Floc Growth Research Articles

A laboratory examination of floc characteristics with regard to turbulent shearing

Turbulent shear generated within the water column is recognised as having a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters, and their respective aggregate break-up. This study examines the inter-relationships between floc characteristics over increasing turbidity (80–200 mg l −1) and turbulent shear (0–0.6 N m −2) environments, by the use of a laboratory flume within which a suspension can be sheared at a controlled rate, and with an unintrusive macro-lens miniature video camera mounted in a viewing port on the flume channel wall. The camera enables the direct simultaneous measurement of both floc size and settling velocity, from which accurate estimates of floc effective density and porosity can be made. Measurements were made 120 s after the induced turbulence has ceased. The instrument has an upper viewing turbidity limit of 210 mg l −1, and a lower resolution of 20 μm. The sediment was collected from the inter-tidal mudflats at Weir Quay on the Tamar Estuary in Devon, Southwest England. The results indicated that increasing turbidity at low shear levels encouraged floc growth, but the effect of the increasing turbulent shear (0.35 N m −2) together with increasing concentration in suspension causes disruption rather than enhancing the flocculation process. At shears up to 0.35 N m −2, the largest size and settling velocity flocs were produced at high concentrations, whereas above 0.35 N m −2 disruption caused smaller flocs at higher concentrations. The use of algorithms which were based either on a single floc characteristic (i.e., size or settling velocity) or a mean fractal dimension, were seen not to accurately approximate the experimental data. A multiple regression analysis of the experimental data produced the following formula, based on mean values of the 20 largest flocs sampled under each of the imposed environmental conditions (referred to as max20size mean values): settling velocity=0.301−0.00337 rms of the gradient in turbulent velocity fluctuations−0.000606 SPM+0.00335 floc size, which proved to be the most accurate representation with an R 2 value of 0.95. A similar formula was determined for the average value of the four fastest settling flocs within each sample-group (max4W S). This highlights the importance of modelling algorithms that are developed from data that take into account effective density variations (i.e., simultaneous size and settling velocity measurement).

The variability of settling velocities of suspended fine-grained sediment in the Ems estuary

Settling velocities of suspended fine-grained sediment in estuaries vary over a range of several orders in magnitude. Variations in the suspended sediment concentration are often put forward as the principal cause. However, comparison of settling velocities from a number of estuaries shows that even in the case of the same suspended sediment concentration, large variations in the settling velocities can occur of up to two orders in magnitude. From measurements in the Ems estuary we found that even within a single estuary such large variations can occur. Field measurements and complementary laboratory experiments demonstrate that `other factors' can affect the settling velocity in the same order as the assumed effects of the suspended sediment concentration. To address these `other factors', which include physical–chemical and biological effects, the concept of `flocculation ability' is introduced, as a measure of the effectiveness of the collisions between suspended particles for floc growth. On the basis of the results from the Ems estuary, it is hypothesised that variations in the flocculation ability of the suspended fine-grained sediments are at the root of the large differences in settling properties of suspended fine-grained sediment in estuaries.

The feasibility of flocculation in a storage sedimentation basin

Fot the Flemish situation, storage sedimentation basins ate one of the best ‘end-of-pipe’ solutions to reduce the impact of combined sewer overflows on the receiving waters. In some cases, when the spilled pollution load is too high, the settling efficiency of the basin has to be improved. Adding coagulants could be a reasonable alternative for building larger basins. To estimate the effect of enhancing the settling by flocculation, a floc growth and break-up model is worked out and is implemented in the numerical model Phoenics. The evolution of the floc dimensions and the sedimentation behaviour of the particles in the basin is calculated for different inflow rates and initial settling velocity profiles. Finally, the efficiency increase by mixing (creating a higher velocity gradient) in the agitation chamber or by adding coagulants in the trunk sewer upstream of the basin is also investigated.

