Floc Growth Research Articles

The coagulation behavior and mechanism of low-coagulability organic matter (LCOM)

Hydrophobic and high-molecular-weight dissolved organic matter (DOM) can be efficiently removed via coagulation, but DOM with a low hydrophobicity and low molecular weight cannot. These can be respectively classified as high-coagulability organic matter (HCOM) and low-coagulability organic matter (LCOM). Most coagulation studies have focused on HCOM (such as humic acid) but not LCOM. Therefore, the coagulation behavior and mechanism of LCOM were revealed by comparing with HCOM in this study. The removal efficiency of HCOM via coagulation was high (>75%), while that of LCOM was very low (<15%). Charge neutralization (CN) and sweep flocculation (SF) were the primary mechanisms of DOM coagulation. Based on the results of fluorescence measurements, zeta potential, and floc growth processes, the CN mechanism was very efficient in HCOM coagulation, but it did not apply to LCOM coagulation. Since LCOM has a smaller molecular size, it bound with coagulants to form soluble complexes in contrast to the flocs formed during HCOM coagulation. Both HCOM and LCOM could be removed via the SF mechanism, but the interaction between LCOM and coagulant was far weaker. According to the analysis of floc properties, HCOM strongly bound with the coagulant to form flocs with a loose structure and uneven surface. The properties of LCOM flocs were very similar to those of hydroxide flocs formed without organics in terms of surface charges, fractal structure, recoverability, surface morphology, and contents of Al/O components of flocs. In conclusion, HCOM coagulation was controlled by the interaction between HCOM and coagulant, while LCOM coagulation was dominated by coagulant hydrolysis and polymerization. This study complements the existing understanding of LCOM coagulation and provides insights into the enhancement strategy.

An optimization strategy for highly efficient flocculation and capture of algal cells: Controlling dosing patterns of modified clay

Modified clay technology is the most effective emergency treatment method for harmful algal blooms. In this study, a multiple dosing scheme was proposed to control the maximum number of flocculated algal cells in aluminium-modified clay, aiming to enhance the removal efficiency of algal cells by modified clay; simulation experiments were conducted to investigate the effect of dosing frequency on the flocculation and capture of algal cells by modified clay under the same total dosage. The mechanism analysis confirmed that the multiple dosing method maximized the number of algal cells removed per gram of clay, thereby reducing the self-flocculation of the modified clay. Compared with the single-dosing method, multiple dosing can increase the positive charge of the modified clay system by 40% and prolong the growth and residence time of flocs, which can increase the removal efficiency of algal cells by 20%–30%. Response surface methodology (RSM) was used to optimize the range of each controlled operating parameter. The experimental results showed that the maximum removal efficiency of algal cells can be achieved when the dosing frequency is controlled at 3–5 times, where the dosing interval and the concentration of stock solution are the key factors affecting the removal efficiency of algal cells under the multiple dosing method; the optimal interval time was 20–40 min, and the concentration of stock solution was 10-20 g/L. The results of this study provide a methodological reference for further improving the field application efficiency of modified clay.

Atomic-Level Structural Differences between Fe(III) Coprecipitates Generated by the Addition of Fe(III) Coagulants and by the Oxidation of Fe(II) Coagulants Determine Their Coagulation Behavior in Phosphate and DOM Removal.

