Thin film composite membrane compaction in high-pressure reverse osmosis

Direct Quantification of Ion Partitioning and Diffusion Resistances in Reverse Osmosis Membranes via Electrochemical Impedance Spectroscopy

Polyamide (PA) reverse osmosis (RO) membranes are crucial for water desalination and purification, where salt ion transport is governed by partitioning and diffusion through the PA film. Despite extensive research, decoupling these two steps and quantifying their relative contributions remain challenging due to the lack of reliable characterization methods. Here, we develop a rapid, reproducible electrochemical impedance spectroscopy (EIS) protocol incorporating advanced electrical equivalent circuits to directly quantify partitioning and diffusion resistance. Its validity is verified through membrane filtration experiments and activation energy analysis. Our findings reveal that diffusion dominates ion transport resistance, with values 4.5 to 6.0 times higher than partitioning resistance across diverse monovalent cations. However, we discovered a critical concentration-dependent behavior where partitioning resistance becomes increasingly significant at lower electrolyte concentrations, eventually equaling diffusion resistance near 0.1 mM. We also uncovered that the anomalously low rejection of NH4+ of RO membranes stemmed from significantly reduced diffusion resistance, likely due to moderate hydrogen-bonding interactions with membrane pores or its tetrahedral geometry. This quantitative insight into transport resistance mechanisms establishes new design principles for next-generation RO membranes, enabling tailored strategies for applications ranging from high-salinity desalination to the removal of low-concentration micropollutants.

Role of Transmembrane Pressure and Water Flux in Reverse Osmosis Composite Membrane Compaction and Performance

This study explores the compaction behavior of thin-film composite reverse osmosis (TFC RO) membranes for different combinations of transmembrane pressure (TMP) and transmembrane water flux. Operating a crossflow system at constant feed pressure (60 bar) but different feed solution osmotic pressures enabled adjusting the TMP─the difference between hydraulic and osmotic pressure─and water flux. The extent of membrane compaction increases as TMP (and flux) increases. Both commercial and hand-cast TFC RO membranes showed substantial compaction at high TMP (up to 30% compaction at 50 bar TMP) compared to less than 10% at 10 bar TMP. Scanning electron microscope (SEM) images reveal a direct relationship between TMP and polysulfone (PSU) support layer compaction, while molecular dynamics (MD) simulations confirmed decreased porosity and reduced thickness in the polyamide (PA) active layer as TMP increases. Combined findings from wet-testing and MD simulations confirm a hydraulic pressure drop occurs across both the PA active layer and the meso-to-macro-porous support layer; higher TMP exacerbates compaction in both layers resulting in lower water permeability but higher water flux, observed salt rejection, and salt permeability. Transitioning from high TMP to low TMP or vice versa did not notably alter the extent of membrane compaction. This observation is attributed to the highly cross-linked PA active layer's ability to recover after pressure is released, whereas the compaction in the PSU support layer is largely irreversible. While TMP dictates the overall pressure gradient, our findings suggest that flux-induced frictional forces play a crucial role in compaction dynamics. Specifically, higher flux generates additional drag forces on the polymer matrix of both the PSU support layer and the PA selective layer, intensifying structural deformation. Overall, our findings offer critical insights into the mechanisms of membrane compaction, providing a foundation for optimizing RO membrane performance and advancing next-generation membrane technologies.

Nanofoamed Polyamide Membranes: Mechanisms, Developments, and Environmental Implications

Thin film composite (TFC) polyamide membranes have been widely applied for environmental applications, such as desalination and water reuse. The separation performance of TFC polyamide membranes strongly depends on their nanovoid-containing roughness morphology. These nanovoids not only influence the effective filtration area of the polyamide film but also regulate the water transport pathways through the film. Although there have been ongoing debates on the formation mechanisms of nanovoids, a nanofoaming theory─stipulating the shaping of polyamide roughness morphology by nanobubbles of degassed CO2 and the vapor of volatile solvents─has gained much attention in recent years. In this review, we provide a comprehensive summary of the nanofoaming mechanism, including related fundamental principles and strategies to tailor nanovoid formation for improved membrane separation performance. The effects of nanovoids on the fouling behaviors of TFC membranes are also discussed. In addition, numerical models on the role of nanovoids in regulating the water transport pathways toward improved water permeance and antifouling ability are highlighted. The comprehensive summary on the nanofoaming mechanism in this review provides insightful guidelines for the future design and optimization of TFC polyamide membranes toward various environmental applications.

