Floc Settling Research Articles

Flocculation of kaolin and kanto loam by methylated soy protein

In this study, methylated soy protein (MeSP) was applied to the solid–liquid separation or flocculation of kaolin and kanto loam (deposits of volcanic ash) suspensions (3.0g/L). The flocculation performance of MeSP was evaluated in terms of supernatant clarity (jar test method) and floc settling rate (sedimentation balance method), and compared with those of chitosan and polyaluminum chloride (PAC). At pH 7, the optimum dosage of MeSP, chitosan and PAC were almost the same (about 1.5wt.% of the suspended solids), but there was a remarkable difference in the floc settling rate. At the optimum dosage, the settling rate of the floc formed by MeSP was 6–7mm/s, while those formed by chitosan and PAC were about 2mm/s. MeSP showed the highest flocculation performance at pH 3–7 among these flocculants. At pH 8–12, the flocculation performance of MeSP was much higher than that of PAC, but lower than that of chitosan. The flocculation performance of MeSP for kanto loam suspension was strongly affected by the addition of seawater, while the effect of seawater concentration on the flocculation performance of MeSP for kaolin was moderate.

Characteristic analysis on morphological evolution of suspended particles in water during dynamic flocculation process

The evolution of floc morphology during constant-speed and variable-speed flocculation was examined to understand floc growth mechanisms. Flocculation-test results were reported in terms of floc average size and fractal dimension, derived from in situ optical sampling and image analysis. The morphological evolution was also described using a fractal growth model, which defined flocculation as the combined processes of aggregation and restructuring. During constant-speed flocculation, aggregation rate increased with increasing shear, but breakage became significant. Also, a decrease was observed after reaching the peak of size, possibly due to floc settling in low shear and the irreversibility of breakage in high shear. The development of floc morphology in variable-speed flocculation indicated that the surface nature of initial flocs was critical to form larger final flocs, because aggregates with irregular shape had more connection spots than those with smooth surface, thus producing a higher aggregation...

Nutrient conditions and reactor configuration influence floc size distribution and settling properties

Floc formation and settleability is critical for effective solid-liquid separation in many wastewater treatment processes. This study aimed to investigate the relationship between particle size distribution and nutrient conditions in different bioreactor configurations. Size distribution profiles of flocs that formed in continuous (B1), continuous with clarifier and return sludge (B2) and SBR (B3) reactors were investigated in parallel under identical nutrient conditions. An eight-fold dilution of the influent COD of a synthetic dairy processing wastewater resulted in a 'feast and famine' regime that triggered significant effects on the biomass and flocculation characteristics. Floc size analysis of reactor MLSS revealed a shift in floc sizes when reactors were fed with the minimum (famine) COD wastewater feed (0.61 g L(-1)). Increasing floc size distributions were detected for all reactors during the minimum COD feed although different size patterns were observed for different reactor configurations. These increases corresponded with variations in aggregation and EPS quantities. The SBR yielded comparatively larger flocs when operated under both COD feeds as indicated by d(0.9) values (90% of particles ≤ d in size). Overall the results indicated that floc formation and floc size are mediated by nutrient concentrations and represents an important step towards improved solid-liquid separation.

Chitosan 응집제를 이용한 준설토 부유물질 처리

The objective of this study is to development of IDFIS system, that are consisted of hydrocyclone, rapid flocculation and inclined settler with chitosan coagulant. As the results of Jar test, a chitosan optimum dosage of 40 mg/L for river sediment, and 5 mg/L for tunnelling wastewater sediment, which these conditions leaves of residual turbidity of less than 5 NTU. Because of the effectiveness of chitosan in removing turbidity was independent on pH, the operation of IDFIS system would be simple. The synthesized turbidity was made with clay particles, river sediment, river suspended sediment, and tunnelling wastewater sediment. Results indicate that the mean overall removal efficiency of turbidity, SS, COD and TP were 98%, 99%, 85% and 95%, respectively. Chitosan is very efficient in removing turbidity in the entire turbidity range examined. IDFIS system would have possibility with compact design, because of the increase of floc size favours the floc settling speed and reduces the settling time.

