Floc Properties Research Articles

Optimization of microalgal-bacterial consortium flocculation through chitosan-diatomite composite materials: A sustainable approach to biomass harvesting

Chitosan-based flocculants are emerging as a promising technology for microalgae harvesting. However, the implementation of cost-effective and sustainable chitosan-inorganic composites within algal-bacterial symbiosis (ABS) systems for harvesting microalgal-bacterial consortiums (MBC) is still underdeveloped. This study addresses this gap by synthesizing and evaluating chitosan-diatomite composite materials (CTS/DTE), with a focus on their concentration impact on MBC flocculation. Optimal biomass production and flocculation performance were achieved at a CTS/DTE concentration of 60 mg/L during MBC co-culture. Comprehensive characterizations revealed that the largest particle size significantly contributed to superior flocculation efficiency. This efficacy was primarily attributed to the synergistic effects of net trapping and adsorption cross-linking, facilitated by the unique interaction between CTS and DTE. These interactions not only enhanced the bond between the flocculant and microalgal cells but also promoted the structural stabilization of the MBC flocs. Additionally, tryptophan-like proteins in the extracellular polymeric substances (EPS), as identified through three-dimensional fluorescence (EEM) analysis, were found to be the primary contributor to the hydrophobic properties of MBC flocs, significantly enhancing their adhesion and aggregation thermodynamics. Collectively, this research elucidates innovative strategies and provides both theoretical and practical insights for the efficient and cost-effective harvesting of MBC, underscoring the potential of CTS/DTE composites in enhancing the sustainability of ABS systems.

Measurements of frazil ice flocs in rivers

Abstract. Frazil floc sizes and concentrations have been investigated in a small number of laboratory studies, but no detailed field measurements have been reported previously. In this study, a submersible camera system was deployed a total of 11 times during the principal and residual supercooling phases in the North Saskatchewan, Peace, and Kananaskis rivers to capture time-series images of frazil ice particles and flocs. Images were processed to accurately identify flocs and to calculate their sizes and concentrations. Key hydraulic and meteorological measurements were collected, and air–water heat fluxes were estimated to investigate their influence on floc properties. A lognormal distribution was found to be a good fit for the floc size distribution. The mean floc size ranged from 1.19 to 5.64 mm and the overall mean floc size was 3.80 mm. The mean floc size decreased linearly as the local Reynolds number increased. The average floc number concentration ranged from 1.80×10-4 to 1.15×10-1 cm−3. The average floc volumetric concentration ranged from 2.05×10-7 to 4.56×10-3 and was found to correlate strongly with the fractional height above the river bed. No significant correlations were found between the air–water heat flux and floc properties. Time series analysis showed that during the principal supercooling phase, floc number concentration and mean size increased significantly just prior to peak supercooling and reached a maximum near the end of principal supercooling. During the residual supercooling phase, the mean floc size did not typically vary significantly even 2.5 h after the residual phase had ended and the water temperature had increased above 0 °C.

Treatment process of pre-coagulated waters involving polyethylene (PE) microplastics by ultrafiltration membranes coupled without or with pre-deposited aggregate-based layer

In recent years, environmental and ecological issues caused by microplastics pollution have received widespread attention. The coexistence of polyethylene (PE) microplastics with other soluble and particulate contaminants in water environments is considered to pose a new challenge for water treatment engineering. This study first focused on the combined effects of microplastic size (13 and 150 μm) and concentration (0, 20 and 100 mg/L) of floc properties in the coagulation-flocculation process (CF). The results showed that the involvement of PE microplastics hindered larger floc formation but enhanced the floc strength. The presence of either smaller microplastic particles at a fixed concentration (100 mg/L), or a higher concentration of 13 μm microplastic particles resulted in smaller and more highly-branched floc aggregates. During ultrafiltration of pre-coagulated water (CF-UF), the “13 μm + 100 mg/L” case exhibited less severe membrane fouling than the other cases (“without PE”, “13 μm + 20 mg/L” and “150 μm + 100 mg/L”). Comparing the CF-UF process in the “without PE” and “13 μm + 100 mg/L” cases, the pre-deposited aggregate-based layer could give a lower degree of membrane flux decline and mitigate membrane fouling during ultrafiltration of pre-coagulated water (CF-UFPL). Moreover, both the involvement of PE microplastics and the properties (thickness and compactness) of the pre-deposited layer affected membrane filterability and fouling degree. Lastly, the influencing mechanisms of coexisting microplastics in UF, CF-UF and CF-UFPL processes were explored by comparatively analyzing the correlation of pre-coagulated floc physicochemical characteristics with membrane filterability, water purification efficiency and membrane fouling degree in both the absence and presence of microplastic particles.

