Model Foulants Research Articles

Monitoring UF membrane performance treating surface-groundwater blends: Limitations of FEEM-PARAFAC on the assessment of the organic matter role

The decrease of water quantity and quality in water scarcity areas is palliated by improving water treatments with membrane technologies. System performance and efficiency, and then cost, is mainly affected by membrane fouling, which is still not well understood and controlled appropriately. In this study, the influence of content and composition of dissolved organic matter (DOM) on a membrane ultrafiltration (UF) stage from a full-scale UF stage in a drinking water treatment plant (DWTP) fed with surface water, groundwater (or blends of them) was investigated. Excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) was used to characterize and assess DOM changes in water samples Water streams feeding the UF stage showed high variability in DOM content and composition. FEEM-PARAFAC analysis allowed the differentiation of seven different organic components. Additionally to the characterization and monitoring of DOM in the full-scale UF stage, a bench scale UF pilot was run to experimentally correlate the impact of water quality with membrane performance. The experiments included testing synthetic solutions of model foulants (synthetic humic acid and bovine serum albumin) and blends of complex waters. To quantify fouling, the total fouling index (TFI) and the hydraulically irreversible fouling index (HIFI) were calculated for each filtration run. According to the results obtained, the correlation plots between the PARAFAC components and the fouling indices pointed at microbial byproducts (C1) and humic-like components (C2, C4, C5) as the ones showing higher correlations.

Antifouling hybrid ultrafiltration membranes with high selectivity fabricated from polysulfone and sulfonic acid functionalized TiO2 nanotubes

In this work, antifouling hybrid ultrafiltration membranes were manufactured using polysulfone (PSf) and sulfonic acid functionalized TiO2 nanotubes (TNTs-SO3H). TNTs-SO3H were first synthesized by performing a coupling reaction between 3-mercaptopropyl trimethoxysilane and hydroxyl groups on TiO2 nanotubes with in situ oxidation using 30wt% H2O2 solution. Hybrid membranes containing TNTs-SO3H up to 5wt% were then prepared from the blend solutions of PSf/TNTs-SO3H via a non-solvent induced phase separation (NIPS) method. The hybrid membranes were characterized using contact angle, Fourier transform infrared spectroscopy, scanning electron microscopy and surface zeta potential studies. The antifouling capability of hybrid membranes with bovine serum albumin and humic acid (HA) as model foulants were investigated in detail. Hybrid membranes exhibited better water permeability and antifouling performance during filtration of foulant solutions. The ultrafiltration of HA solutions of different concentrations was carried out at pH=7 and 1bar feed pressure, with the removal of HA from aqueous solution being controlled through the charged behaviour, porosity and exclusion of HA by hybrid membranes. The maximum removal of HA from 20ppm aqueous solution was achieved using the hybrid membrane MTS-5 containing 5wt% TNTs-SO3H.

The removal of typical pollutants in secondary effluent by the combined process of powdered activated carbon–ultrafiltration

This paper focused on the effects of powdered activated carbon (PAC) dosage on ultrafiltration (UF) membrane flux caused by natural organic matter (NOM). Three model foulants, humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA), were adopted to represent different NOM fractions in secondary effluent treated by the combined process of PAC-UF. Moreover, the membrane fouling resistance and fouling mechanism were also analyzed. The results indicated that the best PAC dosage for the membrane flux variation was 20 mg/L for HA and SA, and 10 mg/L for BSA. SA caused the most serious membrane fouling, which was mainly reversible fouling. The membrane fouling caused by HA and BSA was mainly irreversible membrane fouling. The membrane fouling caused by organics happened mainly at the initial stage of filtration. Because the filter cake layer formed by a moderate amount of PAC could intercept organics, the membrane fouling, especially the irreversible fouling, could be reduced.

Effect of Hydration Forces on Protein Fouling of Ultrafiltration Membranes: The Role of Protein Charge, Hydrated Ion Species, and Membrane Hydrophilicity.

