Thin Film Composite NF Membranes Research Articles

Synergistic interfacial polymerization between hydramine/diamine and trimesoyl chloride: A novel reaction for NF membrane preparation

Polyester-amide (PEA) thin film composite (TFC) NF membranes have rapidly evolved towards a competitive performance, benefiting from their remarkable antifouling capability and superior chlorine resistance. In this report, a new concept of synergistic interfacial polymerization is explored, which promptly triggers the reaction between hydramines and trimesoyl chloride (TMC) in the presence of a trace amount of diamines. This rapid-start mode enables the formation of defect-free PEA films without the requirement of catalysis. A comprehensive characterization of physicochemical properties using high-resolution mass spectrometer (HRMS) reveals that the recombination and formation of a “hydramine-diamine” coupling unit plays a decisive role in activating the synergistic interfacial polymerization reaction with TMC molecules. Taking the pair of serinol and piperazine (PIP) as an example, the PEA-NF membrane fabricated with 0.1 w/v% serinol mixed with 0.04 w/v% PIP as water-soluble monomer and 0.1 w/v% TMC as oil phase monomer was found to have a pure water permeability (PWP) of 18.5 L·m−2·h−1·bar−1 and a MgSO4 rejection of 95.5 %, which surpasses almost all the reported PEA NF membranes. Findings of the current research provide more possibilities for the low-cost and rapid synthesis of high-performance PEA membranes aiming for water purification.

Polyamide/polyethylene thin film composite (PA/PE-TFC) NF membranes prepared from reverse-phase interface polymerization (RIP) for improved Mg(II)/Li(I) separation

Polyamide/polyethylene thin film composite (PA/PE-TFC) NF membranes were prepared from reverse-phase interface polymerization (RIP) for improved Mg(II)/Li(I) separation. RIP processes between 1,3,5-benzenetricarbonyl chloride (TMC) in bottom organic phase and piperazine (PIP) in upper aqueous phase were developed to form continuous and intact surface PA separation layers on PE porous substrates thinner than traditional polysulfone (PSF) based substrates. After activation with isopropyl alcohol (IPA), PA/PE-TFC-IPA NF membranes were formed, whose NF performance could be tuned by adjusting the preparation conditions. When TMC concentration was kept at 0.15 % w/v, RIP with low PIP concentration (0.025 wt%) yield PA/PE-TFC-IPA NF membranes with negatively charged surfaces, high Na2SO4 rejection rate (95 % ± 3 %) and water permeance (13–19 L·m−2·h−1·bar−1). RIP with high PIP concentration (0.3 wt%) formed PA/PE-TFC-IPA NF membranes with less negative surfaces, high MgCl2 rejection rate (94.3 ± 0.7 %), low water permeance (4.8 ± 0.3 L·m−2·h−1·bar−1), and high Mg(II)/Li(I) separation coefficient SLi/Mg (18 ± 1) for 1000 ppm LiCl and MgCl2 mixed solution. PA/PE-TFC-IPA NF membranes also showed good long-term stability and improved anti-fouling performance for positively charged pollutants, bearing high potential for applications in the lithium extraction processes.

Polyamide nanofiltration membrane fabricated with ultra-low concentration triaminoguanidine showing efficient desalination performance

Tuning the reactivity and diffusion of amine monomers in interfacial polymerization (IP) process could efficiently regulate the formation and structure of the polyamide (PA) selective layer. Herein, novel thin-film composite NF membranes with dense and ultrathin PA selective layer were fabricated using 1,3,5-benzenetricarbonyl trichloride (TMC) and ultra-low concentration triaminoguanidine (Tg) as reactive monomers. Fabrication parameters, including Tg concentration, NaOH concentration and reaction time, were systematically explored to optimize the desalination performance of PA-Tg NF membrane. During the stability test over 48 h, PA-Tg-0.06 membrane displayed a water permeance of ∼12 L m−2 h−1·bar−1 and Na2SO4 rejection of ∼99%. Meanwhile, detailed comparisons were made between PA-Tg NF membrane and a typical NF membrane fabricated with PIP as an aqueous monomer, including characteristics of the monomers as well as the structure, water permeance, salt rejection and antifouling property of the membranes. Molecular dynamics simulation and interfacial diffusion experiments indicated that the Tg monomer exhibited a higher reactivity but a lower diffusivity than the PIP monomer, making it feasible for the fabrication of PA-Tg NF membrane with ultra-low concentration of Tg monomer. In addition, compared with the PA-PIP membrane, the PA-Tg membrane exhibited a smoother, more hydrophilic and negatively charged surface, as well as a thinner and denser selective layer, thereby showing a higher water permeance, salt rejection and stronger fouling resistance.

