Commercial Reverse Osmosis Membranes Research Articles

Combined Effects of Surface Roughness, Solubility Parameters, and Hydrophilicity on Biofouling of Reverse Osmosis Membranes

The fouling of protein on the surface of reverse osmosis (RO) membranes is a surface phenomenon strongly dependent on the physical and chemical characteristics of both the membrane surface and the foulant molecule. Much of the focus on fouling mitigation is on the synthesis of more hydrophilic membrane materials. However, hydrophilicity is only one of several factors affecting foulant attachment. A more systematic and rationalized methodology is needed to screen the membrane materials for the synthesis of fouling-resistant materials, which will ensure the prevention of the accumulation of foulants on the membrane surfaces, avoiding the trial and error methodology used in most membrane synthesis in the literature. If a clear correlation is found between various membrane surface properties, in combination or singly, and the amount of fouling, this will facilitate the establishment of a systematic strategy of screening materials and enhance the selection of membrane materials and therefore will reflect on the efficiency of the membrane process. In this work, eight commercial reverse osmosis membranes were tested for bovine serum albumin (BSA) protein fouling. The work here focused on three surface membrane properties: the surface roughness, the water contact angle (hydrophilicity), and finally the Hansen solubility parameter (HSP) distance between the foulant understudy (BSA protein) and the membrane surface. The HSP distance was investigated as it represented the affinities of materials to each other, and therefore, it was believed to have an important contribution to the tendency of foulant to stick to the surface of the membrane. The results showed that the surface roughness and the HSP distance contributed to membrane fouling more than the hydrophilicity. We recommend taking into account the HSP distance between the membrane material and foulants when selecting membrane materials.

Multiscale insights into polyamide membrane fouling during reverse osmosis of rare earth wastewater

The rare earth elements (REEs) industrial wastewater is characterized by high ammonium nitrogen and low-strength organic compounds. Reverse osmosis (RO) process is effective for the REEs wastewater treatment. However, membrane fouling deteriorated the RO process. In this work, the fouling mechanism during RO process of REEs wastewater was elucidated via multiscale methods. A series of bench-scale fouling tests with simulated REEs wastewater containing high NH4+–N and different concentrations of 2-ethylhexyl phosphonic acid mono-(2-ethylhexyl) ester (P507) were performed with a commercial RO membrane to evaluate the fouling extent of the RO process. A critical P507 concentration (0.25 mg L−1) was observed where the fouling pattern changed qualitatively. When the P507 concentration was lower than 0.25 mg L−1, the relative flux increased and the membrane surface became more hydrophilic. When P507 reached this critical point, severe fouling occurred accompanied with a more hydrophobic membrane surface. Multiscale simulations [i.e., molecular dynamics (MD) and dissipative particle dynamics (DPD)] revealed that the fouling layer network varied with P507 concentration. This work provides in-depth insights into membrane fouling mechanism in the REEs wastewater, and has enlightening significance for fouling control strategies and the innovation of anti-fouling membrane materials.

The effect of functional group of Graphene-based woven filter on the desalination performance

Abstract Graphene-based braided filter membranes (GWFM) are becoming increasingly crucial in seawater desalination because of their excellent performance, moving from research to practical use and helping to tackle water scarcity. However, the mechanisms associated with GWFMs functionalized by functional groups are unclear. This paper demonstrates that graphene strips are intricately integrated into a filtration membrane characterized by X-functional groups (GWFM-X). Molecular dynamics (MD) simulations are effective for calculating water flux, assessing salt discharge effects, and understanding the physical mechanisms in seawater desalination, making them crucial for optimizing and advancing desalination technology and salt rejection rate of four types of GWFM-X, namely GWFM-1.2nm-COOH, GWFM-1.2nm-NH2, GWFM-1nm-COOH, and GWFM-1nm-NH2, which were designed with different functional groups and nanopore diameters. These results demonstrate the superior performance of GWFM-X in water filtration applications. Salt rejection efficiency of four distinct variants of GWFM-X, namely GWFM-1.2nm-COOH, GWFM-1.2nm-NH2, GWFM-1nm-COOH, and GWFM-1nm-NH2, is designed with different functional groups and nanopore diameters. The test membrane shows exceptional water flux, significantly outperforming current commercial reverse osmosis membranes, with measurements of 30.00±0.49 L cm−2 day−1 MPa−1, 35.41±0.71 L cm−2 day−1 MPa−1, 9.31±0.31 L cm−2 day−1 MPa−1, and 20.66±0.21 L cm−2 day−1 MPa−1, respectively. The free energy profiles of GWFM-X reveal a pronounced disparity in free energy barriers between water molecules and Na+/Cl− ions, with a difference of up to 14.26 kBT. The simulation results show that the seawater desalination system maintains over 75% efficiency at pressures between 10 and 500 MPa, highlighting its effectiveness and versatility. Moreover, advancements in nanofabrication technology are paving the way for new membrane materials that could improve desalination methods and help address water scarcity.

