Effects of feed solution characteristics and membrane fouling on the removal of THMs by UF/NF/RO membranes

Esterified Chlorine-Resistant Nanofiltration Membranes with Enhanced Removal of Disinfection Byproducts for Efficient Water Purification

The permeance-selectivity trade-off and chlorine sensitivity of conventional polyamide membranes limit further efficiency improvement and cost reduction of nanofiltration (NF) processes for drinking water treatment. To overcome these challenges, this study proposed a reconstruction-esterification strategy for the development of advanced NF membranes. Results showed that the combination of Na3PO4 solution post-treatment and polyol molecule grafting generated a thinner active layer with smaller and more uniform pores. More importantly, the critical role of alkaline post-treatment in reducing the residual amine groups of polyamide layers was revealed, which enhanced the chlorine resistance of membranes jointly with the effect of surface esterification. In comparison with the surface water purification performance of several commercial NF membranes, the obtained esterified membrane showed excellent selectivity between natural organic matter and salts, along with a reasonable water permeance. Moreover, the higher and stable removal capacity of the esterified membrane for disinfection byproducts and their precursors demonstrated its application advantage in the potential chlorination-NF-coupled process. The developed chlorine-resistant membrane and initially attempted NF filtration of chlorinated water in this study can help promote process innovation and highlight more benefits of NF technology for drinking water treatment.

Molecular dynamics simulations reveal efficient heavy metal ion removal by two-dimensional Cu-THQ metal-organic framework membrane

Two-dimensional (2D) metal-organic frameworks (MOFs) have been extensively utilized across various research areas. However, the application of 2D MOF-based membranes for the removal of heavy metal ions remains largely unexplored, despite their potential as suitable candidates due to their inherent porosity. In this study, we employed molecular dynamics (MD) simulations to investigate the capacity of a typical 2D MOF, Cu-THQ, for the separation of heavy metal ions, including Cd²⁺, Cu²⁺, Hg²⁺, and Pb²⁺. Our MD results demonstrate that single-layered Cu-THQ MOF membranes exhibit excellent performance in heavy metal ion removal, with nearly 100% ion rejection while also allowing high water permeability. Free energy calculations confirm that water transport through the Cu-THQ membrane is energetically more favorable compared to the transport of heavy metal ions. Further simulations of multilayered Cu-THQ membranes indicate that increasing the number of Cu-THQ MOF layers hinders water molecule transport, resulting in a reduction in water permeability due to a more widespread adsorption, that is primarily driven by electrostatic interactions within the membrane pores. Therefore, our simulations not only identify a promising MOF membrane candidate for efficient heavy metal ion removal but also suggest an optimal MOF construction scheme, which provide beneficial information for future applications in the sieving field.

Recycling scale inhibitor wastes into pH-responsive complexes to treat wastewater produced from spent lithium-ion battery disposal

A large amount of organophosphorus-containing wastewater is produced in spent lithium-ion battery disposal. Forward osmosis (FO) offers unique advantages in purifying this kind of wastewater if suitable draw solutes - the core of FO technology, are available. Herein we synthesize several pH-sensitive zinc complexes, namely ZnATMP-iNa (i = 0, 1, 2, 3, 4), from ZnSO4 and amino tris(methylene phosphonic acid) (ATMP) obtained from scale inhibitor wastes for organophosphorus-containing wastewater remediation. Among these ZnATMP-iNa, ZnATMP-3Na best meets the standards of an ideal draw solute. This makes ZnATMP-3Na outperform other reported draw solutes. 0.6 M ZnATMP-3Na produces a water flux of 12.7 LMH, 136 % higher than that of NaCl and a solute loss of 0.015 g/L, lower than that of NH4HCO3 (0.83 g/L). In organophosphorus-containing wastewater treatment, ZnATMP-3Na has higher water recovery efficiency (8.3 LMH) and sustainability than NaCl and NH4HCO3, and is sufficient to handle large quantities of wastewater. Remarkably, the pH-responsive property allows ZnATMP-3Na to be readily recovered through pH-control and reused in FO. The ionic property, expanded cage-like structure and easy-recycling make ZnATMP-3Na achieve sustainable FO separation and superior to other draw solutes. This study provides inspiration for draw solute design from wastes and extends FO application to organophosphorus-containing wastewater remediation.