Second interfacial polymerization decorating defects of TFC NF membrane formed by 1D nanochannels for improving separation performance

Generating nanovoids/nanochannels in the polyamide rejection layer is an effective method to enhance the separation performance of thin-film composite (TFC) membranes. However, the excessive nanovoids/nanochannels formed in the rejection layer will cause defects and reduce the selectivity. Here we report a secondary interfacial polymerization (SIP) method to decorate the defects caused by one-dimensional (1D) nanochannels, which were generated by etching 1D copper hydroxide nanorods (CuNRs). The obtained SIP-1D NF membrane had a pure water permeability of 8.2 L·m−2·h−1 bar−1, while the divalent salts rejection was 92.0 ± 2.6% of Na2SO4, 91.3 ± 0.3% of MgCl2, and 91.8 ± 0.2% of MgSO4. Our work provides experimental basis for 1D nanochannels regulate the structures and properties of NF membrane.

Fluoride Polluted Groundwaters in Calabria Region (Southern Italy): Natural Source and Remediation

Excessive ingestion of fluoride through the consumption of F−-rich drinking water could cause adverse effects to human health. For this reason, the WHO has fixed 1.5 mg/L as the maximum F- concentration for drinking water. In this work, a detailed geochemical characterization was performed to define the source of natural pollution of two groundwaters (samples Pc and Bg) coming from deep crystalline aquifers located in the Calabria region (southern Italy) and to define and optimize the most appropriate water treatment strategy. The samples were classified as a F− enriched NaHCO3 type of water. In particular, the F− concentrations observed were 30 mg/L and 8.9 mg/L for the Pc and Bg samples, respectively. Based on the acquired geochemical characterization knowledge, the groundwaters were treated by two thin-film composite NF membranes, namely SPR 10113 and SPR 10114 which have so far not been used for water defluoridation. It was found that the SPR 10114 membrane was able to guarantee water permeates with F− contents lower than the threshold value of 1.5 mg/L for both treated waters, whereas the fluoride content remained above the threshold value when the Pc sample was treated using the SPR 10113 membrane. The obtained permeates were characterized by a low ionic load and were not suitable for long-term consumption as drinking water. However, all of the produced waters did not need any further re-mineralizing processes for agricultural irrigation or other purposes.

Direct generation of an ultrathin (8.5 nm) polyamide film with ultrahigh water permeance via in-situ interfacial polymerization on commercial substrate membrane

Generation of ultrathin polyamide (PA) film has been recognized as an effective strategy for fabricating thin film composite (TFC) NF membranes with ultrahigh water permeance and desirable selectivity. However, direct synthesis of an ultrathin PA film via conventional in-situ interface polymerization (ISIP) on commercial substrate membranes has been widely considered impossible. Here we demonstrated this challenge can be surmounted by simultaneously optimizing the monomer concentration and substrate membrane selection. A defect-free PA film with thickness of 8.5 nm was directly formed on a commercial substrate membrane with smooth and hydrophilic surface, as well as medium pore size, only by using a high trimesoyl chloride (TMC) concentration of 0.1 w/v% and a low piperazine (PIP) concentration of 0.05 w/v %. The optimized TFC NF membrane prepared via this facile strategy exhibited an ultrahigh pure water permeance of 46.6 L m−2 h−1 bar−1 and a desirable Na2SO4 rejection of 98.1%. As far as we know, this performance can exceed all the reported TFC NF membranes fabricated with conventional ISIP technique, and also be comparable to most state-of-the-art TFC NF membranes prepared with other complicated ex-situ interfacial polymerization (ESIP). The novel insight and feasible avenues of this study are expected to pave the way for the practical production and application of TFC NF membranes with ultrahigh water permeance.

A facile and economic route assisted by trace tannic acid to construct a high-performance thin film composite NF membrane for desalination

A trace amount of green tannic acid assisted in the construction of thin film composite nanofiltration membrane for desalination.

