Floc Breakage Research Articles

Dual coagulation with floc breakage to alleviate ultrafiltration membrane fouling caused by algae organic matter

A novel coagulation strategy, where floc property was regulated by varying dosing sequence of dual coagulants of aluminum sulfate (AS) and chitosan (CS) with floc breakage, was applied as the pre-treatment of ultrafiltration to treat algae-laden water. The river water (TW), and TW spiked with intracellular organic matter (IOM) and extracellular organic matter (EOM) respectively (TW-IOM and TW-EOM) were tested. The results indicate that floc breakage applied in successive dosing of AS and CS in TW and TW-IOM water samples formed compact flocs through charge neutralization and bridging, generating dense cake layer to control irreversible fouling; and mitigated irreversible fouling (Rir) caused by protein-like and humic-like species in the molecular weight (MW) of 950–3000 Da. For TW-EOM water sample, the successive addition of CS and AS can relieve Rir by removing protein-like species and soluble by-products in the MW of 500–3000 Da via charge neutralization, sweeping flocculation and bridging together, while the applied breakage aggravated membrane fouling. The obtained results provide a novel coagulation strategy to control membrane fouling during algae-laden water treatment.

Formation, breakage, and re-growth of quartz flocs generated by non-ionic high molecular weight polyacrylamide

Flotation of fine mineral particles is one of the major challenges faced by the minerals industries worldwide. Flocculation of these fine particles prior to flotation, to form separate hydrophobic and hydrophilic flocs, is one of the methods to tackle this challenge. In order to gain a better understanding of the properties of hydrophilic polymer flocs for the intended flocculation flotation, this paper reports testwork completed on the formation, breakage, and re-growth of quartz flocs when non-ionic high molecular weight polyacrylamide (PAM) was used as a flocculant under different test conditions in an alkaline environment. Focused beam reflectance measurement (FBRM) particle size analyses, coupled with sedimentation tests and optical microscope observations were performed to monitor the real-time evolution and change of quartz flocs. In addition, adsorption experiments and zeta potential measurements were carried out and used as parameters in the DLVO theory interaction energy estimation. The results of DLVO theory interaction energy estimation, settling experiments and optical microscope observations showed that the quartz particles dispersed well in slurry at pH 9. PAM had a strong flocculation ability to quartz particles, the peak values of floc size were affected by both the stirring rate and PAM concentration. In addition, the degree of floc breakage depended on the increased stirring rate and the shear time. Floc re-growth occurred to different extents according to the breakage durations and PAM dosages.

Enhanced coagulation by two-stage alum addition: the role of solution pH, floc breakage and assistant of non-ionic polyacrylamide

ABSTRACT Coagulation process is highly influenced by solution pH, floc breakage, and coagulant aid. In this work, two-stage addition of alum-based coagulants, polyaluminum chloride (PACl) and aluminum sulfate (Al2(SO4)3), was compared to their one-stage addition with the same total dosage. The dose ratio of primary and secondary addition was optimised. The effects of solution pH, floc breakage and dosing timing of coagulant aid of non-ionic polyacrylamide (NPAM) on coagulation efficiency and floc property were investigated. The results showed that two-stage addition of PACl showed less sensitivity with the variation of pH. Compared to PACl, two-stage addition of Al2(SO4)3 was more sensitive to the solution pH, especially when the pH ranged from 7 to 10. For all cases, the presence of floc breakage improved the removal of colloidal particles. The coagulation mechanism of primary coagulant was charge neutralisation, while other mechanism was involved with the secondary addition of coagulant. Compared to one-stage addition, the settling performance of aggregates and particulates removal in two-stage addition was more effective with the variation of dosage or dosing timing of NPAM. The present work provides detailed operating parameters for this coagulation strategy.

