Floc Properties Research Articles

Effects of charge density and molecular weight of papermaking sludge-based flocculant on its decolorization efficiencies

The charge densities (CD) and molecular weights (MW) of the flocculants are closely related to their application performances, but seldom researches focus on the effects of flocculant CD and MW on decolorization efficiencies. Herein, a series of flocculants with various CD and MW levels, named as PBF1–9, were designed and synthesized from papermaking sludge. The physicochemical characteristics of the PBF1–9 were measured by fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), X-ray photoelectron spectroscopy (XPS) and particle charge density analyzer (PCD). The efficiencies of PBF1–9 were studied in the reactive blue (RB) dye removals by flocculation under different process conditions. The operation costs of the flocculants were evaluated at their optimal dosages. Also, the pH-independences and ion-tolerances of the aforementioned flocculants were studied in terms of the molecular levels. The experimental results exhibited that the flocculants CD or MW values were relevant to their flocculation behaviors and operation costs. CD values played a dominant role in color removal efficiencies and the costs, whereas MW values were critical to the floc structure. The pH or ion-independences of the flocculants were significantly dependent on the CD and MW values. However, some conclusions, conflicted with prior studies, were observed in this work. For instance, flocculant with the highest CD and MW levels was not the most effective one in enduring pH variation and the coexisting ions. The floc properties, including floc size, resistance and recovery ability, were relatively insensitive to flocculant intrinsic CD and MW levels when the flocculants were used at their optimal dosages. Furthermore, the possible relevance between CD or MW levels and the flocculation mechanisms have been proposed in this work. Exploring the effects of flocculants CD and MW levels could precisely control the flocculant characteristics to achieve satisfactory decontamination efficiencies with low costs.

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.

Enteromorpha prolifera polysaccharide based coagulant aid for humic acids removal and ultrafiltration membrane fouling control

Polyacrylamide (PAM) has been used as a coagulant aid in water treatment process for past decades, but it has caused great damages to human nervous system. Developing new coagulant aid with high biological safety is urgently demanded. This study provides a natural biomacromolecule coagulant aid with good biosecurity-Enteromorpha prolifera polysaccharide (Ep). Its coagulant aid efficiency and mechanism were investigated in terms of organics removal, floc properties and membrane fouling degree. In addition, contrast experiments were conducted with PAM to evaluate its potential of industrial applications. Results showed that organics removal could be increased by 23% when 0.3 mg/L Ep was used, which exhibited comparable aid effects to PAM. Due to the bridging-sweep aid role of Ep, flocs sizes, growth rate and recovery factor reached 470 μm, 62.6 μm/min and 0.492, respectively, while only 170 μm, 14.0 μm/min and 0.326 were obtained by PAM. Additionally, flocs exhibited more porous and multi-branched structures when Ep was applied, which caused less ultrafiltration membrane fouling (eventual J/J0 value = 0.52). As a result, Ep could be considered as a potential substitute of PAM, since better biosecurity, higher organics removal and lower membrane fouling could be obtained simultaneously by Ep addition.

Characterization and application of poly-ferric-titanium-silicate-sulfate in disperse and reactive dye wastewaters treatment

A novel coagulant poly-ferric-titanium-silicate-sulfate (PFTS) was synthesized and employed to treat two typical kinds of dye wastewaters—disperse blue and reactive yellow. The results indicated that PFTS with a Si/Fe molar ratio of 0.02 exhibited superior coagulation performance, especially under alkaline condition. The residual turbidity after coagulation by PFTS was only half of that after coagulation by poly-ferric-titanium sulfate (T-PSF). The sludge volume index was also reduced by PFTS compared to T-PSF in reactive dye treatment. Through the structure and morphology investigation of PFTS, it was found that new bonds of Si–O–Fe, Si–O–Ti and Fe–OH (Si–OH) were formed, and multi-branched structures and expanded surface area were generated. Additionally, compared with T-PSF, the floc strength and the floc size were also enhanced by PFTS, which was attributed to the polymerization between polysilicic acid and Fe/Ti which formed multi-branched structures, and finally adsorption and bridging ability of the coagulant was improved. Furthermore, the floc formed in reactive yellow wastewater treatment was larger and looser than that formed in disperse blue wastewater, with poorer strength and recovery ability, which can also interpret the better coagulation efficiency in disperse dye water treatment. From the results of coagulant characterization, zeta potential and flocs properties, it can be inferred that charge neutralization by the positive charged hydrolysate of coagulant was identified as the critical effect in disperse dyes removal, while the sweep and adsorption of metal hydroxyl compound formed during the hydrolysis of coagulants were considered to play a key role in reactive dye removal.