In situ Fe(III) coprecipitation from Fe2+ oxidation is a widespread phenomenon in natural environments and water treatment processes. Studies have shown the superiority of in situ Fe(III) (formed by in situ oxidation of a Fe(II) coagulant) over ex situ Fe(III) (using a Fe(III) coagulant directly) in coagulation, but the reasons remain unclear due to the uncertain nature of amorphous structures. Here, we utilized an in situ Fe(III) coagulation process, oxidizing the Fe(II) coagulant by potassium permanganate (KMnO4), to treat phosphate-containing surface water and analyzed differences between in situ and ex situ Fe(III) coagulation in phosphate removal, dissolved organic matter (DOM) removal, and floc growth. Compared to ex situ Fe(III), flocs formed by the natural oxidizing Fe2+ coagulant exhibited more effective phosphate removal. Furthermore, in situ Fe(III) formed through accelerated oxidation by KMnO4 demonstrated improved flocculation behavior and enhanced removal of specific types of DOM by forming a more stable structure while still maintaining effective phosphate removal. Fe K-edge extended X-ray absorption fine structure spectra (EXAFS) of the flocs explained their differences. A short-range ordered strengite-like structure (corner-linked PO4 tetrahedra to FeO6 octahedra) was the key to more effective phosphorus removal of in situ Fe(III) than ex situ Fe(III) and was well preserved when KMnO4 accelerated in situ Fe(III) formation. Conversely, KMnO4 significantly inhibited the edge and corner coordination between FeO6 octahedra and altered the floc-chain-forming behavior by accelerating hydrolysis, resulting in a more dispersed monomeric structure than ex situ Fe(III). This research provides an explanation for the superiority of in situ Fe(III) in phosphorus removal and highlights the importance of atomic-level structural differences between ex situ and in situ Fe(III) coprecipitates in water treatment.

Cathodic microenvironment reconstruction boost electrocoagulation treatment of photovoltaic wastewater

Electrocoagulation has emerged as a rapidly advancing area in the field of wastewater treatment due to its capability to eliminate pollutants that are typically resistant to filtration or chemical treatment methods, as well as its overall effectiveness, which is often impeded by the formation and growth of flocs. In this study, simulated photovoltaic wastewater was treated using the electrocoagulation method, with cathodic water reduction employed to induce a highly efficient electrocoagulation rate towards the removal of SiO2 and F−. Our results demonstrate that the optimal conditions for this process involved the use of an Al anode, Nickel foam as cathode, an inter-electrode distance of 1.0 cm, a current density of 30 mA cm−2, a stirring speed of 300 rpm, sodium chloride as supporting electrolyte, and an initial pH of 7. Our experimental findings indicated satisfactory performance with respect to EC, as evidenced by the reduction of F− content from 25 mg L−1 to 3.68 mg L−1, and the complete removal of SiO2 from 16 mg L−1. Furthermore, electrochemical analysis and texture characterization revealed that the overall rate of flocculation was dependent on the cathodic material, with in-situ generated OH− in the electrolysis cell influencing the microenvironment of iron hydroxide floc formation and sedimentation. Our study offers conclusive evidence that the optimal cathodic material capable of intensifying OH− concentration from water dissociation is key to achieving superior electrocoagulation performance.

The Effect of Use of Commercial Probiotics in Feed on Water Quality and Growth Performance of Bawal Fish (Colossoma Macropomum) Bioflok System

Biofloc consists of special substances called probiotics, where probiotics contain beneficial microorganisms in cultivation activities. Probiotics can be given through feed and water (media). Probiotics can break down metabolic waste and stimulate an immune response so that fish health increases and affects growth. Probiotics have an effect on the growth of ti- lapia, but there is no information for freshwater pomfret. This study aims to determine the ef-fect of probiotic types on biofloc volume, water quality, and fish growth in pomfret culture (Colossoma macropomum) biofloc system. The method used was giving different types of probiotics, treatment A control, B EM4, C Probiotic-7, D Aquaenzym. The results of the re-search showed that the lowest pH of the system was 7.85, namely the pH of the probiotic-7 type. The mean floc growth measurement results for treatment A was 12.93, treatment B was 17.54, treatment C was 17.59, treatment D was 15.98. Aquaenzym probiotic has a fish surviv-al rate of 100% compared to other probiotics. The type of probiotic that has the best results in increasing the growth performance of freshwater pomfret (Colossoma macropomum) is probiotic-7.

