New insights into effect of alkaline cleaning on fouling behavior of polyamide nanofiltration membrane for wastewater treatment

Fouling characteristics and cleaning strategies of reverse osmosis membranes at different stages in a wastewater reclamation process

Membrane fouling is the main issue impeding reverse osmosis (RO), a crucial technology for the wastewater reuse process. This study systematically investigated the composition of membrane foulants at different positions on the RO element and the main causes contributing to membrane fouling using membrane autopsy in a pilot-scale RO process for wastewater reclamation. Membrane foulants were analyzed quantitatively and qualitatively using two fouled RO membrane elements from various vessels. The results revealed that the lead RO membrane suffers more severe membrane fouling, which mainly consists of organic matter (aromatic protein-like and soluble microbial product-like) and biofilm. The inorganic fouling, such as Ca, Fe, and Na, is the major foulant composition of the tail RO membrane. The physical cleaning process showed insufficient flux recovery in both fouled RO membranes. The alkaline cleaning was more effective for the lead-fouled membrane, while the acid cleaning was more effective for the tail-fouled membrane. This study offers control strategies for wastewater reclamation as well as a thorough comprehensive knowledge of the composition of membrane fouling and its major contributors at various locations along the RO membrane.

Effect of time delay after alkaline cleaning treatment on the properties of polyelectrolyte-coated end-of-life polyamide membranes

End-of-life (EoL) reverse osmosis (RO) membranes were regenerated by an extended alkaline cleaning treatment (ACT) followed by polyelectrolyte (PE) deposition (coating). The effect of time delay between the ACT and PE coating on the membranes' stability and filtration properties was investigated. The permeance of the membranes increased more than twofold compared to the value exhibited by the EoL membrane before the ACT. Additionally, the surface charge decreased from -45 mV to -99 mV at pH 7.7, due to the ACT. However, the ACT-induced effects were predominantly time-dependent and were partially reversed over time. When the membrane was coated with one layer of polydiallyldimethylammonium chloride (PDADMAC) immediately after the ACT, the resulting membrane had approximately 800 g/mol molecular weight cut-off (MWCO) value and 30 L/(m2 h bar) pressure-corrected flux (PCF). In comparison, if the membrane was stored in deionized (DI) water for five hours between the ACT and coating, the resulting membrane had again approximately 30 L/(m2 h bar) PCF but a much higher 2,900 g/mol MWCO. Furthermore, the promptly coated membranes showcased better replicability and stability during storage, in comparison to the samples that were kept in water prior to coating.

Nanofiltration Membranes for Efficient Lithium Extraction from Salt-Lake Brine: A Critical Review

The global transition to clean energy technologies has escalated the demand for lithium (Li), a critical component in rechargeable Li-ion batteries, highlighting the urgent need for efficient and sustainable Li+ extraction methods. Nanofiltration (NF)-based separations have emerged as a promising solution, offering selective separation capabilities that could advance resource extraction and recovery. However, an NF-based lithium extraction process differs significantly from conventional water treatment, necessitating a paradigm shift in membrane materials design, performance evaluation metrics, and process optimization. In this review, we first explore the state-of-the-art strategies for NF membrane modifications. Machine learning was employed to identify key parameters influencing Li+ extraction efficiency, enabling the rational design of high-performance membranes. We then delve into the evolution of performance evaluation metrics, transitioning from the traditional permeance-selectivity trade-off to a more relevant focus on Li+ purity and recovery balance. A system-scale analysis considering specific energy consumption, flux distribution uniformity, and system-scale Li+ recovery and purity is presented. The review also examines process integration and synergistic combinations of NF with emerging technologies, such as capacitive deionization. Techno-economic and lifecycle assessments are also discussed to provide insights into the economic viability and environmental sustainability of NF-based Li+ extraction. Finally, we highlight future research directions to bridge the gap between fundamental research and practical applications, aiming to accelerate the development of sustainable and cost-effective Li+ extraction methods.

