Free-settling Test Research Articles

Study on Terminal Velocity of Particles in the Teetered Bed Separator

Several mathematical models describing particle separation and predict the separations in TBS are available. Because of the complexity of these models, Galvin’s model and Das’s model are of most interest. Equations of Galvin’s model, Das’s model and Alien’s law are used to predict the values of the terminal velocities. Free settling tests of 25 samples were undertaken. The average deviation of Galvin’s model is 0.019043, the values predicted by equations of Galvin’s model are most closes to the experimental data. Therefore, correlation equations of Galvin’s model should be chosen to calculate the terminal velocity of particles in the TBS.

Settling Properties of Asphaltene Aggregates

Stokes law is the fundamental equation to describe the settling velocity of a particle for small Reynolds number flow and is applicable when the particle is spherical and impermeable. However, this equation is not suitable to describe the settling of porous aggregates found in many industrial applications, because of the nonsphericity and flow through the pores of the aggregates. Free-settling tests were used in the present work to estimate asphaltene aggregates porosity and effective density. The settling velocities of individual asphaltene aggregates are measured in a quiescent column via a photographic technique. Data analysis was conducted with a video frame grabber and digital imaging software, to determine the particle size, shape, and settling velocity. Results show that the aggregate size (area-equivalent diameter) determined from the video images ranged from 20 μm to 200 μm and the measured settling velocities varied from 100 μm/s to 564 μm/s. Settling velocity measurements indicated an inverse r...

Fractal structure of asphaltene aggregates

A photographic technique coupled with image analysis was used to measure the size and fractal dimension of asphaltene aggregates formed in toluene–heptane solvent mixtures. First, asphaltene aggregates were examined in a Couette device and the fractal-like aggregate structures were quantified using boundary fractal dimension. The evolution of the floc structure with time was monitored. The relative rates of shear-induced aggregation and fragmentation/restructuring determine the steady-state floc structure. The average floc structure became more compact or more organized as the floc size distribution attained steady state. Moreover, the higher the shear rate is, the more compact the floc structure is at steady state. Second, the fractal dimensions of asphaltene aggregates were also determined in a free-settling test. The experimentally determined terminal settling velocities and characteristic lengths of the aggregates were utilized to estimate the 2D and 3D fractal dimensions. The size–density fractal dimension ( D 3 ) of the asphaltene aggregates was estimated to be in the range from 1.06 to 1.41. This relatively low fractal dimension suggests that the asphaltene aggregates are highly porous and very tenuous. The aggregates have a structure with extremely low space-filling capacity.

Comparison of Dewaterability and FLOC Structure of Synthetic Sludge with Activated Sludge

This work compared the appearance, dewaterability, floc size, fractal dimensions derived from free-settling test and light-scattering test, and detailed structural information of floc interior including porosity, Sierpinski carpet fractal dimension, and the fractal dimension of the pore boundaries of a synthetic sludge and an activated sludge. The present synthetic sludge could probe the change in filterability and in fractal dimensions extracted from small angle light scattering test and from free-settling test after conditioning, however, failed to reproduce the detailed structural information of sludge flocs. The use of specific synthetic sludge depends on the field of interest. For instance, the present synthetic sludge is not appropriate to simulate the intrafloc transport of solute owing to its incapability to reproduce the detailed floc structure or to its change after conditioning.

Effect of pre-hydrolysis on floc structure

Morphological changes in flocs were elucidated by small-angle light scattering, free settling tests and microscopic observation to determine the effects of pre-hydrolysis (alkaline treatment and ultrasonication) on two sludges—primary raw sludge from the Malabar Sewage Treatment Plant, Sydney (sludge M) and biological sludge collected at the nitrogen removal unit of St Marys Sewage Treatment Plant in Sydney (sludge S). Ultrasonication or alkaline treatment released a marked amount of insoluble organic matter in a soluble form. The latter treatment was more efficient than the former. Meanwhile, the pre-hydrolyzed flocs had more compact structures than the original ones, as shown by their higher free-settling fractal dimension, lower areal porosity and smaller internal pores. In particular, alkaline treatment yielded flocs with more compact interiors than did ultrasonication. Such detailed structural information could not be elicited by monitoring the change in floc size. The greater resistance to mass transfer of the hydrolyzed flocs thus produced does not reduce the efficiency of subsequent digestion, indicating that the surface reaction rate on the solid surface might have dominated since the pre-hydrolysis steps modified the local chemical environment to promote digestion.

