compact flocs settling more quickly as a result of the significant extent of flow through the bacterial assemblages.     

Impact of Aggregate Size and Structure on Biosolids Settleability

Settling is an important solid–liquid separation process in wastewater treatment. While solids concentration is an important factor influencing settling velocity of particles, other solids characteristics including particle size, shape, and structure also play an important role in settling rate. The “compactness” of bacterial aggregates in particular is recognized to exert a great influence on solid phase dynamic behavior since it has a substantial effect on fluid flow through the aggregate, which, in turn, affects the particles buoyancy. With the recognition that biosolids structure can be described by fractal methods, we now have a convenient means of parameterizing aggregate “compactness.” In this article, we examine the fractal nature of bacterial aggregates (which are the main component of the solids in wastewater treatment processes) using small angle light-scattering methods and assess the impact of aggregate compactness (as described by fractal dimension) on settling velocity of both single aggregates and large aggregate clusters. The results indicate that settling velocity is strongly dependent on both size and aggregate structure, with the larger and less compact flocs settling more quickly as a result of the significant extent of flow through the bacterial assemblages.     

CFD approach for simulation of bitumen froth settling process - Part I: Hindered settling of aggregates

Bitumen or heavy oil aggregates are formed when bitumen emulsions, consisting of emulsified water droplets dispersed solids and precipitated asphaltenes, are treated with aliphatic solvents. While settling, the aggregates exhibit zone settling mode with the development of a sharp oil and settling zone interface. Previous research [Long Y, Dabros T, Hamza H. Structure of water/solids/asphaltenes aggregates and effect of mixing temperature on settling rate in solvent-diluted bitumen. Fuel 2004;83:823-32] provides settling experimental data for bitumen aggregate settling and a Richardson-Zaki approximation was proposed by the authors with modified exponents for simulating the settling behavior of aggregates. However, the need for modified exponents and their dependence on aggregates and solvents used was not explained. Since the aggregates exhibit hindered settling, where the settling rate is different from that of individual particles or aggregates, numerous settling models have been proposed to correlate particle swarms to single particle via drag correlations [Richardson JF, Zaki WN. Sedimentation and fluidization: Part I. Trans Inst Chem Eng 1954;32:35-53; John G, Maan RA. Velocity-voidage relationships for fluidization and sedimentation in solid-liquid systems. Ind Eng Chem Proc Des Dev 1977;16:206-14]. The compaction zone, due to high concentrations of solids and the effect of their resultant weight during settling is also often ignored. In this work, the hindered settling behavior of bitumen aggregates is first studied in a CFD framework using two models: (a) modified Richardson-Zaki approximation by Long et al. [Richardson JF, Zaki WN. Sedimentation and fluidization: Part I. Trans Inst Chem Eng 1954;32:35-53] and (b) the Syamlal-O’Brien model [John G, Maan RA. Velocity-voidage relationships for fluidization and sedimentation in solid-liquid systems. Ind Eng Chem Proc Des Dev 1977;16:206-14]. To address the limitations of the two models, a new model is proposed that incorporates the irregular (fractal) structure of aggregates by considering the aggregates as porous liquid-filled solids that are fractal in nature. Results, from the new fractal model, are found to be in good agreement with empirical data.     

Aggregates Settling Velocity‐Direct In Situ Measurement

Abstract

Settling velocity and size of aggregates were observed by the method where the aggregates were illuminated by light sheet (plane) and their movement and size were evaluated from image analyses of obtained images. The main aim was to measure settling velocities of aggregates in situ. The used model system was an agitated vessel where the aggregation process took place. Such determined settling velocity could advantageously serve as a complex parameter for scale up of separation equipment.     

改性超细氢氧化镁上浮式固液相 分离器设计的基础理论

改变氢氧化镁表面特性和提高氢氧化镁胶体状浆料的固液分离性能,是液相化学法产业化制备高纯超细改性氢氧化镁工艺的关键技术。为了设计超细氢氧化镁湿式改性工艺中的关键设备--固液相分离器,假设了被改性超细氢氧化镁的存在形式,分析了改性集合体的沉浮条件,在建立上浮模型的基础上,提出了改性超细氢氧化镁上浮式固液相分离器设计的基础理论。     

Effect of Volume Fraction on Transient Structural Behavior of Aerosol Particles Using Off-Lattice Kinetic Monte Carlo Simulation

