Estimate of sludge floc permeability

This paper provides an estimate of the waste-activated sludge floc interior permeability on the basis of observing the motion of individual floc moving vertically towards an impermeable flat plate. The fluid flow fields surrounding and inside the floc were modeled numerically, from which the hydrodynamic drag force was calculated. The experimental data correlate with the numerical solutions regardless of the floc Reynolds number. Over the floc size range investigated in this paper, the permeability is noted to be approximately proportional to the square of floc size, ranging from 2.5×10⁻⁹ to 9×10⁻⁶ m² for floc size ranging from 150 to 10 000 μm. The structure of waste-activated sludge floc is proposed as a multi-fractal. Permeability measured in this work is thereby mainly attributed to the global structure level of the flocs.     

Floc model and intrafloc flow

The three-dimensional pore structures in waste activated sludge floc were identified using the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM) images. The meshes of the three-dimensional porous configuration of a sludge floc were constructed from the CLSM series images. The intrafloc flow field was simulated for the constructed floc model when it was subjected to a uniform flow field, based on which the Darcy's permeability was estimated. The permeability (k_DL) of original floc was estimated as . Flocculated flocs had higher k_DL due to their large pore size, while the corresponding values of k_DL of the freeze/thawed flocs were lower. The calculated results indicate that a few large pores in the floc determine the permeability. The fractal dimension and compactness, however, are not correlated with the permeability of the flocs.     

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.     

Two-dimensional unsteady flow of power-law fluids over a cylinder

The unsteady flow of incompressible power-law fluids over an unconfined circular cylinder in cross-flow arrangement has been studied numerically. The two-dimensional (2-D) field equations have been solved using a finite volume method based solver (FLUENT 6.3). In particular, the effects of the power-law index (0.4?n?1.8) and Reynolds number (40?Re?140) on the detailed kinematics of the flow (streamline, surface pressure and vorticity patterns) and on the macroscopic parameters (drag and lift coefficients, Strouhal number) are presented in detail. The periodic vortex shedding and the evolution of detailed kinematics with time are also presented to provide insights into the nature of flow. The two-dimensional flow transits from steady to unsteady behaviour at a critical value of the Reynolds number Re∼(40-50) and the von-Karman vortex street is observed beyond the critical Reynolds number (Re). Obviously, both the lift coefficient and Strouhal number values are zero for the steady flow, but their values increase with the increasing Reynolds number (Re) in the unsteady flow regime. For highly shear-thickening fluids (n=1.8), the flow becomes unsteady at Re=40 and unsteadiness in the flow appears at Re=50 for all values of power-law index (n). As expected, the evolution of the kinematics and vortex shedding show a complex dependence on the flow parameters near the transition in the flow. For a fixed value of the Reynolds number (Re), the drag coefficient increases and lift coefficient decreases with increasing value of the power-law index (n). For a fixed value of the power-law index (n), the drag coefficient gradually increases with the Reynolds number (Re). Similar to the drag coefficient, lift coefficient also shows a complex dependence on the power-law index (n) near the transition zone. The value of the Strouhal number (St) decreases with the increasing value of the power-law index (n) at a fixed value of the Reynolds number (Re).     

Sedimentation of Permeable Floc

Sedimentation is an essential step to dewatering of wet materials. The present work utilized the computerized axial tomography scanner (CAT scan) to probe the sedimentation behavior of suspension with different initial solid concentrations. Not as assumed by traditional Kynch theory, the solid flux was not only a function of local solid fraction, but also of the settling time and the initial concentrations. Hence, the interior structure of floc was explored by the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM). Subsequently, the three-dimensional floc model was built up and intrafloc flow fields numerically solved. The area-weighted average pressure drop in the floc and mean flow rates estimated the floc permeability, based on Darcy's law, which were noted to vary along different principal axes and for different flocs. Local structure determines local flow field, hence determining the settling behavior of floc. Floc of large pores with low tortuosity produced easy flow-through paths, giving fast settling. Use of the average properties, such as porosity, cannot describe the sedimentation behavior of permeable flocs.     

