Micromechanics of wastewater sludge floc: force-deformation relationship at cyclic freezing and thawing

This study examined shape changes in two typical wastewater flocs subjected to cyclic freezing and thawing and the associated force exerted by the ice front. While freezing, the engulfing ice front gradually pulled the floc apart. Subsequent thawing only partially restored the floc's shape. By the Maxwell model, used to interpret gross shape deformations, both flocs were visco-elastic objects exhibiting time-varying rheological characteristics which were more viscous than elastic. Detailed observations of floc 1 deformation demonstrated a two-stage force-displacement relationship. Following 1 cycle of freezing and thawing, the interior structure of the floc deteriorated and the force required to elongating a unit length of floc decreased by 60%. The original floc 2 had a dense "core" and loose "tail"; the core was more resistant to deformation under normal stress than the loose tail. Although both flocs had similar shapes and sizes and were acquired from the same activated sludge stream at a wastewater treatment plant, their rheological behaviors differed substantially. A comprehensive theoretical model for freezing and thawing processes should incorporate these rheological characteristics as they corresponded to observed structural changes and reduction in bound water content in sludge following a cyclic treatment of freezing and thawing.     

Advective flow and floc permeability

This work monitored advection flow through a floc by bubble tracking. Close examination of the motion of a swarm of hydrogen bubbles that passed over a free-falling floc allowed the extent of advection flow to be estimated at 53% for the original activated sludge floc, and 12% for the flocculated floc. The interior permeability of the sludge flocs was estimated from this information. The fluid force exerted on the falling floc was also considered.     

Force of a gas bubble on a foreign particle in front of a freezing interface

We monitored the formation and development of a single gas bubble on the surface of a spherical particle of size 1.676 mm under unidirectional freezing and thawing (4.6-5.0 microm/s) and for the first time quantitatively estimated the force exerted on this particle by measuring the deformation of an attached elastic stick. The bubble would nucleate and grow on the particle surface closest to the ice front, while the force curve for a freezing-thawing cycle presented a hysteresis characteristic. This force was much greater than in the case without a bubble, and hence it dominated the engulfment process in the present freezing tests. The bubble force increased with increasing bubble size and was shown to be mainly attributable to the elastic force by the deformed bubble shape. Comments were made on the need to incorporate the role of bubbles in predicting the critical velocity to freeze a suspension with high dissolved gas content.     

Hydrodynamic drag force exerted on activated sludge floc at intermediate Reynolds number

We hung the activated sludge flocs on an elastic nylon stick and then subjected it to a uniform water flow and measured its displacement. The hydrodynamic drag force exerted on the floc was subsequently estimated, both for cationic flocculated flocs and for flocculated and then frozen/thawed flocs. A confocal laser scanning microscope (CLSM) was employed to probe the interior structure of flocs. Polyelectrolyte flocculation leads to a compact global structure, and hence high drag force exerted on the floc by water. The corresponding C(D)Omega value at Re=12-27 for flocs ranges from 1.58 to 3.61. Fast freezing would little affect the hydrodynamic drag force. Slow freezing, in contrast, considerably consolidated the floc structure and hence presented impermeable sphere-like behavior of the slowly frozen/thawed flocs.     

Advective flow in spherical floc

Numerous structural models of flocs, such as homogeneous model or radially-varying model, were proposed in literature for predicting the extent of advective flow on the intrafloc transport processes. This work probed the three-dimensional structure of original and chemically flocculated wastewater flocs using the fluorescence in situ hybridization (FISH) and the confocal laser scanning microscope (CLSM) techniques, from which the spherical mesh model on real floc structure was constructed. Simulation results revealed that if an average characteristic of sludge floc, such as porosity or drag force correction factor of sludge floc is of concern, both homogeneous or radially-varying models may be able to apply, particularly for those flocs that were closely compacted. However, the detailed flow patterns inside real floc are much more tortuous than those of the homogeneous or radially-varying models. If local hydrodynamic environment within the floc is of interest, then only the complicated structural model with real floc could be applicable.     