Compaction of Pressure-Driven Water Treatment Membranes: Real-Time Quantification and Analysis

Water treatment membranes play crucial roles in applications such as desalination, wastewater treatment, and potable water reuse. In a prior study, we introduced a novel method, combining electrical impedance spectroscopy with dynamic mechanical analysis, to quantify single-layer homogeneous membrane compaction up to 12.5 psi. Now we extend the method's capabilities to quantify real-time compaction of multilayer heterogeneous nanofiltration and reverse osmosis (RO) membranes up to 330 psi. Our findings demonstrate that membrane compaction does not solely occur in the support/backing layer. The air pockets between the polysulfone support and the polyester backing layers, which were not discussed previously, account for up to 18% and 14% of total membrane compaction for the nanofiltration and RO membranes. For the nanofiltration membrane, the majority of compaction (up to 45%) occurs in the void spaces of the backing layer, while for the RO membrane, the majority of compaction (up to 40%) occurs in the solid material of the backing layer. We also confirm, with experimental results, the importance of using compressive testing instead of tensile testing to accurately characterize compaction. Membrane fatigue is characterized by experimental trends including: increasing irrevocable compaction, increasing creep/instantaneous compaction ratios, and increasing strains in hysteresis experiments.

Thin-Film Composite Membrane Compaction: Exploring the Interplay among Support Compressive Modulus, Structural Characteristics, and Overall Transport Efficiency

Water scarcity has driven the demand for water production from unconventional sources and the reuse of industrial wastewater. Pressure-driven membranes, notably thin-film composite (TFC) membranes, stand as energy-efficient alternatives to the water scarcity challenge and various wastewater treatments. While pressure drives solvent movement, it concurrently triggers membrane compaction and flux deterioration. This necessitates a profound comprehension of the intricate interplay among compressive modulus, structural properties, and transport efficacy amid the compaction process. In this study, we present an all-encompassing compaction model for TFC membranes, applying authentic structural and mechanical variables, achieved by coupling viscoelasticity with Monte Carlo flux calculations based on the resistance-in-series model. Through validation against experimental data for multiple commercial membranes, we evaluated the influence of diverse physical parameters. We find that support polymers with a higher compressive modulus (lower compliance), supports with higher densities of "finger-like" pores, and "sponge-like" pores with optimum void fractions will be preferred to mitigate compaction. More importantly, we uncover a trade-off correlation between steady-state permeability and the modulus for identical support polymers displaying varying porosities. This model holds the potential as a valuable guide in shaping the design and optimization for further TFC applications and extending its utility to biological scaffolds and hydrogels with thin-film coatings in tissue engineering.

Modeling Framework for Cost Optimization of Process-Scale Desalination Systems with Mineral Scaling and Precipitation

Cost-optimization models are powerful tools for evaluating emerging water treatment processes. However, to date, optimization models do not incorporate detailed chemical reaction phenomena, limiting the assessment of pretreatment and mineral scaling. Moreover, novel approaches for high-salinity and high-recovery desalination are typically proposed without direct quantification of pretreatment needs or mineral scaling. This work addresses a critical gap in the literature by presenting a modeling framework that includes complex water chemistry predictions with process-scale optimization. We use this approach to conduct a technoeconomic assessment on a conceptual high-recovery treatment train that includes chemical pretreatment (i.e., soda ash softening and recarbonation) and membrane-based desalination (i.e., standard and high-pressure reverse osmosis). We demonstrate how to develop and integrate accurate multidimensional surrogate models for predicting precipitation, pH, and mineral scaling tendencies. Our findings show that cost-optimal results balance the costs of pretreatment with reverse osmosis system design. Optimizing across a range of water recoveries (i.e., 50-90%) reveals multiple cost-optimal schemas that vary the chemical dosing in pretreatment and the design and operation of reverse osmosis. Our results reveal that pretreatment costs can be more than double the cost of the primary desalination process at high recoveries due to the extensive pretreatment required to control scaling. This work emphasizes the importance of and provides a framework for including chemistry and mineral scaling predictions in the evaluation of emerging technologies in high-recovery desalination.