Effect of pre-hydrolysis on floc structure

Morphological changes in flocs were elucidated by small-angle light scattering, free settling tests and microscopic observation to determine the effects of pre-hydrolysis (alkaline treatment and ultrasonication) on two sludges—primary raw sludge from the Malabar Sewage Treatment Plant, Sydney (sludge M) and biological sludge collected at the nitrogen removal unit of St Marys Sewage Treatment Plant in Sydney (sludge S). Ultrasonication or alkaline treatment released a marked amount of insoluble organic matter in a soluble form. The latter treatment was more efficient than the former. Meanwhile, the pre-hydrolyzed flocs had more compact structures than the original ones, as shown by their higher free-settling fractal dimension, lower areal porosity and smaller internal pores. In particular, alkaline treatment yielded flocs with more compact interiors than did ultrasonication. Such detailed structural information could not be elicited by monitoring the change in floc size. The greater resistance to mass transfer of the hydrolyzed flocs thus produced does not reduce the efficiency of subsequent digestion, indicating that the surface reaction rate on the solid surface might have dominated since the pre-hydrolysis steps modified the local chemical environment to promote digestion.

Structure of conditioned sludge flocs

Free settling tests, small-angle light scattering, microtome-slicing techniques, and confocal laser scanning microscopy were performed to examine how the cationic flocculation or freezing and thawing affected the floc structure. The floc size, internal pore size, mass fractal dimensions determined from free-settling test or small angle light scattering test, aeral porosity, boundary fractal dimension and Sierpinski carpet fractal dimension of pore boundary from 2D slices, and the volume porosity, compactness, and the pores’ box-counting fractal dimension from 3D reconstructed image, were estimated and compared. Cationic flocculation would produce large flocs with internal pores of shape resembling a long “tube” with rough surface. Freezing and thawing would produce flocs with internal pores with lower aspect ratio and a smoother boundary.

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.

Estimation of Floc Permeability and Porosity (short communication)

This paper briefly reviewed the progress made recently to the estimation of hydrodynamic permeability of sludge floes, which is essential for determining the extent of advection flow through the floc interior. The large pores mainly contribute the hydrodynamic permeability of sludge floc through which the fluid could readily flow. Permeability models that assume a homogeneous floc interior normally yield marked uncertainties in model outputs. Wu et al (l998) proposed a hydrodynamic approach for estimating the hydrodynamic permeability that considers the response of the sludge floc when moves toward a flat plate. The floc permeability for waste activated sludge was noted higher than the cupric hydroxide sludge, while the polyelectrolyte flocculation and fluid shear could reduce the permeability. The bubble-tracking technique describes the fluid flow field around and through the floc interior. The existence of advection flow is noted unexceptional. The misuse of permeability model has been demonstrated with a case study leading to the floc porosity based on free-settling test.

Coagulation of humic-kaolin-pacl aggregates

Water treatment residual flocs are fractal-like aggregates made of many initial aggregates. We investigated in this study the coagulation dynamics for the humic-mineral-polyaluminium chloride (PACI) aggregates using small-angle light scattering techniques and the free-settling test. In contrast to reports in the literature, the presence of humic acid did not lead to a loose floc. Not only the time evolution of the coagulation dynamics, but also the final floc characteristics are only mildly affected by the humic acid. However, the strength of the formed floc does decline with humic acid, which leads to a turbid supernatant with high level of organics.