Effect of Al species of polyaluminum chlorides on floc breakage and re-growth process: Dynamic evolution of floc properties, dissolved organic matter and dissolved Al

Influence of floc breakage and re-growth on the release of natural dissolved organic matter (DOM) and dissolved Al was explored. Results indicated that Al species including monomeric species (Ala), medium polymer species (Alb), and colloidal or solid species (Alc) in polyaluminum chlorides (PACls) played significant role. At lower doses ranged from 5 to 20 mg/L, floc breakage damaged Ala-NOM bonds for AlCl3, causing obvious release of DOM and dissolved Al. After re-growth, dissolved Al mainly connected with broken flocs, rather than released DOM. Thus, after re-growth, DOM release was still remarkable, but additional removal of dissolved Al was observed. At higher doses above 20 mg/L, more Ala transformed to Alb and Alc. Due to the enmeshment effect induced by Alc coagulation, fewer DOM and dissolved Al were released after breakage, and additional removal of DOM and dissolved Al were attained after re-growth. For PAClAl13 which mainly contained Alb, at optimal dose, floc breakage generated the most severe release of DOM and dissolved Al, while the result after re-growth was just reverse. This was ascribed to stronger charge neutralization ability of Alb. Furthermore, the influence of floc breakage and re-growth on DOM and dissolved Al for PAClC was similar to that for AlCl3. The reason was fully analyzed in this research. This study may give further indication regarding reaction mechanisms of floc breakage and re-growth for PACls.

The Impact of Local Hydrodynamics on High-Rate Activated Sludge Flocculation in Laboratory and Full-Scale Reactors

High rate activated sludge (HRAS) processes have a high potential for carbon and energy recovery from sewage, yet they suffer frequently from poor settleability due to flocculation issues. The process of flocculation is generally optimized using jar tests. However, detailed jar hydrodynamics are often unknown, and average quantities are used, which can significantly differ from the local conditions. The presented work combined experimental and numerical data to investigate the impact of local hydrodynamics on HRAS flocculation for two different jar test configurations (i.e., radial vs. axial impellers at different impeller velocities) and compared the hydrodynamics in these jar tests to those in a representative section of a full scale reactor using computational fluid dynamics (CFD). The analysis showed that the flocculation performance was highly influenced by the impeller type and its speed. The axial impeller appeared to be more appropriate for floc formation over a range of impeller speeds as it produced a more homogeneous distribution of local velocity gradients compared to the radial impeller. In contrast, the radial impeller generated larger volumes (%) of high velocity gradients in which floc breakage may occur. Comparison to local velocity gradients in a full scale system showed that also here, high velocity gradients occurred in the region around the impeller, which might significantly hamper the HRAS flocculation process. As such, this study showed that a model based approach was necessary to translate lab scale results to full scale. These new insights can help improve future experimental setups and reactor design for improved HRAS flocculation.

Experimental approaches for identifying the impact of enhanced flotation technology using hollow microspheres

Hollow microspheres should be characterized in terms of physicochemical aspects to understand the flotation effect principle. There have been insufficient studies on the effect of hollow microspheres in water treatment in terms of flotation. In this study, various analytical and experimental approaches were utilized to identify the flotation characteristics and understand the effect of hollow microspheres on flotation. These approaches included analytical methods such as particle count and zeta potential measurements, scanning electron microscopy-energy dispersive X-ray spectroscopy analysis, X-ray photoelectron spectroscopy, a floc breakage experiment, and a collision efficiency model. The hollow microspheres were spherical shape with an average size of 50 μm. The microspheres were identified to have a higher negative charge (−69.1 mV) than microbubbles (−51.1 and −30.5 mV). The binding energy of Si–O from the hollow microspheres showed the highest peak at 103.18 eV and 61,503.7 counts/s. Si–O binding structure and the molecular structure of the SixOx series were considered to be a structure in which Si32− is bonded to oxygen ion. The optimum floc breakage condition was determined to be 15 min. The number of particles increased because the average particle size is increased with the concentration of hollow microspheres injection. The turbulent flocculation (TF) model was confirmed to be consentient to the experimental data in comparison with the white-water bubble blanket (WWBB) model. As a result, the hollow microspheres had the characteristic of increasing floc size so that flocs can be floated easily.