Effects of surfactants, ion valency and solution temperature on PFAS rejection in commercial reverse osmosis (RO) and nanofiltration (NF) processes

Per- and polyfluoroalkyl substances (PFAS) have garnered attention as a pressing environmental issue due to their enduring presence and suspected adverse health effects. This study assessed the rejection or removal efficacy of PFAS by commercial reverse osmosis (RO) and nanofiltration (NF) membranes and examined the impacts of surfactants, ion valency and solution temperature that are inadequately explored. The results reveal that the presence of cationic surfactants such as cetyltrimethylammonium bromide (CTAB) increased the rejection of two selected PFAS compounds, perfluorooctanoic acid (PFOA) and perfluorobutanoic acid (PFBA), by binding with negatively charged PFAS and preventing them from passing through membrane pores via size exclusion, whereas the presence of anionic surfactants such as sodium dodecyl sulfate (SDS) increased the PFAS rejection because the increased electrostatic repulsion prevented PFAS from approaching and adsorbing onto the membrane surface. Moreover, aqueous ions (e.g., Al3+ and PO43−) with higher ion valency enabled higher rejection of PFOA and PFBA through increased effective molecular size and increased electronegativity. Finally, only high solution temperature at 45 °C significantly reduced PFAS rejection efficiency because of the thermally expanded membrane pores and thus the increased leakage of PFAS. Overall, this research provides valuable insights into the various factors impacting PFAS rejection in commercial RO and NF processes. These findings are crucial for developing efficient PFAS removal methods and optimizing existing treatment systems, thereby contributing significantly to the ongoing efforts to combat PFAS contamination.

High-temperature reverse osmosis and molecular separation with robust polyamide-ceramic membranes

Polyamide thin-film composite (TFC) membranes are widely used for reverse osmosis (RO), but most commercial RO membranes have a limited operating temperature of <45 °C. This has constrained the broader applications of RO in industries, where process and water feeds with high temperature >50 °C are common. In this study, robust polyamide-ceramic TFC membranes were developed for high-temperature RO (HT-RO). RO-type polyamide thin-film was successfully synthesized on the inner surface of ceramic tubular membranes via interfacial polymerization. The polyamide-ceramic membranes exhibited excellent thermal stability, maintaining high NaCl rejection (>98 %) and steady water permeability (∼5 LMH/bar) during HT-RO at 70 °C. The molecular weight cut-off (MWCO) of the membranes increased slightly from 90 to 140 Da at 70 °C, and the surface charge played an important role in maintaining the high salt rejection. Long-term stability tests showed that polyamide thin-films may undergo thermal hydrolysis at 80 °C. It was also found that polymeric substrates of flat-sheet RO membranes may experience serious compaction at high temperature, which could subsequently affect the performance and stability of the polyamide layer. The ceramic substrates provide strong support at high temperature, and the highly stable polyamide-ceramic membranes demonstrated huge potential for RO applications under more challenging conditions.

Surface re-modification based on poly-(triazine-amine) from commercial nanofiltration membrane to reverse osmosis membranes with improved antifouling property

Reverse osmosis (RO) has been limited by membrane fouling, which reduces membrane performance, shortens membrane service life, and increases operating costs. This study designed a new poly-(triazine-amine) reverse osmosis (PTA1-RO) membrane that uses a commercial membrane as the substrate for remodification with better anti-fouling performance based on the interfacial polymerization (IP) reaction between piperazine (PIP) and cyanuric chloride (CC). The PTA1-RO membrane has a rejection of 97.8 % for NaCl and a water permeance of 1.56 L m−2·h−1 bar−1. The PTA coating filled the “ridge-valley” structure of the polyamide membranes, reducing surface roughness and enhancing antifouling property. As much as 90 % and 100 % flux recovery of BSA and HA were achieved by PTA1-RO membrane, significantly higher than the current state-of-the-art commercial RO membranes SW30 and BW30.