Changes in the suspended floc properties by bed erosion in a macrotidal muddy flat: Case of Asan Bay, Korea

Intertidal flats have closely connected sediment transport systems with tidal channels. The properties of suspended sediments derived by horizontal advection and local erosion were examined under various current and wave. A field campaign was conducted over two weeks (October 17–31, 2021) in the Asan intertidal flat. Suspended sediments were in the form of flocs, which are clumps of particles, with an average floc size (davg) of 70–275 μm. The maximum current velocity and water level at each tidal cycle continuously decreased from 0.19 to 0.04 m s−1 and from 5 to 2 m, respectively. In accordance with the hydrodynamic conditions, continuous decrease in davg from spring to neap tides was superimposed by flood-ebb variations in davg. Geological heterogeneity between channels and flats can be significantly influenced by tide and wind conditions. Under tide-dominant conditions, the spring tidal current was strong enough to transport macroflocs with davg of 200–500 μm into the intertidal flats. The grown macroflocs up to 1.6 times of davg at early flood tide were deposited in the flats during the slack tide and transported back to the channels by ebb tidal current. Under wind-dominant conditions, the suspended flocs originated primarily from the bed by erosion. The deposited macroflocs or microflocs were entrained into water column and transported to adjacent tidal channel as microflocs or flocculi. The structure of the advected flocs had more varied density in the range of 42–147 kg m−3, compared to the eroded flocs. Under severe erosion, the structure of suspended flocs was similar to that of advected flocs despite their smaller size. This suggests that bed erosion supplied more porous flocs into the water column than those transported by advection. Consequently, the intertidal flats are at an increased risk of erosion due to the diverse structure of flocs.

Evaluation of activated sludge settling characteristics from microscopy images with deep convolutional neural networks and transfer learning

Timely assessment and prediction of changes in microbial compositions leading to activated sludge settling problems, such as filamentous bulking (FB), can reduce water resource recovery facilities (WRRFs) upsets, operational challenges, and negative environmental impacts. This study presents a computer vision approach to assess activated sludge-settling characteristics based on Microscopy Images (MIs). We utilize MIs to train deep convolutional neural networks (CNN) using transfer learning to investigate the morphological properties of flocs and filaments. The methodology was tested on the offline MI dataset collected over two years at a full-scale industrial WRRF in Belgium. Various CNN architectures were tested, including Inception v3, ResNet18, ResNet152, ConvNeXt-nano, and ConvNeXt-S. The sludge volume index (SVI) was used as the final prediction variable, but the method can be easily adjusted to predict any other settling metric of choice. The best-performing CNN, ConvNeXt-nano, could predict SVI values with MAE (37.51 ± 4.02), MTD (11.65 ± 1.94), MAPE (0.18 ± 0.02), and R2 (0.75 ± 0.05). The model was tested in real-life FB events, where it identified early indicators of bulking by predictive surges in SVI values. We used an explainable AI technique, Eigen-CAM, to discover key morphological indicators of sludge bulking transitions. The findings highlight the SVI multimodality issue, where SVI readings as a unidimensional metric could not capture delicate shifts from good to poor sludge settling, while the model detected these subtle changes. The key morphological attributes of threshold conditions leading to FB were identified, which can provide actionable insight for preemptive WRRF management.

