Coagulation Ultrafiltration Research Articles

The role of shear conditions on floc characteristics and membrane fouling in coagulation/ultrafiltration hybrid process – the effect of flocculation duration and slow shear force

The impact of shear conditions during coagulation on the ultrafiltration permeate flux in a coagulation–ultrafiltration (C–UF) process was investigated.

Effects of magnetization on Fe(iii) species in magnetically enhanced coagulation ultrafiltration processes

The effects of magnetization on floc properties and membrane fouling in magnetically enhanced coagulation ultrafiltration (MEC-UF) processes for micro-polluted water treatment were investigated in this study.

Comparison of epichlorohydrin–dimethylamine with other cationic organic polymers as coagulation aids of polyferric chloride in coagulation–ultrafiltration process

Epichlorohydrin–dimethylamine (DAM–ECH) copolymer was acquired by polycondensation of hazardous reagents: epichlorohydrin (analytical reagent, A.R.) and dimethylamine (A.R.) with ethanediamine (A.R.) as cross-linker. Its coagulation and membrane performance as coagulation aid of polyferric chloride (PFC) was evaluated by comparing with other two cationic coagulation aids: poly dimethyl diallyl ammonium chloride (PDMDAAC) and polyacrylamide (PAM) in humic acid–kaolin (HA–Kaolin) simulated water treatment. Firstly, optimum dosages of PFC&DAM–ECH, PFC&PDMDAAC and PFC&PAM were identified according to their coagulation performance. Then their impacts (under optimum dosages) on membrane fouling of regenerated cellulose (RC) ultra-membrane disc in coagulation–ultrafiltration (C–UF) process were reviewed. Results revealed that small addition of DAM–ECH was the effective on turbidity and DOC removal polymer. Furthermore, in the following ultra-filtration process, external membrane fouling resistance was demonstrated to be the dominant portion of the total membrane fouling resistance under all circumstances. Meanwhile, the internal membrane fouling resistance was determined by residual of micro-particles11Micro-particles: Humic acid molecules, Kaolin and micro-floc formed in coagulation process that cannot be intercepted by cake layer or ultrafiltration membrane. that cannot be intercepted by cake layer or ultrafiltration membrane.

Effect of under-dosing coagulant on coagulation–ultrafiltration process for treatment of humic-rich water with divalent calcium ion

The strategy of under-dosing coagulant was firstly applied in a coagulation–ultrafiltration (UF) process for treatment of humic-rich water in this study. When polyaluminum chloride (PACl) dosage was set as 0.025mM (as alum), the removal efficiency of organic matter and residual turbidity were slightly inferior to 0.1mM (as alum), but four times dosage lowered than the optimal-dosing. Although permeate flux of ultrafiltration membrane was larger at the optimal-doing than that of under-dosing, the less proportion of irreversible fouling and large proportion of reversible fouling were proved that under-dosing strategy has more meaningful engineering application value for deep investigation. The impacts of Ca2+ on coagulation and ultrafiltration performance are also investigated under different coagulant doses. Removal of humic-like substance, floc size and incompactness were appreciably enhanced at under-dose with increasing Ca2+ concentration, but did not distinctly change at the optimal-dosing. Besides, lower positive charge of flocs produced at under-dosing hindered the foulants transport to membrane surface. The interactions between humic substance, positively-charged coagulants, and structure of cake layer firstly systemically investigated the impacts on the performance of ultrafiltration in our study. The results shown that the technology of under-dosing with addition of Ca2+ has dramatically increased the UV254 removal efficiency, formed loose and porous flocs still outside of the critical electrostatic distance of membrane surface, which significantly remit the membrane fouling and improve the reversibility of membrane fouling.