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 (0-10 mM). 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.

2D Covalent Organic Framework Membranes for Liquid-Phase Molecular Separations: State of the Field, Common Pitfalls, and Future Opportunities

2D covalent organic frameworks (2D COFs) are attractive candidates for next-generation membranes due to their robust linkages and uniform, tunable pores. Many publications have claimed to achieve selective molecular transport through COF pores, but reported performance metrics for similar networks vary dramatically, and in several cases the reported experiments are inadequate to support such conclusions. These issues require a reevaluation of the literature. Published examples of 2D COF membranes for liquid-phase separations can be broadly divided into two categories, each with common performance characteristics: polycrystalline COF films (most >1 µm thick) and weakly crystalline or amorphous films (most <500 nm thick). Neither category has demonstrated consistent relationships between the designed COF pore structure and separation performance, suggesting that these imperfect materials do not sieve molecules through uniform pores. In this perspective, rigorous practices for evaluating COF membrane structures and separation performance are described, which will facilitate their development toward molecularly precise membranes capable of performing previously unrealized chemical separations. In the absence of this more rigorous standard of proof, reports of COF-based membranes should be treated with skepticism. As methods to control 2D polymerization improve, precise 2D polymer membranes may exhibit exquisite and energy efficient performance relevant for contemporary separation challenges.

Integrating reverse osmosis and forward osmosis (RO-FO) for printing and dyeing wastewater treatment: impact of FO on water recovery

Reverse osmosis (RO) alone has low water recovery efficiency because of membrane fouling and limited operating pressure. In this study, a combined reverse osmosis-forward osmosis (RO-FO) process was used for the first time to improve the water recovery efficiency of secondary effluent in printing and dyeing wastewater. The effects of operating pressure and pH on water recovery and removal efficiency of RO-FO were investigated. The results showed that the optimum conditions were an operating pressure of 1.5 MPa and a feed solution pH of 9.0. Under optimal operating conditions, most of the organic and inorganic substances in the wastewater can be removed, and the rejection of total organic carbon (TOC), Sb, Ca, and K were 98.7, 99.3, 97.0, and 92.7%, respectively. Fluorescence excitation-emission matrices coupled with parallel factor (EEM-PARAFAC) analysis indicated that two components (tryptophan and tyrosine) in the influent were effectively rejected by the hybrid process. The maximum water recovery (Rw, max) could reach 95%, which was higher than the current single RO process (75%). This research provided a feasible strategy to effectively recover water from printing and dyeing wastewater.

A review of the development in shale oil and gas wastewater desalination

The development of the shale oil and gas extraction industry has heightened concerns about shale oil and gas wastewater (SOGW). This review comprehensively summarizes, analyzes, and evaluates multiple issues in SOGW desalination. The detailed analysis of SOGW water quality and various disposal strategies with different water quality standards reveals the water quality characteristics and disposal status of SOGW, clarifying the necessity of desalination for the rational management of SOGW. Subsequently, potential and implemented technologies for SOGW desalination are reviewed, mainly including membrane-based, thermal-based, and adsorption-based desalination technologies, as well as bioelectrochemical desalination systems, and the research progress of these technologies in desalinating SOGW are highlighted. In addition, various pretreatment methods for SOGW desalination are comprehensively reviewed, and the synergistic effects on SOGW desalination that can be achieved by combining different desalination technologies are summarized. Renewable energy sources and waste heat are also discussed, which can be used to replace traditional fossil energy to drive SOGW desalination and reduce the negative impact of shale oil and gas exploitation on the environment. Moreover, real project cases for SOGW desalination are presented, and the full-scale or pilot-scale on-site treatment devices for SOGW desalination are summarized. In order to compare different desalination processes clearly, operational parameters and performance data of varying desalination processes, including feed salinity, water flux, salt removal rate, water recovery, energy consumption, and cost, are collected and analyzed, and the applicability of different desalination technologies in desalinating SOGW is qualitatively evaluated. Finally, the recovery of valuable inorganic resources in SOGW is discussed, which is a meaningful research direction for SOGW desalination. At present, the development of SOGW desalination has not reached a satisfactory level, and investing enough energy in SOGW desalination in the future is still necessary to achieve the optimal management of SOGW.