An Explicit Full-Range Settling Velocity Equation for Mud Flocs

Abstract A simple explicit model for calculating the settling velocity of mud flocs in both the viscous and inertial settling range is presented. The model is based on a nondimensional settling velocity and floc diameter, and the basic structure of the model follows that of Ferguson and Church (2004) with modifications for the fractal structure of flocculated sediment. The explicit form of the equation simplifies settling-velocity calculations and makes it easier to code into larger computational models of river plume sedimentation and morphodynamics. Input parameters needed for calculating settling velocity include: the floc size, the size of the primary particles, the fractal dimension of the floc, the specific gravity of the sediment, and values for two coefficients which incorporate effects due to floc shape, permeability, and flow separation on drag. The behavior of the model using a constant and variable fractal dimension over a range of parameter space is presented. The model is also compared to historic settling-velocity data from in situ measurements and laboratory settling-column data. For much of the field and laboratory data examined, floc settling remained viscous and is best described with a constant fractal dimension. Flocs with fractal dimensions equal to and greater than 2.5 are interpreted as impermeable aggregates, and calculated drag coefficients are larger than those of a smooth solid sphere of equivalent-volumetric diameter. It is suggested that the increased drag is due to the increased surface area of a floc in contact with the fluid relative to a smooth sphere of equivalent diameter. For flocs with fractal dimensions less than 2, calculated drag coefficients are on the order of those for a solid particle of equivalent diameter, even though it is likely that flocs have a larger relative surface area; this phenomenon is interpreted as being the outcome of flow passing through the more permeable flocs as they settle. Estimates of settling-velocity model coefficients are given for these two broad sets of floc settling behavior, and a recommended methodology for using measured settling velocity data to calibrate the model is presented.

Shear-induced floc structure changes for enhanced dewatering of coal preparation plant tailings

The handling and disposal of industrial waste slurries containing colloidal particulate matter, with particular reference to coal and mineral processing plant tailings with a high proportion of clay materials present significant challenge. To develop effective disposal methods requires an improved understanding of floc structure that promote better water recovery and higher sediment density during settling and consolidation of flocculated tailings. In this paper, the effect of floc structure on coal tailings dewatering and the impact of shear-induced modification of floc network structure on additional sediment consolidation and densification have been examined. Four real tailings and a high molecular weight anionic polymeric flocculant were used. Flocculation tests were conducted under reproducible hydrodynamic conditions in batch mode. Flocculated aggregates have complex structures with different sizes, shapes and voidages. The structures of the flocs were characterised by fractal analysis using light scattering technique and the dewatering characteristics of the flocs were investigated in terms of settling rates and sediment consolidation. The fractal analysis by laser scattering identified optimum mixing speed and flocculant dosage required for the production of flocs with compact structure of fractal dimensions greater than 2. It was determined that hydrodynamic conditions during flocculation have a profound impact on floc settling rates and to a lesser extent sediment consolidation. Low mixing speeds produced large flocs with high settling rates, while very high mixing speed during flocculation resulted in irreversible floc degradation leading to much reduced settling rates with some additional sediment consolidation. Application of shear to pre-sedimented flocs resulted in significant additional sediment consolidation for the tailings samples and flocculants studied. This approach would allow flocculation to be optimised for fast settling with subsequent application of shear to the settled sediment to promote the removal of inter and intra-floc liquor to improve consolidation and sediment density.

Effects of humic acid on physical and hydrodynamic properties of kaolin flocs by particle image velocimetry

The physical and hydrodynamic properties of kaolin flocs including floc size, strength, regrowth, fractal structure and settling velocity were investigated by in situ particle image velocimetry technique at different humic acid concentration. Jar-test experimental results showed that the adsorbed humic acid had a significant influence on the coagulation process for alum and ferric chloride. Kaolin flocs formed with the ferric chloride were larger and stronger than those for alum at same humic acid concentration. Floc strength and regrowth were estimated by strength factor and recovery factor at different humic acid concentration. It was found that the increased humic acid concentration had a slight influence on the strength of kaolin flocs and resulted in much worse floc regrowth. In addition, the floc regrowth after breakage depended on the shear history and coagulants under investigation. The changes in fractal structure recorded continuously by in situ particle image velocimetry technique during the growth-breakage-regrowth processes provided a supporting information that the kaolin flocs exhibited a multilevel structure. It was proved that the increased humic acid concentration resulted in decrease in mass fractal dimension of kaolin flocs and consequently worse sedimentation performance through free-settling and microbalance techniques.