Effects of epichlorohydrin–dimethylamine on coagulation and membrane performance of ferric chloride in coagulation–ultrafiltration hybrid process

Cationic polymer epichlorohydrin–dimethylamine (DAM–ECH) was applied as coagulation aid of ferric chloride (FC) in treatment of humic acid–kaolin (HA–Kaolin) simulated water in this study. Impacts of DAM–ECH on coagulation performance, flocs characteristics and membrane fouling of FC in coagulation–ultrafiltration (C–UF) process were investigated under different pH conditions. Results showed that the removal rates of UV254 (ultraviolet adsorption at 254nm) and DOC (dissolved organic carbon) of FC/DAM–ECH was much better than that of FC at same dosage. The optimum coagulation condition was FC at 10mg/L with FC/DAM–ECH mass ratio (MR) of 3:1 based on the consideration of treatment cost and performance. Moreover, turbidity, UV254 and DOC removal rates increased when pH increased from 4 to 6 and then decreased as pH grew from 7 to 10. Thus, the optimum pH for coagulation was also determined to be 6 accordingly. Addition of DAM–ECH also increased sizes of produced flocs and made them looser in structure comparing with FC–HA flocs. Meanwhile, results obtained from ultrafiltration experiments showed that membrane fouling would be reduced by introducing DAM–ECH in coagulation treatment; the optimum membrane performance was given by FC/DAM–ECH under pH 6. In conclusion, proper dose of FC/DAM–ECH under pH 6 could ensure an ideal HA removal rate and meanwhile increase operating duration of C–UF system by restricting membrane fouling.

Influence of flocs breakage process on membrane fouling in coagulation/ultrafiltration process—Effect of additional coagulant of poly-aluminum chloride and polyacrylamide

The additional coagulant dosing strategy was applied in a coagulation–ultrafiltration (UF) process for treatment of drinking water. When poly-aluminum chloride (PACl) dose was set as the optimal coagulant dosage (alum 0.15mM), the additional PACl and polyacrylamide (PAM) were added after breakage to further examine their benefits and impact on membrane fouling. The average size of flocs re-formed with additional non-ionic PAM was larger than that of additional PACl. The fractal dimension of flocs (dpf) was proved to play an important role on membrane fouling. Broken flocs re-formed with additional non-ionic PAM has more convoluted boundaries and lower effective density than the one with additional PACl, which is available to form loose and porosity cake layer on surface of membrane. Microscopic observations of membrane surface demonstrated that there was obvious correlation between the structure of cake layer and the fractal dimension of flocs. Besides, reversible fouling (RF) and irreversible fouling (IF) were also investigated in our study. The additional PACl dosing could lower membrane fouling, but irreversible fouling did not reduce because much more positive charged of flocs may adsorb on the surface of membrane under electrostatic attraction. As for non-ionic PAM, both of RF and IF were simultaneously decreased. As a result, the structural characteristics and surface electrical of flocs are two main mechanisms caused irreversible fouling.

Effect of pH on floc properties and membrane fouling in coagulation – Ultrafiltration process with ferric chloride and polyferric chloride

Impact of pH on coagulation–ultrafiltration (C–UF) process was investigated with respect to coagulation efficiency, floc characteristics and membrane fouling in this study. Ferric chloride (FeCl3) and polyferric chloride with basicity of 1.0 and 2.2 (denoted as PFC10 and PFC22) were used as coagulants and Fe (III) species in them was measured by a timed complexation spectroscopy method. Floc properties under four pH conditions were evaluated using a laser diffraction particle sizing device. Ultrafiltration experiments were conducted by a dead-end batch unit. The results showed that organic matter removal efficiency was higher under acidic conditions than under other pH conditions and turbidity removal efficiency was higher under alkaline condition. At same pH, FeCl3 containing higher monomeric and polymeric species (Fea and Feb) had better organic matter removal and higher turbidity removal efficiency was obtained by coagulants with larger percentage of polymer or colloidal species (Fec). Flocs formed under acidic ranges were larger, weaker and looser. At pH 4.0, 7.0 and 9.0, flocs by FeCl3 were larger and weaker than these by PFC10, followed by PFC22. In case of FeCl3 and PFC10, acidic pH conditions were helpful to reduce membrane fouling. For PFC22, permeate fluxes were less sensitive to pH variations.