Effect of Grewia spp. biopolymer on floc properties of coagulated laterite suspension

Effect of Grewia spp. biopolymer on floc properties of coagulated laterite suspension

Coagulation/flocculation prior to low pressure membranes in drinking water treatment: a review

Coagulation/flocculation has been grouped into three typical configurations and the impact of each examined in terms of floc properties and membrane fouling.

Efficient removal of phosphorus from turbid water using chemical sedimentation by FeCl3 in conjunction with a starch-based flocculant

Phosphorus removal is an important issue in the field of water and wastewater treatment. In this study, a starch-based flocculant (St-CTA), as an assisting agent, was fed after the application of traditional inorganic precipitant of ferric chloride (FeCl3) to remove inorganic and organic phosphorus from their respective simulated turbid wastewaters. The effects of various influencing factors, including CTA content of St-CTA, dosage, initial pH, initial turbidity, and initial total phosphorus (TP), were investigated systematically. This modified chemical sedimentation process assisted by St-CTA not only showed high efficient in removing TP and turbidity but also evidently reduced the required dosage of FeCl3. This combination exhibited better efficiency in removing organic TP than in removing inorganic TP from water. The synergistic mechanisms of FeCl3 and St-CTA were discussed in detail by combination of apparent removal effects and floc properties. St-CTA exhibits strong charge neutralization and bridging flocculation effects and can thus efficiently aggregate and precipitate various phosphorus and iron complexes previously formed through chemical sedimentation, adsorption, and chelation. Besides, the validation of this combination usage was further confirmed by treating a real wastewater, and half of the required FeCl3 dosage and one third of the total cost were saved after addition of only 0.4 mg/L St-CTA when 90% TP and 95% turbidity were reduced. This study thus provided a novel technique for the diminution of TP and turbidity in turbid wastewater, considerable reduction of FeCl3 dosage and final cost, and lowering of secondary pollution risk.

The impact of polymer selection and dose on the incorporation of ballasting agents onto wastewater aggregates

Ballasted flocculation is an efficient high-rate sedimentation process getting more attention as an advanced P removal technology for levels below 0.1 mg/L. The process is well-known yet only very few studies have investigated the interactions, within the matrix of wastewater, of coagulant, polymer and ballast, especially when it comes to polymer doses and types which are, in the industry, rather based on recommendations than scientific evidence. In this work, the impact of anionic and cationic polymers has been investigated on P removal and floc properties. Anionic polymers showed to be superior to cationic ones when it comes to P removal and doses even as low as 0.01 mg/L yield better results than coagulant alone. There appears to be a “best-case” floc size with which very good P removal (>90%) can be achieved and flocs of sufficient strength can be generated.

Influence of sedimentation with pre-coagulation on ultrafiltration membrane fouling performance

Coagulation plays an important role in effectively alleviating ultrafiltration (UF) membrane fouling due to the existence of flocs. In comparison to traditional filtration, direct filtration (no sedimentation tank) is generally considered to result in less membrane fouling due to the looser cake layer formed on the membrane surface. However, cake layer characteristics are easily influenced by operating conditions (e.g., sedimentation time, temperature), resulting in different fouling loads and the still very limited knowledge about the fouling behavior. Here, a detailed investigation of UF membrane performance was carried out to analyze the differences between traditional filtration and direct filtration. The results showed that a critical settling time indeed existed when flocs gradually settled, leading to severe membrane fouling induced by a dense and thick cake layer. Therefore, the traditional filtration performed worse when the settling time was shorter than the critical time, while it performed better when the settling time was longer. In comparison to room temperature water, the proportion of membrane fouling caused by the cake layer increased at low water temperature due to the contracted membrane pore size, and the cake layer thickness became the main fouling load. The longer the settling time, the fewer pollutants reached the membrane surface, the thinner the cake layer, and the lower the potential for membrane fouling. Thus, traditional filtration performed better than direct filtration, and the critical settling time was not obvious. Although solution pH played an important role in floc properties, the fouling load was influenced little and traditional filtration still performed better. This work shows that the cake layer properties/fouling loads vary with the settling time during the coagulation-UF process, and this finding is of great significance to the improvement of membrane technology in drinking water plants.

Floc properties and membrane fouling in coagulation/ultrafiltration process for the treatment of Xiaoqing River: The role of polymeric aluminum-polymer dual-coagulants.