Enhancing solid-liquid separation performance of copper tailings by regulating settling floc properties

The effective improvement of dewatering performance of fine tailings slurry is a common challenge in tailings disposal and mining environmental protection worldwide. Flocculation is a commonly used method for solid-liquid separation of tailings, and the properties and structure of tailings flocs play a key role in sedimentation and filtration performance. This study proposed to improve the dewatering efficiency of tailings by regulating the floc properties and structure through polycarboxylate ether/ester (PCE). It simultaneously enhanced dewatering speed and reduced filter cake moisture. Dewatering experiments results showed that the assistance of PCE increased the initial filtration rate (IFR) of tailings by approximately 60 % and reduced filter cake moisture by 4.16 %. Moreover, the increase of shear strength in the slurry had a detrimental effect on dewatering performance. The dynamic evolution process of flocs was studied using focused beam reflectance measurement (FBRM) technology. The results showed that the addition of PCE caused a significant increase in floc size and counts, while the structural strength and regrowth ability of flocs also improved. Floc density calculations and scanning electron microscope (SEM) observations revealed that PCE caused densification of flocs, with particles aggregating more compactly. Contact angle analysis indicated that the adsorption of PCE on particles surface weakened their hydrophilicity. This study could conduce to understand the mechanism of dewatering performance enhancement in solid-liquid separation.

Collective effects on the settling of clay flocs

In this work a high-magnification digital video camera in combination with a settling column is used to study in a first part the influence of the amount of flocs transferred into the settling column on their settling velocity. In a second part, the setup was used to study the properties of flocs prepared at different clay concentrations but at same flocculant to clay ratio (2.5mgg−1). Illite clay was used and flocculated in a 1 L jar with an anionic polyacrylamide (flocculant). Results show that the average settling velocity of flocs is a function of the amount of transferred flocs. It was also found that floc size and settling velocity depend on clay concentration. This is attributed to the fast aggregation happening in the jar when flocculant and clay are mixed: at higher clay concentrations, larger flocs are created in the first minutes of the experiment, with low densities that prevent them from settling to the bottom of the jar.

Study on the impact of APAM residue on the settlement characteristics of coal slurry agglomerates

ABSTRACT Residual dosage has a significant impact on the settling characteristics of coal slurry flocs and the sedimentation efficiency of coal slurry. This study conducted sedimentation stirring experiments to measure the supernatant grayscale, floc size, settling velocity, and residual anionic polyacrylamide (APAM) under varying energy inputs. The investigation focused on the correlation between different levels of APAM residual and the settling characteristics of coal slurry flocs. The results show that having APAM residuals in the first three stages helps with sedimentation. This leads to lower residual levels, higher grayscale in the supernatant, bigger floc sizes, and better settling velocities. However, in the fourth stage, as APAM residual levels increased once more, the supernatant grayscale decreased linearly, floc breakup became severe, and settling velocities neared stagnation. In the second stage, a correlation was noticed between higher APAM residual levels and better settling properties of coal slurry flocs. By studying residual dosage and energy input, guidance on how to optimize the sedimentation efficiency of the thickener and reduce dosage can be provided.

Chitosan-Based Grafted Cationic Magnetic Material to Remove Emulsified Oil from Wastewater: Performance and Mechanism