Advective flow in a sludge floc

The interior of sludge floc is highly heterogeneous, while the large pores in the floc control the advective flow. This work for the first time numerically details fluid flow and mass transfer processes in pores of activated sludge floc. The dimensionless permeabilities and mass dispersion coefficients were contoured against pore size ratio and the floc Reynolds number. With a pore size less than 20% of the floc size, the commonly adopted homogeneous model overestimates the floc permeability, and pore velocity is less than 2% of the bulk velocity. This is particularly true for flocs with low porosity. Although the convective flux is low, the dispersive mass transfer rate can be much higher than the diffusional rate, attributable to the strong Taylor dispersion effect. The three-dimensional pore structures in waste activated-sludge floc were identified using confocal laser scanning microscope (CLSM) images. Large pores were used to numerically estimate the permeability and dispersion coefficient for these pores. The permeability and the dispersion coefficient of the tortuous pores can be one order of magnitude lower than those for the equivalent straight pores. Besides the dispersion effect, the pore tortuosity appeared as the most important geometrical factor retarding the advective flow in the sludge pores. In addition, the small side pores connected to the large pore had only a mild effect on the flow process, and can be neglected in analysis.     

Morphology of optically anisotropic agarose hydrogel prepared by directional freezing

Agarose hydrogels which showed optical anisotropy were obtained by the directional freezing of starting isotropic gels under a temperature gradient. The directional freezing caused a crystallization of many isolated ice crystal phases, leaving a honeycomb-like gel phase with a higher polymer content. The crystallographic c-axis of the ice crystals was directed to the temperature gradient. X-ray and optical analyses showed that agarose chains had a strong planar orientation along the walls'side surfaces, which were parallel to the equatorial planes of the ice crystals.Scanning electron microscopy showed that the wall consisted of a large number of sheets stacked along the wall thickness; in each sheet, agarose fibrillar structures were found to be densely aligned. With the application of repeated freezing and thawing, the anisotropy of the segregated gel phases increased.     

Drag force on a porous, non-homogeneous spheroidal floc in a uniform flow field

The force acting on a porous spheroidal floc having a nonhomogeneous structure in a uniform flow field is evaluated theoretically. Here, the floc is simulated by an entity having a two-layer type of structure, and its porous nature is mimicked by varying the relative magnitudes of the permeabilities of its inner and outer layers. The results of numerical simulation reveal that, for the same volume-averaged permeability, the drag coefficient of a spheroidal floc with a nonhomogeneous structure is much larger than that of a floc with a homogeneous structure for both prolate and oblate spheroids. This is true regardless of the relative magnitudes of the permeability of the inner layer and that of the outer layer. While the drag coefficient of a homogeneous prolate is the same as that of a homogeneous oblate the drag coefficient of a nonhomogeneous prolate is larger than that of a nonhomogeneous oblate. For the same volume-averaged size, the more nonhomogeneous the structure of a spheroidal floc the easier for the relation between the drag coefficient and the Reynolds number to deviate from a Stokes-law-like relation. For a fixed volume-averaged permeability, the effective drag coefficient increases with the increase in the ratio (polar radius of inner layer/polar radius of floc), regardless of whether its inner layer is less permeable than its outer layer or not.     

Deformation of Individual Aggregates and Flocs

Abstract

The production of flocs or aggregates is important to many solid–liquid separation processes; well‐known examples are found in water treatment and mineral processing. When sedimentation is used to remove these aggregates from suspension, a voluminous sludge results. Reduction of the liquid content of such sludges may be key to process economics, either through improved product quality or decreased disposal charges. Although consolidation of sludge blankets has been studied extensively, the deformation of individual aggregates and flocs has not. Indeed, little quantitative information is available regarding the force required to alter the physical conformation of an aggregate or floc. In this study, a technique has been developed to measure compressive force and aggregate volume simultaneously during deformation. These data show that different compression strategies can significantly affect the work requirement per unit volume of interstitial fluid expelled. The technique reported here makes it possible to quantify the force and work requirements for floc deformation, and also to identify changes in the physicochemical environment (for flocculation) that improve overall process efficiency.     

Evolution of Porosity by Freeze Casting and Sintering of Sol-Gel Derived Ceramics

Freeze cast of aqueous ceramic powder slurries is described as a versatile process to fabricate complex-shaped ceramic parts. Since freezing of aqueous sols or powder suspensions include the nucleation and growth of ice crystals the evolving microstructure in particular the pore characteristics which are left behind after elimination of the solvent can be controlled by the freezing process. The freezing kinetics have then to be used to manifest the conditions for the formation of the intended porosity. The temperature profile in the freezing slurry was measured and calculated, in particular the movement of the freezing front through the slurry was determined. The results show that a homogeneous microstructure is reached in the surface region of the consolidated part. Individual ice crystals are detected within a distance of some hundred micrometers from the surface. The final pores are dendritic in shape with an elliptical cross section. The pores can grow up to several millimeters in length under the process conditions used in this study. The limits of freeze-sensitive slurry compositions should be investigated in further studies and the approach should be followed to increase the porosity by additional foaming steps.     