To investigate the influence of hydration forces on the protein fouling of membranes and the major influence factors of hydration forces during the ultrafiltration process, bovine serum albumin (BSA) was chosen as model foulant. For various pH levels and hydrated ion and membrane species, the membrane-BSA and BSA-BSA interaction forces, and fouling experiments with BSA, as a function of ionic strength, were measured. Results showed that hydration forces were a universal phenomenon during the membrane filtration process, when the levels of pH, ion species, and membrane performances were appropriate. First, for the BSA negatively charged or neutral, hydration forces caused a decrease in the membrane fouling. Conversely, for the BSA positively charged, the hydration forces were absent because the counterions were not hydrated, and membrane fouling was enhanced. For different hydrated ions, the smaller the radii of the ions were, the stronger the hydration forces that were produced, and the membrane fouling observed was less, indicating that hydration forces are closely correlated with the size of the hydrated ions. Moreover, in comparison with a hydrophobic membrane, it is more difficult to observe hydrophilic membrane-BSA hydration forces because the hydrophilic membrane surface adsorbs water molecules, which weakens its binding efficiency to hydrated ions.

In situ formation of copper nanoparticles in carboxylated chitosan layer: Preparation and characterization of surface modified TFC membrane with protein fouling resistance and long-lasting antibacterial properties

Abstract In this study, copper nanoparticles (CuNPs) modified polyamide thin-film composite (TFC) membranes with long-lasting antibacterial properties and anti-protein-fouling properties were prepared by surface coating method. During modification, carboxylated chitosan (CCTS), a hydrophilic natural polymer, was coated onto the membrane surface, which was then treated with CuCl 2 aqueous solutions and cross-linking agent glutaraldehyde to reduce the Cu 2+ to CuNPs in situ and fix them in the cross-linked coating layer. The formation of CuNPs was confirmed by SEM, EDS and XPS analysis. Copper ion release measurement showed CuNPs modified TFC membranes (PA-CCTS-Cu) could release Cu 2+ in a slow and sustainable way due to chelating effect between CCTS and Cu 2+ . PA-CCTS-Cu had antibacterial efficiency of above 99% and could maintain it after 90 days’ immersion in water. PA-CCTS-Cu also exhibited better anti-protein-fouling performance than the pristine TFC membrane, with lower reduction of flux after fouling and higher recovery after cleaning in fouling test using bovine serum albumin as model foulant. This enhanced anti-protein-fouling performance was ascribed to the CCTS coating layer, which had enhanced hydrophilicity as assessed by contact angle measurement.

Effect of fluidized granular activated carbon (GAC) on critical flux in the microfiltration of particulate foulants

The fluidization of Granular Activated Carbon (GAC) particles has received much attention in recent years as a promising method for mitigating membrane fouling particularly in membrane bioreactors (MBRs). In particular, the fluidized GAC particles are acknowledged to be beneficial in terms of adsorption of organic foulants, and mechanical scouring and surface shear of the membrane surface. Both lab-scale and pilot-scale studies have affirmed the efficacy of GAC fluidization in maintaining a depressed extent of membrane fouling in anaerobic fluidized bed - membrane bioreactors (AnFMBRs). More in-depth studies have also correlated the hydrodynamics of fluidized GAC particles with the effectiveness of membrane fouling mitigation. This study aimed at understanding the improvement of fine (micron-sized) particle critical flux in the presence of fluidized GAC via the Direct Observation Through the Membrane (DOTM) technique. The model foulant was a suspension of polystyrene particles (5µm sized), and the parameters investigated included GAC particle diameter (dp), superficial liquid velocity (Ul) and thereby power requirement (Pr), and height along the vertically aligned membrane. Results indicate that: (i) fluidized GAC increased the critical flux by an order-of-magnitude relative to that with tangential liquid shear alone; (ii) the overall critical flux (Jcritical, overall) expectedly increased with power input (Pr), but the relationship between local critical flux (Jcritical) and power input (Pr) depended on the position along the membrane height; (iii) at the same Pr, although the larger GAC particles were more effective locally (i.e., higher Jcritical) due to greater particle inertia, the smaller GAC particles were more effective overall (i.e., higher Jcritical, overall) due to greater bed expansion enabling scouring of more membrane heights; and (iv) a higher power input (Pr) was required for sufficient bed expansion to enable more consistent critical flux (Jcritical) values over the whole membrane height.