Recovery of the Properties of Spent Commercial Reverse Osmosis Membrane Elements and Ways of their Reuse

Recovery of the Properties of Spent Commercial Reverse Osmosis Membrane Elements and Ways of their Reuse

Novel membrane modifications for pressure retarded osmosis as a new way for sustainable power generation from salinity gradients

Pressure retarded osmosis (PRO) is a novel technology that allows power to be generated from high-salinity resources. To achieve a high power density, a high-salinity solution should be used on the draw side together with high hydraulic pressure, thus the PRO process requires a membrane that has high salt rejection and high pressure resistance. The reverse osmosis (RO) membranes can be potential candidates for the PRO process. In this study, commercial RO membranes with different surface modifications were examined: O2 plasma, polydopamine (PDA) and tannic acid (TA). Improved water permeability and salt rejection were obtained after modification. The membranes were also tested with a high salinity solution (175 g/L NaCl) in a PRO mode. Due to the improved water permeability and high salt rejection of the surface-modified membrane, the commercial RO membrane showed enhanced power density (3 W/m2) in comparison to the pristine membrane (2 W/m2).

Are Hansen solubility parameters relevant in predicting the post-treatment effect on polyamide-based TFC membranes?

This study investigates the impact of solvent post-treatment on polyamide-based thin film composite (TFC) membranes, specifically examining the effect on commercial nanofiltration (NF) and reverse osmosis (RO) membranes. Na2SO4 rejection and increase in pure water permeance (PWP) were considered as the output parameters. The disparity in Hansen solubility parameters (HSP) between the post-treatment solution and the polyamide layer of the TFC membrane, denoted by Ra, is well adapted to understand the enhancement in water permeance through the membranes upon treatment. Aqueous solutions of dimethylformamide with a Ra value of 4, acetonitrile with a Ra value of 8.3, and ethanol with a Ra value of 12.7 were used as the post-treatment solutions. Our experimental design, based on the Box-Behnken design of Response Surface Methodology, incorporates variables such as the concentration of the solvent in the solution (% v/v), Ra value, and treatment time (s). Our findings demonstrate that the effect of post-treatment on the TFC membranes is not governed by the Ra value. Notably, while the post-treatment with the aqueous solution of acetonitrile, 80% v/v for 30 s, had considerable effects on NF membranes (124.5% enhancement in PWP; reduction of 3.5% in Na2SO4 rejection), its impact on RO membranes was negligible. Several factors explain this discrepancy, including the limitations of the HSP model for composite polymers, the inaccuracy of the PWP or salt rejection as a swelling indicator, variations in the HSP values of the polyamide layers for different membranes, and possible modifications in the interface between the support membrane and the polyamide layer. In summary, our study provides insights into the complex interactions between solvents and composite membranes, indicating that HSP alone is not a decisive factor in predicting post-treatment effects on polyamide-based TFC membranes.

Evaluation of commercial nanofiltration and reverse osmosis membrane filtration to remove per-and polyfluoroalkyl substances (PFAS): Effects of transmembrane pressures and water matrices.

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are now widely found in aquatic ecosystems, including sources of drinking water and portable water, due to their increasing prevalence. Among different PFAS treatment or separation technologies, nanofiltration (NF) and reverse osmosis (RO) both yield high rejection efficiencies (>95%) of diverse PFAS in water; however, both technologies are affected by many intrinsic and extrinsic factors. This study evaluated the rejection of PFAS of different carbon chain length (e.g., PFOA and PFBA) by two commercial RO and NF membranes under different operational conditions (e.g., applied pressure and initial PFAS concentration) and feed solution matrixes, such as pH (4-10), salinity (0- to 1000-mM NaCl), and organic matters (0-10mM). We further performed principal component analysis (PCA) to demonstrate the interrelationships of molecular weight (213-499 g·mol-1 ), membrane characteristics (RO or NF), feed water matrices, and operational conditions on PFAS rejection. Our results confirmed that size exclusion is a primary mechanism of PFAS rejection by RO and NF, as well as the fact that electrostatic interactions are important when PFAS molecules have sizes less than the NF membrane pores. PRACTITIONER POINTS: Two commercial RO and NF membranes were both evaluated to remove 10 different PFAS. High transmembrane pressures facilitated permeate recovery and PFAS rejection by RO. Electrostatic repulsion and pore size exclusion are dominant rejection mechanisms for PFAS removal. pH, ionic strength, and organic matters affected PFAS rejection. Mechanisms of PFAS rejection with RO/NF membranes were explained by PCA analysis.