Sedimentation of Permeable Floc

Sedimentation is an essential step to dewatering of wet materials. The present work utilized the computerized axial tomography scanner (CAT scan) to probe the sedimentation behavior of suspension with different initial solid concentrations. Not as assumed by traditional Kynch theory, the solid flux was not only a function of local solid fraction, but also of the settling time and the initial concentrations. Hence, the interior structure of floc was explored by the fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM). Subsequently, the three-dimensional floc model was built up and intrafloc flow fields numerically solved. The area-weighted average pressure drop in the floc and mean flow rates estimated the floc permeability, based on Darcy's law, which were noted to vary along different principal axes and for different flocs. Local structure determines local flow field, hence determining the settling behavior of floc. Floc of large pores with low tortuosity produced easy flow-through paths, giving fast settling. Use of the average properties, such as porosity, cannot describe the sedimentation behavior of permeable flocs.     

Effects of Fe(III) on floc characteristics of activated sludge

The effects of Fe(III) on floc characteristics of activated sludge were investigated in nine parallel sequencing batch reactors (SBRs). The results showed that Fe(III) improved the quality of organic matters in the effluent of reactors. Concentrations of Fe(III) up to 23.8 mg dm^?3 decreased suspended solids and turbidity in effluent but overdosage resulted in deterioration of these parameters. Activated sludge floc size measurements indicated that Fe(III) led to a shift in the size distribution from large to small flocs. Concentrations of Fe(III) less than 23.8mg dm^?3 did not significantly change the proportion of larger flocs, but overdosage of Fe(III) markedly decreased the fraction of larger flocs and produced a large number of smaller flocs, which may be responsible for the deterioration of effluent suspended solids and turbidity. Scanning electronic microscopic (SEM) observation suggested high Fe(III) concentrations lead to significant changes in floc morphology and reduction of filamentous microorganisms available for the formation of large aggregates. Copyright © 2005 Society of Chemical Industry     

Effects of Recycling-Sludge Operation on the Structure and Moisture Content of Floc in Water Treatment Plant

Abstract

Recycling-sludge operation is applied in the Feng-Yuan Water Treatment Facility to treat low turbidity source water. By employing fractal dimension analysis and the dilatometry technique, the effects of recycling-sludge operation on the structure and moisture content of sludge flocs are studied. As the results show, the recycling-sludge operation can decrease the average bound water content by nearly 13–24% and increase the effective floc density, thereby forming a compact structure with a high fractal dimension and low floc porosity. The results also indicate that a significant correlation arises between the fractal dimensions and the bound water content of the sludge flocs.     

Properties of flocs in sludge conditioning by a novel amphoteric polymer with different anionic degrees

This paper aimed to examine how the amphoteric conditioner poly (acrylamide‐′acryloyloxyethyl trimethyl ammonium chloride ?2‐acrylamido‐2‐methyl‐propane sulfonate) (PADA) with different anionic degrees (AD) affected the properties of sludge flocs in the conditioning. The floc properties were characterized by morphological parameters (floc size distribution, fractal dimension, specific surface area, and pore volume), physical properties (floc strength and surface charge density), and chemical constituents (Fe³⁺, Al³⁺ and extracellular polymeric substances (EPS), including the polymeric proteins and carbohydrates). The results of this investigation revealed that (1) morphological properties of flocs were associated with anionic degree, particularly in the range of 0–4%, where the anionic degree led to a shift of the particle size toward groupings of larger diameter, meanwhile better regularity and increased compactness of floc structure formed. (2) The introduction of the anionic groups indeed had bad effects on flocs in terms of its stability and charge neutralization, but the downtrend could be inhibited by adjusting the anionic degree of polymer to a reasonable level. (3) The dissolved EPS for sludge followed a role of decreasing firstly then increasing with increased anionic degree, but the content of bound EPS kept nearly constant. (4) The polymer with anionic groups had apparent effects on enrichment of metal ions. POLYM. ENG. SCI., 57:197–205, 2017. © 2016 Society of Plastics Engineers     

Reynolds number effects in a simple planetary mixer

Planetary mixers are widely used in a diverse range of industrial applications. This paper presents an experimental investigation of mixing in a planetary mixer, and a comparison with numerical simulations based on a simple mathematical model of the flow. The model allows an exact expression for the velocity field in the Stokes flow regime, apparently the first for a mixer with genuinely moving parts, which permits accurate numerical tracking of material interfaces. Experiments performed at low Reynolds number (Re?1) show good agreement with corresponding numerical simulations, but as the Reynolds number is increased, the agreement between experiments and Stokes-flow numerics worsens, in a manner that reflects improving experimental mixing quality. Specifically, we find that islands of poor mixing shrink as Re increases. Our results suggest that, while numerical simulations in the Stokes flow regime may be used as a ‘sieve’ to select good mixing protocols at small Re, experiments or computational fluid dynamics simulations are required properly to evaluate mixing protocols operated at finite Reynolds numbers.     