Surface patterning of polymeric membranes and its effect on antifouling characteristics

ABSTRACTSurface roughness of membranes is often perceived by many as a factor that promotes fouling during filtration, and thus is undesirable. Almost all liquid-based separation membranes display flat surfaces with an intrinsic surface roughness that is associated with the membrane manufacturing process. Recently, polymer ultrafiltration and thin film composite membranes containing regular, periodic surface patterns were fabricated using cost-effective lithographic methods. Here, we review the work to date on the fabrication and characterization of these patterned membranes with a focus on processing–structure–performance relationships. In addition, the antifouling performance of these membranes against model foulants including colloidal suspensions and protein solutions is also highlighted.

Organic fouling behaviour of structurally and chemically different forward osmosis membranes – A study of cellulose triacetate and thin film composite membranes

The HTI cellulose triacetate (CTA) and novel thin film composite (TFC) membranes are used to study the multifaceted interactions involved in the fouling and cleaning of forward osmosis (FO) membranes, using calcium alginate as a model foulant. Results show that fouling on the TFC membrane was more significant compared to CTA, arising from a variety of factors associated with surface chemistry, membrane morphology and structural properties. Interestingly, it was observed that in FO mode, membrane surface properties dominated over fouling layer properties in determining fouling behaviour, with some surface properties (e.g. surface roughness) having a greater effect on fouling than others (e.g. surface hydrophilicity). In pressure retarded osmosis (PRO) mode, structural properties of the support played a more dominant role whereby fouling mechanism was specific to the foulant size and aggregation as well as the support pore size relative to the foulant. Whilst pore clogging was observed in the TFC membrane due to its highly asymmetric and porous support structure, fouling occurred as a surface phenomenon on the CTA membrane support layer. Besides pore clogging, the severe fouling observed on the TFC membrane in PRO mode was due to a high specific mass of foulant adsorbed in its porous support. It was observed that a trade-off between enhanced membrane performance and fouling mitigation is apparent in these membranes, with both membranes providing improvement in one aspect at the expense of the other. Hence, significant developments in their surface and structural properties are needed to achieve high anti-fouling properties without compromising flux performance. Measured fouling densities on the studied surfaces suggest that there is not a strong correlation between foulant-membrane interaction and fouling density. Cleaning results suggest that physical cleaning was more efficient on the CTA membrane compared to the TFC membrane. Further, they implied that despite different mechanisms of fouling and quantities of foulant adsorbed in FO membranes, FO is a resilient process with high cleaning efficiencies and fouling reversibility.

Polyvinylidene fluoride membrane blended with quaternary ammonium compound for enhancing anti-biofouling properties: Effects of dosage

Biofouling is one of the major obstacles to the widespread application of membrane technology in water and wastewater treatment, and use of anti-biofouling membrane materials is an effective approach to suppress biofouling. In this study, anti-biofouling polyvinylidene fluoride (PVDF) membranes were prepared by blending quaternary ammonium compound (QAC). Effects of QAC dosages on the physicochemical properties and anti-biofouling behaviours of the modified membranes were systematically investigated. The results showed that QAC was successfully present on the membrane surfaces, as revealed by X-ray photoelectron spectroscopy, Zeta potential and contact angle analyses. Anti-fouling tests using individual organic model foulants showed that the modified membranes had higher membrane fouling rate against organic matters, which is consistent with the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. In contrast, the modified membranes demonstrated evident anti-bacterial behaviours when individual model bacteria were used, which cannot be evaluated by this theory. Interestingly, the modified membrane showed a clear anti-fouling activity in a membrane bioreactor (the sludge mixture contained both organic foulant and microorganisms), suggesting its concurrent efficiency in controlling fouling induced by both organic matters and bacteria. This might be attributed to the contact-killing effects of QAC and the dual interactions between bacteria and organic foulants in the mixed liquor of membrane bioreactor. The study offers an effective way to enhance membrane anti-biofouling ability through the addition of antibacterial agent QAC.