In order to remove high-concentration emulsified oil from wastewater, a chitosan-based magnetic flocculant, denoted as FS@CTS-P(AM-DMC), was employed in this present study. The effects of factors including the magnetic flocculant dose, pH values, and coexisting ions were investigated. A comparative dosing mode with the assistance of polyacrylamide (PAM) was also included. The evolution of floc size was studied using microscopic observation to investigate the properties of flocs under different pH values and dosing modes. Particle image velocimetry (PIV) and extended Deryaguin–Landau–Verwey–Overbeek models were utilized to illustrate the distribution and velocity magnitude of the particle flow fields and to delve into the mechanism of magnetic flocculation. The results showed that FS@CTS-P(AM-DMC) achieved values of 96.4 and 74.5% for both turbidity and COD removal for 3000 mg/L of simulated emulsified oil. In the presence of PAM, the turbidity and COD removal reached 95.7 and 71.6%. In addition, FS@CTS-P(AM-DMC) demonstrated remarkable recycling and reusability performances, maintaining effective removal after eight cycles. The strength and recovery factors of magnetic flocs without PAM reached 69.3 and 76.8%, respectively. However, with the addition of PAM, they decreased to 46.73 and 51.47%, respectively. During the magnetophoretic processes, FS@CTS-P(AM-DMC) and oil droplets continuously collided and aggregated, forming three-dimensional network aggregates. Moreover, the magnetic floc generated a swirling motion, and the residual emulsified oil droplets could be further captured. Emulsified oil droplets were primarily removed through charge neutralization under acidic conditions. Under neutral and alkaline conditions, magnetic interactions played a major role in magnetic flocculation.

Understanding the mechanism of polyethyleneimine-mediated cell disintegration and protein extraction in E. coli: The role of floc network formation and PEI molecular weight

Cell disintegration and protein extraction are crucial steps in downstream process development for biopharmaceuticals produced in E. coli. In this study, we explored the extraction mechanism of polyethyleneimine (PEI) at the cellular level and characterized the floc network that is formed upon PEI addition by Focused Beam Reflectance Measurement and Dispersion Analyzer. PEI disintegrates the cells by detachment of the outer membrane allowing protein to diffuse into the interspace of the flocs. Protein release into the supernatant occurs by diffusion out of the floc network. We could show that the type and concentrations of PEIs with varying molecular weight determines the floc properties and thus the extraction efficiency. We could demonstrate why optimal conditions, using 70 kDa PEI at 0.25 g/g cell dry mass, lead to efficient extraction while at suboptimal conditions extraction is almost negligible. Our findings provide valuable insights into the relationship between floc properties and PEI-driven protein extraction, with potential applications in bioprocessing and biotechnology.

Degradation of fluoride in groundwater by electrochemical fixed bed system with bauxite: performance and synergistic catalytic mechanism.

Fluoride pollution in water has garnered significant attention worldwide. The issue of fluoride removal remains challenging in areas not covered by municipal water systems. The industrial aluminum electrode and natural bauxite coordinated defluorination system (IE-BA) have been employed for fluoride removal. The experiment investigated the effects of pH, current density, and inter-electrode mineral layer thickness on the defluorination process of IE-BA. Additionally, the study examined the treatment efficiency of IE-BA for simulated water with varying F- concentrations and assessed its long-term performance. The results demonstrate that the defluorination efficiency can reach 98.4% after optimization. Moreover, irrespective of different fluoride concentrations, the defluorination rate exceeds 95.2%. After 72 hours of continuous operation, the defluorination rate reached 91.9%. The effluent exhibited weak alkalinity with a pH of around 8.0, and the voltage increased by 2.0 V compared to the initial moment. By analyzing the characterization properties of minerals and flocs, this study puts forward the possible defluorination mechanism of the IE-BA system. The efficacy of the IE-BA system in fluoride removal from water was ultimately confirmed, demonstrating its advantages in terms of defluorination ability under different initial conditions and resistance to complex interference. This study demonstrates that the IE-BA technology is a promising approach for defluorination.

High efficiency regulating sedimentation and rheological properties of copper tailings using polycarboxylate superplasticizers

The recovery of low grade and fine particle copper ore usually requires sufficient dissociation, which reduces the particle size to the submicron level, presenting new challenges in subsequent copper tailings disposal. Flocculants can improve tailings sedimentation efficiency, but they also change the rheological properties of the slurry, resulting in low efficiency and high energy consumption during long-distances pumping. To address this issue, this study introduced polycarboxylate ether (PCE) superplasticizers as auxiliary additives for tailings treatment to improve fine particles sedimentation efficiency while enhancing slurry flowability. The results showed that compared to non-ionic polyacrylamide (NPAM) treated slurries, the synergistic effects of PCE and NPAM increased the initial sedimentation rate (ISR) by up to 3.4 times while decreasing the yield stress by up to 8 times and the thixotropic loop area by 10.5 times. DLVO theory calculations showed that PCE mainly affects particle interactions through a significant decrease in electrostatic repulsion. By in-situ monitoring with a focused beam reflectance measurement (FBRM) device, it was demonstrated that the synergistic effect of PCE improved the flocculation ability, strength, and regrowth ability of flocs. Furthermore, strong correlations were found between floc properties and fluid rheological properties. Overall, this study indicated that PCE additive was a promising reagent for fine particles slurry rapid settling and flowability enhancement, providing a new approach for copper tailings disposal.