Organic modifier of poly-silicic-ferric coagulant: Characterization, treatment of dyeing wastewater and floc change during coagulation

Organic modifier (PSFD) of poly-silicic-ferric (PSF) coagulant was prepared by adding dimethyldiallylammonium chloride (DMDAAC) as additives in PSF, and was characterized by Scanning Electron Microscopy (SEM), Photon Correlation Spectroscopy (PCS) and Fourier Transform Infrared Spectroscopy (FT-IR). Color removal and floc change during coagulation process in treating a simulated dyeing wastewater by PSFD were investigated compared to those of PSF. The results show that a similar silicate of cross-linking matter was generated in PSFD, and PSFD has greater adsorbtion capacity than PSF. The size of PSFD hydrolysis products was larger than that of PSF over a given concentration range. The size of PSFD hydrolysis products tended to change continuously and the small size of products disappeared after its hydrolysis. PSFD has better color removal at given dosage range than PSF, with the largest difference of 30%. The floc growth and settling velocity of PSFD were larger than those of PSF. Larger flocs of PSFD were formed from its small initial flocs over a very short period of time. The floc density of PSFD was greater than that of PSF, leading to a much less amount of PSFD flocs in the finished water than that of PSF.

Experimental assessment of Athabasca River cohesive sediment deposition dynamics

Polycyclic aromatic hydrocarbons (PAHs) originating from natural sources, and potentially from the Athabasca Oil Sands development, are of concern for the Athabasca River and Lake Athabasca delta ecosystems. In order to model the transport of fine sediments (and associated PAHs), it is important to describe the sediment dynamics within the river system. Flocs possess different settling characteristics compared to individual particles. A key aspect in modelling floc settling behaviour is the mathematical linkage of the floc density to floc size. In this paper, a rotating annular flume is used to determine the settling characteristics of Muskeg River (a tributary of the Athabasca River) sediments under different shear conditions. Simulations of the settling and flocculation behaviour of these sediments were used to calibrate a density vs. floc size model. A relationship of the parameters relating floc size and density with the fractal dimension F shows that as diameter increases flocs become weaker. Recommendations for the practical application of the model are further formulated in this paper. The deposition tests offer a quantitative measure of the relative amount of sediment that is likely to be transported through the river for given flow conditions.

Settlement Properties of the Suspended Sediment using Micro Videoscope in the Chikugo River Estuary

Under water micro video scope was developed to measure floc settling velocity and applied to the suspended sediment floc in the Chikugo River estuary. The effective densities of the flocs were measured successfully. The floc density distribution function was proposed from the floc settling speed and their diameters. The floc density distribution function varied with tide. In ebb tide, floc densites were smaller than in flood tide. This fact shows that the flocs in flood tide mainly consists of the mud which was suspended from the river bed. Mass settling flux was also calculated. When suspended sediment contains algae or algal residuals, the settling velocity showed lower value. On the other hand, containing high concentration of extracellular polymeric substances (EPS) made settlement velocity high.

Floc properties and settling velocity of San Jacinto estuary mud under variable shear and salinity conditions