Effects of powdered activated carbon on the coagulation-flocculation process in humic acid and humic acid-kaolin water treatment

The addition of powdered activated carbon (PAC) to remove micropollutants is a commonly used technology to improve drinking water quality. However, the effects of PAC dosing strategy on the coagulation-flocculation process of water treatment have not been well understood, especially for water with low amounts of inorganic particles. Therefore, the current research aimed to comprehensively study the effects of simultaneous addition of PAC and aluminum sulfate (AS) coagulants (denoted as PAC-AS) or adding PAC 2 h before coagulation (denoted as PAC2h-AS) on the coagulation behavior in humic acid (HA) and HA-kaolin water treatment. The results showed that the floc size, growth rate, breakage factor, and fractal dimension were all enhanced by PAC-AS and PAC2h-AS for HA but not for HA-kaolin water treatment. In HA water treatment, PAC-AS reached a larger floc size and faster growth rate, while PAC2h-AS achieved a larger floc breakage factor and fractal dimension value. For PAC2h-AS, the pre-adsorption of HA onto PAC would lower the initial particle concentration and reduce the collision probability during HA water coagulation process; thus, the DOC removal efficiency, floc size, and growth rate of PAC2h-AS were relatively smaller than those of PAC-AS. For the floc strength and floc fractal dimension, the pre-adsorption of HA onto PAC contributed to formation of stronger inter-particle bonds; thus, stronger and more compact flocs were formed by PAC2h-AS compared with those of PAC-AS. The addition of PAC had a smaller impact on the floc properties in HA-kaolin water treatment owing to its higher initial particle concentration.

Effect of aeration rate on the anti-biofouling properties of cellulose acetate nanocomposite membranes in a membrane bioreactor system for the treatment of pharmaceutical wastewater

In this study, the effect of aeration rate in terms of specific aeration demand per membrane area (SADm) on the anti-biofouling properties of cellulose acetate (CA) nanocomposite membranes (CA/ND-NH2) in a membrane bioreactor system was investigated. The amount of EPS and soluble EPS under high aeration rate conditions was observed to be higher than under low aeration rate conditions. The results obtained showed that either lower or higher aeration rates had a negative impact on membrane permeability. The high aeration rate resulted in a severe breakage of sludge flocs, and promoted the release of soluble EPS from the microbial flocs to the bioreactor tank. By increasing the aeration rate, the COD removal increased and decreased respectively for the membranes and the activated sludge. It was finally concluded that higher anti-biofouling properties of neat CA and nanocomposite membranes were obtained under optimal aeration rate conditions (SADm = 1 m3 m−2 h−1).

Better understanding the polymerization kinetics of ultrasonic-template method and new insight on sludge floc characteristics research

As one of the core technologies employed in the field of sludge conditioning, flocculation has the ability to improve the sludge dewatering performance and reduce its volume and amount, which can accordingly result in lower costs in sludge transportation as well as subsequent disposal. Therefore, the development of new and high-efficiency flocculants is a hot topic in this field. The template copolymer (TPAD) of acryloyloxyethyl trimethyl ammonium chloride (DAC) and acrylamide (AM) was successfully synthesized through ultrasonic-template copolymerization using sodium-polyacrylate (NaPAA) as a template. The analysis of FTIR, 1H (13C) NMR, TG/DSC and SEM revealed that TPAD had a conspicuously significant cationic segmental structure. In addition, the results obtained from the analysis on the association constant (KM) and the kinetics of the template reaction indicated that the ultrasonic-template was a free radical initiated polymerization and the polymerization mechanism was I Zip-up (ZIP), and which once again confirmed the formation of the cationic fragment structure. This novel cationic fragment structure in TPAD greatly enhanced the ability of charge neutralization, electric patching, adsorption and bridging, thus improving the active sludge conditioning and dewatering performance (FCMC: 72.9%, SRF: 4.0 × 1012 m·kg−1, d50: 228.604 μm, Df: 2.02 at 400 r/min). The floc breakage and regeneration experiments showed that the cationic fragment structure in TPAD could make great contribution to the formation of large and dense floc structures, and these flocs were able to regenerate rapidly after breakage. Finally, it was also known that these large and compact floc structures were beneficial to the creation of more channels and voids, thereby decreasing sludge resistance (SRF) and improving sludge dewatering performance.