In-depth study of the removal of Mn(II) by Fe(VI) treatment and the profound influence of NOM on floc formation and properties

The presence of manganese(II) in drinking water sources poses a significant treatment difficulty for water utilities, thus necessitating the development of effective removal strategies. Treatment by Fe(VI), a combined oxidant and coagulant, has been identified as a potential green solution; however, its effectiveness is hampered by natural organic matter (NOM), and this underlying mechanism is not fully understood. Here, we investigated the inhibitory effect of three different types of NOM, representing terrestrial, aquatic, and microbial origins, on Mn(II) removal and floc growth during Fe(VI) coagulation. Results revealed that Fe(VI) coagulation effectively removes Mn(II), but NOM could inhibit its effectiveness by competing in oxidation reactions, forming NOM-Fe complexes, and altering floc aggregation. Humic acid was found to exhibit the strongest inhibition due to its unsaturated heterocyclic species that strongly bond to flocs and react with Fe(VI). For the first time, this study has presented a comprehensive elucidation of the atomic-level structure of Fe(VI) hydrolysis products by employing Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). Results demonstrated that NOM strengthened single-corner and double-corner coordination between FeO6 octahedrons that were consumed by Mn(II), resulting in an increased contribution of γ-FeOOH in the core-shell structure (γ-FeOOH shell and γ-F2O3 core), thereby inhibiting coagulation effects. Furthermore, NOM impeded the formation of stable manganite, resulting in more low-valence Mn(III) being incorporated in the form of an unstable intermediate. These findings provide a deeper understanding of the complex interplay between Fe coagulants, heavy metal pollution, and NOM in water treatment and offer insight into the limitations of Fe(VI) in practical applications.

Salt-wedge intrusion-retreat cycle induced sediment floc dynamics in bottom boundary layer (BBL) of a micro-tidal estuary

Suspended sediment floc dynamics, particularly in the bottom boundary layer (BBL), has long been recognized as a complex and crucial factor influencing flow hydrodynamic and bed morphodynamics in tidal estuarine regions. Periodic salt-water intrusion in micro-tidal estuaries plays a crucial role due to its direct impacts on ecosystem, biological habitats, and human activities. To gain deeper understanding of the interactions between sediment flocculation and salt-water dynamics in estuaries, a continuous 115-h in-situ field survey onboard was conducted in a micro-tidal estuary, specifically the Huangmaohai estuary in China. This survey involved the measurements of suspended sediment concentrations (SSCs), floc properties, tidal current, turbulence and salinity in the BBL, utilizing state-of-art instrumentation such as the LISST-200X and ADV. Principal component analysis (PCA) was employed to examine the characteristics of sediment flocculation and analyze the influences of dynamic and structural factors on flocculation. The results reveal that, with the movement of the salt-wedge, two distinct stages of sediment flocculation process can be observed. Stage 1 occurs when the water column is vertically stratified, and a salt-wedge forms. Despite a reduction in SSC, this stage is characterized by a flocculation-dominated regime, in which large-sized, low density and low fractal dimension flocs settle to the bed. The presence of a stable salt-wedge enhances the flocculation by facilitating the formation of large-sized flocs with low density, low fractal dimension and high settling velocity while minimizing the disruptive effects of turbulence in the BBL. Statistical analysis reveals that saline water stratification contributes 19.76% to flocculation efficiency during this stage. Stage 2 occurs when the water column undergoes vertical mixing, causing the salt-wedge to dissipate. The regime transitions to a de-flocculation-dominated phase, leading to the resuspension of fine particles from the bed surface due to intense turbulence shear in the BBL. Consequently, flow shear becomes the key factor controlling factor for flocculation in the BBL with a contributing rate of 18.82%, statistically. A comprehensive conceptual model is presented to elucidate the intricate interactions between salt-wedge intrusion-retreat cycles and sediment flocculation within the BBL of a micro-tidal estuary.