The flocculation properties of a natural silt–clay mud taken from the San Jacinto estuary near Houston, TX were investigated over a range of suspended sediment concentrations, salinities, and turbulent shear rates. The study was conducted in a laboratory using a paddle mixer to create a turbulent shear field for driving the flocculation process; floc settling velocity and size attributes were measured in a settling column with a camera system and image analysis. In general, maximum floc sizes were observed at turbulent shear rates less than 30 s −1. For turbulent shear rates less than 50 s −1, flocs originating in saline water at 10 and 15 ppt were larger and more loosely packed than those created in freshwater at identical turbulent shear rates and suspended sediment concentrations. At turbulent shear rates of 50 s −1, floc properties of size, submerged specific gravity and fractal dimension were approximately equal for all conditions. Two ranges of behavior were observed in regards to the fractal dimension of flocs at a constant turbulent shear rate. In the first region, for floc sizes less than 200 μ m , a variable fractal dimension was needed to describe the submerged specific gravity as a function of floc size. It is suggested that this variability in fractal dimension may physically be the result of ploysized primary particles. In the second region, for floc sizes greater than 200 μ m , a constant fractal dimension was found to suffice in describing the submerged specific gravity. The constant fractal dimension for this second region was n f =2.3 for freshwater flocs and n f =1.95 for saltwater flocs. Based on the observation of the two-region behavior, a new variable fractal dimension model is presented using a simple exponential decay which asymptotically approaches a constant fractal dimension at a specified floc size. The model itself is based on the uniform-size primary particle formulation of the relationship between floc size and fractal dimension, but the equation was able to model the average shift from variable to constant fractal dimension, which was thought to primarily be due to polysized primary particles.

Application of fractal dimensions to study the structure of flocs formed in lime softening process

The use of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. Fractal dimensions of flocs were measured directly on floc images and indirectly from their settling velocity. An optical microscope with a motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The directly determined fractal dimensions of the lime softening flocs were 1.11–1.25 for floc boundary, 1.82–1.99 for cross-sectional area and 2.6–2.99 for floc volume. The fractal dimension determined indirectly from the flocs settling rates was 1.87 that was different from the 3D fractal dimension determined directly on floc images. This discrepancy is due to the following incorrect assumptions used for fractal dimensions determined from floc settling rates: linear relationship between square settling velocity and floc size (Stokes’ Law), Euclidean relationship between floc size and volume, constant fractal dimensions and one primary particle size describing entire population of flocs. Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>50 μm) lime softening flocs better than Stokes’ Law. Settling velocities of smaller flocs (<50 μm) could still be quite well predicted by Stokes’ Law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster–cluster aggregation for small flocs (<50 μm) and diffusion-limited aggregation for large flocs (>50 μm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc formation mechanisms.

Comparison of Acid-Soluble and Water-Soluble Chitosan as Coagulants in Removing Bentonite Suspensions

Chitosan is a biodegradable cationic polymer that may be a potential substitute for aluminum salts in water treatment systems. In our study, we compared the coagulation performances of chitosan with those of coagulant mixtures of chitosan and aluminum sulfate and chitosan and poly-aluminum chloride, respectively. The coagulation efficiency was evaluated in terms of coagulant dosage, solution pH, settling velocity of flocs, floc diameter, and water turbidity. The optimum dosages for acid-soluble and water-soluble chitosan required for removal of a bentonite suspension (100 NTU) were only 1.25 and 1.50 mg/l, respectively, at a respective efficiency of 99.2% and 95.8%. The optimal dosage range for water-soluble chitosan was broader than that for acid-soluble chitosan. The coagulation of bentonite decreased with increasing pH when acid-soluble chitosan was the coagulant. In contrast, the coagulation efficiency of bentonite was not affected at pH 5–9 when water-soluble chitosan was the coagulant. The mixing of chitosan with alum or PAC in a 1:1 mass ratio significantly improved the coagulation process in terms of preventing the occurrence of re-stabilization. The highest floc settling velocity occurred at a dosage of 5–6 mg/l of the coagulant mixtures, which was also the highest coagulation efficiency. Given the relatively high cost of chitosan and the good coagulation performance of the coagulant mixtures, we suggest that a 1:1 mass ratio of chitosan mixed with alum or PAC may be an alternative method to the use of pure chitosan in water treatment systems.