具有壁面滑移特性的水煤浆流经局部管件的阻力特性

以水煤浆的流动特性(壁面滑移特性和流变特性)研究为基础,在中试规模的浆体输送装置上研究水煤浆流经90°弯管、突缩管和突扩管的阻力损失。以表征浆体流动特性的广义Reynolds数(Re_g)作为参数,建立了局部阻力系数关联式;考察了弯径比、管径和Re_g对弯管阻力损失的影响,分析了浆体流动特性对突缩管和突扩管阻力损失的影响。结果表明：除突缩管外,采用Re_g作为入口流动状态参数可使流动特性不同的浆体的局部阻力系数试验数据一致;不同弯管的阻力系数随Re_g的增加具有相似的变化趋势,在弯管的流动阻力损失方面不存在动力学相似性。当Re_g<350,突扩管的阻力系数随Reg的增加迅速降低;当Re_g >350,突扩管的阻力系数达到稳定值。在高Reynolds数下,突缩管的阻力系数与浆体的流动特性之间存在显著的相关性,即阻力系数随浓度的增加而增加。     

Predicting turbulence modulations at different Reynolds numbers in dilute-phase turbulent liquid–particle flow simulations

The present work examines the predictive capability of two-fluid CFD model based on the kinetic theory of granular flow in capturing the Reynolds number (Re) dependence of fluid-phase turbulence modulations in dilute-phase turbulent liquid–particle flows. The model predictions are examined using turbulent liquid–particle flow data in a vertical pipe at Re=17,000, 48,000, 65,000, and 76,000 in the particle concentration range of between 0.5% and 4.0% (v/v). The experimental data indicate that the fluid-phase turbulence intensities are enhanced with respect to the single-phase flow at Re≤48,000 but are attenuated at Re≥65,000. The simulation results indicate that the CFD model can successfully predict the turbulence modulations at Re=17,000, 65,000, and 76,000 both qualitatively and quantitatively, but not at the intermediate Re of 48,000. In this regard, (1) different drag correlations to describe the fluctuating drag force are needed to accurately predict the trends in the turbulence modulations as a function of Re, and (2) appropriate combinations of the drag correlations and turbulence closure models to describe the long-range fluid–particle interactions must be identified in each phase at different Re in order to accurately predict the turbulence modulation, granular temperature, and particle radial concentration profile.     

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.     

液体横掠热圆柱体时旋涡脱落的频谱分析

引言 流体(液体)横掠柱体时的工况是许多工业设备特别是换热器工作的主要工况.在这种工况下,流体横掠柱体的旋涡脱落特性、流动参数的分布、柱体受力及流动阻力等均是设备运行和设计的关键问题.而这些问题均源自流体横掠柱体时旋涡的脱落,因此,对流体横掠柱体旋涡脱落特性特别是其频谱特性的研究,对设备的高效节能、安全和稳定运行有重要意义.     

Drag force on a floc in a flow field: two-layer model

The behavior of a floc in a flow field is analyzed theoretically; in particular, the force it experienced is estimated. Here, a floc is simulated by an entity having a two-layer type of structure, and its porous nature mimicked by varying the relative magnitudes of the permeability of its inner layer and that of its outer layer. The results of numerical simulation reveal that, for the same volume-averaged permeability, the drag coefficient of a floc with a heterogeneous structure is always much larger than that of a floc with a homogeneous structure. This is true regardless of the relative magnitudes of the permeability of the inner layer and that of the outer layer. The drag coefficient of a floc is mainly determined by the part having a less porous structure. We show that for the same volume-averaged permeability, the more heterogeneous the structure of a floc is, the easier for the relation between the drag coefficient and the Reynolds number to deviate from a Stokes’ law-like correlation.     