Salinity Impacts on Floc Size and Growth Rate With and Without Natural Organic Matter

AbstractDue to the flocculation process, suspended mud aggregates carried by rivers to the coastal ocean are thought to undergo changes in size and shape in response to environmental drivers such as turbulence, sediment concentration, organic matter (OM), and salinity. Some have assumed that salt is necessary for floc formation, and that mud, therefore, reaches the estuary unflocculated. Yet mud flocs exist in freshwater systems long before the estuarine zone, likely due to the presence of OM acting as a floc‐promoting binder. Therefore, it is important to consider how salinity affects flocculation, if at all, in the presence of OM. Here, we used experiments to examine the flocculation of a natural mud with and without OM. Results showed that the rate of floc growth and equilibrium size both increase with salinity regardless of the presence or absence of OM. However, the response of both to salinity was stronger when OM was present. In deionized water, natural sediment with OM was seen to produce large flocs. However, the size distribution of the suspension tended to be bimodal. With the addition of salt, increasing amounts of unflocculated material became bound within flocs, producing a more unimodal size distribution. Here, the enhancing effects of salt were noticeable at even 0.5 ppt, and increases in salinity past 3–5 ppt only marginally increased the floc growth rate and final size. Data from the experiment were used to develop a salinity‐dependent model to account for changes in floc growth rate and equilibrium size.

Mutual enhancement of metal coagulant and peroxymonosulfate for ciprofloxacin removing and the multiple-mode mechanisms

Metal coagulants, which are widely employed in water and wastewater treatments, have been recently found to have catalyzation effect on oxidation, while chemical oxidation, in turn, could possibly impose impacts on coagulation process. In this study, a hybrid coagulation/oxidation process was developed using four metal-based coagulants, i.e., aluminum sulfate (Al2(SO4)3), ferric chloride (FeCl3), titanic sulfate (Ti(SO4)2), zirconium oxychloride (ZrOCl2) and peroxymonosulfate (PMS) as the oxidant, for removing ciprofloxacin (CIP), which is a representative emerging organic contaminant (EOC) with small molecular weight (MW). The results showed PMS (0.2 mM) improved the CIP removals by Al2(SO4)3 and FeCl3 from 13% and 21% respectively to around 92% at the coagulant dose of 0.02 mM, and for Ti(SO4)2 and ZrOCl2, the CIP removal was increased from about 21% to 70∼75%, which was also more efficient than separate PMS oxidation (around 42%). The results of zeta potential, dynamic floc growth and residual coagulants demonstrated PMS led to the destabilization of coagulation suspensions, where lag time for floc formation was largely shortened and also, residual metal coagulants in terms of dissolved and colloids/particles were decreased. PMS alleviated the negative effect of crystallization on colloids and clots aggregation via reducing the quantity and crystallinity of nanocrystal according to the high-resolution transmission electron microscopy (HR-TEM) analysis, which facilitated the flocculation. The results of radicals scavenging experiments verified metal coagulants could activate PMS in turn, where •OH played a dominant role in CIP degradation. While, the four kinds of coagulants behaved significantly different when combined with PMS. Due to the different hydrolysis and polymerization properties, coupling PMS with aluminum/iron salt were more efficient than with titanium/zirconium. Nevertheless, the synthetic effect was suppressed as the coagulant dose increased to 0.1 mM and above, where restabilization occurred. Therefore, coagulation played a major role in coagulant/PMS process, which could provide a promising technology for CIP removals.

Application of laminarin as a novel coagulant aid to improve coagulation-ultrafiltration efficiency

Polyacrylamide (PAM) is the most commonly used coagulant aid in coagulation-ultrafiltration (C-UF) systems; however, its hydrolyzed monomer is harmful to the human nervous system. In this study, laminarin (LA), was extracted from Laminaria japonica and used as a novel coagulant aid to improve coagulation efficiency and reduce membrane fouling during the C-UF process. Optimal LA usage conditions were systematically examined and compared with those of PAM to evaluate their potential for industrial applications. The results revealed that coagulation efficiency could be enhanced by 15–35% with moderate LA addition, which exhibited comparable aid effects to PAM. LA exhibited the highest coagulation aid effect at pH 8–9, and under this condition, turbidity and natural organic matter (NOM) removal achieved 82% and 54%, respectively. Compared with a one-time LA dosing strategy, the pollutant removal capacity of batch dosing was superior. Even in lower water temperatures (5–15 °C), coagulation efficiency was still satisfied, which exhibited a good practical application perspective. The coagulation aid role of LA should be attributed to its long-chain molecular structure, which enhances the bridging role between micro flocs and assists floc growth, thus facilitating the formation of large flocs. In addition, LA adsorption on floc surface was conducive to the direct electrostatic repulsion effect of electronegative membrane, which resulted in a more porous cake layer and higher membrane flux. Therefore, LA exhibits excellent application potential for eliminating NOM while simultaneously reducing membrane fouling through the C-UF process.