Observations and analyses of floc size and floc settling velocity in coastal salt marsh of Luoyuan Bay, Fujian Province, China

In coastal environments, fine-grain sediments often aggregate into large and porous flocs. Electro-Magnetic Current Meters (EMCM) and Laser In Situ Scattering and Transmissometry (LISST-ST) have been deployed within a Spartina alterniflora marsh of the Luoyuan Bay in Fujian Province, China, to measure the current velocity, the floc size and the settling velocity between 15 and 22 January 2008. During the observations, the near-bed water was collected in order to obtain the suspended sediment concentration (SSC) and constituent grain size. Data show that: (1) the nearbed current velocities vary from 0.1 to 5.6 cm/s in the central Spartina alterniflora marsh and 0.1–12.5 cm/s at the edge; (2) the SSCs vary from 47 to 188 mg/dm3. The mean grain size of constituent grains varies from 7.0 to 9.6 µm, and the mean floc sizes (MFS) vary from 30.4 to 69.4 µm. The relationship between the mean floc size and settling velocity can be described as: ws=adb, in which ws is the floc settling velocity (mm/s), a and b are coefficients. The floc settling velocity varies from 0.17 to 0.32 mm/s, with a mean value of 0.26 mm/s, and the floc settling velocity during the flood tide is higher than that during the ebb tide. The current velocity and the SSC are the main factors controlling the flocculation processes and the floc settling velocity.

Flocculation of diatomite by a soy protein-based bioflocculant

A novel bioflocculant, methylated soy protein (MeSP), has been developed and its flocculation performance was tested with diatomite suspensions in freshwater (pH 2–11) and in seawater. The flocculation performance of MeSP was much higher than that of commercial polyaluminum chloride (PAC) in terms of supernatant clarity and floc settling velocity. In freshwater and at a fixed flocculant dosage of 60 ppm, MeSP could reduce the relative absorbance to 0.1 within 1 min over a wide pH range (pH 3–10), while PAC was effective only at around pH 5. In seawater (pH 8), there was a striking difference in flocculation performance between MeSP and PAC. MeSP (20–50 ppm) could reduce the relative absorbance to less than 0.1, while PAC was ineffective. The settling velocity of the floc formed by MeSP in freshwater at pH 7 and in seawater was about 4 mm/s. MeSP was applied to the flocculation of a real wastewater generated from an andesite quarry. The relative absorbance could be reduced to less than 0.05 by the addition of 40 ppm of MeSP. The floc settling velocity was about 5 mm/s.

Observations of Floc Sizes in a Muddy Estuary

Measurements are presented of median floc diameters and associated environmental data over spring-tide tidal cycles at two stations in the muddy Tamar Estuary, UK, for winter, spring and summer conditions. The particulate organic carbon and particulate total carbon contents of mudflats and SPM (suspended particulate matter) at the stations, together with other evidence, indicates that much of the SPM was derived from mud sources that were located between the two stations during winter and spring, and from very mobile sediment sources in the upper estuary during summer. Observed in-situ median floc sizes varied widely, from <50 to >500 μm and rapid settling of particles close to HW and LW (high and low water) left only the smaller flocs in suspension. Time-series of depth-averaged median floc sizes generally were most closely, positively, correlated with depth-averaged SPM concentrations. Floc diameters tended to reach maximum median sizes near the time when SPM concentrations were highest. These high concentrations were in turn largely generated by resuspension of sediment during the fastest current speeds. Although such correlations may have arisen because of SPM-driven floc growth - despite fast tidal currents - there is also the possibility that tough aggregates were eroded from the intertidal mudflats and mudbanks. Although a hypothesis, such large aggregates of fine sediment may have resulted from the binding together of very fine bed particles by sticky extracellular polymeric substances (EPS) coatings, produced by benthic diatoms and by other biologically-mediated activity. A rapid reduction of SPM occurred at the up-estuary station within 2.5 h of HW on the flood, when decelerating currents were still relatively fast. It appears that at least two processes were at work: localised settling of the largest flocs and up-estuary transport in which large flocs were transported further into the estuary before settling into the Tamar's ETM (estuarine turbidity maximum) over the HW-slack period. Up-estuary advection of large flocs and their eventual settling would place the down-estuary edge of the ETM above the upper-estuary station during summer, spring-tide conditions. This position of the ETM was observed close to HW during longitudinal surveys of the estuary.