How to fabricate a negatively charged NF membrane for heavy metal removal via the interfacial polymerization between PIP and TMC?

The potential toxicity of heavy metals arising from the discharge of industrial wastewater has become a serious public concern. Fabrication of a thin film composite (TFC) NF membrane with desirable rejection of heavy metal ions is an attracting route to solve this problem. Hereon, we proposed a facile strategy for fabricating a TFC NF membrane with excellent heavy metal removal. The ratio of PIP and TMC was greatly enlarged by decreasing the concentration of TMC to an exceptional low extent. Thus a polyamide film with weak electronegativity and small pore size was successfully constructed on an ultrafiltration substrate membrane. Comprehensive characterizations were adopted to reveal the micro-variation of the resulted NF membranes when regulating TMC concentration. The optimized membrane prepared only with PIP and TMC can exhibit a high pure water permeability of 14.6 L m−2 h−1 bar−1, desirable rejection of over 98% against Ni2+, Zn2+, Cu2+, and >92.7% against Cd2+. This performance could go beyond most of the reported candidates which were prepared with other complicated methods. The simplicity, low cost and scaling up feasibility of the current strategy is expected to well meet the requirements of industrial production of TFC NF membrane for heavy metal removal.

Feasible Preparation of a Thin‐Film Composite Nanofiltration (TFC NF) Membrane with Enhanced Skin–Substrate Adhesion and Compaction Resistance: In Situ Construction of Rigid–Flexible Polymer Composited Microspheres (CPs) in the Casting Solution

AbstractNovel microspheres (CPs) composited by rigid and flexible polymers are synthesized and embedded in the supporting membranes to enhance both the skin–substrate adhesion and compaction resistance of the thin‐film composite (TFC) nanofiltration membranes. The CPs are in situ formed in the casting solution after the rigid poly(p‐phenylene terephthamide) (PPTA) is produced in the flexible poly(m‐phenylene isophthalamide) (PMIA) solution. Then the PPTA/PMIA in situ blending membranes are prepared by using the NIPs method, and the TFC NF membranes are fabricated via interfacial polymerization on them. The CPs are characterized via polarizing microscopy and TEM. The surface morphology and chemical composition of the blended membranes are characterized by using FESEM, AFM, FTIR, and WCA, respectively. As the results show, the supporting membrane with higher PPTA content exhibits higher permeability, thermal stability, and compaction resistance. Moreover, the adhesion strength between the TFC functional layer and the supporting membrane is improved significantly. It is proposed that this improvement can be attributed to the CPs that are exposed on the top surface of the supporting membrane, which leads to a great enhancement because of the anchoring effect between the functional layer and the CPs.

Tannic acid assisted interfacial polymerization based loose thin-film composite NF membrane for dye/salt separation

In this work, a novel tannic acid assisted interfacial polymerization (TAIP) method was developed for preparing loose nanofiltration membranes (LNMs). Piperazine (PIP) and tannic acid (TA), two kinds of common monomer for interfacial polymerization (IP), were employed to deposit on substrate for the TAIP process. The TAIP process resulted in enlarged pores in the skin layer due to the longer monomers, thus achieved considerable permselectivity in separating salts from dye/salt mixture solution. The optimal TAIP membrane (M4) exhibited high permeability (32.57 LMH·bar−1 to Congo Red solution), reasonable rejection to dyes (99.40% and 99.19% for Congo Red and Rose Bengal, respectively) and satisfactory transport to salts (90.59% to Na2SO4 and 97.75% to NaCl). Meanwhile, this membrane retained high performance in dye/salt separation (7.16 LMH·bar−1 of permeability, 97.47% of CR rejection and 2.55% of NaCl retention) under even as high as 6 wt% of salt concentration. Moreover, TAIP LNM showed high stability throughout a 30-hour-long running test to separate salts from dye/salt mixture solution. We anticipated the TAIP methodology, together with the consequential LNM, to provide a potential candidate for dye/salt separation application.