Precise Cation Separations with Composite Cation-Exchange Membranes: Role of Base Layer Properties

Separation of specific ions from water could enable recovery and reuse of essential metals and nutrients, but established membrane technologies lack the high-precision selectivity needed to facilitate a circular resource economy. In this work, we investigate whether the cation/cation selectivity of a composite cation-exchange membrane (CEM), or a thin polymer selective layer on top of a CEM, may be limited by the mass transfer resistance of the underlying CEM. In our analysis, we utilize a layer-by-layer technique to modify CEMs with a thin polymer selective layer (∼50 nm) that has previously shown high selectivity toward copper over similarly sized metals. While these composite membranes have a CuCl₂/MgCl₂ selectivity up to 33 times larger than unmodified CEMs in diffusion dialysis, our estimates suggest that eliminating resistance from the underlying CEM could further increase selectivity twofold. In contrast, the CEM base layer has a smaller effect on the selectivity of these composite membranes in electrodialysis, although these effects could become more pronounced for ultrathin or highly conductive selective layers. Our results highlight that base layer resistance prevents selectivity factors from being comparable across diffusion dialysis and electrodialysis, and CEMs with low resistance are necessary for providing highly precise separations with composite CEMs.

Water transport in reverse osmosis membranes is governed by pore flow, not a solution-diffusion mechanism

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate that the solution-friction model, in which transport is driven by a pressure gradient, can describe water and solvent transport in RO membranes.

Assessment of graphene-based polymers for sustainable wastewater treatment: Development of a soft computing approach

Since graphene possesses distinct electrical and material properties that could improve material performance, there is currently a growing demand for graphene-based electronics and applications. Numerous potential applications for graphene include lightweight and high-strength polymeric composite materials. Due to its structural qualities, which include low thickness and compact 2D dimensions, it has also been recognized as a promising nanomaterial for water-barrier applications. For barrier polymer applications, it is usually applied using two main strategies. The first is the application of graphene, graphene oxide (GO), and reduced graphene oxide (rGO) to polymeric substrates through transfer or coating. In the second method, fully exfoliated GO or rGO is integrated into the material. This study provides an overview of the most recent findings from research on the use of graphene in the context of water-barrier applications. The advantages and current limits of graphene-based composites are compared with those of other nanomaterials utilized for barrier purposes in order to emphasize difficult challenges for future study and prospective applications.

Seawater Reverse Osmosis Performance Decline Caused by Short-Term Elevated Feed Water Temperature

The shortage of fresh water resources has made the desalination of seawater a widely adopted technology. Seawater reverse osmosis (SWRO) is the most commonly used method for desalination. The SWRO process is energy-intensive, and most of the energy in SWRO is spent on pressurizing the seawater to overcome the osmotic barrier for producing fresh water. The pressure needed depends on the salinity of the seawater, its temperature, and the membrane surface properties. Membrane compaction occurs in SWRO due to hydraulic pressure application for long-term operations and operating temperature fluctuations due to seasonal seawater changes. This study investigates the effects of short-term feed water temperature increase on the SWRO process in a full-scale pilot with pretreatment and a SWRO installation consisting of a pressure vessel which contains seven industrial-scale 8" diameter spiral wound membrane elements. A SWRO feed water temperature of 40 °C, even for a short period of 7 days, caused a permanent performance decline illustrated by a strong specific energy consumption increase of 7.5%. This study highlights the need for membrane manufacturer data that account for the water temperature effect on membrane performance over a broad temperature range. There is a need to develop new membranes that are more tolerant to temperature fluctuations.