Nano aluminum-based hybrid flocculant: Synthesis, characterization, application in mine drainage, flocculation mechanism

Given the characteristics of high turbidity and trace emulsified oil in mine drainage, which leads to the problems of low settling efficiency and poor treatment efficiency in the coagulation/flocculation treatment process, there is an urgent need to develop efficient, economical, and stable flocculants. In this study, a new hybrid flocculant Al-P(AM-DMDAAC-BA) was synthesized by the solution copolymerization method, and its viscosity, molecular weight, and cationic degree were 1492.93 mL/g, 7.44 × 106 Da and 16.97%, respectively. The physical and chemical properties of Al-P(AM-DMDAAC-BA) were characterized by Fourier transform infrared spectroscopy (FTIR), Nuclear magnetic resonance (1H and 13C NMR), Transmission electron microscopy (TEM), X-ray diffraction (XRD), Scanning electron microscopy (SEM), Thermogravimetric (TG), and Zeta potential. SEM, Zeta potential, and laser particle size measurement characterized the properties of flocs. Batch wastewater treatment experiments analyzed the flocculation treatment efficacy. The results of the analysis are as follows. Aluminum hydroxyl complexes were attached to organic polymers via -Al-O-C- bonds. Acrylamide (AM), dimethyl diallyl ammonium chloride (DMDAAC) and butyl acrylate (BA) were linked to each other by Carbon-carbon double bond (-CC-). The aluminum-based complex is dispersed as nano-microspheres in the hybrid flocculant and cross-linked to the organic branched chains. Al-P(AM-DMDAAC-BA) is thermally stable and less susceptible to thermal decomposition. It possesses strong electrical neutralization ability and rapid floc growth ability. It has excellent turbidity removal (reduced to 7.96 ± 1.15 NTU) and emulsified oil removal (reduced to 0.23 ± 0.01 mg/L) performance. The flocculation mechanism is mainly attributed to the electro-neutralization of the hybrid flocculants and the adsorption bridging effect of organic branched chains.

On the importance of temporal floc size statistics and yield strength for population balance equation flocculation model

Many aquatic environments contain cohesive sediments that flocculate and create flocs with a wide range of sizes. The Population Balance Equation (PBE) flocculation model is designed to predict the time-dependent floc size distribution and should be more complete than models based on median floc size. However, a PBE flocculation model includes many empirical parameters to represent important physical, chemical, and biological processes. We report a systematic investigation of key model parameters of the open-source PBE-based size class flocculation model FLOCMOD (Verney, Lafite, Claude Brun-Cottan and Le Hir, 2011) using the measured temporal floc size statistics reported by Keyvani and Strom (2014) at a constant turbulent shear rate S. Results show that the median floc size d50, in terms of both the equilibrium floc size and the initial floc growth, is insufficient to constrain the model parameters. A comprehensive error analysis shows that the model is capable of predicting three floc size statistics d16, d50 and d84, which also reveals a clear trend that the best calibrated fragmentation rate (inverse of floc yield strength) is proportional to the floc size statistics considered. Motivated by this finding, the importance of floc yield strength is demonstrated in the predicted temporal evolution of floc size by modeling the floc yield strength as microflocs and macroflocs giving two corresponding fragmentation rates. The model shows a significantly improved agreement in matching the measured floc size statistics.