The variation of flocs activity during floc breakage and aging, adsorbing phosphate, humic acid and clay particles

The mechanism of removal of humic acid, phosphate and kaolin particles by coagulation with alum and PACl or adsorption by their pre-formed precipitates was investigated, and it was found that the coagulation mechanisms for monomeric Al at neutral pH and polymeric Al13 at alkaline pH were very similar. The removal of phosphate and humic acid by coagulation with alum or PACl did not change with stirring time (between 1 min and 15 min), independent of the dose and species of coagulants. However, for adsorption of these impurities by pre-formed precipitates, the results were significantly different. Both Al3+ and nano-sized Al13 could precipitate and form aggregates at pH 7 and pH 9, respectively, and their precipitates became less active (fewer binding sites on the surface of precipitate) with the increase of shear time or shear rates before adsorbing pollutants. Thus, although the total surface area increased (the average size of flocs became smaller) at higher applied shear rates or longer shear time, the removal efficiency of humic acid and phosphate decreased. Also, from the MW distributions, it was confirmed that less humic acid was removed by the adsorption on alum precipitate pre-formed with longer shear time. Chemical groups (OH2and OH) on the surface of precipitate determined the removal efficiency of phosphate and humic acid, and the activity of precipitate become lower as a result of higher applied shear and longer shear time. This is confirmed be due to some crystallization of the amorphous precipitate, forming inactivated hydroxyl. When kaolin was added 10 min after the alum or PACl precipitate formed, the precipitates captured kaolin particles only on their surface, whereas when alum was added to kaolin suspensions particles were trapped within the growing flocs. When alum/kaolin flocs were broken at high shear rate re-growth of flocs decreased with increasing shear time, but after a short breakage period, long aging of broken flocs had little effect on floc regrowth.

Effects of ballasting particles on settling rate of iron ore tailings

The high density or large size grains are proposed to be added as ballasting particles to achieve a high settling rate and low supernatant turbidity for fine particle flocs during the settlement of ore tailings. Settling tests on iron ore tailings were carried out to evaluate the effects of size, density, and dosage of ballasting particles on initial settling rate and turbidity. The results showed that low density (2.65 g/cm3) particles and medium density (4.67 g/cm3) particles contributed to the increase in settling rate of the tailing slurry at all size fractions from 0.074 to 0.5 mm, with the peak performance obtained between 0.125–0.2 mm, whereas high density (6.14 g/cm3) particles only accelerated the process with the particle size not larger than 0.2 mm. It is supposed that there is a close relationship between ballasting particle properties and floc strength. The acceleration effect dominates when flocs are strong enough to bear the ballasting particles; deceleration occurs when ballasting particles are oversize, over-density, or overdosed, causing the breakage of flocs.

Optimal hydraulic shear strength and mechanism of activated sludge floc re-growth after breakage.

In order to evaluate the effects of hydraulic shearing action on activated sludge floc aggregation, floc aggregation, breakage and re-growth performances under different shear strengths were systematically examined. Performances were evaluated by measuring the floc size variations using a laser particle size analyser on-line monitoring technique. The flocs after breakage were characterised by investigating the composition of extracellular polymer substances (EPS), floc size, substances released due to surface erosion and chemical structures using Fourier transform infrared spectroscopy (FTIR) to clarify the breakage model and re-growth mechanism. The results showed that activated sludge flocs broken at a hydraulic shear strength GT value (the product of the velocity gradient and time) of 56,280 exhibited enhanced re-growth performance compared with the control. The substances released from sludge increased with shear strength, indicating that the floc breakage mode gradually transitioned from fragmentation to surface erosion. FTIR revealed that, after breakage, the spectrum of loosely bound EPS in sludge tended to be similar to that of tightly bound EPS. The results suggest that the breakage of activated sludge flocs under moderate hydraulic shear strength can fragment flocs into smaller particles without surface erosion and promote the exposure of inner tightly bound EPS, thereby improving re-growth performance.

Performance of Freshly Generated Magnesium Hydroxide (FGMH) for Reactive Dye Removal

Coagulation-flocculation (CF) is an essential process for water and wastewater treatment plants. Whilst compared with the conventional coagulants, the freshly generated magnesium hydroxide (FGMH) is environmental friendly within its coagulative and adsorptive jellied sediment. However, decoloration of dye wastewater using FGMH has not been well understood yet. In this paper, operating variables such as initial pH, coagulant dosage, agitation speed and duration were examined for efficient reactive dye removal. The flocs breakage and regrowth capacity after slow agitation of original CF phase were monitored for mechanism analysis. The mechanism analysis showed that the charge neutralization was the predominance during coagulation, and the precipitate enmeshment and adsorption bridging dominated the flocculation stage. In comparison with the initially formed flocs, though the regenerated flocs were considerably smaller after the second rapid agitation, the bridging interaction of loose flocs could absorb the small residual particles during the slow agitation stage.

