Coagulation Of Suspensions Research Articles

Container size effects on the validity for the concept of sedimentation turbulence studied using coagulated suspension of Na-montmorillonite in the semi-dilute regime

In our previously proposed concept of sedimentation turbulence, the feed-forward mechanism of flocculation induced by a sequential cohesive collision between settling flocs during the sedimentation process resulted in the monotonous increase of the settling velocity with increments of the settling cylinder height. However, the monotonous increase of the settling velocity and the size of flocs onset of the sedimentation turbulence are considered to meet across their limiting values. In order to confirm this conjecture, the effects of container sizes, i.e., the diameter and the height of the settling cylinder, on the validity of this concept were investigated on the basis of maximum floc size, which was maintained during the sedimentation process. The sedimentation experiments of highly coagulated Na-montmorillonite in the semi-dilute regime were extended using cylinders taller in height and smaller in diameter. The results obtained demonstrate that the maximum velocity becomes much faster than that of the free settling of a single floc. A notable observation was made when a cylinder with a diameter of 10 mm was used and in which the corresponding floc size was in the same order of magnitude. In this case, the clogged large floc essentially stopped its sedimentation motion. The crude estimation of the fluid force exerted on the floc during the sedimentation process was found to be consistent with this drastic change. • Sedimentation rate was enhanced by a collision of flocs resulted from a feed-forward mechanism. • The validity of sedimentation turbulence concept against the diameter and the height of the settling cylinder was examined. • Limiting behavior of sedimentation was found in the relation of cylinder diameter and size of developed flocs. • Crude estimation of the fluid force exerted on the floc was discussed.

USE OF MODIFIED REAGENTS SOLUTIONS WHEN PREPARING DRINKING WATER

Currently, attention is paid to the intensification of the process of natural and wastewater treatment, improvement of technology, development of new effective methods of intensification of water treatment. This will simplify the existing technology of water treatment, reduce labor-intensive processes of preparation and dosing of reagents, reduce the cost of operating treatment plants, increase their productivity, improve quality and reduce the cost of treated water. Treatment of water with a modified coagulant solution allows to increase the hydraulic size of the coagulated suspension. The strongest effect of the modified coagulant solution has on the hydraulic suspension size of 0.2 mm/s and less, i.e. the smallest and the suspension, which is difficult to remove, which creates conditions for more intensive sedimentation in settling tanks and improve the quality of water clarification and fed to fast filters. It has been experimentally determined that the treatment of water with a modified solution of aluminum sulfate coagulant should be performed when the content of suspended solids in the clarified water is up to 100–150 mg / dm3. The color of the clarified water during treatment with a modified solution of aluminum sulfate coagulant does not depend on the content of suspended solids and is 1.5-1.6 times lower than the color than when using a conventional coagulant solution. It is established that the use of a modified solution of aluminum sulfate coagulant in water purification can reduce the residual aluminum content in clarified water by an average of 50-60%. The results of researches of the modified solutions of reagents which are used at preparation of drinking water are presented. It is established that water treatment with a modified solution of aluminum sulfate coagulant allows to improve the quality of drinking water preparation by suspended solids, color, reduce the content of aluminum salts in the clarified water, thereby increasing the environmental safety of drinking water.

Front-face fluorescence excitation-emission matrix (FF-EEM) for direct analysis of flocculated suspension without sample preparation in coagulation-ultrafiltration for wastewater reclamation

Fluorescence spectroscopy has been suggested as a promising online monitoring technique in water and wastewater treatment processes due to its high sensitivity and selectivity. However, a pre-filtration is still indispensable in fluorescence measurement for removing ubiquitous particles and flocs in real samples to eliminate the strong light scattering that could attenuate fluorescence detection significantly. This study proposed a front-face fluorescence spectroscopy, which could characterize the liquid sample with suspended solids directly without pre-filtration. Front-face excitation-emission matrix (FF-EEM) coupled with parallel factor (PARAFAC) analysis was used for analyzing fluorescence components and to probe coagulation of secondary effluent and fouling in the subsequent ultrafiltration (UF), and conventional right-angle fluorescence EEM (RA-EEM) was also compared. The results showed that FF-EEM was less susceptible to turbidity (induced by standard particles) in the secondary effluent compared to RA-EEM. FF-EEM could successfully measure dissolved fluorophores in coagulated suspension without pre-filtration, while conventional RA-EEM was undermined significantly due to the existing flocs. FF-EEM coupled with PARAFAC could accurately probe dissolved organic matter and fouling in coagulation- UF wastewater reclamation processes. Therefore, it was demonstrated that this front-face fluorescence without any sample preparation step might be highly promising in real-time online fluorescence monitoring in multi water and wastewater treatment processes.