Mechanism of freezing of water in contact with mesoporous silicas MCM-41, SBA-15 and SBA-16: role of boundary water of pore outlets in freezing

The freezing mechanism of water contacted with mesoporous silicas with uniform pore shapes, both cylindrical and cagelike, was studied by thermodynamic and structural analyses with differential scanning calorimetry (DSC) and X-ray diffraction (XRD) together with adsorption measurements. In the DSC data extra exothermic peaks were found at around 230 K for water confined in SBA-15, in addition to that due to the freezing of pore water. These peaks are most likely to be ascribed to the freezing of water present over the micropore and/or mesopore outlets of coronas in SBA-15. Freezing of water confined in SBA-16 was systematically analysed by DSC with changing the pore size. The freezing temperature was found to be around 232 K, close to the homogeneous nucleation temperature of bulk water, independent of the pore size when the pore diameter (d) < 7.0 nm. Water confined in the cagelike pores of SBA-16 is probably surrounded by a water layer (boundary water) at the outlets of channels to interconnect the pores and of fine corona-like pores, which is similar to that present at the outlet of cylindrical pores in MCM-41 and of cylindrical channels in SBA-15. The presence of the boundary water would be a key for water in SBA-16 to freeze at the homogeneous nucleation temperature. This phenomenon is similar to those well known for water droplets in oil and water droplets of clouds in the sky. The XRD data showed that the cubic ice I(c) was formed in SBA-16 as previously found in SBA-15 when d < 8.0 nm.     

Evaluation of thermoporometry for characterization of mesoporous materials

The accuracy of thermoporometry (TPM) in terms of the characterization of SBA-15 is examined based on a model that classifies the water in the mesopores into two different types: freezable pore water, which can form cylindrical ice crystals, and nonfreezable pore water, which cannot undergo a phase transition during a differential scanning calorimetry (DSC) measurement. Applying the empirical relationship between the sizes of the ice crystals formed in the mesopores and the solidification temperature of the freezable pore water to a thermogram (a recording of the heat flux during the solidification of the freezable pore water) yielded a size distribution of the ice crystals. The size of the ice crystals increased slightly with repetitive freezing, indicating that the mesopores were enlarged by formation of the ice crystals. Adding the thickness, t(nf), of the nonfreezable pore water layer to the ice crystal-size distribution calculated from the thermogram allowed for the determination of the porous properties of SBA-15. The porous properties attained from TPM experiments were compared with the results attained through the combination of Ar gas adsorption experiments and nonlocal density functional theory (NLDFT) analysis. The porous properties determined by TPM were confirmed to be quite sensitive to the t(nf) value.     

Molecular dynamics simulations of ice nucleation by electric fields

Molecular dynamics simulations are used to investigate heterogeneous ice nucleation in model systems where an electric field acts on water molecules within 10-20 ? of a surface. Two different water models (the six-site and TIP4P/Ice models) are considered, and in both cases, it is shown that a surface field can serve as a very effective ice nucleation catalyst in supercooled water. Ice with a ferroelectric cubic structure nucleates near the surface, and dipole disordered cubic ice grows outward from the surface layer. We examine the influences of temperature and two important field parameters, the field strength and distance from the surface over which it acts, on the ice nucleation process. For the six-site model, the highest temperature where we observe field-induced ice nucleation is 280 K, and for TIP4P/Ice 270 K (note that the estimated normal freezing points of the six-site and TIP4P/Ice models are ～289 and ～270 K, respectively). The minimum electric field strength required to nucleate ice depends a little on how far the field extends from the surface. If it extends 20 ?, then a field strength of 1.5 × 10(9) V/m is effective for both models. If the field extent is 10 ?, then stronger fields are required (2.5 × 10(9) V/m for TIP4P/Ice and 3.5 × 10(9) V/m for the six-site model). Our results demonstrate that fields of realistic strength, that act only over a narrow surface region, can effectively nucleate ice at temperatures not far below the freezing point. This further supports the possibility that local electric fields can be a significant factor influencing heterogeneous ice nucleation in physical situations. We would expect this to be especially relevant for ice nuclei with very rough surfaces where one would expect local fields of varying strength and direction.     