The feasibility of flocculation in a storage sedimentation basin

For the Flemish situation, storage sedimentation basins are one of the best ‘end-of-pipe’ solutions to reduce the impact of combined sewer overflows on the receiving waters. In some cases, when the spilled pollution load is too high, the settling efficiency of the basin has to be improved. Adding coagulants could be a reasonable alternative for building larger basins. To estimate the effect of enhancing the settling by flocculation, a floc growth and break-up model is worked out and is implemented in the numerical model Phoenics. The evolution of the floc dimensions and the sedimentation behaviour of the particles in the basin is calculated for different inflow rates and initial settling velocity profiles. Finally, the efficiency increase by mixing (creating a higher velocity gradient) in the agitation chamber or by adding coagulants in the trunk sewer upstream of the basin is also investigated.

Is sweep-flocculation relevant to saponite aggregation?

Synthetic saponites with various Cationic Exchange Capacities were aggregated at pH 8.0 in a stirred vessel by hydrolysed aluminum species. Floc growth was followed by Fraunhoffer laser diffraction and the efficiency of clarification was assessed from conventional residual turbidity and sediment volume measurements. We show that the aggregation mechanisms are layer-charge dependent and can not be described by the widely accepted sweep-flocculation model.

Is sweep-flocculation relevant to saponite aggregation?

Synthetic saponites with various Cationic Exchange Capacities were aggregated at pH 8.0 In a stirred vessel by hydrolysed aluminum species. Floc growth was followed by Fraunhoffer laser diffraction and the efficiency of clarification was assessed from conventional residual turbidity and sediment volume measurements. We show that the aggregation mechanisms are layer-charge dependent and can not be described by the widely accepted sweep-flocculation model.

The variability of suspended aggregates on the Amazon Continental Shelf

Underwater photographs taken as part of the AmasSeds project from 1989 to 1992 reveal the abundance of suspended aggregates on the Amazon Continental Shelf. Computerized image analysis of photographs from specific layers within the water column at inner shelf, midshelf, outer shelf, and rivermouth anchor stations allowed description of the in situ characteristics of the aggregates, or flocs. Floc sizes varied from the lower limit of detection (125 μm) to a maximum greater than 2 mm, with a mean diameter of 518 μm and a modal diameter of 646 μm for the complete data set. Additionally, large, elongate, comet-shaped aggregates (stringers) as large as 2.5 cm were observed in individual photographs. Results of floc-size analyses (in terms of the maximum floc size, d MAX) were compared to salinity, particle concentration, and current velocity. Additionally, d MAX was also compared to shear ( G) and the Kolmogorov microscale (λ). Results suggest that salinity has little effect on d MAX ( r 2 = 0.06), while particle volume concentration demonstrates a strong relationship with d MAX ( r 2 = 0.92). The maximum floc size increased with increased current velocity and G up to a critical value (floc growth stage) and then decreased as the current velocity and G continued to increase (floc breakup stage). The Kolmogorov microscale, which describes the size of the smallest turbulent eddies (and, thus, should limit the d MAX observed) also demonstrates a relationship with the observed maximum floc size. The results suggest that d MAX is approximately one-half the magnitude of the Kolmogorov microscale (λ).

Dependence of separation properties on flocculation dynamics of kaolinite suspension

Abstract Solid–liquid separation is widely done by polymeric flocculation. But very little fundamental information is available for sheared suspension. In this work, online measuring of the particle number concentration, floc size and floc size distribution has been done in stirred suspension to understand the effects of addition of flocculant in stages, agitation speed and time, pH and solids loading in a kaolinite suspension on formation, growth, breakage and conformation of flocs and finally on separation properties, namely, turbidity of the suspernatant, capillary suction time (CST) and specific resistance to filtration (SRF). The exact amount of flocculant required for optimum results can be determined by stage addition of flocculant. The best separation properties are obtained from the flocs which can withstand shear for a long time. Higher agitation speed reduces the mean floc size and narrows the size distribution.