Applying a modified aluminum sulfate solution in the processes of drinking water preparation

This paper reports a study on the application of aluminum sulfate solution, modified by the magnetic field and electrocoagulation, in the processes of drinking water preparation. The modification of the coagulant solution makes it possible to intensify water purification processes, to reduce reagent consumption by 25–30 %. It has been found that a dose of the modified aluminum sulfate solution of 28–30 mg/dm 3 improves the efficiency of removal of suspended substances and coloration by 35–40 %. The dosage of the conventional reagent solution was 40 mg/dm 3 while reaching the same purification parameters. Modifying a solution of aluminum sulfate with the magnetic field and electrocoagulation increases the hydraulic size of the coagulated suspension. A change in the hydraulic size in the suspension has been studied at different periods of the year. In winter, when treating water with the modified aluminum sulfate solution, there a decrease in the suspension content whose hydraulic size is 0.1 mm/s and less, from 89 % to 22 %. In this case, the content of suspended substances at settling decreases from 8.5–12.5 mg/dm3 to 5.6–8.3 mg/dm 3 . In spring, when using the modified coagulant solution, the content of suspension whose size is 0.1 mm/s and less decreased from 55 % to 15 %. In summer, there is an increase in the content of suspension whose size is 0.3–0.5 mm/, from 58 % (a conventional reagent solution) to 66 % (the modified reagent solution). This indicates an intensification of the coagulation of impurities and the clarification of water. The experimental data testify to an increase in the effectiveness of discoloration of natural low-turbid colored water to 63.3–63.9 % for the modified reagent solution at 45.5 % for a conventional reagent solution. A change in the bacteriological parameters has been determined: the effectiveness of the decrease in a microbial number grows from 11.6‒18.7 % to 18.6–25.1 %. In terms of a coli-index, the efficiency of purification grows from 16.6‒23.1 % to 23.0–29.5 %

Changes in Chara hispida L. morphology in response to phosphate aluminium coagulant application

Abstract Progressing eutrophication of waterbodies requires measures to be undertaken that aim at halting or reversing negative changes in the environment. Chemical restoration is one of the most common methods used for lake treatment, where iron or aluminium phosphate coagulants are applied. However, their chemical qualities pose the risk of acidification and aluminium ion release, which become toxic in acidic conditions. The influence of coagulants on aquatic plants, including charophytes that are very valuable from the ecological perspective, is little recognised. For this reason, the aim of the research was to define changes in the growth pattern of the charophyte Chara hispida under the influence of an aluminium coagulant. The research was carried out in mesocosms (0.8 m3) located in situ in a lake. Polyaluminium chloride was applied once to each chamber in doses of 50.0, 100.0 and 200.0 ml m−3. Coagulant concentrations reflected aggressive restoration aimed at precipitation of phosphates, suspension and water colour at the same time. It was proved that the coagulant had inhibited the growth and slightly reduced the length of branchlets, and simultaneously elongated internode cells. Changes in the total length as well as the length of branchlets were caused by a strong pH decrease of the environment which simultaneously induced higher aluminium solubility and toxicity. Elongation of internode cells was caused by reduced light availability, resulting from high water turbidity in the first stage of coagulant’s application, and then from the charophytes’ thallus being covered by a coagulated suspension precipitated from water.