Comprehensive Characterization of Commercial Reverse Osmosis Membranes through High-Temperature Cross-Flow Filtration.

Developing thermally stable reverse osmosis membranes is a potential game-changer in high-temperature water treatment. In this work, the performance of three commercial reverse osmosis membranes was evaluated with a series of high-temperature filtrations. The membranes were tested with different filtration methodologies: long-term operation, cyclic tests, controlled stepwise temperature increment, and permeability tests. The morphological and physiochemical characterizations were performed to study the impact of high-temperature filtration on the membranes' chemical composition and morphological characteristics. An increase in the temperature deteriorated the membrane performance in terms of water flux and salt rejection. Flux decline at high temperatures was recognized as the primary concern for high-temperature filtrations, restricting the applications of commercial membranes for long-term operations. This research provides valuable insights for researchers aiming to thoroughly characterize reverse osmosis membranes at high temperatures.

Evaluation of nanofiltration and reverse osmosis membranes for efficient rejection of organic micropollutants

Nanofiltration (NF) and reverse osmosis (RO) membranes have revolutionized water treatment processes. However, emerging separations, such as the removal of organic micropollutants (OMPs), pose a challenge for traditional RO and NF membranes due to either low water permeance (RO) or inadequate OMP rejection (NF). In this work, we investigated the rejection of thirteen frequently detected OMPs at environmentally relevant low feed concentration of 12 μg/L by using a wide range of commercial RO and NF polyamide thin-film composite (TFC) membranes and an in-house prepared NF membrane. The membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), contact angle, and surface charge measurements. We investigated removal capabilities of OMPs at standard brackish water RO operating conditions (i.e., 15.5 bar transmembrane pressure, pH 8) and also examined the effect of feed solution transmembrane pressure on membrane performance. The rejection of ionic OMPs was strongly influenced by electrostatic repulsion effects, whereas rejection of non-ionic solutes was primarily driven by size exclusion and hydrophobic/hydrophilic interactions. The best performing TFC membranes — a lab-made semi-aromatic piperazine-based NF polyamide membrane and a fully aromatic commercial polyamide NF membrane (NF1) — displayed excellent rejection of OMPs comparable to a commercial RO seawater membrane (RO4), but exhibited ∼2-fold and ∼6-fold higher water permeance, respectively.

Development of high-integrity reverse osmosis membranes for enhanced removal of microorganisms

Improved integrity of reverse osmosis (RO) membranes for pathogenic microorganisms can enhance the microbiological safety of potable water reuse. This study aimed to produce a high-integrity RO membrane that is less prone to defects or failures of the polyamide skin layer. The commercial track-etched (TE) microfiltration membranes with uniform pore sizes of 0.2 μm were adopted as a robust support layer. The polyamide skin layer was successfully formed by creating a thin m-phenylenediamine (MPD) layer using a spin coater before the interfacial polymerization. The best TE-based RO membranes had a pure water permeability of 1.1 L/m2hbar and a salt rejection of 97 %. The field emission scanning electron microscopy revealed that the skin layer thickness of the TE-based membrane (approx. 31 nm) was equivalent to a commercial RO membrane. The removal of bacteria-sized particles (0.5 μm fluorescent microspheres) by the fabricated TE-based membrane (8.6-log) was greater than that by the commercial RO membrane taken out from an RO membrane element (7.8-log). Although complete removal of bacteria-sized particles was not achieved with small TE-based RO membrane coupons, this study established a viable protocol for producing a high-integrity RO membrane by forming the polyamide skin layer on the TE support layer.

Bioinspired humic acid-based membranes for water desalination: Mechanistic insights from molecular simulations

Bioinspired membranes have received considerable interest for separation, but their design is challenging because of lack in fundamental understanding of separation mechanism involved. Herein, we explore renewable humic acid (HA)-based bioinspired membranes for water desalination via molecular simulations. Different HA membranes (monomeric, Y- and star-crosslinked) with microscopic “ridge-and-valley” patterned pore structures are constructed. With the existence of intermolecular hydrogen bonds, the HA membranes possess good mechanical strength with Young's moduli ranging from 0.44 to 1.64 GPa. Water desalination performance of the HA membranes is comprehensively examined in 12 salt solutions with different cations (Mg2+, Ca2+, Li+, Na+, K+ and NH4+) and anions (CO32−, SO42−, F−, OH−, Cl− and NO3−), respectively. Water and ion in the HA membranes are revealed to follow a dynamical “hopping-transport” mechanism. The HA membranes exhibit superior desalination performance, comparable to commercial reverse osmosis membranes and graphene oxide membranes. Interestingly, water flux in the monomeric HA membrane is enhanced by tailoring the membrane structure from monomeric to crosslinked architecture. Upon varying cation or anion in a salt solution, salt rejection drops when ion hydration level decreases. From bottom-up, the simulations provide mechanistic insights into water desalination and would facilitate the development of new membranes for high-performance water desalination.