The Effect of Impeller Type on Floc Size and Structure during Shear-Induced Flocculation

The effect of impeller type and shear rate on the evolution of floc size and structure during shear-induced flocculation of polystyrene particles with aluminum sulfate is investigated by image analysis. One radial flow (six-blade Rushton turbine) and two axial flow (three-blade fluid foil, four-blade 45° pitch) impeller configurations are examined. The steady state average floc size is shown to depend on the frequency of recirculation to the impeller zone and its characteristic velocity gradient. The concepts of fractal geometry are used to characterize the floc structure. For all impellers, the two-dimensional floc fractal dimension,Dpf, increases during floc growth, indicating formation of more open structures. Later on,Dpflevels off at a steady state value as breakage becomes significant and the floc size distribution approaches steady state. The shear rate does not affect the steady stateDpfof the flocs within experimental uncertainty.

Recent Progress in the Numerical Modelling of Wastewater Sedimentation Tanks

The recent progress in numerical modelling techniques applied to sedimentation tanks in wastewater treatment is reviewed to determine their usefulness. The models reviewed range from empirical models to sophisticated computational fluid dynamics (CFD) models and the conclusions are as follows. Empirical models are still widely used today to predict mainly the characteristics of the effluent and return sludge from sedimentation tanks but cannot model the flow pattern or solids distribution within the tank. The solids mass flux model is used to perform a mass balance on secondary sedimentation tanks for new designs or for the audit of a poor process and to determine the return sludge flow. It can sometimes be used to monitor the height of the sludge blanket. The lumped parameter model is usually used to determine the characteristics of the effluent from primary sedimentation. CFD models are used to predict the flow pattern and suspended solids distributi on with in the tank and are normally applied in research to find the relationship between the tank hydraulics and process performance. Modelling using CFD in the water industry has not been widespread because of the associated costs and the unfamiliarity with the mathematical models. The paper suggests that the important CFD modelling criteria for the settling of suspended solids in sedimentation tanks are the velocity distribution, settling velocity distribution of suspended solids, turbulent mass diffusion of suspended solids, resuspension of settled solids from tank base, temperature effects, flow variation, effect of flow on floc growth or breakup, wind effects on the water surface and movement of scrapers. Gaps in the literature on CFD modelling can be identified for the particle density, particle size, particle flocculation, the turbulent mass diffusion of solids, the effect of the flow on particle growth or breakup, denitrification, wind on the water surface, baffle designs, inlet pipe geometries, the side wall depth, the slope of the tank, the hopper design, peripheral weirs and in-board launders. Where possible experimental data should be entered in these numerical models to give realistic parameters and be used to validate these models.

カチオン性ポリ電解質を用いたセラミックス粉体スラリーの凝集挙動 凝集スラリー中のフロックサイズとレオロジー特性及び表面電位との関連性について

The flocculation of particles in a condensed ceramic powder slurry was investigated as a function of the concentration of cationic polyelectrolyte from rheological and colloid chemical points of view. The results obtained are summarized as follows;(1) Flocculation occurs in two stages. In the first stage, rapid growth of flocs occurs showing that a consistency index (K) decreases toward the minimum value. Then a zeta potential(ζ) becomes zero. There is a rather moderate growth of flocs in the second stage compared to the first stage indicating increases in (K) and (ζ).(2) There are two types of behavior in the second stage. One is that the flocs grow showing the increase in (K); the other is that the flocs show the saturation in the value of (K).(3) The important factors for flocculation control are particle diameter and its corresponding polymer concentration in a given solid loading.(4) Consistency index (K) is effective as a measure for an evaluation of the change of the interaction force among flocs in flocculated slurries with the polymer.

Effects of Macromolecular Conformation upon Solid-Liquid Separation and Water Treatment Plant Residuals

The efficiency of solid-liquid separation in coagulation -sedimentation processes depends greatly upon the performance of the coagulant with respect to the production of suitable floc properties. When high-molecular weight polymers (that function primarily by bridging between proximate primary particles) are used as coagulant-aids, the dynamic processes of polymer adsorption and macromolecular rearrangement figure prominently in the success of floc formation. Ideally, one would like to have the macromolecule adsorb quickly upon the primary particle surface in an extended, rod-like conformation; this configuration should persist throughout the collisional process to produce rapid floc growth. A period of macromolecular relaxation would then ensue, with the formation of additional affiliations between functional groups and surface sites. Recently, there have been speculations that the relative speeds of these processes may not necessarily conform to the common perception that sorption is quick and that the ...