Inline coagulation–ultrafiltration as the pretreatment for reverse osmosis brine treatment and recovery

To help resolve issues associated with reverse osmosis (RO) brine handling for inland water reuse plants, RO brine treatment and recovery from NEWater factory was demonstrated in a laboratory using a low cost, less footprint and low energy consumption pretreatment method of inline coagulation–ultrafiltration (UF). Polyaluminum chloride (PACl), aluminum chlorohydrate (ACH) and ferric chloride (FeCl3) were chosen as the coagulants. PACl and ACH attained dissolved organic carbon (DOC) removal of 31% and 27% respectively at the optimal dosage (0.556mM as Al), while FeCl3 outperformed PACl and ACH at this molar dosage, reaching up to 60% of DOC removal. At the optimal pH (pH7), FeCl3 was again more superior with the highest DOC removal efficiency of 55%. FeCl3 also presented the highest phosphate (>99%) and silica removals (14%) at its optimum dosage and pH. Liquid chromatography-organic carbon detector (LC-OCD) and fluorescence excitation–emission matrix (EEM) were used to characterize the DOC fractions removed by inline coagulation-UF. For LC-OCD, FeCl3 revealed higher removal efficiency for almost all the DOC fractions ranging from low to high molecular weight. These results suggest that the inline-coagulation (FeCl3)-UF pretreatment is potential in reducing the fouling tendency of downstream RO because of its high DOC and phosphate removal efficiency.

Coagulation efficiency, floc properties and membrane fouling of polyaluminum chloride in coagulation–ultrafiltration system: The role of magnesium

The influence of magnesium on coagulation–ultrafiltration process was investigated regarding coagulation efficiency, floc properties and the subsequent impact on membrane fouling. PACl with different basicity values (B, OH/Al3+molar ratio) were used as coagulants. Fulvic acid (FA) water and high hardness FA water (containing magnesium) were used as original water in this study. Floc characteristics were investigated using a laser diffraction particle sizing device while membrane fouling was evaluated by a dead-end ultrafiltration batch unit. The results showed that PACl with higher OH/Al3+ value was more efficient for FA removal. Moreover, the addition of magnesium could improve its coagulation efficiency. Flocs generated in FA-magnesium water was smaller than that of FA water and showed a lower Df value, which indicated that these flocs had open structures. Results of ultrafiltration test showed that membrane fouling became severe as OH/Al3+ value increasing in FA water, but could be reduced apparently when magnesium was added. In addition, the permeate pH increased as OH/Al3+ values growing whereas the permeate pH of FA-magnesium solution was higher than that of FA solution regardless of the original pH and coagulants.

Influence of Enteromorpha polysaccharides on variation of coagulation behavior, flocs properties and membrane fouling in coagulation–ultrafiltration process

Enteromorpha polysaccharides (Ep) were used as a new coagulant aid together with polyaluminum chloride (PACl) in coagulation–ultrafiltration process to purify Yellow River water. The evolution of flocs size, growth rate, strength, recoverability and fractal structure due to Ep addition were systematically studied in this paper. On this basis, membrane fouling caused by the coagulation effluents of PACl and Ep were also investigated. Results indicated that Ep addition lead to 20% increase in coagulation performance, and meanwhile generate flocs with bigger sizes, faster growth rates and higher recovery abilities. Additionally, the flocs formed by PACl presented more compact structure with a larger Df value, while much looser flocs were obtained when Ep was added. Results of ultrafiltration experiments implied that with Ep addition, membrane fouling could be significantly reduced due to large size and loosely structures of flocs in coagulation effluents. Considering both the coagulation efficiency and ultrafiltration membrane performance, 0.2mg/L Ep was determined as the optimal dosage in coagulation–ultrafiltration process in this study.