Macro-initiator mediated surface selective functionalization of ultrafiltration membranes with anti-fouling hydrogel layers applicable to ready-to-use capillary membrane modules

A new process for surface selective graft modification of ultrafiltration (UF) membranes with protective hydrogel layers was developed. The process uses a random copolymer of n-butylmethacrylate and N,N-dimethylaminoethylmethacrylate as a redox co-initiator (“macro-initiator”). Due to its molecular weight, the macro-initiator is completely rejected by the used Multibore® polyethersulfone UF membranes. Zwitterionic [3-(methacryloylamino)propyl]dimethyl(3-sulfopropyl) ammonium hydroxide and bifunctional N,N′-methylenebis(acrylamide) were used as monomers for “macro-initiator”-mediated, surface selective cross-linking polymerization toward anti-fouling hydrogel layers. The functionalization comprises two main steps; i) adsorption of the macro-initiator to the barrier layer surface of the membrane, e.g. during a short ultrafiltration; ii) grafting of the hydrogel layer after bringing the membrane in contact with a solution containing monomer(s) and a dissolved initiator (here ammonium persulfate) which is complementary to the co-initiator for a predetermined reaction time at room temperature. Hydrogel-grafted flat sheet and capillary UF membranes showed dextran sieving curves shifted to lower molecular weight values, increased total organic carbon rejection and improved anti-fouling behaviour in filtration tests with flower soil extract as model foulant. Furthermore, stability tests with sodium hydroxide and hydrogen peroxide solutions showed good chemical stability of graft-functionalized membranes. The obtained results are very promising for future applications, since the presented technique can be applied in ready-to-use membrane modules and capillary membranes easily.

Effect of ultrasonic irradiation on membrane fouling and membrane wetting in direct contact membrane distillation process

Membrane distillation (MD) is a novel separation process that have drawn attention as an affordable alternative to conventional desalination processes. However, membrane fouling and pore wetting are issues to be addressed prior to widespread application of MD. In this study, the influence of ultrasonic irradiation on fouling and wetting of MD membranes was investigated for better understanding of the MD process. Experiments were carried out using a direct contact membrane distillation apparatus Colloidal silica was used as a model foulants in a synthetic seawater (35,000 mg/L NaCl solution). A vibrator was directed attached to membrane module to generate ultrasonic waves from 25 kHz (the highest energy) to 75 kHz (the lowest energy). Flux and TDS for the distillate water were continuously monitored. Results suggested that ultrasonic irradiation is effective to retard flux decline due to fouling only in the early stage of the MD operation. Moreover, wetting occurred by a long-term application of ultrasonic rradiation at 75 kHz. These results suggest that the conditions for ultrasonic irradiation should be carefully optimized to maximize fouling control and minimize pore wetting.

Comparison of fouling propensity and physical cleaning effect in forward osmosis, reverse osmosis, and membrane distillation

AbstractMembrane-based desalination technologies including reverse osmosis (RO), forward osmosis (FO), and membrane distillation (MD) hold promise as efficient methods to produce fresh water from saline water sources. However, the fouling properties of these membranes are quite different due to the difference in the driving forces among them. Accordingly, the objective of this study is to compare fouling behavior and physical cleaning efficiency for these three membranes under similar operating conditions. Colloidal silica and alginate were used as model foulants and NaCl was added to feed solutions. Laboratory-scale experiments were carried out to compare fouling rates and recovery of flux by physical cleaning. Results showed that fouling propensity was the highest in FO membrane and the lowest in MD membrane, which may be attributed to the effect of cake-enhanced concentration polarization. On the other hand, physical cleaning was more efficient to recover flux in FO and RO than in MD, suggesting that t...