Effect of impeller clearance on floc growth behaviors in a baffled square stirred-tank reactor: Flocculation-test and CFD-aided studies

The impeller clearance (C) and shear rate (Gave) effects on aggregation and breakage behaviors in the whole process of floc growth were investigated by applying lab-scale flocculation tests with the aid of computational fluid dynamics (CFD) simulations. For a baffled square stirred-tank flocculating reactor, the recirculating times for water flow to the impeller region and intensity of turbulence occurring within this region, together with the non-uniform nature of turbulence for water flow above the rotating impeller, were found to be more or less different in all C-value cases at a constant Gave, thereby affecting the development of floc morphology. Besides, the importance of impeller clearance (C) to floc morphological evolution seemed to be largely determined by predominant growth behavior(s), likely as a result of distinct occurring degrees of either aggregation or breakage for each combination of C and Gave; and interestingly, a certain compaction for the above effect took place at a relatively high shear, possibly attributed to increasingly pronounced occurrence of floc breakage and irreversibility of this behavior. To produce fractal flocs with favorable morphological characteristics, the impeller should be placed at an appropriate distance from the reactor bottom within the flocculating system, under whether aggregation-dominated or breakage-dominated circumstances, for weakening the role of reactor bottom and water surface on floc growth behaviors during flocculation. This research would be beneficial for thoroughly understanding floc formation mechanisms and optimally designing impeller-based flocculating reactors.

Modeling the growth of cohesive sediment flocs in three-dimensional space

This paper presents a model to simulate the growth of cohesive sediment flocs in 3-D space, which takes into account: Brownian motion contribution, gravity contribution, flow contribution, flow shear stress, floc breakage, and non-uniform initial sediment size distribution. Results of validation tests indicate the model is feasible to simulate the growth of cohesive sediment floc under different contributions. Floc structure in different water depths and the influence of initial sediment size distribution were tested using the new model, taking fractal dimension as an index. The simulated results show that, the sediment floc at the lower region possesses a larger fractal dimension than that of the upper region. It is mainly due to the reproduction of new stronger, denser flocs from the breakage and reform under the higher flow shear stress in the lower region. Additionally, on the assumption condition that the initial sediment size distribution follows normal distribution, an increase in the standard deviation results in the fractal dimension presents a trend from increasing to decreasing.

Dewatering characteristics of sludge conditioned by surfactant with bioleaching pretreatment

AbstractIn view of the merits of bioleaching and surfactant for sludge treatment, the possibility of surfactant with bioleaching pretreatment applied to improve sludge dewaterability was investigated in this work. The results showed that cetyl trimethyl ammonium chloride (CTAC) with bioleaching pretreatment was highly efficient in dewatering sludge. The optimal CTAC dosage and bioleaching pH for this combination were 120 mg/g (dry solids) and 3.55, respectively, under which the water content of filtrated cake was 68.94% and the specific resistance to filtration was 0.12 × 1013 m/kg with a reduction of 94.92%. Although CTAC and bioleaching both had positive effects on sludge dewatering, their combination was more efficient. The significant enhancement of sludge dewaterability was mainly contributed from the breakage and charge neutralization of sludge flocs, and the release of extracellular polymeric substances (EPS). In addition, the positive correlation between supernatant EPS concentration and sludge dewaterability was observed under experimental conditions.

Effect of kaolin on floc properties for reactive orange removal in continuous coagulation process.

Magnesium hydroxide was used as a coagulant for treating reactive orange wastewater in a real continuous process. Effects of kaolin on coagulation performance and floc properties were investigated with controlled experiments through floc size distribution, zeta potential, scanning electron microscopy and Fourier transform infrared spectroscopy. Kaolin had significant influence on magnesium hydroxide-reactive orange floc formation and growth. The results showed that average floc size reached 16.31, 12.88 and 20.50 μm, respectively, in the rapid mixer, flocculation basin and sedimentation tank when kaolin concentration was 10 mg/L and reactive orange initial concentration was 0.25 g/L. The floc size tended to increase with the increase of kaolin suspension to 10 mg/L. All of the flocs under investigation showed that floc breakage led to decreased average floc size and remained stable in the flocculation basin. Reactive orange and kaolin could be removed effectively in the continuous coagulation process. Reactive orange was adsorbed in the surface of magnesium hydroxide through charge neutralization and adsorption.