Fabrication of high performance nanofiltration membrane on a coordination-driven assembled interlayer for water purification

A facile coordination-driven assembly strategy is proposed to construct a multifunctional PSS-metal ions complex interlayer on the substrate for the fabrication of the high performance thin-film composite (TFC) NF membrane with crumpled morphology. The PSS-metal ions interlayer not only acts as nanoscaffold role to provide a more uniform interfacial polymerization (IP) platform that is favorable for forming a thinner and free-defect PA layer with less PA intruded into the pores of support, but also works as macromolecule addition to induce the diffusion-driven instability of IP process that causes the creation of the crumpled PA layer with increased surface area, of which dual roles in regulating the IP process are confirmed by the comprehensive characterization of the top and back surface morphologies and structure of the selective layer. As such, the obtained TFC membrane exhibits a four folds of permeance (up to 22.15 ± 1.14 L m−2 h−1 bar−1) as compared with traditional TFC-0 membrane, while maintaining a competitive rejection for Na2SO4 (about 99.3%). Moreover, owing to the increased surface negative charges and decreased effective pore size, the novel NF membrane shows a simultaneous high rejection of trace organic compounds (TrOCs) and low retention of divalent cation (Ca2+/Mg2+) in a real tap water, suggesting that it has great potential in drinking water treatment. This study provides a new direction to prepare the high-performance TFC membrane for water purification.

Preparation of thin film composite nano-filtration membranes for brackish water softening based on the reaction between functionalized UF membranes and polyethyleneimine

A new method was developed to prepare a low-pressure nano-filtration membrane based on the membrane surface reaction (functionalized UF blend membrane) with polyethyleneimine (PEI) to fabricate thin film composite (TFC) membranes for water softening applications. The TFC selective layer was prepared by a reaction between the carboxyl functional groups of a carboxylated polyether sulfone (C-PES)/PES UF blend membrane and PEI. Hyper-branched PEI (HPEI) and PEI were used as poly-cations for fabricating the TFC NF membrane. Glutaraldehyde was used to cross-link the PEI on the membrane surface. The effects of the poly-cation concentration, crosslinker concentration, reaction time, reaction temperature, pH, and NaCl concentration as the supporting polyelectrolyte on the water permeability (WP) and rejection were evaluated. The addition of C-PES lead to the formation of finger-like structures and increased the water flux. While the effect of the polycation solution pH value was found to be the dominant parameter, optimization of the reaction time and concentration was necessary to obtain a membrane with acceptable water filtration capability. An NF membrane with WP as high as 10.1 LMH/bar and MgCl2 (1000 ppm) rejection of 90% was obtained.

Preparation and characterization of low-pressure and high MgSO4 rejection thin-film composite NF membranes via interfacial polymerization process

Low-pressure nanofiltration membrane produced by FILMTEC-DOW is generally considered by membrane industries as the commercial standard; according to its specification, for a 2000 ppm MgSO4(aq) feed, it yields 98.2% MgSO4 rejection and 22 GFD permeate flow at 70 psi applied cross-flow filtration pressure. Here we want to report our nanofiltration membrane development works on improved reactant formulation and fabrication process to produce our nanofiltration membrane having 98.2% MgSO4 rejection and 33 GFD permeate flow for the similar test conditions as FILMTEC-DOW did. Our near 50% improvement on water permeate flux over FILMTEC-DOW low-pressure nanofiltration membrane was due to the fact that an adduct salt named TEACSA was added to the aqueous piperazine aqueous solution during our membrane fabrication process. Besides evaluating the nanofiltration performances of our NF membranes, characterizations of the polyetherimide (PEI) microporous supports fabricated in our laboratory in the areas of pure water permeate fluxes at 40 psi cross-flow filtration pressure, wet–dry studies of hydrophobic porous polyetherimide and polysulfone supports, AFM and SEM were performed.

The anti-biofouling properties of thin-film composite nanofiltration membranes grafted with biogenic silver nanoparticles

Biofouling is still one of the most challenging issues of nanofiltration. One of the practical strategies to reduce biofouling is to develop novel anti-biofouling membranes. Herein, biogenic silver nanoparticles (BioAg0-6) with the averaged diameter of only 6nm were firstly grafted on the surface of polyamide NF membrane. The effect of grafted BioAg0-6 on the performance of thin-film composite (TFC) NF membranes was systematically investigated with a comparison to the grafted chemical AgNPs. BioAg0-6 grafted membrane (TFC-S-BioAg) increased the hydrophilicity of the TFC membrane and water permeability, while maintaining the relatively high salt rejection. The result of silver leaching experiment indicated that the grafted BioAg0-6 had a better stability on the membranes, the ratio of remained silver in the TFC-S-BioAg membrane was 95%, after soaked in pure water for 50days. After 4month immersion, the rejection of TFC-S-BioAg membrane remained more than 90% of initial rejection. The results of disk diffusion test revealed that both of TFC-S-BioAg membrane and TFC-S-ChemAg membrane showed effective anti-bacterial ability to inhibit Pseudomonas aeruginosa and Escherichia coli growth, the TFC-S-BioAg membrane showed more excellent and longer lasting antibacterial property. Therefore, BioAg0-6 grafted TFC membranes could be potential as an effective strategy to decrease biofouling in nanofiltration process.