Fractal-like aggregates exhibit interesting properties that determine their physicochemical advantages, and thus, the control and prediction of aggregation is critical for many applications. An off-lattice kinetic Monte Carlo (KMC) simulation was performed to investigate the aggregate evolution from primary particles to three-dimensional fractal aggregates, at three different volume fractions. We have found that at low volume fraction, aggregation kinetics is slow, and aggregate morphology is widely open and stringy, with fractal dimension of (D_f) 1.8, in which the system is constantly preserved in the dilute regime. In denser volume fractions, however, the aggregation kinetics appears to be accelerated and aggregate morphology is more compact and less stringy due to the transition from dilute to dense regimes. Moreover, the volume fractions determine what kind of coagulation mechanism may occur to produce aggregates with different morphologies. At low volume fraction, coagulation is predominated by coagulation between aggregates in which the maximum probability of interpenetration event is only 18%. This suggests that aggregates at low volume fraction can maintain their self-similarity behavior. While at high volume fraction, coagulation is predominated by two subsequent coagulation mechanisms, namely, primary particle–aggregate and aggregate–aggregate interaction. The probability of interpenetration event increases up to 40%. In addition, the interpenetration process as well as the primary particle–aggregate coagulation, particularly in the dense regime, could produce superaggregates with a hybrid structure with a high fractal dimension at large size scales and a low fractal dimension at small scales. A detail mechanism for the formation of superaggregates was discussed.

Copyright © 2015 American Association for Aerosol Research     

淘洗六水氯化铝过程中晶体沉降速度与纯化规律分析

针对粉煤灰酸法制氧化铝工艺中六水氯化铝结晶纯度难以控制、需进一步纯化的问题建立液固淘洗过程模拟实验,考察了液相密度、黏度、颗粒粒度及体积分数对颗粒沉降速度的影响,并用液体对颗粒的抵抗阻力对其修正,建立了关于沉降速度的计算模型,相对误差小于10%,能够较好地预测该体系下颗粒的沉降速度。按照淘洗过程模拟实验条件及对应的颗粒沉降速度设计静态实验,考察了钠、钙、镁和钾4种杂质离子随淘洗时间的变化规律,得到较优淘洗时间为60s。实验结果表明,杂质离子脱除率依次为Na⁺>Ca²⁺>Mg²⁺>K⁺,且最高脱除率均可达80%以上,杂质离子脱除率随着液固比的增大而提高,与沉降速度计算模型趋势一致,研究结果可为淘洗工艺的设计计算和优化提供参考。     

铁尾矿干排应用倾斜板浓缩机的研制

重力浓缩是固液分离的第一道工序,为满足铁尾矿干排处理工艺中含有大量微细颗粒且浓度较低的矿浆对浓缩技术的要求,基于"浅层沉降"原理对浓缩机的结构进行改进,利用一组表面光滑的特制倾斜板面,改变固体颗粒的沉降路径,缩短沉降距离,减少沉降时间,提高沉降速度,改善液流条件,强化浓缩机工作效率,增加浓缩机自然沉淀面积及单位面积生产能力,提出一种新型高效倾斜板浓缩机。     

Monte Carlo simulation of the diffusion-limited aggregating process of particle suspension systems

The aggregating process of particle suspension systems is a very universal phenomena and crucial for various processes both in nature and in industry. In this paper, the aggregating process was simulated with offlattice diffusion-limited cluster-cluster aggregation (DLCA) Monte Carlo programs. The self-similar fractal structures of aggregates have been clearly demonstrated by the statistical analysis of gyration radius distribution and the existence of a scaling distribution of the reduced cluster size. The fractal dimension determined from the relationship between mass and gyration radius of aggregates was 1.80 or so. The fractal dimension of the aggregates drawn from the radial distribution function and structure factor of a single aggregate is about 1.90–2.10. It was also showed that, along with the increasing of particle volume fraction, the fractal dimension will increase in a nearly square root manner, and the spatial range of the fractal structure appearing becomes narrower. Also, the gelation transition can only occur in a particle suspension system where the particle volume fraction is greater than a critical value.     

Monte Carlo simulation of the diffusion-limited aggregating process of particle suspension systems

The aggregating process of particle suspension systems is a very universal phenomena and crucial for various processes both in nature and in industry. In this paper, the aggregating process was simulated with off-lattice diffusion-limited cluster-cluster aggregation (DLCA) Monte Carlo programs. The self-similar fractal structures of aggregates have been clearly demonstrated by the statistical analysis of gyration radius distribution and the existence of a scaling distribution of the reduced cluster size. The fractal dimension determined from the relationship between mass and gyration radius of aggregates was 1.80 or so. The fractal dimension of the aggregates drawn from the radial distribution function and structure factor of a single aggregate is about 1.90–2.10. It was also showed that, along with the increasing of particle volume fraction, the fractal dimension will increase in a nearly square root manner, and the spatial range of the fractal structure appearing becomes narrower. Also, the gelation transition can only occur in a particle suspension system where the particle volume fraction is greater than a critical value.     