The impact of PAC-loaded polymer membrane thickness on chloroform removal and comparison of solvent and thermal membrane regeneration methods

Powdered activated carbon (PAC) has better adsorption performance than granular activated carbon (GAC) and is widely used in water purification. In most cases, PAC is dosed into water directly, then precipitated as sludge, and landfilled. In this study, PAC was mixed with a polymer and dissolved in dimethylformamide (DMF) solvent to form a PAC-loaded membrane, which was then tested for chloroform removal. The chloroform adsorption capacity of the PAC membrane increased with increasing membrane thickness because of higher carbon loading. However, regardless of membrane thickness, the flux of the PAC membranes was similar since flux resistance predominantly occurred at the top dense polymer surface. This dense surface can be removed by sandpaper polishing, where the adsorption capacity of the polished PAC membranes was 20% higher than the unpolished membranes because of more even distribution of feed water on the polished surface. Removal of the dense surface via polishing increased the flux by 97% to 130%, exceeding the flux of typical household carbon block filters. Using DMF to regenerate the membrane recovered 48% to 66% of the initial adsorption capacity. Thermal regeneration of the exhausted PAC membrane at 250°C was more effective than DMF regeneration (both in terms of cost and performance), with 83% to 94% PAC membrane regeneration efficiency over four regeneration recycles. After four thermal regeneration cycles, flux increased by 300% and the membrane became brittle because of thermal aging of the polymer, indicating that a total of 6 h of regeneration time (equivalent to three cycles in this study) was the limit for effective PAC membrane performance. PRACTITIONER POINTS: Powdered activated carbon was immobilized on a membrane to remove chloroform from water. Thicker membranes increased adsorption capacity but did not impact flux. Flux and capacity increased using polishing to remove the dense polymer surface and more evenly distribute flow across the membrane. Thermal regeneration of the membrane at 250°C was effective for up to three cycles and outperformed solvent-based regeneration. PAC-loaded filters are relevant for applications such as household carbon block filtration.

Control of chlorination disinfection by-products in drinking water by combined nanofiltration process: A case study with trihalomethanes and haloacetic acids

Disinfection by-products (DBPs) are prevalent contaminants in drinking water and are primarily linked to issues regarding water quality. These contaminants have been associated with various adverse health effects. Among different treatment processes, nanofiltration (NF) has demonstrated superior performance in effectively reducing the levels of DBPs compared to conventional processes and ozone-biological activated carbon (O3-BAC) processes. In this experiment, we systematically investigated the performance of three advanced membrane filtration treatment schemes, namely "sand filter + nanofiltration" (SF + NF), "sand filter + ozone-biological activated carbon + nanofiltration" (SF + O3-BAC + NF), and "ultrafiltration + nanofiltration" (UF + NF), in terms of their ability to control disinfection by-product (DBP) formation in treated water, analyzed the source and fate of DBP precursors during chlorination, and elucidated the role of precursor molecular weight distribution during membrane filtration in relation to DBP formation potential (DBPFP). The results indicated that each treatment process reduced DBPFP, as measured by trihalomethane formation potential (THMFP) and haloacetic acid formation potential (HAAFP), with the SF + O3-BAC + NF process being the most effective (14.27 μg/L and 14.88 μg/L), followed by the SF + NF process (21.04 μg/L and 16.29 μg/L) and the UF + NF process (26.26 μg/L and 21.75 μg/L). Tyrosine, tryptophan, and soluble microbial products were identified as the major DBP precursors during chlorination, with their fluorescence intensity decreasing gradually as water treatment progressed. Additionally, while large molecular weight organics (60-100,000 KDa) played a minor role in DBPFP, small molecular weight organics (0.2-5 KDa) were highlighted as key contributors to DBPFP, and medium molecular weight organics (5-60 KDa) could adhere to the membrane surface and reduce DBPFP. Based on these findings, the combined NF process can be reasonably selected for controlling DBP formation, with potential long-term benefits for human health.

Pressure-driven membrane filtration technology for terminal control of organic DBPs: A review

Disinfection by-products (DBPs), a series of undesired secondary contaminants formed during the disinfection processes, deteriorate water quality, threaten human health and endanger ecological safety. Membrane-filtration technologies are commonly used in the advanced water treatment and have shown a promising performance for removing trace contaminants. In order to gain a clearer understanding of the behavior of DBPs in membrane-filtration processes, this work dedicated to: (1) comprehensively reviewed the retention efficiency of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) for DBPs. (2) summarized the mechanisms involved size exclusion, electrostatic repulsion and adsorption in the membrane retention of DBPs. (3) In conjunction with principal component analysis, discussed the influence of various factors (such as the characteristics of membrane and DBPs, feed solution composition and operating conditions) on the removal efficiency. In general, the characteristics of the membranes (salt rejection, molecular weight cut-off, zeta potential, etc.) and DBPs (molecular size, electrical property, hydrophobicity, polarity, etc.) fundamentally determine the membrane-filtration performance on retaining DBPs, and the actual operating environmental factors (such as solute concentration, coexisting ions/NOMs, pH and transmembrane pressure) exert a positive/negative impact on performance to some extent. Current researches indicate that NF and RO can be effective in removing DBPs, and looking forward, we recommend that multiple factors should be taken into account that optimize the existed membrane-filtration technologies, rationalize the selection of membrane products, and develop novel membrane materials targeting the removal of DBPs.