Quantification of Floc Growth for Sediment with Mixing Intensity

Pollutants related to water quality often exist in rivers and form clusters. These pollutants adversely affect river environments and ecosystems. In Korea, the public’s interest in water quality has been increasing for decades. Many studies on water quality and pollutants in sewage treatment plants have been conducted; however, studies on the formation of flocs based on the flow characteristics of rivers are insufficient. In general, it is known that floc formation is influenced by hydraulic characteristics, such as velocity and turbulence, and that it combines them with contaminants in the river. However, studies that quantitatively analyze this topic are also insufficient. An analysis of floc formation between sediments must be conducted to understand the formation process of sediments and contaminants. Therefore, in this study, kaolin, which is a cohesive sediment, was used to quantify the floc formation process according to the mixing intensity. Turbidity was analyzed to observe the amount of floc formation, and samples were collected to confirm the concentration. Additionally, the turbidity concentration relationship according to the mixing intensity was quantified using an optical microscope. Regarding the mixing intensity, when the rotation speed was 200 rpm or more, the separation of the flocs was dominant. In contrast, when the rotation speed was 100 rpm or less, turbidity changes due to sedimentation and floc formation were dominant. Analyzing mixing intensities and their association with the flow characteristics of rivers may be useful for the management of contaminants in rivers.

Effects of organic matter on the aggregation of anthropogenic microplastic particles in turbulent environments

Biofilm-coated microplastics are omnipresent in aquatic environments, carrying different organic matter (OM) that in turn influences the flocculation and settling of microplastic aggregates. In this study, the effects of chitosan, guar gum, humic acid, and xanthan gum on the flocculation of anthropogenic microplastics are examined under controlled shear through the mixing chamber experiments. The results show that all of the selected OMs have positive effects on biofilm culturing and thus enhance the growth of microplastic flocs, with more evident promoting effects for cationic and neutral OMs (i.e., chitosan and guar gum) than anionic OMs (i.e., humic acid and xanthan). No critical shear rate is observed in the size vs. shear relationship based on our measurements. In addition, the quadrature-based two-class population balance model is employed to track the development of bimodal floc size distributions (FSDs) composed of small and large microplastic flocs. The model predictions show reasonable agreement with the observed FSDs. The largest error of settling flux from the two-class model is 7.8% in contrast with the reference value measured by the camera-based FSDs with 30 bins. This study highlights the role of different OMs on microplastic flocculation and indicates that a two-class model may be sufficient to describe microplastic transport processes in estuaries.

Effect of microplastic aging degree on filter cake formation and membrane fouling characteristics in ultrafiltration process with pre-coagulation

Microplastic aging has gained more and more attentions in the field of environmental studies. However, it is not clear yet how aged microplastics possessing different aging degrees would affect membrane filterability and associated fouling mechanisms during ultrafiltration of pre-coagulated microplastic-containing waters. Here, four chemically-aged polypropylene (PP) microplastics were individually prepared by immersing pristine PP into either HCl or NaOH solution at two prearranged concentrations (1 and 5 mol/L). The aging degree of each chemically-aged PP was reflected by microplastic surface physicochemical changes after artificially-accelerated aging. All pre-coagulation and ultrafiltration experimental results demonstrated that microplastic aging degree indeed had a critical effect on permeate flux decline and membrane resistance distribution, along with microscopic characteristics of fouled cake layer. These expected effects were mainly induced by the involvement of different chemically-aged PP in microplastic-containing floc growth and subsequent filter cake formation. For HCl-based aging, aged microplastic particles produced at the higher concentration (5 mol/L) appeared to play a stronger skeleton role in filter cake formation than those obtained at 1 mol/L, giving an enhanced filterability of ultrafiltration membrane. In contrast, for the two cases of NaOH-based aging, the lower reagent-solution concentration corresponded to a rather slighter permeate flux decline during ultrafiltration, possibly arising from the predominated role of humic acid molecules in membrane pore blocking/narrowing fouling and steric hindrance effects.

Biotesting as a modern assessment method of the aquatic environment Biofloc quality

Studies were conducted to determine the biofloc media toxicity degree to hydrobionts by biotesting. Environment variability in which biocenosis biofloc is formed can cause optimal C/N ratio values violations in a closed water system, which is considered a critical factor affecting the formation rate of microbial communities and species ratio in them. Test objects use in this study showed no toxic effects on the bioindicator in the environment created by the bacterial communities that make up the Biofloc system. Of course, the data obtained cannot be considered exhaustive, but it should be noted that aquatic plant use as a bioindicator provides new opportunities for developing biofloc systems potential. The BFT-systems principle is to stimulate the natural growth of macroaggregate flocs, enhance culture water self-nitrification. A prerequisite for the flocs’ functioning as a biofilter analogue, along with supporting them in suspension, is the presence of an easily digestible organic carbon source, as the cultures of bacterial microorganisms in the base of flocs are heterotrophic.