电石渣的处理及回收利用

在采用电石-乙炔工艺生产PVC树脂的过程中，如何处理所产生的大量电石渣是企业迫在眉睫的问题。文中简单介绍了电石渣浆的前期处理方法，包括自然沉降法和机械分离法；扼要阐述了电石渣浆的加工回用方法：废水可经过二次处理循环用于电石反应生产乙炔及中和酸性废水，而干电石渣可制成石灰作为电石的生产原料、与煤渣等煅烧生产电石渣水泥或用作普通建筑材料及防水涂料。     

Simulation of solid-liquid flows using a two-way coupled smoothed particle hydrodynamics-discrete element method model

We introduce a coupled smoothed particle hydrodynamics-discrete element method (SPH-DEM) to describe the two-way interaction between the two phases of a solid-liquid flow. To validate the model, we simulated two test problems: a solid-liquid counter-flow in a periodic box and particle settlement. The simulations correctly predicted the dynamics, and the results showed good agreement with the theory. The developed model was then applied to simulate the slurry coagulation process to examine the coagulation efficiency. When the rotational speed exceeded the normal range, the coagulation rate decreased with time, even though the rate was high during the early stage due to the size separation effect of the particles. Given this result, overly fast stirring appears to have an adverse effect on the coagulation efficiency. The model is applicable to the design of various types of solid-liquid flows.     

Hydrodynamics in a gravity settling vessel: CFD modelling with LDA validation

Vertical gravity settling vessels, usually referred to as primary separation vessels (PSV), are used in separating bitumen aggregates from slurry containing sand and fine clays. The hydrodynamics in the PSV influences the separation efficiency of recovered bitumen through the overall mean flow and turbulent interaction. In order to deepen our understanding of the hydrodynamic conditions in such vessels, this paper presents a combined study of the flow field using Laser Doppler Anemometry (LDA) to measure the velocity field, and computational fluid dynamics (CFD) simulations to validate the CFD model. The investigation shows that the flow geometry has a significant influence on the overall flow pattern in such vessels. It also demonstrates that the CFD simulation is a reliable tool in capturing the complex mean flow pattern observed in experiments. Use of different turbulent models such as the standard k‐ε model and Reynolds stress model has very little effect on the mean flow field.     

On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis

Electrical mobility analyzers are usually calibrated for spherical particles, and provide number, area and volume distributions for spherical particles. However, these instruments cannot be directly used to obtain the surface area and volume distributions for aggregates. Aggregates are important in technological applications, such as the manufacture of fine powdered materials, and in air pollution and atmospheric sciences. Thus, nanoparticle chain aggregates of low fractal dimension are another important limiting case, in addition to spheres; a method is described which makes it possible to relate aggregate surface area and volume distributions to the electrical mobility diameter. This is accomplished by equating the migration velocity of an aggregate to that of a sphere. Particles of equal migration velocities will trace similar paths in the mobility analyzer and have the same mobility diameter (neglecting the Brownian diffusive spread). By equating the migration velocities of a sphere and aggregate, the number and size of the primary particles composing the aggregate can be related to the diameter of a sphere with the same migration velocity.The calculation of aggregate surface areas and volumes requires two theoretical “modules”, one for the drag on the aggregates and the other for aggregate charging efficiency. Two modules selected from the literature were used. The results indicate that the surface area distributions of aggregates with random orientation are somewhat over-predicted when calculated directly from the mobility diameter. However, the volume distributions are greatly over-predicted, up to a factor of ten compared with values based on the mobility diameter. The affect of aggregate orientation on surface area estimates was also examined.     

A RECIPE FOR IMAGE CHARACTERIZATION OF FRACTAL-LIKE AGGREGATES

In the present paper a simple and straightforward recipe for characterizing the structural and fractal properties of aggregates from their projected images is presented. Starting from geometrical properties that are directly measured from the projected image—such as primary particle mean diameter, maximum projected length, projected area, and overlap coefficient—important threedimensional properties including number of primary particles in an aggregate, radius of gyration, aggregate surface, or fractal dimensions, D_f and k_g, can be inferred. Expressions proposed in the recipe to relate three dimensional with projected properties were obtained from an extensive investigation of the structure of numerically simulated cluster–cluster fractal-like aggregates. This involved the simulation of statistically significant populations of aggregates having appropriate fractal properties and prescribed numbers of primary particles per aggregate in order to characterize three-dimensional morphological properties of aggregates. Specific ranges of aggregate properties considered were as follows: number of primary particles per aggregate up to 512, fractal dimension, D_f≈1.78, overlap coefficient in the range 0–0.33 and fractal pre factor between 1.5 and 3.1.     