A Case Study on Recycling Industrial Wastewater with Nanofiltration Membrane Separation Technology

As pressure on water resources intensifies and stringent regulations for groundwater and surface water are enacted, wastewater recycling has emerged as a key research objective for many enterprises. In this study, based on the actual wastewater discharged from Eternal Electronic (Suzhou, China) Co., Ltd., the performance differences in different membrane materials in treating this wastewater were analyzed and compared. The NF90 membrane was ultimately selected as the most suitable choice for treating this wastewater with optimal operating conditions of 150 psi, 25 °C, and 1 L/min, respectively. All the indices of wastewater treatment can satisfy the standard of circulating cooling water. In addition, this work is closely combined with the practical applications by using an HCl solution (pH = 3-4) to clean the fouled membranes, thus effectively solving the membrane fouling in the treatment process. This approach not only satisfies the environmental management requirements of enterprises but also provides valuable insights for similar wastewater treatment projects.

Optimized antifouling γ-Al2O3/α-Al2O3 nanofiltration composite membrane for lignin recovery from wastewater

This study focuses on the development of nanofiltration (NF) membranes with enhanced antifouling properties, high flux, and low molecular weight cut-off (MWCO) for the separation of lignin from paper mill wastewater. Using a sol-gel method by dip-coating, alumina hollow fiber membranes were fabricated with an interlayer to reduce surface roughness. The interlayer improved mechanical properties, effectively covering the surface irregularities and allowing for the subsequent application of a thinner functional layer. This approach significantly reduced surface roughness, from 112.6 nm to 62.9 nm, enhancing contamination resistance and lifetime. Characterization techniques, including X-ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM), and water contact angle measurements, confirmed the successful fabrication and enhanced properties of the membranes. The C2T6T3 membrane demonstrated the smallest roughness and the highest flux recovery rate (FRR) of 82.39% after cleaning with a 0.1 M NaOH solution. Performance evaluations showed that the developed membranes maintained high permeability (initial flux of 25.58 L·m⁻²·h⁻¹, decreasing to 14.06 L·m⁻²·h⁻¹ over time), achieved effective lignin rejection (consistently above 80%), and exhibited excellent long-term operational stability over 144 h of operation.

Progress towards Stable and High-Performance Polyelectrolyte Multilayer Nanofiltration Membranes for Future Wastewater Treatment Applications

The increasing demand for nanofiltration processes in drinking water treatment, industrial separation and wastewater treatment processes has highlighted several shortcomings of current state-of-the-art thin film composite (TFC NF) membranes, including limitations in chemical resistance, fouling resistance and selectivity. Polyelectrolyte multilayer (PEM) membranes provide a viable, industrially applicable alternative, providing significant improvements in these limitations. Laboratory experiments using artificial feedwaters have demonstrated selectivity an order of magnitude higher than polyamide NF, significantly higher fouling resistance and excellent chemical resistance (e.g., 200,000 ppmh chlorine resistance and stability over the 0-14 pH range). This review provides a brief overview of the various parameters that can be modified during the layer-by-layer procedure to determine and fine-tune the properties of the resulting NF membrane. The different parameters that can be adjusted during the layer-by-layer process are presented, which are used to optimize the properties of the resulting nanofiltration membrane. Substantial progress in PEM membrane development is presented, particularly selectivity improvements, of which the most promising route seems to be asymmetric PEM NF membranes, offering a breakthrough in active layer thickness and organic/salt selectivity: an average of 98% micropollutant rejection coupled with a NaCl rejection below 15%. Advantages for wastewater treatment are highlighted, including high selectivity, fouling resistance, chemical stability and a wide range of cleaning methods. Additionally, disadvantages of the current PEM NF membranes are also outlined; while these may impede their use in some industrial wastewater applications, they are largely not restrictive. The effect of realistic feeds (wastewaters and challenging surface waters) on PEM NF membrane performance is also presented: pilot studies conducted for up to 12 months show stable rejection values and no significant irreversible fouling. We close our review by identifying research areas where further studies are needed to facilitate the adoption of this notable technology.