Controlling lipid crystallization across multiple length scales by directed shear flow.

The crystallization behavior of lipids is relevant in many fields such as adipose tissue formation and regeneration, forensic investigations and food production. Using a lipid model system composed of triacylglycerols, we study the formation of crystalline structures under laminar shear flows across various length scales by polarized light-, scanning electron-, and atomic force microscopy, as well as laser diffraction spectroscopy. The shear rate during crystallization γ̇cryst influences the acyl-chain length structure and promotes domain growth into the flow direction thereby transforming the crystallites from oblate into prolate particles. Concentration dependent aggregation of crystallites into clusters is the rate limiting step for floc and floc network formation. At high γ̇cryst, fast crystallite cluster formation at smaller equilibrium diameters is promoted. The high crystallite cluster concentration induces their aggregation into flocs which form weak networks. At low γ̇cryst, floc generation is limited by the low amount of crystallite clusters leading to slow growth of larger flocs and forming of strong networks. The findings in this work have potential implications ranging from the design of injectable soft tissue fillers for adipose tissue regeneration, to the crystalline network formation in microorganism derived lipids, up to a more energy-efficient production of chocolate confectionery.

Continuous floc image analyser (C-FIA) for tracking floc particle dynamics during coagulation–flocculation–settling processes

A continuous floc image analyser (C-FIA) was developed for tracking the dynamics of floc particle formation, growth and settling during conventional water treatment. This is based on a new RGB signal for flocculation index determination.

Bisulfite-activated permanganate oxidation plus coagulation as a pretreatment of SWRO desalination lines to enhance boron rejection

To address the concern of high level of residual boron in the filtrate of seawater reverse osmosis (SWRO), bisulfite-activated permanganate technology (PM/BS) combined with post-coagulation/sedimentation (C/S) was evaluated as a pre-treatment strategy. The results revealed that the combination (PM/BS + C/S) can remove 37.1 % of boron under optimal conditions. For PM/BS pretreatment, there was an optimal dosage of BS (450 μM) and PM (90 μM) and pre-oxidation time (5 min) to assist C/S in achieving boron removal. For post-C/S, the optimal flocculant dose (10 mg/L), GT value (GrapidT = 45948 and GslowT = 12480), and precipitation time (20 min) were also determined. BS/PM pretreatment weakened the dependency on strict GT control and shortened the precipitation time required. Moreover, compared to C/S alone, BS/PM + C/S showed a remarkable improvement in the elimination of dissolved organic matter (19.8 %) and nutrients (19.8 % for assimilable organic carbon, 30.8 % for total nitrogen, and 16.0 % for total phosphorus), which reduced membrane fouling. The synergistic mechanism for enhanced boron removal includes destroying the organic coating of colloid or suspended particles, changing the structure of dissolved organic matter, generating MnO2 particles in situ as a boron absorbent and coagulation aid, and regulating the growth and development of flocs. A pilot-scale test integrating PM/BS into a typical SWRO process yielded an average boron rejection rate of 95.7 %. This study indicates that PM/BS + C/S may be a promising pretreatment technology for SWRO to control the amount of residual boron in the filtrate.