Sedimentation of suspended solids in ultrasound field

Physical and chemical effects of aquatic environment that occur in an ultrasonic field change the sedimentation rate of coagulated suspension. This might only happen in case of cavitation of ultrasonic filed that causes a change of potentials of the medium. Research of the influence of ultrasonic vibrations on coagulation of suspended solids within water purification allows expanding their scope of implementation. The objective of the research is to estimate the effect of ultrasound on the sedimentation of the suspended solids, to determine of the efficiency of the process in relation to the dose of the coagulant, and to calculate the numerical values of the constants in the theoretical equation. The experiment condition was held in the water with the clay substances before the introduction of the coagulant. The method of magnetostriction ultrasonic generator was applied to receive ultrasonic vibration. Estimate of concentration of clay particles in water was performed using photometry. As a result of the research, the obtained data allow determining the increase in efficiency of suspended particles sedimentation related to the dose of coagulant, depending on time of ultrasonic treatment. The experiments confirmed the connection between the effect of sedimentation in the coagulation process, the coagulant dose and the time of scoring. Studies have shown that the increase in the duration of ultrasonic treatment causes a decrease of administered doses of coagulant.

Sedimentation of suspended solids in ultrasound field

Physical and chemical effects of aquatic environment that occur in an ultrasonic field change the sedimentation rate of coagulated suspension. This might only happen in case of cavitation of ultrasonic filed that causes a change of potentials of the medium. Research of the influence of ultrasonic vibrations on coagulation of suspended solids within water purification allows expanding their scope of implementation. The objective of the research is to estimate the effect of ultrasound on the sedimentation of the suspended solids, to determine of the efficiency of the process in relation to the dose of the coagulant, and to calculate the numerical values of the constants in the theoretical equation. The experiment condition was held in the water with the clay substances before the introduction of the coagulant. The method of magnetostriction ultrasonic generator was applied to receive ultrasonic vibration. Estimate of concentration of clay particles in water was performed using photometry. As a result of the research, the obtained data allow determining the increase in efficiency of suspended particles sedimentation related to the dose of coagulant, depending on time of ultrasonic treatment. The experiments confirmed the connection between the effect of sedimentation in the coagulation process, the coagulant dose and the time of scoring. Studies have shown that the increase in the duration of ultrasonic treatment causes a decrease of administered doses of coagulant.

Effects of electrolyte concentration and pH on the sedimentation rate of coagulated suspension of sodium montmorillonite

The sedimentation behavior of semi-dilute suspensions of sodium montmorillonite in the flocculation regime was studied as a function of ionic strength at pH 4.0 and 9.5 to elucidate the response of variation of macroscopic sedimentation behavior with microscopic particle association. The maximum velocity of the boundary between the supernatant and sediment and ultimate sediment height were chosen as indexes to evaluate the sedimentation behavior. Increment ratio of effective volume fraction of the particles (α) caused by the formation of flocs and hydrodynamic floc diameter were evaluated by Michaels and Bolger method on the basis of the Richardson⿿Zaki formula. The results clearly demonstrate that the maximum velocity of the boundary is strongly affected by both pH and salt concentration. The two observed patterns of variation of maximum sedimentation rate and ultimate sediment height can be clearly defined using 0.25⿿0.5M NaCl as an inflection range. This range is consistent with the transition point at 0.3M NaCl for the swelling behavior of sodium montmorillonite reported by Norrish. Below 0.25M NaCl, the effect of pH on the sedimentation rate is remarkably enhanced compared with that at higher salt concentration. This effect is ascribed to the dominant appearance of edge⿿face (EF) particle interactions, which can be regarded as negligible at higher salt concentration. The reason for this is that the breakage of weak EF bonds is induced by the dominant appearance of face⿿face particle interactions at higher salt concentrations. That is, the decrease of repulsion strengthening the attraction between neighboring particles is responsible for EF bond breakage. The small influence of pH on ultimate sediment height at high salt concentration can be ascribed to the residual strong EF bonds. The Michaels and Bolger method can be used qualitatively at range of NaCl greater than 0.5M.

Breakup of Agglomerated Clusters of Cellulosic Fines and CaCO3 Particles Exposed to Hydrodynamic Stress

The capacity of fine particles to remain clustered together after being agglomerated by polyelectrolytes plays an important role in papermaking and in the treatment of wastewater. Tests were carried out with agglomerated suspensions of calcium carbonate and primary cellulosic fines in neutral buffer solution. Agglomeration was induced either by a high-charge cationic polyelectrolyte (a coagulant) or by sequential treatment with a coagulant and a very high-mass anionic acrylamide copolymer (a flocculant). Particle size analysis, based on diffraction of laser light, showed that the coagulated suspensions were susceptible to being redispersed by hydrodynamic shear. By contrast, flocculated suspensions were only partly broken up. In a flocculated mixture of CaCO3 and cellulosic fines, only the cellulosic fines could be separated from each other. The intensity of shear was more critical than its duration. Conventional shear stress was more effective for the breakup of the polyelectrolyte-induced agglomerates versus extensional flow or intense ultrasonic vibrations.