In this study, novel lignin-based flocculant (LBF) was prepared in recycling of papermaking sludge. And LBF combined with polyaluminum chloride (PAC) was used in the coagulation/ultrafiltration combined process. Effects of polymer types, pH and ultrafiltration time on the membrane fouling mechanism and resistance distribution were studied based on blocking model and resistance-in-series model. Results showed that strongly-attached external fouling was the primary fouling mechanism in PAC coagulation and dual-coagulation systems. Dual-coagulation achieved the slighter fouling due to the formation of poriferous cake layer. In the PAC system, the fouling mechanism transformed from intermediate fouling to cake layer with the ultrafiltration process. Addition of LBF could accelerate cake layer formation and further control internal fouling. PAC+LBF mainly decreased external fouling resistance and reversible internal fouling resistance to enhance permeation flux. And the total resistance (expect intrinsic membrane resistance) was lowered by more than 50% compared with PAC. The primary combined models in PAC and PAC+LBF systems were cake-standard blocking and cake-intermediate blocking, respectively. Initial pH showed a significant effect on membrane fouling. The slighter membrane fouling was acquired at pH 5.5. Addition of LBF could decrease the influence of pH on the fouling index and distribution.

Large-sized planktonic bioaggregates possess high biofilm formation potentials: Bacterial succession and assembly in the biofilm metacommunity

Wanted and unwanted surface-attached growth of bacteria is ubiquitous in natural and engineered settings. Normally, attachment of planktonic cells to media surfaces initiates biofilm formation and fundamentally regulates biofilm assembly processes. Here, culturing biofilm with planktonic sludge as source community, we found distinct succession profiles of biofilm communities sourced from the size-fractionated sludge flocs (<25; 25–120; >120 μm). Null model analyses revealed that deterministic process dominated in biofilm community assemblies but decreased with decreasing floc size. Additionally, the relative importance of environmental selection increased with increasing floc size of the source sludge, whereas homogenizing dispersal and ecological drift followed opposite trends. Phylogenetic molecular ecological networks (pMENs) indicated that species interactions were intensive in biofilm microbiota developed from large-sized flocs (>120 μm), as evidenced by the low modularity and harmonic geodesic distance and the high average degree. Intriguingly, the keystone taxa in these biofilm ecological networks were controlled by distinct interaction patterns but all showed strong habitat characteristics (e.g., facultative anaerobic, motile, hydrophobic and involved in extracellular polymeric substance metabolism), corroborating the crucial roles of environmental filtering in structuring biofilm community. Taken together, our findings highlight the role of planktonic floc properties in biofilm community assembly and advance our understanding of microbial ecology in biofilm-based systems.

Quantitative analysis of cake characteristics based on SEM imaging during coagulation-ultrafiltration process.

Cake formed by flocs is a crucial factor to affect membrane fouling during coagulation-ultrafiltration process. To investigate the role of floc properties on cake, cake characteristics under various coagulant dosage conditions were calculated by scanning electron microscope (SEM) imaging. Results found that one SEM image with × 5000 magnification could accurately estimate cake porosity with relative error lower than 5.00% for all conditions, whereas more SEM images with × 10,000 magnification or × 20,000 magnification should be applied to calculate cake porosity precisely. This could be explained by different pore information of SEM images with various magnifications. Compared to single SEM image with × 10,000 magnification and × 20,000 magnification, single SEM image with × 5000 magnification contained the most comprehensive pore information and slightly overestimated pore area for pore smaller than 0.4 μm2 due to lower resolution. To verify feasibility by SEM image evaluating cake characteristics, cake porosity calculated by SEM image and Carman-Kozeny equation were analyzed. The results showed that cake porosity estimated by these two methods were nearly the same, proving the feasibility of this method. Moreover, with the increase of coagulant dosage, cake porosity presented similar variation with floc average size, indicating that floc average size was likely to dominate cake porosity in this study. For pore characteristics, pore average characteristic length and pore average area were in accordance with floc fractal dimension, whereas pore fractal dimension and pore amount were consistent with floc average size. This gives specific information about the relation between floc properties and cake characteristics.

Laboratory study of the properties of frazil ice particles and flocs in water of different salinities