Non-equilibrium flocculation characteristics of fine-grained sediments in grid-generated turbulent flow

Results are presented from a series of settling column experiments investigating temporal variations in the flocculation characteristics of purely cohesive (kaolin clay) sediment suspensions and cohesive (kaolin) and non-cohesive (fine sand) sediment fraction mixtures. Experimental runs were conducted under controlled hydrodynamic conditions generated by a rigid array of in-phase oscillating grids. The results indicated that rapid initial floc aggregation occurred under low turbulent shear rates, with peak maximal and root-mean-square (r.m.s.) floc sizes (∼ 400 μm and ∼ 200 μm, respectively) attained after relatively short time periods, before reducing with time. By contrast, lower aggregation rates and smaller floc sizes were observed under higher shear conditions, with flocs retaining suspended in the settling column for longer time scales due to the increased turbulence. The mud input concentration displayed some correlation with maximal and r.m.s. floc sizes at higher shear rates but no correlation was apparent at low shear rates. This observed floc behaviour may be attributed to the differences in concentration gradients at high and low shear rates that affect both floc settling rate and time required for flocs to attain equilibrium size. The addition of the fine sand fraction to the kaolin clay suspension reduced both the initial floc formation (i.e. aggregation) rate and the maximal and r.m.s. floc sizes attained throughout the experiments. The reduction in maximal floc sizes appeared to be enhanced by an increase in the ratio of fine sand to kaolin clay content within the mixture.

Hydrodynamics of fractal flocs during settling

The settling and hydrodynamic properties of 3-D fractal flocs in quiescent water are investigated with a numerical model based on the Lattice Boltzmann Method (LBM), with considering the settling velocity, hydrodynamic drag force and infra-floc flow. The comparisons of floc settling velocities and effective densities indicate that the numerical results present good agreements with observations in field and at laboratory. The results show that the drag force F D increases with the floc size d f according to the relationship F D ∝ d 3 f . Moreover, the intra-floc flow field and movement of the pore water provide a better understanding of the intra-floc flow from the microscopic viewpoint. The results also indicate that the lattice Boltzmann method is a promising approach to reveal the mechanisms of the flocculation in aquatic environments.

Heavy metals removal by flocculation/precipitation using N-(2-carboxyethyl)chitosans

Here we report on solution properties and flocculation performance of polyampholyte chitosan derivatives – N-carboxyethylated chitosans (CEC) – synthesized “in gel” by treatment of chitosan with acrylic acid. We show that carboxyethylation with the substitution degrees (DS) from 0.7 to 1.6 yields water-soluble derivatives, whose isoelectric points (IEP) range from 6.30 to 3.55. At pH > 7.5 interaction of negatively charged CEC derivatives with positively charged colloids of the heavy metal hydroxides (Zn 2+, Cu 2+, Ni 2+), serving as a model of postgalvanic wastewaters, results in lowering of electrokinetic potential of the hydroxide colloids and their precipitation at appropriate flocculant dose. Investigations of CEC flocculation performance depending on pH and DS show that the efficiency of metal removal decreases in the row Cu 2+ > Zn 2+ > Ni 2+ corresponding to the row of the hydroxocomplex stability for these metals. The higher was pH of the system and DS of CEC derivative, the narrower was the flocculation window and the stronger was negative effect of flocculant overdosing on the efficiency of heavy metal removal. Based on the estimation of floc settling rates and the residual metal concentrations, we have concluded that the optimal DS of CEC derivatives for the precipitation of metal hydroxides is 0.7–1.0.

Relationship between floc composition and flocculation and settling properties studied at a full scale activated sludge plant

The variation in activated sludge floc composition, flocculation and settling properties was studied at a full scale plant over a 2-year period. A comprehensive set of process parameters was analysed and related to the floc properties to increase the understanding of the factors affecting floc formation. The composition of the activated sludge showed a seasonal change with higher concentrations of extractable extracellular polymeric substances (EPS) during the winter months. The protein content of the total sludge and EPS increased significantly during the winter. This coincided with higher effluent suspended solids concentrations and increased shear sensitivity of the sludge flocs. Only poor correlations between EPS contents and stirred sludge volume index (SSVI) could be observed. High iron concentrations in the sludge due to dosage of iron salt to precipitate phosphorus were found to have a negative impact on the settling and compaction properties of the sludge, whereas it had a positive impact on floc stability. Higher organic loading due to by-passed primary settlers leads to improved settling and compaction properties.