The effect of different surface conditioning layers on bacterial adhesion on reverse osmosis membranes

Reverse osmosis (RO) biofouling is a problem of biofilm development which is initiated by bacterial attachment to the membrane surface following the surface conditioning. In this study, the impact of the initial fouling layers on bacterial attachment was investigated. Both model foulants and MBR permeate water were used to simulate conditioning layers on RO membranes by performing fouling experiments at constant flux and crossflow velocity. Analysis of the properties of different conditioning layers included zeta potential, contact angle, roughness, and adhesive force. Results in this study show that the bacterial attachment was greater on pre-conditioned surfaces and the load of attached bacteria was closely correlated to the change of adhesive force rather than to the change of zeta potential and hydrophilicity. Additionally, the amount of the pre-conditioning layer also influenced the bacterial attachment. Differences in surface roughness were found to be unimportant. To examine the effect of pretreatment bacterial attachment tests were performed on a secondary effluent filtered by microfiltration or ultrafiltration. The UF pretreatment showed significantly less attachment. Results in this study confirm the role of the initial conditioning layer and highlight which surface characteristics of the conditioning layer are the most important for the initial bacterial attachment.

Influence of colloidal fouling on pressure retarded osmosis

In this study, colloidal fouling behavior in pressure retarded osmosis (PRO) was systematically investigated in terms of the effects of draw solution concentration, applied hydraulic pressure at the draw side, feed solution pH, and particle size. Commercially-available cellulose triacetate (CTA) membranes were fouled with feed solution containing silica colloidal particles. Two different silica particles with mean diameter of 27 and 152nm were used as model foulants. Our findings demonstrated that the colloidal fouling in PRO was dominantly affected by the cake layer buildup at the membrane surface. Fouling was further exacerbated by diffused salts from the draw side because retained salts within the cake layer elevated the salt concentration on the membrane surface, and consequently reduced the driving force of PRO. Substantial flux decline with the smaller particles was attributed to the high cake layer resistance due to the formation of the void-less cake layer. In addition, our approaches to mitigate the colloidal fouling revealed that the hydraulic cleaning by increasing the cross-flow rates was not effective to eliminate the compact cake layer. However, adjusting the feed solution pH showed the high potential to relieve the colloidal fouling resulting from the more stabilization of particles at low solution pH.

Effect of inorganic colloidal water constituents on combined low-pressure membrane fouling with natural organic matter (NOM)

The impact of inorganic colloidal water constituents on fouling in low-pressure membrane systems was evaluated by carrying out ultrafiltration (UF) experiments using synthetic waters containing model foulants. Humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA) were chosen as model natural organic matter (NOM) foulants, and silicon dioxide and α-aluminum oxide were chosen as model inorganic colloidal foulants. The synthetic solutions were filtered using one lab-scale flat sheet UF system (i.e., unstirred filtration cell) and one pilot-scale hollow fiber UF system (i.e., custom made hollow fiber modules), both of which employed polyethersulfone (PES) membranes. Filtering the inorganic colloids by themselves did not result in any significant fouling, as evidenced by the negligible permeability decline and high hydraulic reversibility. However, when combined with NOM, severe permeability decline and hydraulic reversibility reduction were observed. Moreover, the permeability decline and reversibility loss of the combined inorganic colloids and NOM were more severe than the sum of the individual effects from organics and inorganic colloids, suggesting that combining the two acts to enhance fouling (i.e., synergistic fouling effects). Aluminum oxide tended to result in more severe synergistic fouling compared to silicon dioxide, suggesting that the inorganic colloid material character, especially its surface charge, affects the severity of combined fouling. Rejection of TOC and analysis of the fouled membranes by scanning electron microscopy indicated higher NOM adsorption by aluminum oxide colloids, which lead to a larger amount of foulants in the cake layer and a worse reversibility. Both the lab- and pilot-scale systems exhibited similar fouling behavior, thus indicating that the lab-scale system provides a meaningful representation of the pilot-scale system, even though the operating conditions are quite different. This study highlights the importance of interactions between foulants with respect to fouling in low-pressure membrane systems and showed conclusively that inorganic colloids play an important role in fouling of NOM. Moreover, present results underline the impact of colloidal surface charge on the resulting hydraulic and chemical fouling reversibility during ultrafiltration of natural waters containing NOM and inorganic colloids.