A two-phase model that unifies and extends the classical models of membrane transport

Two models describe solvent transport through swollen, nonporous membranes. The pore-flow model, based on fluid mechanics, works for porous membranes, whereas the solution-diffusion model invokes molecular diffusion to treat nonporous membranes. Both approaches make valid arguments for swollen polymer membranes, but they disagree in their predictions of intramembrane pressure and concentration profiles. Using a fluid-solid model that treats the solvent and membrane matrix as separate phases, we show both classical models to be valid, to represent complementary approaches to the same phenomenon, and to make identical predictions. The fluid-solid model clarifies recent reverse osmosis measurements; provides a predictive and mechanistic basis for empirical high-pressure limiting flux phenomena, in quantitative agreement with classic measurements; and gives a framework to treat nonporous but mechanically heterogeneous membrane materials.

A plantar wearable pressure sensor based on hybrid lead zirconate-titanate/microfibrillated cellulose piezoelectric composite films for human health monitoring

Flexible and wearable electronic sensors hold great promise for improving the quality of life, especially in the field of healthcare monitoring, owing to their low cost, flexibility, high electromechanical coupling performance, high sensitivity, and biocompatibility. To achieve high piezoelectric performance similar to that of rigid materials while satisfying the flexible requirements for wearable sensors, we propose novel hybrid films based on lead zirconate titanate powder and microfibrillated cellulose (PZT/MFC) for plantar pressure measurements. The flexible films made using the polarization process are tested. It was found that the maximum piezoelectric coefficient was 31 pC N^-1 and the maximum tensile force of the flexible films was 26 N. A wide range of bending angles between 15° and 180° proves the flexibility capability of the films. In addition, the charge density shows a proportional relation with the applied mechanical force, and it could sense stress of 1 kPa. Finally, plantar pressure sensors are arranged and packaged with a film array followed by connection with the detection module. Then, the pressure curves of each point on the plantar are obtained. Through analysis of the curve, several parameters of human body motions that are important in the rehabilitation of diabetic patients and the detection of sports injury can be performed, including stride frequency, length and speed. Overall, the proposed PZT/MFC wearable plantar pressure sensor has broad application prospects in the field of sports injury detection and medical rehabilitation training.

Confounding Effect of Wetting, Compaction, and Fouling in an Ultra-Low-Pressure Membrane Filtration: A Review

Ultra-low-pressure membrane (ULPM) filtration has emerged as a promising decentralized water and wastewater treatment method. It has been proven effective in long-term filtration under stable flux without requiring physical or chemical cleaning, despite operating at considerably lower flux. The use of ultra-low pressure, often simply by hydrostatic force (often called gravity-driven membrane (GDM) filtration), makes it fall into the uncharted territory of common pressure-driven membrane filtration. The applied polymeric membrane is sensitive to compaction, wetting, and fouling. This paper reviews recent studies on membrane compaction, wetting, and fouling. The scope of this review includes studies on those phenomena in the ULPM and how they affect the overall performance of the system. The performance of GDM systems for water and wastewater treatment is also evaluated. Finally, perspectives on the future research direction of ULPM filtration are also detailed.

A novel ceramic-based thin-film composite nanofiltration membrane with enhanced performance and regeneration potential