High shear breakage of compact polyelectrolyte-bridged flocs: A method for obtaining model-independent breakage rate function data

A method of obtaining model-independent breakage rate or fragmentation kinetic function (FKF) data in the population balance (PB) equation (PBE) was proposed for the high shear breakage of compact polyelectrolyte-bridged alumina flocs. The continuous particle size distribution function (PSDF) data were obtained as a function of fragmentation time. The PSDF data at zero fragmentation time was and must be used as the input in the PB modelling. Computed temporal PSDF curves were obtained by solving the dimensionless log-form PBE using a FKF based only on the fragmenting particle and a daughter size. Another important requirement is that the experimental time must be used in the scaling. Using appropriate values for the parameters in the FKF, an excellent match between the temporal experimental and computed PSDF data was achieved. Upon meeting all these conditions, the FKF values used in the computation should truly represent the experimental values for all father-daughter size combinations encountered in the fragmentation process. The fragmentation occurred in the rotor-stator device under the conditions where the Kolmogorov microscale and characteristics velocity gradient of 37 μm and 1600 s−1. The ratio of the largest to smallest length scale turbulent eddies was 2047.

Experimental study on reversible formation of 2D flocs from plate-like particles dispersed in Newtonian fluid under torsional flow

In this study the reversible aggregation and breakage of two dimensional (2D) flocs in the bulk phase are investigated experimentally in a suspension of platelike particles under a torsional flow between two parallel plates. The suspension was prepared by dispersing platelike particles in a Newtonian fluid. The Newtonian fluid has a viscosity of 1.27 Pa s and the average dimension of the particle face is approximately 1 μm with a thickness of 250 nm. Peclet number of the particle is over 100 and the aggregation is orthokinetic. The microstructure was examined under an optical microscope without disturbing the flow or particle motions. The result shows that 2D flocs of fractal types with the aspect ratio of over 20 between the average face diameter and thickness are formed in the bulk phase by the aggregation of platelike particles and the face of the planar floc is oriented in the same direction of the velocity gradient. The flocs have non-circular faces with size dependent fractal dimensions. The flocs tend to be oriented along the vorticity or the flow direction depending on shear rate and the orientation strongly affects floc breakage and floc size. The breakup of flocs occurs mostly at the middle of the floc after being aligned along the flow direction.

Comparative analysis on flocculation performance in unbaffled square stirred tanks with different height-to-width ratios: Experimental and CFD investigations

The effect of liquid-level height (H) on floc growth during flocculation has been investigated in unbaffled square stirred tanks with a fixed bottom width (D). Firstly, flocculation tests were performed by using an in situ recognition system for floc morphology to evaluate flocculation performance within each tank at three typical shear rates of Gave=10, 30 and 70s−1. Then, turbulent flow fields generated under all flocculation-test conditions were predicted by computational fluid dynamics (CFD) simulations, followed by a detailed discussion based on experimental and numerical data. It was found that stirred tanks with different H values (characterized by H-to-D ratio) caused distinct turbulent flow fields at the same shear rates examined, reflected by different turbulent intensities in the impeller zone and different non-uniform nature of turbulence in the overall flocculating system. Accordingly, the evolution of floc size and structure was greatly affected due to various aggregation and breakage kinetics. Moreover, the effect of H-to-D ratio on floc growth appeared to be related to the predominant growth mechanism(s), i.e., for the shear rates of Gave=10 and 30s−1, little or no significant breakage occurred and more significant non-uniformity of turbulence generated in the stirred tank with a higher H-to-D ratio led to increasingly pronounced behavior of restructuring, forming aggregates with more compact structure caused by primary-particle rearrangement, whereas at Gave=70s−1, breakage dominated over aggregation and resultant aggregates became smaller and more compact with increasing H-to-D ratios, possibly due to the irreversibility of floc breakage. Therefore, an appropriate liquid-level height of tank should be designed to enhance floc formation and subsequent particle removal efficiency. The combined means of flocculation tests and CFD simulations may provide useful insights for optimizing the design and operation of stirred-tank reactors for flocculation.