Experiments and machine learning-based modeling for haloacetic acids rejection by nanofiltration: Influence of solute properties and operating conditions

Because of potential risks to public health, the presence of haloacetic acids (HAAs) in drinking water is a major concern. Nanofiltration (NF) has shown potential for HAAs rejection, and several factors, namely, membrane properties, solute properties, and operating conditions, have been revealed key roles. However, knowledge of NF separation mechanism by quantifying these factors is limited. This study investigated and modeled NF performance on HAAs rejection. NF performance was experimentally investigated under various transmembrane pressure (TMP), cross-flow velocity (CV), temperature, pH, ionic strength (IS), and HAAs initial feed concentration (C_in). We used machine learning (ML) to understand the mechanism from the perspective of HAAs properties and operating conditions. Multiple linear regression (MLR), support vector machine (SVM), multsilayer perceptron (MLP), extreme gradient boosting (XGBoost), and random forest (RF) models were used. The MLP, XGBoost and RF models achieved significant performance with high R² (0.970, 0.973, and 0.980) and low RMSE (4.71, 4.41, and 3.84). These three models were analyzed using the Shapley Additive explanation (SHAP) to quantify relative contributions of HAAs properties and operating conditions. XGBoost-SHAP produced the most logical results and was the best-performing model for selecting optimal input variables combinations. The results showed that Stokes radius (r_s), logarithmic octanol-water partitioning coefficient (logK_ow), molecular weight (MW), pH, TMP, and temperature are key variables for interpreting NF process. The effects of HAAs properties were ranked as r_s > logK_ow > MW, suggesting significance of size exclusion and hydrophobic interaction. The impact of the operational conditions followed the order pH > TMP > temperature, illustrating that pH was the major influencing operating condition. This study demonstrated significant capacity of ML, which reduced amount of experimental work. In addition, the main operating conditions can be evaluated in terms of their contributions, making ML an efficient tool for risk management and process optimization.

Drinking water treatment and associated toxic byproducts: Concurrence and urgence

Reclaimed water is highly required for environmental sustainability and to meet sustainable development goals (SDGs). Chemical processes are frequently associated with highly hazardous and toxic by-products, like nitrosamines, trihalomethanes, haloaldehydes, haloketones, and haloacetic acids. In this context, we aim to summarize the formation of various commonly produced disinfection by-products (DBPs) during wastewater treatment and their treatment approaches. Owing to DBPs formation, we discussed permissible limits, concentrations in various water systems reported globally, and their consequences on humans. While most reviews focus on DBPs detection methods, this review discusses factors affecting DBPs formation and critically reviews various remediation approaches, such as adsorption, reverse osmosis, nano/micro-filtration, UV treatment, ozonation, and advanced oxidation process. However, research in the detection of hazardous DBPs and their removal is quite at an early and initial stage, and therefore, numerous advancements are required prior to scale-up at commercial level. DBPs abatement in wastewater treatment approach should be considered. This review provides the baseline for optimizing DBPs formation and advancements in the remediation process, efficiently reducing their production and providing safe, clean drinking water. Future studies should focus on a more efficient and rigorous understanding of DBPs properties and degradation of hazardous pollutants using low-cost techniques in wastewater treatment.

The role of medium molecular weight organics on reducing disinfection by-products and fouling prevention in nanofiltration

Nanofiltration (NF) is utilized in water treatment for controlling disinfection by-products formation potential (DBPFP) and disinfection by-products (DBPs). Attention regarding NF-based technology has been paid on membrane fouling of NF and the rejection efficiency of contaminants by NF membranes. Natural organic matter (NOM) presenting in surface waters is one main removal target in drinking water treatment by NF-based technology, and is thereby a contributor to the membrane fouling of NF. In application, pretreatments of other membrane filtration (e.g., microfiltration (MF) and ultrafiltration (UF)) has been taken prior to NF, resulting in the separation of NOM of specific molecular weight. Meanwhile, it is well known that NOM is composed of organic compounds of different molecular weights. However, the effect of NOM of specific molecular weight has been seldom investigated from the aspects of membrane fouling and the resulting DBPFP after membrane filtration. By using combinations of MF and UF (molecular weight cut-off of 100K or 20K) as pretreatment prior to NF, the NOM of various molecular weight on DBPFP and DBPs in the NF-treated water were investigated. The experiments were conducted with two real-world surface water samples and one tap water sample. It was found that medium molecular weight NOM, defined as NOM that passed UF100K but did not pass UF20K in this study, reduced fouling of the NF membrane. This is supported by the excitation and emission matrix (EEM) fluorescence spectra, size exclusion chromatography (SEC) and flux analysis. In addition, the medium molecular weight NOM also reduced the DBPFP in the NF treated water and eventually the DBPs by participating in forming a protective layer on the NF surface, blocking the transfer of small molecular weight NOM into the NF filtrate, thereby reducing the DBPFP of the NF filtrate since small molecular weight NOM was the major contributor to DBPFP in this study.