The influence of micro-flocculation on membrane fouling during ultrafiltration of dissolved organic matter

This study investigated the mitigating effect of micro-flocculation as an ultrafiltration (UF) membrane pretreatment process on membrane fouling caused by organic matter in urban micro-polluted river water. The removal characteristics of organic matter by micro-flocculation were analyzed using three-dimensional fluorescence excitation emission matrix (3D-FEEM) and liquid chromatography organic carbon detector (LC-OCD). Furthermore, the variation of floc properties such as flocculation index and fractal dimension of micro-flocs formed under different flocculant dosages and their effects on UF membrane flux were investigated. The experimental results show that micro-flocculation using ferric chloride (FeCl3) can effectively remove large molecular weight organics. The features of flocs, which are influenced by the amount of FeCl3, affect the formation of the cake layer. Micro-flocculation can control the formation of a floc cake layer with a larger size and lower fractal dimension. It is helpful to effectively delay the attenuation rate of membrane flux by reducing membrane pore blockage and forming a filter cake layer with high porosity, which is of great significance for improving membrane technology. After the FeCl3 micro-flocculation treatment, the final specific flux of the membrane increased by 57.0 %. The combination of FeCl3 micro-flocculation and UF is a superior technical and economical treatment scheme for micro-polluted river water.

Functional behaviour of flocs explained by observed 3D structure and porosity

Clay-rich flocculated suspended sediments are an important constituent of estuarine and coastal systems globally. They are responsible for the host, movement and deposition of a variety of pollutants, contaminants and sediment itself. Accurate modelling of the movement of these sediments is crucial for a number of industries including fisheries, aquaculture, shipping and waste management. This requires an accurate and reliable measurements of the physical properties of flocs and their behaviour. Porosity is a key element in floc structures, and this research provides updated 3D quantified porosity and pore space morphological data in relation to influences on floc settling behaviour. We report the questionable relationship between floc size and settling velocity, and explore alternative influences such as floc composition, porosity and pore morphology. These outcomes suggest that a shift in focus from floc size to a combination of factors is necessitated to understand the complex movement behaviour of flocculated suspended sediments.

Coagulation performance and floc characteristics of Fe–Ti–V ternary inorganic coagulant for organic wastewater treatment

A novel Fe–Ti–V ternary inorganic coagulant (VTMS) was prepared from vanadium titanomagnetite (VTM) by two-step sulfuric acid leaching method, which is simple, efficient, and economical. VTMS was used to treat four typical organic wastewaters: synthetic domestic sewage, oily wastewater, surface water, and algae-laden water. The results indicated that it was superior to conventional coagulants. VTMS achieved a turbidity removal rate of nearly 100 % for all four wastewaters. In addition, it performed better in organic matter removal than polyaluminum chloride (PAC) with a larger floc size and higher growth rate. These results were attributed to the hydrolysis of Fe, Ti, and V, which formed a chain structure that improved the adsorption, bridging, and sweeping performances of VTMS. Based on the observations of coagulation performance, floc properties, and zeta potential, the predominant coagulation mechanisms appeared to be charge neutralization in synthetic domestic sewage and bridging flocculation in oily wastewater, surface water, and algae-laden water. Sweep coagulation also played a role in the coagulation processes for all four wastewaters. This study provided efficient technical guidance for large-scale production and application of Ti-based coagulants.

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.