New Methodology for the Analysis of Physical Structure of Solid Aggregates.

The structure of solid aggregates was investigated by using three particle analyzers; an electrical sensing zone, laser light diffraction and electron microscopy. Four particulate materials; aluminum trihydrate, aluminum phosphate, fly ash and iron hydroxide were used for the analysis of particle shape, size and floc structure. One of the primary conclusions of this work is that the floc structure could be characterized by the evaluation of the fractal dimension based on the experimental relationship between floc size and floc density through the free-settling test. It revealed that coarser flocs had a looser structure with a low value of fractal dimension.

Hydrodynamic drag on non-spherical floc and free-settling test

This work numerically investigates the hydrodynamic drag force exerted on a porous spheroid floc moving steadily through a quiescent Newtonian fluid over a wide range of the Reynolds number. The flow patterns for a highly porous spheroid moving at an elevated Reynolds number are basically the same as those at a low Reynolds number, which extends the applicable range of a creeping-flow based correlation to the higher Reynolds number regime. The shape effect becomes more prominent as the spheroid becomes more porous. Using the equivalent diameter, defined as the geometric mean diameter of the principal axes, leads to a universal correlation relating to the drag force, aspect ratio, and interior permeability. In addition, free-settling experiments are performed to estimate how the non-spherical shape affects the three sludge samples. The possible errors in data reduction for the free-settling test are attributed to the a/ b ratio and the internal permeability. The errors range from 16–34% for a/ b=0.6–2.0.

Generation and properties of aerobic granular sludge

A sequencing batch reactor (SBR) was used to investigate the generation of different granules cultured under aerobic and alternating anaerobic/aerobic conditions. The reactor was fed with synthetic wastewater. A substrate loading rate of 3.6 kg COD/(m3 day) was applied. Granules of heterotrophic microorganisms were formed. After the first experimental period of 8 weeks the average granule diameter was 3.2 mm. In the second period, alternating anaerobic/aerobic conditions were applied to form granular sludge with an average diameter of 3.0 mm. An isopycnic centrifugation procedure was used to determine the characteristic density of the aerobic granular sludge. The average density of the granular sludge was 1.044 g/ml and 1.048 g/ml, respectively. In free-settling tests the final settling velocity of single aggregates was examined to estimate porosity. Settling velocities up to 2.0 cm/s could be measured. Calculations based on the experimental results showed an average granula porosity of 72% for the first run and 65% average porosity for the second run. This paper indicates the validity of general assumptions in free-settling tests.

Hydrodynamic drag force exerted on a moving floc and its implication to free-settling tests

This paper numerically evaluates the hydrodynamic drag force exerted on an individual floc moving steadily through an quiescent Newtonian fluid over a wide range of Reynolds numbers. The floc is modelled as a highly porous sphere, whose interior flow field is governed by the combined Darcy–Brinkman model; while the bulk fluid field is governed by Navier–Stokes equations. We used a computational fluid dynamics software, FIDAP 7.5, to solve the fluid field within and around the moving floc, from which the corresponding hydrodynamic drag force exerted on the floc is subsequently obtained. When the floc permeability is rather high, which is usually the case for activated sludge flocs, the drag force was found to be inversely proportional to the floc Reynolds number. The floc density versus size relationship obtained on the basis of the Stokes law-like correlation is thereby applicable to estimate the flocs' fractal dimension, however, is erroneous for estimating the floc density.

On the free-settling test for estimating activated sludge floc density

The feasibility of using free-settling test for estimating activated sludge floc density was discussed in the present work. Some usually applied models or correlations, such as for the primary particle density, the drag coefficient, the correction factor for advection flow through the floc and the estimation for floc permeability, are examined. The results show that floc permeability depends strongly on the models or parameter sets applied. However, the effective floc density, and also the fractal dimension, is insensitive to the floc permeability estimation, but is strongly dependent on the drag coefficient expression employed. Possible valid experimental range is proposed.