Velocity profiles and frictional pressure drop for shear thinning materials in lid-driven cavities with fully developed axial flow

A finite element numerical study has been carried out on the isothermal flow of power law fluids in lid-driven cavities with axial throughflow. The effects of the tangential flow Reynolds number (Re_U), axial flow Reynolds number (Re_W), cavity aspect ratio and shear thinning property of the fluids on tangential and axial velocity distributions and the frictional pressure drop are studied. Where comparison is possible, very good agreement is found between current numerical results and published asymptotic and numerical results. For shear thinning materials in long thin cavities in the tangential flow dominated flow regime, the numerical results show that the frictional pressure drop lies between two extreme conditions, namely the results for duct flow and analytical results from lubrication theory. For shear thinning materials in a lid-driven cavity, the interaction between the tangential flow and axial flow is very complex because the flow is dependent on the flow Reynolds numbers and the ratio of the average axial velocity and the lid velocity. For both Newtonian and shear thinning fluids, the axial velocity peak is shifted and the frictional pressure drop is increased with increasing tangential flow Reynolds number. The results are highly relevant to industrial devices such as screw extruders and scraped surface heat exchangers.     

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.     

Evolution of asphaltene floc size distribution in organic solvents under shear

A mathematical model was developed on the basis of population balance to analyze experimental data on asphaltene floc size distribution in a coagulating suspension. Experiments were carried out in a Couette device under a laminar flow condition. Floc size distributions were measured on-line using optical microscopy and image analysis. The aggregation behavior of asphaltenes was investigated by monitoring the size distribution of flocs for various intensities of agitation (i.e., shear rate, G), solvent composition (i.e., ratio of toluene to n-heptane in the solution, T:H) and particle contents (i.e., volume fraction of particles, ?). The results showed that (i) the floc size distribution can be predicted using a population balance approach, (ii) a steady-state mean floc size is reached for a given shear rate, and (iii) this steady-state floc size increases as ? is increased or T:H is reduced. The relative rates of shear-induced aggregation and fragmentation determine the steady-state size distribution. Similar floc size distributions were obtained at steady state for various shear rates, indicating that the width of the size distribution is independent of shear. However, the experimental observations indicate that the steady-state floc size distribution depends on asphaltene concentration and solvent composition.     

Effect of polymerization method on structure and properties of cationic polyacrylamide

Acrylamide and 2‐(methacryloyloxy)ethyltrimethylammonium chloride (AM/MADQUAT) copolymers were synthesized by solution and inverse microemulsion polymerization using (NH₄)₂S₂O₈/NaHSO₃ as redox initiator at the same feed mole ratio, and their microstructure, such as sequence distribution and composition distribution, was calculated from monomer reactivity ratios of different polymerization methods. The results show that charge distribution is more uniform for copolymer prepared in inverse microemulsion than that in solution, and copolymer composition distribution is close to unity, and maintains approximately at the feed ratio. Furthermore, the influence of the two structures of cationic polyacrylamides on kaolinite floc size and effective floc density, reduction of Zeta potential and floc compressive yield stress had been investigated at pH 7. The results show that the kaolinite floc size and effective floc density are strongly dependent upon copolymer microstructure, with greater floc size and lower effective floc density being observed for copolymer prepared in inverse microemulsion than for that in solution. Copolymer microstructure has a marked effect on the Zeta potential, whose reduction in the magnitude was much greater in the presence of copolymer prepared in inverse microemulsion than that in solution. Greater compressive yield stress was achieved for the strong flocs produced by copolymer prepared in inverse microemulsion than for the weak flocs produced by that in solution. The difference in flocs compressive yield stress may be attributed to flocs structure. Therefore, in this article, a correlation between the cationic polyacrylamide structure and flocculation property for kaolinite suspension was established. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

A note on wall effect on the terminal falling velocity of a sphere in quiescent Newtonian media in cylindrical tubes

The settling velocity of a sphere in a quiescent fluid is significantly reduced by the presence of confining boundaries. The severity of this phenomenon (wall effect) is strongly dependent on the geometrical and kinematic parameters. In this work, the simplest case of a sphere settling on the axis of a long cylindrical tube is considered. The functional dependence of the wall factor on the sphere-to-tube diameter ratio (λ) and the sphere Reynolds number has been examined. The wall factor is a function of λ only both at very low and at very high values of the sphere Reynolds number (Re). On the other hand, it depends on both λ and Re in the intermediate zone. The values of the Reynolds number marking the transitions from the viscous to intermediate and from the transitional to fully inertial flow regimes have been identified as functions of λ. Using these criteria to delineate experimental data, the relative performance of some of the available expressions for the estimation of wall factor has been evaluated against a large body of experimental results.