Ice growth from supercooled aqueous solutions of benzene, naphthalene, and phenanthrene

Classical molecular dynamics (MD) were performed to investigate the growth of ice from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene. The main objective of this study is to explore the fate of those aromatic molecules after freezing of the supercooled aqueous solutions, i.e., if these molecules become trapped inside the ice lattice or if they are displaced to the QLL or to the interface with air. Ice growth from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene result in the formation of quasi-liquid layers (QLLs) at the air/ice interface that are thicker than those observed when pure supercooled water freezes. Naphthalene and phenanthrene molecules in the supercooled aqueous solutions are displaced to the air/ice interface during the freezing process at both 270 and 260 K; no incorporation of these aromatics into the ice lattice is observed throughout the freezing process. Similar trends were observed during freezing of supercooled aqueous solutions of benzene at 270 K. In contrast, a fraction of the benzene molecules become trapped inside the ice lattice during the freezing process at 260 K, with the rest of the benzene molecules being displaced to the air/ice interface. These results suggest that the size of the aromatic molecule in the supercooled aqueous solution is an important parameter in determining whether these molecules become trapped inside the ice crystals. Finally, we also report potential of mean force (PMF) calculations aimed at studying the adsorption of gas-phase benzene and phenanthrene on atmospheric air/ice interfaces. Our PMF calculations indicate the presence of deep free energy minima for both benzene and phenanthrene at the air/ice interface, with these molecules adopting a flat orientation at the air/ice interface.     

Porosity and interior structure of flocculated activated sludge floc

This work estimated the porosities of activated sludge flocs, cationic polyelectrolyte flocculated, based on free-settling tests, buoyant weight measurements, and confocal laser scanning microscope (CLSM) tests. The extent of advective flow was estimated based on bubble-tracking test. The former two measurements suggested a close-to-unity porosity, that is, an extremely void floc interior. Meanwhile, the latter two tests recommended a dense floc interior with a porosity less than 64%. A discrepancy exists between the porosities estimated by various tests. A floc model was proposed based on the understanding that a vast amount of bound water in the floc was regarded as void in buoyant weight measurement, but was impermeable for advective flow. The distribution rather than the mean value of the porosity controls the advective flow. There existed no simple correlation between the porosities measured by different tests.     

Physicochemical properties and stability of activated sludge flocs under temperature upshifts from 30 to 45 degrees C

The impacts of temperature shifts from 30 to 45 degrees C on the structural stability and surface charge of activated sludge flocs were assessed in four sequencing batch reactors (SBRs) treating pulp and paper mill effluent. The improvement in floc stability was tested by sludge magnesium enrichment in one SBR and by operating another reactor at a high sludge retention time (SRT) of 33 days. Floc stability was characterized by dissociation constants with solutions of CaCl(2), KCl, urea, and ethylenediamine tetraacetate (EDTA). Surface charge was assessed by cationic-anionic titration and metals concentrations were also determined. The temperature shift consistently caused an increase in the negative sludge surface charge from approximately -0.180 to -0.300 meq/g MLSS. Magnesium enrichment and a high SRT of 33 days promoted less negatively charged sludge, dampened the increase in negative sludge surface charge, and yielded structurally stronger flocs; however, sludge deflocculation still occurred. Manganese and iron appeared to be released by sludge under the temperature shift. It was concluded that the temperature shift deteriorates the flocculating physicochemical properties of the sludge and that better floc stability achieved by magnesium enrichment and a high SRT is not enough to stop deflocculation. Further research is required to clarify the origin of the increase in negative sludge surface charge, the role of metals, and the governing factors in sludge deflocculation under such temperature shifts.     

Oscillations of exopolymeric composition and sludge volume index in nitrifying flocs

Protein, carbohydrate, and lipid composition of the exopolymer fraction of a nitrifying sludge in steady-state culture was analyzed after dissociation with 50 mM EDTA and dialysis of the nonfilamentous flocs. Steady-state culture was established when the nitrification rate was constant. The nitrification efficiency at that regime was 93%, also constant. In steady state the concentration of exopolymer protein in the nitrifying sludge floc oscillated from 5 (lowest) to 45 (highest) mg/L with a consistent oscillating pattern having a duration period of 10 d each. Carbohydrate and lipid content in the flocs showed no significant variations (30 and 36 mg/L, respectively). Only 20% of the extracellular polysaccharides had molecular weights higher than 10 kDa, suggesting that the floc aggregation depended on smaller fractions of low-molecular-weight carbohydrates. The oscillations in the concentration of exopolymeric protein coincided with parallel variations in the sludge volumetric index (SVI) value (12.2±2.1 mL/g). Analysis of the polymeric substances of the floc and suspended solids corroborated by statistical analysis indicated that the variations in the SVI of the nitrifying nonfilamentous flocs were mainly related to the changes in the exopolymeric protein content.     