The rational design of a ceramic-based nanofiltration membrane remains a significant challenge due to its performance and fabrication cost. Herein, we report a high-performance ceramic-based thin-film composite (TFC) membrane fabricated via a typical interfacial polymerization on an interwoven net substrate assembled by titanium dioxide (TiO₂) nanowires. The chemical properties and morphologies were systematically investigated for ceramic substrates and their corresponding TFC membranes. Due to the significantly improved hydrophilicity of the TiO₂ framework, more reactive amine monomers were uniformly adsorbed on the modified surface of the ceramic substrate, yielding an ultrathin polyamide layer with less resistance. In addition, the smooth surface and decreased pore size of the TiO₂ framework contributed to forming a defect-free polyamide layer. As a result, the obtained ceramic-based TFC membrane evinced high permeance of 26.4 L m^-2 h^-1 bar^-1 and excellent salt rejection efficiency, leading to simultaneous improvements compared with the control TFC membrane without the TiO₂ framework. Notably, the potential regeneration ability of the ceramic-based TFC membrane could be achieved via facile low-temperature calcination and re-polymerization process due to the varied thermostability between the polyamide layer and the robust ceramic substrate. The operation of regeneration helped to prolong the lifetime and decrease the cost for the ceramic-based TFC membrane. This research provides a feasible protocol to fabricate sustainable ceramic-based nanofiltration membranes with enhanced performance for water treatment.

Effects of Benzalkonium Chloride Contents on Structures, Properties, and Ultrafiltration Performances of Chitosan-Based Nanocomposite Membranes

The effects of benzalkonium chloride (BKC) contents on the structure, properties, and ultrafiltration performance of chitosan-based nanocomposite membranes containing poly(ethylene glycol) and multi-walled carbon nanotube (chitosan/BKC/PEG/CNT) were examined. The membranes were prepared by a mixing solution method and phase inversion before being characterized with microscopic techniques, tensile tests, thermogravimetric analysis, water contact angle, and porosity measurements. The performance of the nanocomposite membranes in regard to permeability (flux) and permselectivity (rejection) was examined. The results show that the incorporation of BKC produced nanocomposite membranes with smaller pore structures and improved physico-chemical properties, such as an increase in porosity and surface roughness (Ra = 45.15 to 145.35 nm and Rq = 53.69 to 167.44 nm), an enhancement in the elongation at break from 45 to 109%, and an enhancement in the mechanical strength from 31.2 to 45.8 MPa. In contrast, a decrease in the membrane hydrophilicity (water contact angle increased from 56.3 to 82.8°) and a decrease in the average substructure pore size from 32.64 to 10.08 nm were observed. The membrane rejection performances toward Bovine Serum Albumin (BSA) increased with the BKC composition in both dead-end and cross-flow filtration processes. The chitosan/BKC/PEG/CNT nanocomposite membranes have great potential in wastewater treatments for minimizing biofouling without reducing the water purification performance.

Salt and Water Transport in Reverse Osmosis Membranes: Beyond the Solution-Diffusion Model

Understanding the salt-water separation mechanisms of reverse osmosis (RO) membranes is critical for the further development and optimization of RO technology. The solution-diffusion (SD) model is widely used to describe water and salt transport in RO, but it does not describe the intricate transport mechanisms of water molecules and ions through the membrane. In this study, we develop an ion transport model for RO, referred to as the solution-friction model, by rigorously considering the mechanisms of partitioning and the interactions among water, salt ions, and the membrane. Ion transport through the membrane is described by the extended Nernst-Planck equation, with the consideration of frictions between the species (i.e., ion, water, and membrane matrix). Water flow through the membrane is governed by the hydraulic pressure gradient and the friction between the water and membrane matrix as well as the friction between water and ions. The model is validated using experimental measurements of salt rejection and permeate water flux in a lab-scale, cross-flow RO setup. We then investigate the effects of feed salt concentration and hydraulic pressure on salt permeability, demonstrating strong dependence of salt permeability on feed salt concentration and applied pressure, starkly disparate from the SD model. Lastly, we develop a framework to analyze the pressure drop distribution across the membrane, demonstrating that cross-membrane transport dominates the overall pressure drop in RO, in marked contrast to the SD model that assumes no pressure drop across the membrane.

Effective Parameters on Fabrication and Modification of Braid Hollow Fiber Membranes: A Review

Hollow fiber membranes (HFMs) possess desired properties such as high surface area, desirable filtration efficiency, high packing density relative to other configurations. Nevertheless, they are often possible to break or damage during the high-pressure cleaning and aeration process. Recently, using the braid reinforcing as support is recommended to improve the mechanical strength of HFMs. The braid hollow fiber membrane (BHFM) is capable apply under higher pressure conditions. This review investigates the fabrication parameters and the methods for the improvement of BHFM performance.