Impacts of organic coagulant aid on purification performance and membrane fouling of coagulation/ultrafiltration hybrid process with different Al-based coagulants

Effects of polydimethyldiallylammonium chloride (PD) on coagulation behaviors of different Al-based coagulants were investigated in coagulation–ultrafiltration (C–UF) hybrid process, regarding the impurity removal efficiency, floc properties and membrane foulings. Floc characteristics, including floc size, compact degree, strength and re-growth ability were studied using a laser diffraction particle sizing device. Resistance analyses were implemented to explore the membrane fouling mechanisms. The results indicated that PD aid could increase the purification efficiency of C–UF, especially at low coagulant doses. PD–alum/PD–PACl contributed to large flocs, while alum–PD/PACl–PD gave rise to flocs with high Rf and Df values. The results of ultrafiltration experiments showed that conventional coagulant, i.e., alum led to a flux reduction of 52%; while the reductions for alum–PD and PD–alum were 53% and 34%, respectively. The flux reductions for PACl, PACl–PD and PD–PACl were 60%, 57% and 39%. Flux declines became more severe when the coagulated suspensions were exposed to increased shears and the suspensions coagulated by aluminum plus PD resulted in the least reductions in fluxes.

Wall effects during settling in cylinders

The phenomena of syneresis and shear densification were analysed through the sedimentation of alumina suspensions. Experiments compared cylinder settling tests with a rod inserted in a cylinder against free settling without a rod for the cases of polymer flocculated and salt coagulated suspensions. Additionally, dye intrusion was used to visualise the final bed structure to identify the influence of cylinder walls and the potential presence of aggregate arching structures.It was found that, for various cylinder diameters, polymer flocculated alumina suspensions settled more quickly when a rod was placed in the centre of the settling cylinder. This is attributed to shear induced densification of the flocculated aggregates during the settling process, as it was observed that the solids shrank away from the rod, creating a channel which allowed water to escape more easily than when the rod was absent. The reverse result was observed when a rod was placed in an unflocculated/coagulated alumina suspension; the solids stuck to the rod, resembling the syneresis mechanism of isotropic shrinkage towards the rod. Industrially these results are significant since inserting rods into sedimentation devices such as a thickener can result in a flocculated suspension settling further and at a faster rate, such that rods can provide an alternative to mechanical devices such as rakes.Dye intrusion tests indicated that there was arching in the network bed, resulting in small pockets of water being trapped in the structure. An equation was derived to predict the maximum diameter arch that could be supported, taking into account the shear yield stress of the suspension. The solution to this equation was supported by observed cavities at the cylinder wall of up to 3 mm. These results suggest that cylinder walls may have a negative effect on the extent to which a suspension dewaters.

Pressure-driven flows of concentrated alumina suspensions depending on dispersion states of particles

The flow developments of 25 vol% alumina suspensions in slit channel were visualized and analyzed depending on the dispersion states. For the coagulated alumina suspension, the shear stress showed an N curve that included a region of stress decrease with an increase in shear rate followed by a monotonic increase. Depending on the region in the stress curve, the flow profile changed from a shear-banded profile to a plug-like flow profile similar to the Newtonian fluid. In addition, it was observed that the transient flow behavior over time at high shear rate in liquid state experienced all of the steady state flow profiles at lower shear rates in solid-liquid transition. During the solid-liquid transition, the flow profile was found to be shear banded, and the pressure profile did not reach a steady state but fluctuated with a characteristic time period. In contrast, the well-dispersed suspension showed only a monotonic increase of shear stress in the range of shear rates we could measure, indicating that the suspension was in liquid state. The flow profile was plug-like, and the pressure was fluctuating without any characteristic time period.