Modelling of transmembrane pressure using slot/pore blocking model, response surface and artificial intelligence approach

This work investigates the application of empirical, statistical and machine learning methods to appraise the prediction of transmembrane pressure (TMP) by oscillating slotted pore membrane for the treatment of two kinds of deformable oil drops. Here, we utilized the previous experimental runs with permeate flux, shear rate and filtration time as features, while TMP of crude oil and Tween-20 were two distinct targets. For 87 experimental runs, Response surface methodology (RSM) and Artificial Neural network (ANN) modelling were opted as statistical and machine learning tools, respectively, which were comprehensively compared with empirical slot-pore blocking model (SBM) considering accuracy and generalization. ANN with 10 neurons in the hidden layer could approximate the TMP of both oils better than RSM and SBM, which is reflected by computed performance metrics. Under the given conditions, almost similar analysis were predicted for TMP of both oils except changes in magnitude which were interpreted by (1) line plots, which showed that TMP of crude oil and Tween-20 were linearly related to flux rate and filtration time, and there was an inverse relationship between TMP and shear rate, (2) contour plots, which illustrated the strong interaction effect of flux rate and time on TMP, and (3)- sensitivity analysis, which revealed the influential sequence of variables on TMP as; flux rate > filtration time > shear rate, for both cases. The optimisation of the process showed that minimum TMP can be attained by maintaining higher shear rate and lower flux rate and time. Conclusively, the current findings indicate the utilization of ANN for the accurate assessment of TMP and can be helpful for the process designing and scale up.

Pilot Study on the Combination of Different Pre-Treatments with Nanofiltration for Efficiently Restraining Membrane Fouling While Providing High-Quality Drinking Water

Nanofiltration (NF) is a promising post-treatment technology for providing high-quality drinking water. However, membrane fouling remains a challenge to long-term NF in providing high-quality drinking water. Herein, we found that coupling pre-treatments (sand filtration (SF) and ozone-biological activated carbon (O₃-BAC)) and NF is a potent tactic against membrane fouling while achieving high-quality drinking water. The pilot results showed that using SF+O₃-BAC pre-treated water as the feed water resulted in a lower but a slowly rising transmembrane pressure (TMP) in NF post-treatment, whereas an opposite observation was found when using SF pre-treated water as the feed water. High-performance size-exclusion chromatography (HPSEC) and three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy determined that the O₃-BAC process changed the characteristic of dissolved organic matter (DOM), probably by removing the DOM of lower apparent molecular weight (LMW) and decreasing the biodegradability of water. Moreover, amino acids and tyrosine-like substances which were significantly related to medium and small molecule organics were found as the key foulants to membrane fouling. In addition, the accumulation of powdered activated carbon in O₃-BAC pre-treated water on the membrane surface could be the key reason protecting the NF membrane from fouling.

Rejection of chlorinated, brominated, and iodinated trihalomethanes by multi-stage reverse osmosis: Efficiency and mechanisms

Reverse osmosis (RO), a promising technology for removing inorganic salts and a wide range of trace organic pollutants, is widely used in water treatment industry. In this study, the rejection of chlorinated, brominated, and iodinated trihalomethanes (THMs) by a multi-stage RO system was investigated. The results showed that the multi-stage RO system is effective in rejecting THMs, and THMs with large size, high hydrophobicity and low polarity were highly rejected. In the first stage, high percentage of THMs was adsorbed on RO membrane, and the THM rejection was dominated by both hydrophobic adsorption and size exclusion. The contribution of hydrophobic adsorption to THM rejection decreased significantly along RO stages due to decreased feed concentration, but the enhancement of size exclusion still ensured high rejection efficiencies for most THMs, indicating a compensation effect between two rejection mechanisms. Finally, to further understand the rejection in the multi-RO system from a perspective of THM property, multiple linear regression models were built. The impact of n-octanol-water partition coefficient (Log K_ow) was slightly higher than that of stokes radius in the first stage, which was consistent with the rejection mechanism. But dipole moment played an increasingly important role in the second and third stage, weakening the impact of Log K_ow on THM rejection.