Freezing of water and aqueous NaCl droplets coated by organic monolayers as a function of surfactant properties and water activity

This study presents heterogeneous ice nucleation from water and aqueous NaCl droplets coated by 1-nonadecanol and 1-nonadecanoic acid monolayers as a function of water activity (a(w)) from 0.8 to 1 accompanied by measurements of the corresponding pressure-area isotherms and equilibrium spreading pressures. For water and aqueous NaCl solutions of ~0-20 wt % in concentration, 1-nonadecanol exhibits a condensed phase, whereas the phase of 1-nonadecanoic acid changes from an expanded to a condensed state with increasing NaCl content of the aqueous subphase. 1-Nonadecanol-coated aqueous droplets exhibit the highest median freezing temperatures that can be described by a shift in a(w) of the ice melting curve by 0.098 according to the a(w)-based ice nucleation approach. This freezing curve represents a heterogeneous ice nucleation rate coefficient (J(het)) of 0.85 ± 0.30 cm(-2) s(-1). The median freezing temperatures of 1-nonadecanoic acid-coated aqueous droplets decrease less with increasing NaCl content compared to the homogeneous freezing temperatures. This trend in freezing temperature is best described by a linear function in a(w) and not by the a(w)-based ice nucleation approach most likely due to an increased ice nucleation efficiency of 1-nonadecanoic acid governed by the monolayer state. This freezing curve represents J(het) = 0.46 ± 0.16 cm(-2) s(-1). Contact angles (α) for 1-nonadecanol- and 1-nonadecanoic acid-coated aqueous droplets increase as temperature decreases for each droplet composition, but absolute values depend on employed water diffusivity and the interfacial energies of the ice embryo. A parametrization of log[J(het)(Δa(w))] is presented which allows prediction of freezing temperatures and heterogeneous ice nucleation rate coefficients for water and aqueous NaCl droplets coated by 1-nonadecanol without knowledge of the droplet's composition and α.     

Flux decline behaviors in dead-end microfiltration of activated sludge and its supernatant

The properties of dead-end microfiltration were explored under constant pressure using two types of activated sludge controlled under the condition of different air flow rates. The activated sludge cultured at the air flow rate of 0.15 L min⁻¹ (the anaerobic condition) exhibited a significant flux decline compared with the case of the air flow rate of 2.33 L min⁻¹ (the aerobic condition). It was found from the results of microfiltration of the supernatant separated by centrifugation that the constituents in the supernatant caused a major cake resistance in microfiltration of the activated sludge. The average specific filtration resistance for filtration of the activated sludge was closely consistent with that for filtration of the supernatant at low pressure (49 kPa). However, the cake resistance of the microbial floc in microfiltration of the activated sludge became substantial with increasing filtration pressure because of high compressibility of the microbial floc. Moreover, the foulant and the fouling mechanism in microfiltration of the supernatant were evaluated from both microfiltration test of the supernatant and microfiltration test of the filtrate collected thereby. As a result, the effects of the pore size and material of the microfiltration membrane on the flux decline behaviors in dead-end microfiltration were reasonably elucidated.     

Gelation of cassia gum by freezing and thawing

Aqueous solution of cassia gum (CG), which is categorized as a galactomannan polysaccharide having mannose/galactose ratio = 5/1, forms hydrogels by freezing and thawing. When frozen CG aqueous solution was thawed, transparent sol was separated from a turbid gel, i.e. syneresis occurred. Gel concentration ({(Mass of dry gel) / (Mass of gel)} × 100) increased with increasing CG concentration. Viscoelastic properties of CG hydrogels formed by freezing and thawing were investigated by thermomechanical analysis (TMA) in water using an oscillation mode at 0.05 Hz. Dynamic modulus (E′) increased from 3 kPa to ca. 5 kPa with increasing freezing rate. In contrast, E′ maintained a constant value regardless of repeating number of freezing and thawing. From TMA results, it is concluded that the density of cross-linking network structure depends on the size of ice formed by freezing. At the same time, the low E′ value of CG gels is ascribed to the fact that association of galactosyl side group is disturbed by the stiff chain attributed to the unsubstituted region of CG.