Rheological behavior of dilute imogolite suspensions

Imogolite is one of the most important clay minerals contained in volcanic ash soil. The morphology of imogolite is very unique and comprises thin fibrous tubes with inside and outside diameters of approximately 1 and 2nm, respectively. The chemical structure of imogolite features SiOH groups along the inner walls of the tubes and AlOH groups on the outer surfaces of the tubes. As a result of this morphology and the chemical structure, imogolite coagulates at high pH. In the present study, we focus on the detailed differences of the non-Newtonian behavior of the dilute imogolite suspension in the dispersed and coagulated states. Experiments on the viscosity as a function of volume fraction were performed using a concentric cylindrical viscometer. The applied shear rate was increased from 6.45 to 258s−1 and then decreased to the initial value; this procedure was repeated three times. The flow curves for the dispersed imogolite suspensions showed slight hysteresis. Additionally, shear thinning, which is a decrease of the viscosity with increasing shear rate, emerged; this is thought to be due to the change of the orientation distribution of the imogolite particles. For the coagulated suspensions, the area of the hysteresis loop increased with increasing volume fraction. The flow curves tended to shift upwards and became constant as the shearing cycles increased. Moreover, shear thinning at high pH was more significant than that at low pH. The yield stress of the coagulated state was determined from the intercept of the flow curve. We propose that imogolite at high pH forms flocs that are repeatedly subjected to breakup and re-coagulation by the given shear stress; thus, a stronger structure is formed.

Direct Coagulation Casting of Positively Charged Alumina Suspension by Controlled Release of High Valence Counter Ions from Calcium Phosphate

The idea of controlled release of high valence counter ions was used in direct coagulation casting of ceramics. Calcium phosphate was used as the high valence counter ions source. Influence of the content of calcium phosphate on the viscosity of the suspension with positive charge on particles and the properties of green body were investigated. The results show that the content of calcium phosphate which leads to coagulation of suspension is 0.15–0.2 wt%. The pH changes from acidic regime to basic regime after calcium phosphate addition. Hydrochloric acid is consumed and CaHPO4, Ca(H2PO4)2, and CaCl2 are generated. The electrical double layer of alumina particles is strongly compressed by Ca2+ ions, leading to rapid increase of viscosity. Also, the pH value at high temperature approaches to isoelectric point of alumina and contributes to the instability of suspension. The optimum coagulated temperature is 60°C with coagulation time of 135 min. The compressive strength of green body is up to 2.8 MPa.

Speciation characterization and coagulation of poly-silica-ferric-chloride: The role of hydrolyzed Fe(III) and silica interaction

Abstract The highly efficient inorganic polymer flocculants (IPFs) of the ferric-silica system is a new and promising coagulant. Interactions between ferric species and silica play a large part in the coagulation of suspensions. These effects are quite distinct from those associated with polymeric or colloid silica. However, although these species are key to coagulation efficiency, they have not been comprehensively discussed. A new type of coagulant, poly-silica-ferric-chloride (PFSC), was synthesized by co-polymerization and characterized by time complexation spectroscopy and photon correlation spectroscopy. Compared with traditional ferric salt, the results indicated that PFSC had a higher molecular weight, lesser positive charge, lower Fe b and higher Fe c. The higher the Si/Fe ratio, the higher the silica and lower the silica found. The PFSC with appropriate polysilica acid not only obtained better coagulation/flocculation efficiency in turbidity removal, enhanced the flocculation index (FI) and provided less residual ferric, it also lowered water treatment costs compared to traditional ferric salt. Results showed that PFSC could remove colloid particles in water by charge neutralization and sweeping, adsorption bridging mechanism.

Studies on the Realationship between the Speciation Distribution and Si/Fe Ratio of Poly-Silica-Ferric-Chloride

Interactions between ferric species and silica have been shown to play a large part in the coagulation of suspensions. These effects are quite distinct from those associated with polymeric or colloid silica. The interaction of monosilicic acid with Fe ions has been discussed in this paper. Compared with traditional iron salt, poly-silica-ferric-chloride (PFSC) is characterized by having higher molecular weight, less positive charge, lower Feb and higher Fec. In addition, the Si/Fe is higher and the Silica is higher and Silicc is lower. With increasing Si/Fe ratio the effective diameter of PFSC increases. Along with the increase of Si/Fe ratio, more complicated polymers are formed and the particle size distribution is broadened. Resluts show that some co-polymerized species or aggregates between silica and Fe( ) polymers are formed.