Coagulation performance and membrane fouling of polyferric chloride/epichlorohydrin–dimethylamine in coagulation/ultrafiltration combined process

Polyferric chloride (PFC) and epichlorohydrin–dimethylamine (DAM–ECH) with different PFC (as Fe3+)/DAM–ECH mass ratios (MR) and dose methods were used in the treatment of humic acid–kaolin (HA–kaolin) simulated water under different pH conditions. Coagulation performance, floc characteristics and membrane fouling of PFC/DAM–ECH in coagulation–ultrafiltration (C–UF) process were investigated in this study. Results showed that the optimum coagulation performance was achieved when PFC dose was 15mg/L with PFC/DAM–ECH MR of 2:1 under pH6.0. Opposite adding sequence (DAM–ECH/PFC) would reduce UV254 (ultraviolet adsorption at 254nm) and DOC (dissolved organic carbon) removal efficiency. Addition of DAM–ECH on the basis of PFC produced larger and stronger flocs with better regrowth ability. Proper dosage of DAM–ECH under certain coagulation condition could achieve the best coagulation performance and keep the UF membrane operating under a preferable status. Results of this study would be beneficial for application of composite coagulant PFC–DAM–ECH and DAM–ECH as coagulant aid in water treatment processes.

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.

Coagulation performance and membrane fouling of different aluminum species during coagulation/ultrafiltration combined process

Three species of aluminum coagulants mainly consist of mononuclear Al species (Ala), medium polymeric species (Alb) and colloidal species (Alc) were prepared, separated and purified by organic solvent precipitation method. The effect of different Al species on coagulation performance and membrane fouling were investigated through the coagulation–ultrafiltration hybrid process. The influence of flocs characteristics were also studied to build the relation model between Al species coagulation and membrane fouling mechanisms. The results showed different Al species combined with humic acid (HA) in diverse ways feature distinct floc properties and membrane fouling. Ala coagulant had better UV254 and dissolved organic carbon (DOC) removal efficiency. Furthermore, Ala and Alc were beneficial to form larger flocs with lower strength and looser structure, whereas Alb was prone to form small flocs with higher strength and compact structure. Ultrafiltration experiments indicated that Alb is the most effective Al species to alleviate membrane fouling.

Floc properties and membrane fouling of polyferric silicate chloride and polyferric chloride: the role of polysilicic acid.

Impact of polysilicic acid (pSi) in polyferric silicate chloride (PFSiC) on coagulation-ultrafiltration process was investigated in comparison with polyferric chloride (PFC). The Fe(III) species distribution in PFSiC and PFC was measured by a timed complexation spectroscopy method. Characteristics of flocs produced by PFSiC and PFC were studied using a laser diffraction particle sizing device. Moreover, membrane fouling was evaluated using a dead-end batch ultrafiltration unit under two operation modes, coagulation-ultrafiltration (C-UF) and coagulation-sedimentation-ultrafiltration (CSUF). The results indicated that PFSiC with various Si/Fe ratios had better turbidity removal efficiency but inferior organic matter removal. Flocs formed by PFSiC were larger than those by PFC. In case of PFSiC, floc size increased with Si/Fe ratio increasing. PFSiC with various Si/Fe ratios resulted in more compact and weaker flocs than PFC. Ultrafiltration experiments indicated that under C-UF mode, PFSiC with Si/Fe ratios of 0.07 and 0.10 presented better membrane performance than PFC. Under CSUF mode, addition of pSi could alleviate membrane fouling.