Natural organic matter fouling behaviors on superwetting nanofiltration membranes

Nanofiltration has been widely recognized as a promising technology for the removal of micro-molecular organic components from natural water. Natural organic matter (NOM), a very important precursor of disinfection by-products, is currently considered as the major cause of membrane fouling. It is necessary to develop a membrane with both high NOM rejection and anti-NOM fouling properties. In this study, both superhydrophilic and superhydrophobic nanofiltration membranes for NOM removal have been fabricated. The fouling behavior of NOM on superwetting nanofiltration membranes has been extensively investigated by using humic acid (HA) as the model foulant. The extended Derjaguin−Landau−Verwey−Overbeek approach and nanoindentor scratch tests suggested that the superhydrophilic membrane had the strongest repulsion force to HA due to the highest positive total interaction energy (ΔGTOT) value and the lowest critical load. Excitation emission matrix analyses of natural water also indicated that the superhydrophilic membrane showed resistance to fouling by hydrophobic substances and therefore high removal thereof. Conversely, the superhydrophobic membrane showed resistance to fouling by hydrophilic substances and therefore high removal capacity. Long-term operation suggested that the superhydrophilic membrane had high stability due to its anti-NOM fouling capacity. Based on the different anti-fouling properties of the studied superwetting membranes, a combination of superhydrophilic and superhydrophobic membranes was examined to further improve the removal of both hydrophobic and hydrophilic pollutants. With a combination of superhydrophilic and superhydrophobic membranes, the NOM rejection (RUV254) and DOC removal rates (RDOC) could be increased to 83.6% and 73.3%, respectively.

Effects of pre-ozonation on the ultrafiltration of different natural organic matter (NOM) fractions: Membrane fouling mitigation, prediction and mechanism

The effects of pre-ozonation on ultrafiltration (UF) membrane fouling caused by different natural organic matter (NOM) fractions were investigated. Three typical organic model foulants, humic acid (HA), sodium alginate (SA) and bovine serum albumin (BSA) were selected as representatives of different NOM fractions in natural waters. Moreover, Songhua River water (SRW) was employed as a natural surface water. To predict membrane fouling, the dissolved organic carbon (DOC), ultraviolet absorbance (UV254) and the maximum fluorescence intensity (Fmax) were used to establish correlations with total fouling index (TFI) and hydraulic irreversible fouling index (HIFI). In addition, the fouling mechanisms were preliminarily analyzed. The results indicated that pre-ozonation significantly alleviated membrane fouling caused by HA, SA and SRW. Maximum ozone dose (4.0mg/L) showed the best performance with approximately 39%, 78% and 42% TFI reduction, and 26%, 73% and 39% HIFI reduction for HA, SA and SRW, respectively. However, pre-ozonation exerted little influence on BSA fouling under the tested ozone exposure (0.5, 1.5 and 4.0mg/L). TFI and HIFI were poorly correlated with the DOC contents of HA, SA, BSA and SRW, and the Fmax of BSA, whereas positively correlated with the UV254 of HA and SRW, and the Fmax of HA and each component of SRW. The fouling mitigation mechanisms were attributed to the changes of NOM properties after pre-ozonation. The results are expected to provide relevant information on predicting and controlling UF membrane fouling according to the composition and characteristics of NOM.