高分子絮凝剂对泥沙絮体分形结构的影响

黄河泥沙架桥絮凝沉降实验与电镜实验研究表明,絮体结构具有分形特性。文章讨论了高分子絮凝剂的架桥絮凝机理以及高分子所带电荷种类、高分子的分子量、投加剂量与浓度对絮体分形结构的影响,并用表征絮体结构分形特性的参数“分维D”定量分析了这种影响规律。同时,对比了相同含沙量下无机混凝剂絮体与高分子架桥絮体结构达最佳时的分形特征及所需的混凝控制指标Gt值。     

混床树脂分离效果预测的探讨

根据Ｓｔｏｃｋｓ模型,对混床用阳阴树脂沉降速度、沉降比进行计算和分析。结果表明,符合标准的阳、阴树脂沉降速度、沉降比有很大差别,粒度、密度不同,分离效果就不同,选择和评价混床树脂时,应对阳、阴树脂粒度分布、沉降速度、沉降比进行综合考察,以确定其分离效果。     

Evolution of aggregate size and fractal dimension during Brownian coagulation

Fractal aggregate coagulation is described within a general framework of multivariate population dynamics. The effect of aggregate morphology on the coagulation rate, is taken into account explicitly, introducing in addition to aggregate particle size, the aggregate fractal dimension, as a second independent variable. A simple constitutive law is derived for determining the fractal dimension of an aggregate, resulting from a coagulation event between aggregates with different fractal dimensions. An efficient Monte Carlo method was implemented to solve the resulting bivariate Brownian coagulation equation, in the limits of continuum and free molecular flow regimes. The results indicate that as the population mean fractal dimension goes from its initial value towards its asymptotic value, the distribution of fractal dimension remains narrow for both flow regimes. The evolution of the mean aggregate size in the continuum regime is found to be nearly independent of aggregate morphology. In the free molecular regime however, the effects of aggregate morphology, as embodied in its fractal dimension, become more important. In this case the evolution of the aggregate size distribution cannot be described by the traditional approach, that employs a constant fractal dimension.     

Numerical simulation of aggregate dispersion in different flow fields using discrete element method

An attempt was made to study the aggregate dispersion process in three different flow fields namely; steady shear, elongation flow, and combined shear and elongational flows using the discrete element method. The simulation was performed on two aggregate structures characterized by their fractal dimensions. The predicted results showed that the aggregate break‐up process evaluated in terms of weighted average fragment size 〈w〉 follows a power–law type relation as 〈w〉 = kt^?m in all the three flow fields. The dispersion performance of different flow fields evaluated by dispersing rate and a final steady‐state fragment size was found to be dependent upon the extent of applied stress and flow fields such that at low applied stress levels much smaller steady state values of 〈w〉 could be obtained for the elongational flow. The aggregate structure, characterized by its fractal dimension, was found to have different effects on the aggregate dispersion process depending on the flow field and applied stress level. The results predicted from this simulation could be explained in terms of ability of flow fields in rotating the aggregates and fragments in appropriate position to be broken up and the fractal dimensions of aggregates. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

铁矿烧结烟气半干法脱硫灰除杂及水热非均相低温快速氧化

铁矿烧结半干法烟气脱硫灰因含有大量性质不稳定的亚硫酸钙,无法得到有效利用,脱硫灰的氧化改性是实现其大规模或高值化利用的重要手段。脱硫灰中的碳酸钙是限制其氧化改性的主要因素,碳酸钙与亚硫酸钙两者相互包覆,降低了亚硫酸钙氧化反应的比表面积,限制其氧化速度。利用L(+)-抗坏血酸和冰醋酸两种弱酸作为对碳酸钙的脱除剂,其中浓度为0.4 g/g的冰醋酸可将脱硫中亚硫酸钙含量最高提升至81.17%,最大增幅达69.71%。采用亚临界水热非均相氧化法对不同因素对脱硫灰中亚硫酸钙氧化率的影响进行了研究。结果表明除杂后亚硫酸钙的氧化速率明显增加。在温度140℃、时间30 min、初始压力2 MPa、初始固液比1:30、转速300 r/min条件下,亚硫酸钙氧化率达98.72%,实现了脱硫灰低温快速氧化,产物初步具备制备大长径比硫酸钙晶须生长基础条件。