System dynamics modeling and simulation of a coagulation–ultrafiltration process for the treatment of drinking water

In this work, a system dynamics (SD) model of a coagulation–ultrafiltration (UF) process was developed using Vensim and validated with experimental data. The SD model includes the basic elements and essential structure of the coagulation–UF process system. Numerical simulations were used to optimize the operating parameters of the SD model. The simulation results demonstrated that the absorption resistance (Ra) contributed a small portion of the total resistance (Rt) (≤15%) in the basic experiment, where the initial permeate flux (J0) was 35 m3/(m2 h). Furthermore, the cake layer resistance (Rc) was the main cause of the total resistance (Rt) within the model. The concentration of coagulant and the removal rate of membrane fouling were positively correlated when the coagulant concentration increased from 20 to 35 mg/L. In addition, the contribution of total resistance to membrane fouling was obtained from the SD model. The membrane fouling rate reached 22.5 Pa/h after 16 backwashing cycles which corresponds to the predicted chemical cleaning time. The SD model presented here can provide guidance in optimizing operating parameters through numerical simulations which eschew the need for large amounts of repeated tests.

Floc properties and membrane fouling of different monomer and polymer Fe coagulants in coagulation–ultrafiltration process: The role of Fe (III) species

Importance of Fe (III) species in ferric chloride (FeCl3), polyferric chloride with basicity values (B, OH−/Fe3+ molar ratio) of 1.0 and 2.2 (denoted as PFC10 and PFC22) was investigated in terms of floc properties and membrane fouling during coagulation–ultrafitration process. The Fe (III) species distribution in these three coagulants was measured by a timed complexation spectroscopy method. Floc characteristics by FeCl3, PFC10 and PFC22 were studied using a laser diffraction particle sizing device. Moreover, SEM images and resistance analysis were conducted to explore mechanism of membrane fouling. The results indicated that the monomeric and polymeric species (Fea and Feb) increased as follows: FeCl3 > PFC10 > PFC22. FeCl3 had better coagulation performance in terms of turbidity, UV254 and DOC than PFC10 and PFC22. Meanwhile, FeCl3 produced largest flocs with loosest structure which were, however, weakest with best recoverability. PFC10 took the second place regarding to floc properties. The membrane fouling of PFC10 and PFC22 was more serious than FeCl3. SEM images and resistance analysis showed that cake layer resistance relative to the total resistance produced by FeCl3 was lowest due to the larger and looser flocs with high re-growth ability. Thus, more Fea and Feb could effectively reduce the membrane fouling.

Coagulation and sludge recovery using titanium tetrachloride as coagulant for real water treatment: A comparison against traditional aluminum and iron salts

Coagulation/flocculation performance of titanium tetrachloride (TiCl4), ferric chloride (FeCl3) and aluminum sulfate (Al2(SO4)3) was comparatively investigated for real water treatment. Comparisons were made under different coagulant dose and initial solution pH conditions and their performances measured in terms of UV254 (absorbance at 254nm) and DOC (dissolved organic carbon) removal and residual turbidity. Characteristics of aggregated flocs during the coagulation/flocculation process by the three coagulants were studied using a laser diffraction particle sizing device. The performances of the three coagulants were also assessed in terms of the membrane fouling potential of the ultrafiltration (UF) membrane or during coagulation–ultrafiltration (C–UF) process using a stirred and dead-end batch UF unit. Additionally, the TiCl4 flocculated sludge was recovered to produce titanium dioxide (TiO2) under thermal treatment. The results indicate that the TiCl4 showed superior coagulation performance compared to that of FeCl3 and Al2(SO4)3, with the optimum removal of UV254 and DOC of 54.9% and 55.1%, respectively. The aggregated flocs formed by TiCl4 showed the highest growth rate with the largest size compared to those by FeCl3 and Al2(SO4)3, but with the weakest floc strength and the worst re-growth ability. The TiCl4 and FeCl3 yielded the flocs with comparable degree of compaction, higher than that by Al2(SO4)3. Additionally, the investigation of membrane fouling demonstrated that the severity of flux decline followed the order of Al2(SO4)3<FeCl3<TiCl4. TiCl4 coagulated sludge was also characterized by X-ray diffraction, Thermal analysis and scanning electron microscope.