Experimental study on the effect of hydrodynamic conditions on flocculation and settling properties of fine-grain sediment

The flocculation of cohesive sediment particles is a function of the collision efficiency of sediment particles, which is mainly influenced by local flow hydrodynamics. A detailed study on local hydrodynamic characteristics in a novel stirred tank was done to measure the flow field of a turbulent flow. Instantaneous flow velocity fields were obtained and processed, and the average flow field was computed. Particle image velocimetry (PIV) results measuring the hydrodynamic characteristics (expressed as turbulent kinetic energy (TKE), local dissipation rate of TKE, velocity gradient or shear rate, turbulent shear stress, etc.) were analyzed and discussed. Particle tracking and automated image processing techniques were utilized to analyze the impact of the hydrodynamic conditions on the floc and settling properties. The differences in floc properties under the two turbulent shear states (i.e., steady-state (SS) and unsteady-state (US)) hydrodynamic conditions were examined. The results show that floc size and floc size distribution (FSD) of pure kaolin clay flocs are significantly influenced by the local dissipation rate of TKE and the turbulent intensity states. The floc size, df, 84, at local dissipation rate of TKE of 11 × 10−4, 29 × 10−4, 44 × 10−4, 83 × 10−4, and 142 × 10−4 m2/s3 is 97, 108, 107, 124, and 150 μm, respectively. It was found that by increasing the local dissipation rate of TKE, the FSD was skewed left. The US shear condition generated larger flocs (and in higher proportions) relative to the SS condition. The settling velocity and flux settling velocities of flocs formed under SS conditions are higher than those formed under US conditions. The fractal dimension of SS flocs also is higher than the fractal dimension of US flocs. These results highlight a significant impact of the states of hydrodynamics on floc properties such as floc size and FSD as well as the settling and morphological properties of flocs.

Evaluation of the nanoplastics removal by using starch-based coagulants: Roles of the chain architecture and hydrophobicity of the coagulant

Four starch-based coagulants with similar charge densities but different chain architectures and hydrophilicities were designed and fabricated. These are a linear and hydrophilic coagulant, St-CTA (Starch-3-chloro-2-hydroxypropyl triethyl ammonium chloride); a graft and hydrophilic coagulant, CS-AM-DMC (cationic starch-graft-poly[2-methacryloyloxyethyl trimethyl ammonium chloride and acrylamide]); and two graft and partially hydrophobic coagulants, CS-AM-DML (cationic starch-graft-poly[methacrylic acid 2-(benzyldimethylaminio) ethyl chloride and acrylamide]) and CS-AM-DMR (cationic starch-graft-poly[2-(methacryloyloxy)-N,N-dimethyl-N-alkyl ammonium chloride and acrylamide]) with aromatic and n-octane groups respectively. These starch-based coagulants in conjunction with polysilicic acid were employed in a current major environmental challenge which is to efficiently remove nano-sized plastics (NPs) from water. NPs of polystyrenes, polymethyl methacrylates and polyvinylchlorides were removed from different water sources under various conditions of pH and salinity. The effects of the chain architectures and hydrophilicities of the starch-based coagulants on the NPs removal have been investigated in detail. According to experimental evidence, three graft starch-based coagulants exhibited higher NPs removal efficiencies and better flocs properties than the linear St-CTA. This was attributed to the superiority of the graft chain architecture in the graft starch-based coagulants causing higher charge neutralization efficiencies in coagulative removal of NPs. In the three graft starch-based coagulants, CS-AM-DMR and CS-AM-DML with partially hydrophobic structures were more effectively to remove three NPs than CS-AM-DMC, and produced large, compact, durable and rapidly regrown flocs, due to the synergistic effects of charge neutralization and hydrophobic association. The interaction energies between NPs and the coagulants obtained from the extended Derjaguin-Landau-Verwey-Overbeek theory further confirmed that CS-AM-DMR and CS-AM-DML could achieve faster coagulation and thus higher efficiency in NPs removal. In addition, the NPs removal efficiency increased in acidic and highly saline solutions. These results obtained are of importance in guiding the design and selection of suitable polymeric coagulants for efficiently treating target contaminants in water.