Flocculation of sediment from the Tanshui River estuary

Experiments on the flocculation of fine-grained sediments from the Tanshui River estuary, Taiwan, were carried out. The effects of fluid shear, sediment concentration and salinity on floc growth were investigated. The ranges of these parameters were 12.5 to 400 s-1 for the shear, 50 to 600 mg L-1 for the sediments concentration and 0% to 100% sea water for the salinity. The results showed the following. (1) The time required to reach a steady state ranges from 2 to 44 h and median floc sizes range from 100 to 520 μm. (2) The steady-state median floc size decreases with the increase in sediment concentration. (3) The floc size increases as the shear increases. (4) Increasing the proportion of sea water from 0% to 50% decreases the floc size to one-fourth of that of freshwater flocs, and a further increase in salinity does not decrease floc size significantly. (5) The time required to reach a steady state decreases with the decrease in shear, the increase in sediment concentration and the increase in salinity. (6) The larger the steady-state flocs, the longer they take to form. Comparison of these results with those obtained in similar studies of Lake Erie sediments shows that the flocculation properties of Tanshui sediments are substantially different from those of Lake Erie sediments. Tanshui sediments are much more difficult to aggregate, but once flocs are formed they are larger. The reasons why different types of sediment behave differently should be studied further.

Determining the effect of physico-chemical parameters on floc size using a population balance model

The US Bureau of Mines is investigating the fundamentals of the flocculation process. The objective of this study was to determine the effects of various physico-chemical parameters, namely, shear rate, polymer dosage, and polymer concentration, on floc growth and to determine the applicability of a model for predicting polymer-flocculation behavior. A rapid in-line particle size analyzer was used to follow the progress of both the floc growth and the breakage as a function of time. A genetic algorithm (GA) was used to determine the unknown parameters in the model and to fit a curve to the experimental data. The results showed that the model reproduces the experimental data with only reasonable accuracy and that it was relatively insensitive to the parameters. Results of the flocculation study showed that increases in the polymer dosage had a greater effect on the time required to reach the maximum floc size than on the floc size itself. It was also determined that, given all the above parameters, the degree of slurry mixing during the flocculation process had the greatest bearing on floc growth.

Study of shear-flocculation of silica

The US Bureau of Mines investigated the fundamental aspects of shear-flocculation using a rapid, in-situ particle size analyzer. Silica was stirred in the presence of an aliphatic amine to determine the effects of collector coverage, stirring speed and solids concentration on the kinetics of shear-flocculation. The results showed that shear-flocculation is a two-stage process. In the first stage, which occurs during the first 1–2 min. of the reaction, the mean particle size increases rapidly, and in the second stage, an equilibrium is set up between the rates of floc growth and floc decay. Collector dosage affected the position of the equilibrium by altering the number of surface sites for particle-particle attachment. A minimum stirring speed was necessary for shear-flocculation, which was dependent on solids concentration and collector dosage. This stirring speed was the minimum necessary for particles to overcome the electrostatic energy barrier, and that results in keeping the conditioned particles dispersed. Larger particles flocculated more rapidly. At very high stirring speeds, large flocs became unstable, resulting in a narrowed particle size distribution in the slurry.

Fundamentals of Particle Flocculation and Removal From Water

AbstractThe flocculation of polystyrene particles with aluminum sulfate or alum (Al2 (SO4)3) by turbulent shear was studied as a function of the applied shear rates (63–129 s−1) and flocculant concentrations (11 and 32 mg/L) in a stirred tank. Increasing the shear rate increased the floc growth rate but decreased the maximum attainable floc size. Increasing the concentration of alum increased the floc growth rate and the maximum floc size. A steady state between floc growth and breakage was attained after which the floc size distribution no longer changed. The normalized steady state size distributions allowed evaluation of the relative contributions of shear rate and flocculant concentration to the performance of the process.