Removal of antibiotics and estrogens by nanofiltration and reverse osmosis membranes

The separation behavior of a variety of emerging contaminants, including nine antibiotics and six estrogens commonly reported in natural environment, by four commercial nanofiltration and reverse osmosis (NF/RO) membranes at various water conditions (pH, concentration) was investigated. The contaminant rejection at pH 6.0 followed a decreasing trend of XLE (94%–100%) ≈ NF90 (88%–100%) ＞ NF270 (25%–85%) ＞ DL (16%–75%). The dense structures of NF90 and XLE reflected by their small effective pore radii (0.30–0.31 nm) contributed mainly to their high rejection, demonstrating the important role of size exclusion. For the negatively charged loose NF270 and DL membranes (0.40–0.45 nm), charge repulsion made additional contribution, which is markedly reflected by their greater rejection to charged antibiotics than neutral estrogens (45%–85% vs. 25%–60% by NF270). The correlation between rejection data and normalized molecular sizes at pH 4.0 and 9.0 intuitively demonstrated the individual role of size exclusion and charge repulsion. The adsorption by membranes was mainly responsible for the initial compound reduction in feedwater by 6%–25% within 3 h, while only 0.3%–5.6% was attributed to self-degradation. The adsorption capacity was determined, which might be mainly governed by hydrophobic interaction. The resolved controlling factors and mechanisms will contribute to the accurate prediction and membrane selection for trace contaminant removal by membrane process.

Fluoride Removal and Recovery from Water Using Reverse Osmosis and Osmotic Membrane Crystallization

Fluoride is a concern for human health at high concentrations, but it is also a valuable compound with multiple applications. Thus, having a system that gives the opportunity to remove and recover this valuable element from water is highly interesting. Reverse osmosis (RO) is a promising technology in the removal of fluoride from water. Nevertheless, the residual retentate highly concentrated in fluoride is still a concern. The aim of this study was to evaluate the performance of an integrated process consisting of RO and membrane crystallization to remove fluoride from water and to recover it as a pure fluoride salt. Pure water permeability and fluoride rejection of a commercial RO membrane was tested under different conditions. In addition, the performance of an osmotic membrane crystallization setup was evaluated, considering the effect caused by the flow rates and the concentration of both the feed and the osmotic solution on the mass transfer coefficient. The crystallization process allowed the production of pure NaF crystals with octahedral morphology with a face-centered cubic crystal system.

Orientation matters: Measuring the correct surface of polyamide membranes with quartz crystal microbalance

The surface of polyamide reverse osmosis (RO) membranes which regulates interface-dominated phenomena, such as partitioning and fouling, is the one facing the feed during operation. However, the opposite surface of the polyamide selective layer, the one facing the permeate and in contact with the polysulfone porous support, is commonly analyzed in quartz crystal microbalance (QCM) measurements due to limitations of state-of-the-art transfer methodologies. Such measurements on the back surface cannot be generalized because the polyamide layer is chemically and morphologically asymmetric. Herein, we introduce a simple method to coat QCM sensors with polyamide active layers in the correct orientation (i.e., exposing their front surface) and show that interface-dominated phenomena differ significantly between orientations. We start by describing a transfer protocol to coat any surface with a polyamide layer on its front surface orientation. We then systematically analyze the chemical and morphological differences between the two surfaces of the polyamide layer of a commercial RO membrane. Finally, we demonstrate that interface-dominated phenomena depend on the orientation by showing that NaCl partitioning at pH 6 was 1.3 to 2.3-fold higher on the front surface and that organic fouling with humic acid occurred at a lower rate on this surface. The new method presented herein enables measurements on the front surface of polyamide RO membranes, which should be the standard in any future QCM studies.

High-Flux Nanofibrous Composite Reverse Osmosis Membrane Containing Interfacial Water Channels for Desalination.