Abstract. Measurements of the size and shape of frazil ice particles and flocs in saline water and of frazil ice flocs in freshwater are limited. This study consisted of a series of laboratory experiments producing frazil ice at salinities of 0 ‰, 15 ‰, 25 ‰ and 35 ‰ to address this lack of data. The experiments were conducted in a large tank in a cold room with bottom-mounted propellers to create turbulence. A high-resolution camera system was used to capture images of frazil ice particles and flocs passing through cross-polarizing lenses. The high-resolution images of the frazil ice were processed using a computer algorithm to differentiate particles from flocs and determine key properties including size, concentration and volume. The size and volume distributions of particles and flocs at all four salinities were found to fit log-normal distributions closely. The concentration, mean size, and standard deviation of flocs and particles were assessed at different times during the supercooling process to determine how these properties evolve with time. Comparisons were made to determine the effect of salinity on the properties of frazil ice particles and flocs. The overall mean size of frazil ice particles in saline water and freshwater was found to range between 0.52 and 0.45 mm, with particles sizes in freshwater ∼13 % larger than in saline water. However, qualitative observations showed that frazil ice particles in saline water tend to be more irregularly shaped. The overall mean size of flocs in freshwater was 2.57 mm compared to a mean size of 1.47 mm for flocs in saline water. The average growth rate of frazil particles was found to be 0.174, 0.070, 0.033, and 0.024 mm min−1 and the average floc growth rate was 0.408, 0.118, 0.089, and 0.072 mm min−1 for the 0 ‰, 15 ‰, 25 ‰, and 35 ‰, respectively. Estimates for the porosity of frazil ice flocs were made by equating the estimated volume of ice produced based on thermodynamic conditions to the estimated volume of ice determined from the digital images. The estimated porosities of frazil ice flocs were determined to be 0.86, 0.82, 0.8 and 0.75 for 0 ‰, 15 ‰, 25 ‰ and 35 ‰ saline water, respectively.

Improvements on activated sludge settling and flocculation using biomass-based fly ash as activator

Biomass-based fly ash and wastewater are undesired products of the pulping industry. Recently, the use of biomass-based fly ash as an adsorbent (i.e., a valued material) for constituents of wastewater effluents was reported. In this work, the settling performance and properties of activated sludge were studied in the presence of fly ash. Upon mixing, fly ash increased the zeta potential of the sludge from −31 mV to −28 mV, which was due to the release of cationic ions from fly ash in the sludge suspension. The sludge settling and its flocculation affinity were improved through the complexation of flocs and released cation ions from fly ash. The relationships between the protein/polysaccharide (PN/PS) ratio and the content of extracellular polymeric substances (EPS) as well as the ratio and the properties of the sludge flocs were determined. A correlation between the total loosely bound-EPS (LB-EPS) content and the effluent suspended solids (ESS) (Pearson’s coefficient, rp = 0.83) was observed. The performance of sludge flocculation and settling were much more closely correlated with LB-EPS than with tightly bound EPS (TB-EPS). Scanning electron microscopy (SEM) analysis of sludge flocs before and after EPS extraction showed that the sludge flocs contained a large number of microorganisms, mainly Bacillus and Cocci. The amount of LB-EPS had an adverse influence on bioflocculation, effluent clarification and sludge settling affinity. The sludge properties had a moderate relationship with the PN/PS ratio of LB-EPS. Also, no correlation could be established between the ratio and the TB-EPS content.

US: Chroma Color – plastics masterbatch

In this study, the widely applied modified Smoluchowski model of flocculation kinetics based on the existence of two simultaneous processes, namely aggregation and breakup, is further developed by combining it with the fractal theory. The floc properties including the floc size and fractal dimension are extracted using online data acquisition through light scattering technique and combined into the model. The new method has been carefully examined by three flocculation systems using kaolin suspension as synthetic water and different typical flocculants. The theoretic flocculation kinetic curves are in good agreement with the experimental data with the determination coefficients larger than 0.9 by nonlinear regression. Moreover, the flocculation kinetic characteristics of different flocculation systems under various conditions are discussed based on the kinetic rates calculated from the modified model in detail. In summary, the new method for calculation of flocculation kinetics is more universal and simpler to operate, and is expected to be a useful tool in real water treatment operations.

Evaluation of the structural factors for the flocculation performance of a co-graft cationic starch-based flocculant

Three series of co-graft cationic starch (St)-based flocculants with distinct structural characteristics, namely, charge density (CD), graft-chain length (L), and graft-chain distribution (N), were successfully synthesized through graft copolymerization of [(2-methacryloyloxyethyl) trimethyl ammonium chloride] and acrylamide. These St-based flocculants with different molecular structures were used to flocculate various kaolin suspensions with different initial turbidities and a sodium humate (NaHA) aqueous solution. The experimental results indicated that CD contributed to flocculation evidently, whereas average L and its N were insignificant in experimentally measured ranges. On the basis of phenomenological theory, a second-order polynomial equation was used to further quantitatively analyze the effects of the three structural factors on the flocculation performance of these St-based flocculants, which were fully consistent with the experimental results. Besides, the optimal dose and its corresponding removal rate could be predicted exactly, and the flocculation mechanisms were discussed in detail according to the established models. With the combination of floc properties and zeta potentials, the flocculation mechanisms of these St-based flocculants for flocculation of kaolin suspensions and NaHA aqueous solution were mainly ascribed to charge patching and simple charge neutralization, respectively. These results improve the understanding of the structure–activity relationship of these graft St-based flocculants, which is of significant guidance for the utilization and design of novel flocculants.