The behaviour of floc in the flocculation of kaolinite clay

Several experimental runs were performed to investigate the effect of kaolinite particle size, flocculation time, velocity gradient, coagulants dose and initial turbidity on the solid removal and floc size or floc formation of suspended solid. The coagulants used were alum, polyelectrolyte (cationic) or a combination of both. Different classifications of clay particles are used in the present work of grade less than 10, 20 and 40 μm. The particle size of 10 μ at 200 NTU initial water turbidity was used to study the effect of flocculation time, (1-20 min) on floc formation size and flocs number at optimum velocity gradient. The size distribution of flocs was studied as a function of kaolinite concentration in the range 1-3 mg/1 at 20 min flocculation time in case of alum and polyelectrolyte. The results are presented graphically by three-dimensional and contour plots showing the floc number as a function of coagulant dose and flocculation time. The results are also plotted to show the required turbidity as a function of velocity gradient and flocculation time at optimum coagulant dose to locate the overall optimum working conditions. It is found from various experimental results that floc size or floc formation and growth are the principal criteria that effect the flocculation process.

Floc structure and growth kinetics for rapid shear coagulation of polystyrene colloids

Colloid coagulation in a flowing suspension is governed by floc structure and by the colloidal and hydrodynamic interactions between the aggregates. We have investigated both the internal structure and growth rate of rigid flocs formed by rapid coagulation in a linear shear flow, examining structure by static light scattering and measuring sizes by dynamic light scattering. Comparison of the results for sheared suspensions with Brownian coagulation data reveals a similar structure for the two modes; in both cases the flocs exhibit characteristics of a fractal with dimension d = 1.8 ± 0.1, and for both modes the average number of nearest neighbors is approximately the same. In contrast, the growth kinetics are inherently different, as expected. To model growth in a shear flow, we treat a porous floc as a body with a hydrodynamic radius which is smaller than the capture radius corresponding to floc—floc contact, and we also find it necessary to invoke a criterion for the maximum size a floc can obtain at a given shear rate. Necessary structural details are provided by the static light scattering results. We compare kinetics predicted by this model with data at several shear rates and find qualitative agreement.

Stability of particle aggregates in flocculation with polymers: Stabilität von teilchenaggregaten bei der flockung mit polymeren

The growth of flocs in flowing suspensions is limited by hydrodynamic forces responsible not only for floc formation but also for their destruction. New approaches to find the maximum stable floc size in turbulent flow have been derived from a physical point of view considering both the influence of hydrodynamics and particle adhesion within the floc. Modelling is based on the assumption that fluid eddies of the scale of the aggregate size are responsible for the destruction of the floc. The validity of this floc stability model was proved by studies on glass bead adhesion and flocculation of suspensions of such monodisperse glass particles using high molecular weight polyacrylamides as bridging agents. Polymer bridging forces were determined with a centrifugal technique. Batch flocculation experiments were carried out in turbulent Couette flow with an average energy dissipation of ε - = 0.1–30 W kg −1. Adhesive forces as well as floc properties depend to a great extent on the concentration and molecular weight of the flocculant. The experimental results indicate that the hydrodynamic forces have a definite effect both on particle adhesion and on floc stability as a consequence of conformational changes of the bridging molecules.

AGGREGATE CHARACTERISTICS AND TURBULENCE IN FLOCCULATION BASINS

An investigation has been undertaken using laser-Doppler velocimetry and flow visualization to better understand the interactions of turbulent eddies with the component processes of coagulation. Coagulant-aid activation, floc structure and density, and aggregate integrity are of particular interest. A 20 liter flocculation basin with two different paddle assemblies was constructed and examined extensively. The perforated blades produced a far mote energetic gross circulation at modest rates of rotation than obtainable with the hardware cloth paddles, and a larger initial aggregate growth rate. The hardware cloth paddle assembly appeared to produce greater floc volume fraction at large aggregate diameters, however. The objective of this study was to determine the extent to which the turbulent energy distribution can be manipulated to produce desired floc properties or floc growth patterns.