A nanofibrous composite reverse osmosis (RO) membrane with a polyamide barrier layer containing interfacial water channels was fabricated on an electrospun nanofibrous substrate via an interfacial polymerization process. The RO membrane was employed for desalination of brackish water and exhibited enhanced permeation flux as well as rejection ratio. Nanocellulose was prepared by sequential oxidations of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and sodium periodate systems and surface grafting with different alkyl groups including octyl, decanyl, dodecanyl, tetradecanyl, cetyl, and octadecanyl groups. The chemical structure of the modified nanocellulose was verified subsequently by Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), and solid NMR measurements. Two monomers, trimesoyl chloride (TMC) and m-phenylenediamine (MPD), were employed to prepare a cross-linked polyamide matrix, i.e., the barrier layer of the RO membrane, which integrated with the alkyl groups-grafted nanocellulose to build up interfacial water channels via interfacial polymerization. The top and cross-sectional morphologies of the composite barrier layer were observed by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) to verify the integration structure of the nanofibrous composite containing water channels. The aggregation and distribution of water molecules in the nanofibrous composite RO membrane verified the existence of water channels, demonstrated by molecular dynamics (MD) simulations. The desalination performance of the nanofibrous composite RO membrane was conducted and compared with that of commercially available RO membranes in the processing of brackish water, where 3 times higher permeation flux and 99.1% rejection ratio against NaCl were accomplished. This indicated that the engineering of interfacial water channels in the barrier layer could substantially increase the permeation flux of the nanofibrous composite membrane while retaining the high rejection ratio as well, i.e., to break through the trade-off between permeation flux and rejection ratio. Antifouling properties, chlorine resistance, and long-term desalination performance were also demonstrated to evaluate the potential applications of the nanofibrous composite RO membrane; remarkable durability and robustness were achieved in addition to 3 times higher permeation flux and a higher rejection ratio against commercial RO membranes in brackish water desalination.

Novel method for accurately estimating membrane transport properties and mass transfer coefficients in reverse osmosis

We present a simple and robust method to simultaneously characterize the water and salt permeability (A, B) of reverse osmosis (RO) membranes and mass transfer coefficient (k) in membrane modules. The proposed methodology comprises a set of RO experiments performed at different operating pressures or stages. The measured water and salt fluxes in each stage are simultaneously fitted to the RO transport equations by performing a non-linear regression, using A, B, and k as regression parameters. We first perform a systematic accuracy analysis of the proposed method across the full operational range of RO. The assessment shows that the method accuracy is substantially higher than current methods and increases with number of experimental stages and driving forces. This assessment is used to inform the design of an experimental protocol that minimizes errors in estimated A, B, and k. We then evaluate two commercial RO membranes following the new protocol. For both membranes, A and B parameters decrease by 17% and 15% from the dilute solution to seawater concentrations, whereas the k parameter remains constant. Our study demonstrates that the proposed method, informed by data-driven experimental designs, provides a new approach for accurately characterizing transport phenomenon in membrane processes with feeds of less than 100 g/L total dissolved solids.

Maximum allowable retention for low-salt-rejection reverse osmosis membranes and its effect on concentrating undersaturated NaCl solutions to saturation

For conventional reverse osmosis (RO), the osmotic pressure difference limits the allowable concentration factor for concentrating NaCl solutions. The use of low-salt-rejection RO (LSRRO) membranes could allow much higher concentration factors, provided that these membranes show a sufficient decrease in NaCl retention with increasing NaCl concentration in the retentate. This work quantifies allowable NaCl retentions for LSRRO as function of the retentate concentration for different pressures. For a pressure of 70 bar allowable NaCl retentions start to deviate from retentions reported for commercial RO membranes at NaCl weight fractions below 0.1. NaCl retentions for commercial nanofiltration (NF) membranes are much lower than allowed for LSRRO, leading to high NaCl losses in permeate. The required NaCl retention for LSRRO as function of the NaCl retentate concentration therefore falls in between those reported for commercial RO and NF membranes, implying that LSRRO membrane development is required. Based on the obtained relations required concentration factors for the concentration of purified seawater RO retentate and depleted brine from chlor/alkali production have been determined. The concentration factor for LSRRO for concentrating solutions with a NaCl concentration of either 0.07 kg·kg−1 or 0.18 kg·kg−1 to saturation at 120 bar pressure is 4 – 5 times higher than for a case where full retention would have been allowed. By optimizing the unit lay-out to minimize NaCl loss in permeate, approximately 1.2 m2·kg−1·h membrane area per saturated NaCl solution flow produced would be required for concentrating a 0.18 kg·kg−1 NaCl solution to saturation, assuming a permeance of 0.36 kg·m−2·h−1·bar−1.