Analysis of lead(II) retention from single salt and binary aqueous solutions by a polyamide nanofiltration membrane: Experimental results and modelling

The present paper reports the performance of a polyamide thin-film composite NF membrane (AFC 80) in the removal of highly polluting and toxic Pb(II) ions from single salt and binary aqueous solutions simulating real wastewaters. The effect of the operating variables (pH, transmembrane pressure and feed solution concentration) on the separation process was investigated. It was observed that the rejection of lead ions slightly increases with increasing the transmembrane pressure and slightly decreases with increasing the metal concentration in feed at constant pH of aqueous solutions. The maximum rejection of lead ions in single salt solution was 99.44% for a feed solution containing 50mg Pb/L at pH 5.7. In binary Pb–Cd aqueous solutions the metals rejections are higher than 98.5% for concentrations in the 15–80mg/L range, and the rejection order follows the order of metals hydration energies. The optimum operating conditions selected were: transmembrane pressure in 10–30bars range, feed solutions (polluted wastewaters) at pH 5.7 containing 50–80mg metal/L. The Spiegler–Kedem model ensures a very good description and prediction of the NF processes under study for a wide concentrations range. The model parameters showed that the transport mechanism of the solute is mainly convective, being almost totally hindered by the size of the metal ions which finally determine the salts retention. The AFC 80 membrane proved to be able to perform an effective removal of lead, and also of cadmium and nitrate ions, in very mild and economically advantageous conditions, leading to wastewaters suitable to be discharged into the environment. The structural characteristics of the AFC 80 membrane (effective pore radius and the thickness to porosity ratio) were estimated from uncharged solutes rejection experiments. The charge properties of the membrane surface were assessed by measuring the tangential streaming potential. It was found that the membrane becomes positively charged in lead nitrate solution, and this behaviour was attributed to adsorption of lead ions on the membrane surface. The membrane characteristics thus determined were correlated with the results of lead separation experiments and with the Spiegler–Kedem model parameters.

Structure and performance of nanofiltration membrane prepared in a large-scale at CSIR-CSMCRI using indigenous coating unit

The paper presents the fundamental and performance study of poly (piperazine-trimesamide) based thin film composite NF membranes prepared in a large scale (0.5m×30m scale) in a continuous mode using indigenously developed membrane casting and coating units. Different preparation parameters were optimized. Membrane modules of sizes 4×14 and 2×10 (inches×inches) were fabricated in spiral wound configurations. NF membrane modules showed product water flux up to 73±20l/m2.h at 150psi and MgSO4 rejection of 95±4%. The membranes maintained rejection ratio of 3 to 4.1 between divalent and monovalent anions. Almost a total rejection of ampicillin is observed by these membranes signifying their applicability in purification of pharmaceutically contaminated water.

Fractionation of homologous CD 6 to CD 60 cyclodextrin mixture by ultrafiltration and nanofiltration

This paper investigates the membrane purification and fractionation of a mixture containing the homologous series of cyclodextrins CD 6 to CD 60 obtained by enzymatic conversion of starch. Three commercial polyamide thin film composite NF and UF membranes (HL, GH and GK from GE-Osmonics) were used for this purpose. In a first step, a binary mixture composed of glucose and heptacyclomaltose (β-cyclodextrin, CD 7) was filtered to examine the separation performance of the studied membranes. A mathematical model based on mass balance was proposed for the simulation of the discontinuous diafiltration process assuming that the membrane separation performance is based on a sieving mechanism. A three stage diafiltration cascade (in retentate configuration) was then selected to fractionate the CD 6–CD 60 crude mixture. The experimental composition of the obtained permeate and retentate solutions in the targeted fractions (glucose, CD 6–CD 8, CD 9–CD 21, CD 22–CD 60) fit well with the predicted data indicating that membrane process enables purification and fractionation of the homologous series of large ring CDs. Some discrepancies were however observed implying that other mechanisms such as coupled transport also took place. The most striking effect was the presence of glucose in the GK retentate possibly as a result of the formation of inclusion complexes with the large ring CDs.

Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: II. Membrane physiochemical properties and their dependence on polyamide and coating layers

The physiochemical properties of 17 widely used commercial RO and NF polyamide (PA) membranes were fully characterized by atomic force microscopy, transmission electron microscopy, contact angle measurement, streaming potential analysis, and flux and rejection performance tests. The surface properties (roughness, hydrophilicity, and surface charge) and bulk properties (permeability and rejection) were demonstrated to be highly inter-dependent, as all these were determined by the polyamide chemistry and any associated surface coating layer. The 1,3-benzenediamine and trimesoyl chloride based fully aromatic membranes had surface roughness on the order of 100 nm, an order of magnitude rougher than the semi-aromatic poly(piperazinamide) membranes. Furthermore, the uncoated fully aromatic membranes were significantly more hydrophobic (contact angles 43–49°) than the semi-aromatic ones (~30°). The presence of a neutral polyvinyl alcohol (PVA) coating layer can significantly enhance hydrophilicity and reduce surface charge and roughness for fully aromatic PA membranes, while its effect was only marginal for semi-aromatic poly(piperazinamide) membranes. The selectivity of a membrane appeared to be inversely related with its permeability. The highly permeable piperazine based membranes were much less selective than the fully aromatic ones. The salt rejection of a membrane was enhanced upon coating with a PVA layer, at the expense of reduced permeability. The current study suggests that the physiochemical properties can be used to diagnose the polyamide and coating chemistry, in addition to the conventional spectroscopic methods. Understanding such dependence of membrane properties and performances on their structure and chemistry might also be important for membrane synthesis, modification, and their applications in water and wastewater treatment.

Influence of temperature and cleaning on aromatic and semi-aromatic polyamide thin-film composite NF and RO membranes

Although cleaning removes fouling, it may change the properties of membranes and consequently influence their performance. We have evaluated the combined effect of acidic/alkaline cleaning and temperature on the performance of two aromatic membranes, one a reverse osmosis membrane (HR98PP) and the other a nanofiltration membrane (NF90), two semi-aromatic polypiperazine membranes (NF200, NFT-50) and a Desal-5DK nanofiltration membrane. At the retention minimum of each membrane, the KCl retention and the water permeability were in the order: NFT-50 < NF200 < Desal-5DK < NF90 < HR98PP and HR98PP < NF90 < NF200 < Desal-5DK < NFT-50, respectively. After nanofiltration at elevated temperatures acidic and alkaline cleaning increased the water permeability and decreased the retention of the Desal-5DK membrane. Similar behaviour was observed after acidic cleaning of the aromatic polyamide membranes and with alkaline cleaning of the semi-aromatic polyamide (polypiperazine) membranes. Increased hysteresis was seen in the membrane performance with increasing temperature. The hysteresis behaviour was dependent on the cleaning procedure and could be related to swelling and shrinkage of the active polyamide layer. At 40 °C the reverse osmosis membrane (HR98PP) showed the same performance as the NF90 nanofiltration membrane at 20 °C.

Influence of biofouling on boron removal by nanofiltration and reverse osmosis membranes

Excess of boron in water poses a problem due to adverse effects on crop production as well as human health and aquatic life. This study examined the influence of biofouling of NF and RO membrane on the performance of the membranes in removing boron from a synthetic wastewater effluent. Accelerated laboratory-scale biofouling experiments were carried out with commercial thin film composite NF and RO membranes under controlled conditions. Permeate flux decline, down to less than 25% of its initial value, and substantial decrease in boron rejection were attributed to extensive biofilm growth on the membranes. For the RO membrane, boron rejections declined by 45 and 34% of the initial values for influent boron concentrations of 5.5 and 1.1 mg B/L, respectively, whereas the corresponding declines in boron rejection for the NF membrane were 44 and 13% of the initial values. These adverse effects of biofilm growth on permeate water flux and boron rejection are attributed to both an increase in hydraulic resistance to permeate flow due to bacterial extracellular polymeric substances (EPS) and a biofilm-enhanced concentration polarization near the membrane surface.