Planting Anion Channels in a Negatively Charged Polyamide Layer for Highly Selective Nanofiltration Separation

A nanofiltration (NF) membrane with high salt permeation and high retention of small organics is appealing for the treatment of high-salinity organic wastewater. However, the conventional negatively charged NF membranes commonly show high retention of divalent anions (e.g., SO₄^2-), and the reported positively charged NF membranes normally suffer super low selectivity for small organics/Na₂SO₄ and high fouling potential. In this work, we propose a novel "etching-swelling-planting" strategy assisted by interfacial polymerization and mussel-inspired catecholamine chemistry to prepare a mix-charged NF membrane. By X-ray photoelectron spectroscopy depth profiling and pore size distribution analysis, it was found that such a strategy could not only deepen the positive charge distribution but also narrow the pore size. Molecular dynamics confirm that the planted polyethyleneimine chains play an important role to relay SO₄^2- ions to facilitate their transport across the membrane, thus reversing the retention of Na₂SO₄ and glucose (43 vs 71%). Meanwhile, due to the high surface hydrophilicity and smoothness as well as the preservation of abundant negatively charged groups (-OH and -COOH) inside the separation layer, the obtained membrane exhibited excellent antifouling performance, even for the coking wastewater. This study advances the importance of vertical charge distribution of NF membranes in separation selectivity and antifouling performance.

Incorporating Ag@RF core-shell nanomaterials into the thin film nanocomposite membrane to improve permeability and long-term antibacterial properties for nanofiltration

Ag@resorcinol-formaldehyde resin (Ag@RF) core-shell nanomaterials were prepared by Stöber method, and introduced into polyamide (PA) selective layer of thin-film nanocomposite (TFN) membranes through the interfacial polymerization (IP) process. Due to the abundant hydroxyl groups on the surface and suitable particle size, Ag@RF nanoparticles (Ag@RFs) could be uniformly dispersed in the piperazine aqueous solution and participate in the IP process to precisely regulate the microstructure of the PA selective layer. The resulting "crater structure" and irregular granular structure enlarged the permeable area and contributed to the surface hydrophilicity. For the nanofiltration application, the water flux of TFN membrane modified by Ag@RFs to Na₂SO₄ solution reached 150 L·m^-2·h^-1 which was 87.5% greater than TFC, and salt rejection was maintained. The antibacterial efficiency of the prepared TFN membrane on E. coli reached 99.6% in the antibacterial experiment. In addition, due to the special structure of Ag@RFs, the TFN membrane also showed an expected slow-release capability of Ag⁺, allowing for long-term anti-biofouling properties. This work demonstrates that Ag@RF core-shell nanoparticles with high compatibility of organic nanoparticles and antibacterial properties of Ag nanoparticles could be used as promising nanofillers for designing functional nanofiltration TFN membranes.

Fouling investigation of cartridge filter (CF) used as "firewall" in a nanofiltration drinking water plant

The cartridge filter (CF) as a "firewall" is crucial between pretreatment and nanofiltration (NF) units, but CF fouling with risk has received limited attention. The systematic autopsy for CFs (CF₁ and CF₂) applied in a NF drinking water plant was conducted to reveal CF fouling profile. Herein, scale blocks, irregular-shaped particles, and stacked-floc clusters were observed as the main morphologies of foulants. The major elements from foulants included Fe, Ca, Al, Mg, Na, P, and Si. The dissolved matters especially bioproducts resulted in the secondary pollution of permeated water. Biofouling was mainly caused by Proteobacteria phyla, and consisted of a large proportion of polysaccharides (11% and 25.1%), proteins (10.3% and 22.7%), lipids (21.7% and 22.4%), respectively. In addition, an obvious contrast was observed regarding the antifouling performance of CFs. The surface scaling degree of CF₁ with horizontal irregular loose-pleats was more serious than CF₂ with vertical regular compact-pleats, while the latter with high-density pleats appeared the higher fouling potential due to a greater capacity for organic foulants in the inner layers of "firewall" and better bio-diversity and bio-evenness of microbial communities. This study provides a deeper insight into CF fouling and contributes to the application of CFs.