High-impact innovations for high-salinity membrane desalination

Reducing the cost of high-salinity (>75 g/L total dissolved solids) brine concentration technology would unlock the potential for vast inland water supplies and promote the safe management of concentrated aqueous waste streams. Impactful innovation will target component performance improvements and cost reductions that yield the highest impact on system costs, but the desalination community lacks methods for quantitatively evaluating the value of innovation or the robustness of technology platforms relative to competing technologies. This work proposes a suite of methods built on process-based cost optimization models that explicitly address the complexities of membrane-separation processes, namely that these processes comprise dozens of nonlinearly interacting components and that innovation can occur in more than one component at a time. We begin by demonstrating the merit of performing simple parametric sensitivity analysis on component performance and cost to guide the selection of materials and manufacturing methods that reduce system costs. A more rigorous implementation of this approach relates improvements in component performance to increases in component costs, helping to further discern high-impact innovation trajectories. The most advanced implementation includes a stochastic simulation of the value of innovation that accounts for both the expected impact of a component innovation on reducing system costs and the potential for improvements in other components. Finally, we apply these methods to identify innovations with the highest probability of substantially reducing the levelized cost of water from emerging membrane processes for high-salinity brine treatment.

Clinical Autopsy of a Reverse Osmosis Membrane Module

The desalination of seawater using reverse osmosis membranes is an attractive solution to global freshwater scarcity. However, membrane performance is reduced by (bio)fouling. Membrane autopsies are essential for identifying the type of fouling material, and applying corrective measures to minimize membrane fouling. Information from full-scale membrane autopsies guiding improved plant operations is scant in the formal literature. In this case-study, a reverse osmosis membrane from a full-scale seawater desalination plant with a feed channel pressure drop increase of about 218% over the pressure vessel was autopsied. The simultaneous determination of microbial cells, ATP, and total organic carbon (TOC) abundances per membrane area allowed estimating the contributions of biofouling and organic fouling. The abundance of microbial cells determined by flow cytometry (up to 7 × 108 cells/cm2), and ATP (up to 21,000 pg/cm2) as well as TOC (up to 98 μg/cm2) were homogeneously distributed on the membrane. Inorganic fouling was also measured, and followed a similar coverage distribution to that of biofouling. Iron (∼150 μg/cm2, estimated by ICP-MS) was the main inorganic foulant. ATR-FTIR spectra supported that membrane fouling was both organic/biological and inorganic. High-resolution SEM-EDS imaging of cross-sectioned membranes allowed assessing the thickness of the fouling layer (up to 20 μm) and its elemental composition. Imaging results further supported the results of homogeneous fouling coverage. Moreover, imaging revealed both zones with and without compression of the polysulfone membrane layer, suggesting that the stress due to operating pressure was heterogeneous. The procedure for this membrane autopsy provided a reasonable overview of the diverse contributors of fouling and might be a starting point to building a consensus autopsy protocol. Next, it would be valuable to build a RO membrane autopsy database, which can be used as a guidance and diagnostic tool to improve the management and operation of RO desalination plants.

Composite Membranes with Nanofibrous Cross-Hatched Supports for Reverse Osmosis Desalination

A novel membrane structure composed of cross-hatched electrospun nanofibers is developed. We illustrate that this novel structure allows for much higher water permeability when used as a support for reverse osmosis thin-film composite membranes. Reinforcement and lamination of the aligned nanofibers generates mechanically robust structures that retain very high porosity and low tortuosity when applied to high pressure desalination operations. The cross-hatched nanofiber layers support the polyamide active layer firmly and reduce resistance to water flow due to the high porosity, low tortuosity, high mechanical strength, and minimal thickness of the structures. The nanofiber composite membrane gives a water flux significantly greater than when a traditional support layer is used, at 99 ± 5 m^-2 h^-1 with NaCl rejection of 98.7% at 15.5 bar.