Removal of natural organic matter by titanium tetrachloride: The effect of total hardness and ionic strength

This study is the first attempt to investigate the effect of total hardness and ionic strength on coagulation performance and the floc characteristics of titanium tetrachloride (TiCl4). Membrane fouling under different total hardness and ionic strength conditions was also evaluated during a coagulation–ultrafiltration (C–UF) hybrid process. Coagulation experiments were performed with two simulated waters, using humic acid (HA, high molecular weight) and fulvic acid (FA, relatively low molecular weight), respectively, as model natural organic matter (NOM). Results show that both particle and organic matter removal can be enhanced by increasing total hardness and ionic strength. Floc characteristics were significantly influenced by total hardness and ionic strength and were improved in terms of floc size, growth rate, strength, recoverability and compactness. The results of the UF tests show that the pre-coagulation with TiCl4 significantly improves the membrane permeate fluxes. Under different total hardness and ionic strength conditions, the membrane permeate flux varied according to both NOM and floc characteristics. The increase in total hardness and ionic strength improved the membrane permeate flux in the case of HA simulated water treatment.

Effect of Fe (III) species in polyferric chloride on floc properties and membrane fouling in coagulation–ultrafiltration process

Impacts of Fe (III) species on floc properties and membrane fouling in coagulation–ultrafiltration (C-UF) hybrid process were investigated. Fulvic acid (FA)-kaolin synthetized water was used to simulate the actual surface water coagulated by polyferric chloride (PFC) with four different basicity (B) values. Four PFC coagulants with B values of 0.5, 1.0, 1.5 and 2.0 were denoted as PFC05, PFC10, PFC15 and PFC20, respectively. The distribution of Fe (III) species in PFC was determined by a timed complexation spectroscopy method. The results implied that the sum of monomeric and polymeric species (Fea and Feb) in PFC decreased as B values of PFC increased. PFC with higher Fea and Feb had better dissolved organic carbon (DOC) removal efficiency and higher zeta potential, which made charge neturalization stronger. Fea and Feb in PFC were beneficial to form larger and looser flocs. The permeate fluxes of PFC with different B values decreased in the following order: PFC05>PFC20>PFC10=PFC15 and membrane fouling index (MFI) had opposite order. PFC05 displayed a flux advantage due to larger and looser flocs formed by it. In addition, cake resistance (Rc) relative to the total resistance was in the order of PFC20<PFC05<PFC10<PFC15.

Characterization of coagulation behavior of titanium tetrachloride coagulant for high and low molecule weight natural organic matter removal: The effect of second dosing

A two-stage titanium tetrachloride (TiCl4) dosing strategy is applied for investigating the coagulation effect on simulated water containing humic acid (HA) and fulvic acid (FA) and these represent the dissolved organic matter (DOM). The influence of second TiCl4 dose on coagulation efficiency, flock re-growth and the fractal dimension properties is studied for the beneficial effect of the second coagulant dose. Moreover, the impact of increased shear on the ultra filtration permeate flux in coagulation–ultra filtration (C–UF) hybrid process is explored. A small second dose essentially decreases the residual turbidity and improves the organic matter removal. Also, flocks with larger size and better recoverability result and having more compact structure than those obtained without the second TiCl4 coagulant dose. At the same total TiCl4 coagulant concentrations, the two-stage dosing mode reveals significant advantage over the single coagulant dosing in terms of turbidity removal, flock size and recoverability although organic matter removal and flock compactness degree are more or less deteriorated during the two-stage dosing process. The second TiCl4 dose contributes to larger re-generated flocks with better re-growth ability than those obtained without the second dose within a wide range of high shear rate and even with prolonged breaking period. Two-stage dosing mode could improve the ultra filtration permeate flux under certain shear conditions. An extension in breakage time leads to much severer flux decline than short breakage periods. In case of FA simulated water, the ultra filtration permeate flux was less sensitive to increasing shear rate than in case of HA simulated water.