Coagulation pre-treatment to reduce membrane fouling in the microfiltration process of Nakdong River water

The effects of coagulation pre-treatment on microfiltration process were investigated with the Nakdong River source water. In this study, the potential membrane-fouling materials presented in the raw water were mainly attributed to particulate matters and UV254-absorbing materials. Coagulation pre-treatment mitigated the effects of these components on membrane fouling because it induced a change in the characteristics of the cake layer and a decrease in the content of UV254-absorbing materials. Even though the coagulated suspensions were not removed before MF membrane filtration, their impact on resistance to filtration was insignificant when compared with that caused by the suspensions without pre-coagulation. However, insufficient dosages of coagulant or the improper controls of coagulation pH might cause severe membrane fouling in the treatment of low turbidity water (i.e. turbidity below 10 NTU). It appeared that a selection of coagulant dosage that focused on the reduction of specific cake resistance was possibly the best way to achieve the optimal condition in this study. Also, the coagulation pre-treatment process at pH values between 5.3 and 6.8 was found to be most effective in providing the lowest specific cake resistance as well as residual UV254 absorbance.

Plastic Forming of Zirconia from Coagulated Suspensions

A new method of plastic forming was carried out for zirconia ceramics, which was based on more uniformly mixed and de-agglomerated pastes produced by a process thatinitial dispersion of suspension and subsequent coagulated at a sufficient high solid loading. Through comparing of three approaches or conditions, our results proved that this plastic forming process can give rise to components of higher strength compared to those produced by existing processing route.

Electrically enhanced dewatering (EED) of particulate suspensions

Electrically enhanced dewatering has been characterised from first principles using model kaolinite suspensions in both the coagulated and dispersed state and sludge from a potable water treatment plant. The dewatering properties, namely the compressive yield stress or extent of dewatering (quantified as the applied pressure at an equilibrium solids concentration) and the hindered settling function or the resistance of fluid flow (quantified as the rate of fluid expression at an equilibrium solids concentration), have been measured as a function of electric field strength. For both the dispersed and coagulated kaolinite suspensions, the rate of dewatering was found to improve at all applied pressures and with increasing applied electrical field strength, up to 1250 V m −1. Improvements in the extent of dewatering were also observed but only for the coagulated suspension. The greatest improvements in dewatering were observed at the lowest applied pressures. Improvement in suspension compressibility is only predicted for the specific case where the feed to the process is both coagulated and at a neutral or low pH. For the potable water sludge, improvements in the rate of dewatering were observed at all pressures above 10 kPa. At lower pressures, the low particle surface charge caused a slow onset of electro-osmotic effects. In general, the results indicate that the application of an electric field in situ during dewatering primarily helps to increase the rate of dewatering and the benefit of this methodology is predicted to be for suspensions that exhibit low permeability at low solids concentrations in applications where the pressure is low or the process is gravity driven.

Low-pressure membrane filtration with unconventional coagulation regimes

The objective of the research was to evaluate in-line coagulation to improve performance during ultrafiltration (UF). In-line coagulation means use of coagulants without removal of coagulated solids prior to UF. Performance was evaluated by removal of contaminants (water quality) and by resistance to filtration and recovery of flux after hydraulic or chemical cleaning (water production). We hypothesized that coagulation conditions inappropriate for conventional treatment, in particular under-dosing conditions that produce particles that neither settle nor are removed in rapid sand filters, would be effective for in-line coagulation prior to UF. A variety of pre-treatment processes for UF have been investigated including coagulation, powdered activated carbon (PAC) or granular activated carbon (GAC), adsorption on iron oxides or other pre-formed settleable solid phases, or ozonation. Coagulation pre-treatment is often used for removal of fouling substances prior to NF or RO. It has been reported that effective conventional coagulation conditions produced larger particles and this reduced fouling during membrane filtration by reducing adsorption in membrane pores, increasing cake porosity, and increasing transport of foulants away from the membrane surface. However, aggregates produced under sweep floc conditions were more compressible than for charge neutralization conditions, resulting in compaction when the membrane filtration system was pressurized. It was known that the coagulated suspension under either charge-neutralization or sweep floc condition showed similar steady-state flux under the cross-flow microfiltration mode. Another report on the concept of critical floc size suggested that flocs need to reach a certain critical size before MF, otherwise membranes can be irreversibly clogged by the coagulant solids. The authors were motivated to study the effect of various coagulation conditions on the performance of a membrane filtration system.