Characterization of activated sludge flocs in membrane bioreactor: stable and unstable flocs.

In this study, the properties of unstable and stable flocs were investigated under the steady operation of a membrane bioreactor (MBR). The extracellular polymeric substances (EPS) composition, surface charge, and hydrophobicity of unstable and stable flocs were examined and compared. Interfacial interactions of the membrane with unstable flocs, unstable flocs themselves, and unstable and stable flocs were assessed using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) models. Cake layer resistance was found to contribute more than 80% of total resistance under steady operating conditions. Compared with stable flocs, unstable flocs possessed a higher level of EPS, more diverse protein, more negative charge, weaker hydrophobicity, and higher fouling potential. Thermodynamic analyses showed that unstable flocs had a higher adhesive strength (- 63.4mJ/m2) with the membrane, lower self-cohesive strength (- 18.3mJ/m2), and higher cohesive strength (- 54.3mJ/m2) with stable flocs. Therefore, some unstable flocs remained on the membrane surface to form the cake layer due to their poor cohesion strength.

Characterization and influence of floc under different coagulation systems on ultrafiltration membrane fouling

A hybrid system was developed in this study consisting of different coagulation systems and ultrafiltration (UF). Property and effect of flocs formed in different coagulation systems on ultrafiltration membrane fouling control were investigated. All three coagulation systems, as pretreatment of UF, were effective in improving membrane flux and reducing membrane resistance within an appropriate range of natural organic matters (NOM) concentration. At high initial NOM concentration, the performance of polyaluminum chloride (PAC) on NOM removal and fouling control was severely limited. For PAC-poly dimethyl diallyl ammonium chloride (PAC-PolyDMDAAC) coagulation system, the limitation of initial NOM concentration on removing NOM and alleviating membrane fouling was slightly weakened, indicating composite flocculant PAC-PolyDMDAAC produced larger flocs through combined action of charge neutralization and adsorption bridging. In PAC + PolyDMDAAC dual coagulation system, the combined action of adsorption-bridging effect, sweeping effect, and charge neutralization were the mechanisms under both low and high initial NOM concentration. Although the flocs formed in PAC + PolyDMDAAC dual coagulation system had poor recovery ability compared with those formed in PAC and PAC-PolyDMDAAC coagulation system, flocs formed through adsorption-bridging and sweeping had large size and higher ability to resist shear force, resulting in the formation of cake layer with porous and fluffy structure and less blockage in membrane pore in PAC + PolyDMDAAC dual coagulation system. These results demonstrated that dual coagulation system combined PAC coagulation and PolyDMDAAC flocculation as a pretreatment of UF process can improve the characteristics of flocs and structure of cake layer for improving NOM removal and controlling membrane fouling.

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.

Synchronous removal of CuO nanoparticles and Cu2+ by polyaluminum chloride-Enteromorpha polysaccharides: Effect of Al species and pH

Copper oxide nanomaterials have been extensively applied and can have serious impacts when discharged into the aquatic environment, especially when complexed with humic acid (HA) to form composite contaminants. As an innovative recycled coagulant aid, Enteromorpha polysaccharides (Ep) were associated with polyaluminum chloride (PACl) (denoted as PACl-Ep) to simultaneously remove CuO nanoparticles, Cu2+ and HA in this study. The influence of different Al species coagulants (AlCl3, PAClb and PAClc) and water pH on coagulation performance, floc properties and reaction mechanisms was investigated in detail. Results showed that in the three PACl-Ep systems, PAClb-Ep gave the highest removal efficiencies for turbidity and Cu2+, and the best UV254 removal effect was reached by using PAClc-Ep. Higher contents of Alb and Alc contributed to great coagulation performance because of their stronger bridging and sweeping effects. For all the Al species coagulants, alkalescent conditions were more conducive to removing Cu and HA compared to acidic conditions. Additionally, smaller and more agminated flocs with great recovery ability were formed by PAClb-Ep and PAClc-Ep systems (bridging and enmeshment effects cooperated with the chelated reticular structure formed by the Ep and Al species). Similarly, due to the increased hydrolysis and hydroxide precipitates, flocs formed under the condition of alkalescence were smaller, denser and stronger compared with weakly acidic conditions.