Enhancing the Antifouling Ability of a Polyamide Nanofiltration Membrane by Narrowing the Pore Size Distribution via One-Step Multiple Interfacial Polymerization

Application of nanofiltration membranes in industries still has to contend with membrane fouling that causes a significant loss of separation performance. Herein, an innovative approach to design antifouling membranes with a narrowed pore size distribution by interfacial polymerization (IP) assisted by silane coupling agents is reported. An aqueous solution of piperazine anhydrous (PIP) and γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560) is employed to perform IP with an organic solution of trimesoyl chloride and tetraethyl orthosilicate (TEOS) on a porous support. In accordance with the results of molecular dynamics and dissipative particle dynamics simulations, the reactive additive KH560 accelerates the diffusion rate of PIP to enrich at the reaction boundary. Moreover, the hydrolysis/condensation of KH560 and TEOS at the aqueous/organic interface forms an interpenetrating network with the polyamide network, which regulates the separation layer structure. The characterization results indicate that the polyamide-silica membrane has a denser, thicker, and uniform separation layer. The mean pore size of the polyamide-silica membrane and the traditional polyamide membrane is 0.62 and 0.74 nm, respectively, and these correspond to the geometric standard deviation (namely, pore size distribution) of 1.39 and 1.97, respectively. It is proved that the narrower pore size distribution endows the polyamide-silica membrane with stronger antifouling performance (flux decay ratio decreases from 18.4 to 3.8%). Such a membrane also has impressive long-term antifouling stability during cane molasses decolorization at a high temperature (50 °C). The outcomes of this study not only provide a novel one-step multiple IP strategy to prepare antifouling nanofiltration membranes but also emphasize the importance of pore size distribution in fouling control for various industrial liquid separations.

Chemoenzymatic Cascade Reaction for Green Cleaning of Polyamide Nanofiltration Membrane

Chemical cleaning is indispensable for the sustainable operation of nanofiltration (NF) in wastewater treatment. However, the common chemical cleaning methods are plagued by low cleaning efficiency, high chemical consumption, and separation performance deterioration. In this work, a chemoenzymatic cascade reaction is proposed for pollutant degradation and polyamide NF membrane cleaning. Glucose oxidase (GOD) enzymatic reaction in this cascade system produces hydrogen peroxide (H₂O₂) and gluconic acid to trigger the oxidation of foulants by Fe₃O₄-catalyzed Fenton reaction. By virtue of the microenvironment (pH and H₂O₂ concentration) engineering and substrate enrichments, this chemoenzymatic cascade reaction (GOD-Fe₃O₄) exhibits a favorable degradation efficiency for bisphenol A and methyl blue (MB). Thanks to the strong oxidizing degradation, the water flux of the NF10 membrane fouled by MB is almost completely recovered (∼95.8%) after a 3-cycle fouling/cleaning experiment. Meanwhile, the chemoenzymatic cascade reaction improves the applicability of the Fenton reaction in polyamide NF membrane cleaning because it prevents the membrane from damaging by high concentration of H₂O₂ and inhibits the secondary fouling caused by ferric hydroxide precipitates. By immobilizing GOD on the aminated Fe₃O₄ nanoparticles, a reusable cleaning agent is prepared for highly efficient membrane cleaning. This chemoenzymatic cascade reaction without the addition of an acid/base/oxidant provides a promising candidate for sustainable and cost-effective cleaning for the polyamide NF membrane.

Progress in Research and Application of Nanofiltration (NF) Technology for Brackish Water Treatment

Brackish water is a potential fresh water resource with lower salt content than seawater. Desalination of brackish water is an important option to alleviate the prevalent water crisis around the world. As a membrane technology ranging between UF and RO, NF can achieve the partial desalination via size exclusion and charge exclusion. So, it has been widely concerned and applied in treatment of brackish water during the past several decades. Hereon, an overview of the progress in research on and application of NF technology for brackish water treatment is provided. On the basis of expounding the features of brackish water, the factors affecting NF efficiency, including the feed water characteristics, operating conditions and NF membrane properties, are analyzed. For the ubiquitous membrane fouling problem, three preventive fouling control strategies including feed water pretreatment, optimization of operating conditions and selection of anti-fouling membranes are summarized. In addition, membrane cleaning methods for restoring the fouled membrane are discussed. Furthermore, the combined utilization of NF with other membrane technologies is reviewed. Finally, future research prospects are proposed to deal with the current existing problems. Lessons gained from this review are expected to promote the sustainable development of brackish water treatment with NF technology.