Effect of Suspended Solids and Organic Matter in Water on the Removal of ZnO-NPs by Coagulation

Background: Zinc oxide nanoparticles (ZnO-NPs) have been shown to have a non-negligible impact on the environment Objective: Kaolin and humic acid were used in the aqueous environment to study their effects on the removal of ZnO-NPs. Method: In this work, polyaluminum ferric chloride (PAFC)/cationic polyacrylamide (CPAM) coagulants were used together with kaolin and humic acid were used to study their effects on the removal of ZnO-NPs and to analyze their mechanism of action. Results: The results showed that the removal rate of ZnO-NPs in the humic acid system decreased by about 30% compared to that in the pure water system, and increasing the ionic strength and humic acid concentration was not conducive to removing ZnO-NPs. On the other hand, the ZnO-NPs removal rate in the kaolin system was up to 96.28%, and increasing the ionic strength and kaolin concentration contributed to the removal of ZnO-NPs. In the humic acid and kaolin systems, the effects of coagulant dosage and pH on the removal of ZnO-NPs were about the same as in the pure water system. Moreover, 5 mg/L humic acid inhibited floc growth during removal of ZnO-NPs by coagulation with PAFC/CPAM. In contrast, 5 mg/L kaolin promoted flocs growth, resulting in stronger and more stable flocs and a 5.25% increase in the fractal dimension compared to the pure water system. Conclusion: These results suggested that suspended solids and natural organic matter in the water could directly affect the effectiveness of coagulation to remove ZnO-NPs.

Study on the characteristics and mechanism of the flocculation behaviour in a novel fluidized bed flocculator

Fluidized bed flocculator is a novel and efficient flocculation reactor. It has an efficiency that directly affected by flow field parameters. However, the structural characteristics of flocs and their influence on subsequent settling performance in fluidized bed flocculators are not clear. In this study, a multi-stage velocity gradient fluidized bed flocculator (MGF) was constructed for the continuous flocculation-sedimentation of kaolinite to investigate the growth and sedimentation behaviour of flocs. Additionally, MGF was compared with traditional mechanical stirring flocculation (MSF). Results showed that MGF achieved a higher removal efficiency (97.6 ± 0.7 %) with less coagulant and flocculant than MSF. Moreover, flocs generated in the MGF yielded a faster settling performance, which can reduce the sedimentation time by 60 times. Comparative analysis of floc structural characteristics, as well as their relationship with the settling velocity, demonstrated that flocs in MGF were more compacted than those in MSFs and thus achieved higher settling velocity. The settling velocity was collaboratively dominated by the size and fractal dimension of flocs. A modified Stokes’ settling velocity equation introducing the structural characteristics of flocs was derived, and verified by actual data to accurately and quantitatively describe the settling velocity of irregular flocs. These results indicated that MGFs have a better flocculation and sedimentation performance than MSFs for solid–liquid separation, and may lay a theoretical foundation for the design of efficient fluidized bed flocculators and popularize their application in practical engineering.

Internal interaction between chemically-pretreated polypropylene microplastics and floc growth during flocculation: Critical effect on floc properties and flocculation mechanisms

Microplastic pollution in drinking water resources has attracted increasing concerns due to the potential harm of microplastic particles to the health of either human or other living creatures. To better understand how the presence of microplastic contaminants would affect floc morphological properties and associated flocculation mechanism, various chemically-pretreated polypropylene (PP) microplastics were firstly produced by exposing pristine ones to HCl or NaOH solution at two concentrations of 1 and 5 mol/L. SEM and FTIR analyses demonstrated that microplastic physicochemical properties (including surface morphological details and oxygen-containing functional groups) were related to the type and concentration of chemical reagent for pretreatment. When flocculating model PP-containing water samples with coagulant of polymeric ferric sulfate (PFS), the involvement of PP microplastics after NaOH-based pretreatment was found more beneficial for the formation of larger floc aggregates during flocculation than those after HCl-based pretreatment at a fixed chemical-reagent concentration, whereas for either chemical pretreatment, PP microplastics produced at a higher solution concentration seemed to facilitate the process of larger PP-containing floc growth. Also, the structure, strength and recovery ability of resultant floc aggregates at steady state of flocculation appeared quite different, possibly attributed to distinct interaction patterns between preformed floc aggregates and microplastic particles, under all circumstances of chemically-pretreated PP microplastics investigated. Furthermore, it was suggested that the growth of PP-containing flocs during flocculation was partially determined by not only charge neutralization, but also the incorporation of humic acid (HA) molecules into PP-containing floc aggregates via the formation of PFS-HA and/or PFS-HA-PP complexes.