Amphiphobic PVDF composite membranes for anti-fouling direct contact membrane distillation

This study aimed to develop an effective method to fabricate the amphiphobic polyvinylidene fluoride (PVDF) composite membranes for membrane distillation (MD) with excellent tolerance to various organic foulants. A facial surface modification method was explored to obtain amphiphobic membranes with mechanical and thermal robustness by dynamically forming perfluorooctyl trichlorosilane (PFTS) and coating SiO2 nanoparticles onto the membrane surface. A variety of techniques such as environmental scanning electron microscopy (ESEM), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), liquid entry pressure (LEP) measurement and contact angle goniometry were applied to examine the effects of surface modification on surface chemistry, morphology and wettability of the derived membranes. The surface modification conferred the modified membrane superhydrophobicity and oleophobicity, stemmed from micro-fluorinated SiO2 particles covering the membrane surface. The anti-fouling property of the pristine and modified membranes were examined in a direct contact membrane distillation (DCMD) process using sodium chloride solution containing three model foulants (hydrophobic, hydrophilic or amphiprotic). The dynamically formed SiO2-PFTS/PVDF-2 membranes exhibited good thermal and mechanical resistance for DCMD operation. DCMD test showed that the surface modification did not sacrifice the permeate flux and the salt rejection. By adding the organic foulants, the pristine membrane displayed severe permeate flux decay and salt penetration. In contrast, the modified membrane presented a stable permeate flux and high salt rejection with the presence of three foulants respectively. The anti-fouling and anti-wetting properties of the modified membrane could be attributed to the enhanced amphiphobicity of membrane surface.

High-flux and fouling-resistant reverse osmosis membrane prepared with incorporating zwitterionic amine monomers via interfacial polymerization

Zwitterionic monomers have aroused great attraction for membrane preparation due to their tremendous hydrophilicity and antifouling properties. Zwitterionic diamine monomer N-aminoethyl piperazine propane sulfonate (AEPPS) was added into m-phenylenediamine (MPD) aqueous solution to react with trimesoyl chloride (TMC) via interfacial polymerization to fabricate zwitterionic thin film composite reverse osmosis membrane (TFCMZs). The membrane preparation conditions, including AEPPS content, TMC concentration and curing temperature, were systematically evaluated. The membrane surface hydrophilicity and antifouling properties of TFCMZs were greatly enhanced via introducing AEPPS into membranes. When tested with brackish water desalination conditions (2000ppm NaCl feed solution at 1.5MPa and 25°C), the water flux of TFCMZs containing 10% AEPPS was significantly increased to be 54.5±3.2L·m−2·h−1, which is about an 82% increase compared with the pristine membrane, while the NaCl rejection remains above 98%. Furthermore, using tannic acid (TA) and sodium alginate (NaAlg) as model foulants, the TFCMZs displayed outstanding fouling-resistant and easy-cleaning properties. TFCMZs present both admirable separation and fouling-resistance performance, which indicates that TFCMZs have wide application potential for brackish water desalination.

Quantitative determination of the electrical properties of RO membranes during fouling and cleaning processes using electrical impedance spectroscopy

Electrical impedance spectroscopy (EIS) has been used as a non-invasive means of characterising the electrical properties of a reverse osmosis (RO) membrane during fouling and cleaning processes. The crossflow RO unit used in this study was equipped with suitable electrodes and used in conjunction with a high resolution impedance spectroscope that allowed changes in the electrical properties of the membrane and foulant layer to be monitored. The internal structure of the membrane could be represented by a series combination of parallel conductance and capacitance elements. The deduced support layer thickness matched optical measurements and the effective polyamide skin layer thickness matched recently reported data using high resolution TEM. During the filtration process using silica and alginate as model foulants, the conductance of all the electrical elements initially decreased with time and then increased as fouling progressed. The increase was associated with a local increase in salinity due to the development of cake enhanced concentration polarization. The capacitance of the skin layer decreased as the fouling process progressed and this could be evidence of the built up of foulant layers on membrane surface. By flushing the fouled membrane with NaCl solution for about 17h, the membrane returned back to its original state, with its capacitance and conductance values similar to the unfouled membrane.