Loose nanofiltration membranes for selective rejection of natural organic matter and mineral salts in drinking water treatment

Adsorption of natural organic matter and divalent cations onto / inside loose nanofiltration membranes: Implications for drinking water treatment from rejection selectivity perspective

Loose nanofiltration (LNF) membranes hold great promise for the selective rejection of natural organic matter (NOM) while maintaining mineral salts to produce high-quality drinking water. Nevertheless, the rejection selectivity performance is not only determined by the inherent properties of membranes but also influenced by the feed water compositions. This study explored the inevitable adsorption of NOM and inorganic ions onto and inside membrane materials, which in turn altered the charge properties of LNF membranes, thereby affecting the rejection selectivity. Zeta potential measurements, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry technique were employed to characterize solute adsorption on the membrane surface and within the membrane pores. Filtration experiments using synthetic and natural waters were conducted to assess the contribution of electrostatic effects and evaluate the membrane rejection performance. Results revealed that LNF membrane surfaces during filtration were readily coated by NOM molecules, probably via hydrophobic interactions, which in turn adsorbed divalent cations that actually determined the net charge density on the membrane surface. Additionally, NOM adsorption within the membrane pores largely altered pore charge properties, particularly of the sulfonated polyethersulfone membranes (e.g. NTR7450), where deprotonated sulfonic groups otherwise contributed to a high charge density. These interactions among NOM, divalent cations and membrane materials greatly reduced charge density on the membrane surface and largely diminished charges in pores, leading to decreased rejection of both NOM and mineral salts, as well as the mitigation of co-ion competition effects. Nevertheless, the UA60 membrane, having a molecular weight cut-off of ∼1000 Da, rejected NOM by ∼70 % while maintaining ∼95 % bicarbonate and ∼65 % hardness ions in the treated water, demonstrating fairly good selectivity. These findings offer valuable insights for optimizing LNF membranes to improve the safety, chemical stability and palatability of treated drinking water.

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.

Nano and microplastics occurrence in wastewater treatment plants: A comprehensive understanding of microplastics fragmentation and their removal

Nano/microplastic (NP/MP) pollution is a growing concern for the water environment. Wastewater treatment plants (WWTPs) are considered the major recipients of MP before discharging into local waterbodies. MPs enter WWTPs mainly from synthetic fibers through washing activities and personal care products. To control and prevent NP/MP pollution, it is essential to have a comprehensive understanding of their characteristics, fragmentation mechanisms, and the effectiveness of the current treatment processes used in WWTPs for NP/MP removal. Therefore, the objectives of this study are to (i) understand the detailed mapping of NP/MP in the WWTP, (ii) understand the fragmentation mechanisms of MP into NP, and (iii) investigate the removal efficiency of NP/MP by existing processes in the WWTP. This study found that fiber is the dominant shape of MP, and polyethylene, polypropylene, polyethylene terephthalate, and polystyrene are the major polymer type of MP in wastewater samples. Crack propagation and mechanical breakdown of MP due to water shear forces induced by treatment facilities (e.g., pumping, mixing, and bubbling) could be the major causes for NP generation in the WWTP. Conventional wastewater treatment processes are ineffective for the complete removal of MPs. Although these processes are capable of removing ∼95% of MPs, they tend to accumulate in sludge. Thus, a significant number of MPs may still be released into the environment from WWTPs on a daily basis. Therefore, this study suggested that using DAF process in the primary treatment unit can be an effective strategy to control MP in the initial stage before it goes to the secondary and tertiary stage.

Chromium (III) and chromium (VI) removal and organic matter interaction with nanofiltration

Chromium (Cr) is a toxic inorganic contaminant for drinking water, in which the concentration has to be controlled for human health and safety. Cr retention was investigated with stirred cell experiments using sulphonated polyethersulfone nanofiltration (NF) membranes of different molecular weight cut-off (MWCO). Cr(III) and Cr(VI) retention follow the order of the MWCO of the studied NF membranes; HY70-720 Da > HY50-1000 Da > HY10-3000 Da with a pH dependency, especially for Cr(III). The importance of the charge exclusion was highlighted when Cr(OH)₄^- (for Cr(III)) and CrO₄^2- (for Cr(VI)) was the predominant species in the feed solution. In presence of organic matter, namely humic acid (HA), Cr(III) retention increased by 60 %, while no influence of HA was observed for Cr(VI). HA did not induce major modifications on the membrane surface charge for these membranes. Solute-solute interaction, in particular Cr(III)-HA complexation, was the responsible mechanism for the increase in Cr(III) retention. This was confirmed by asymmetric flow field-flow fractionation, coupled with inductively coupled plasma mass spectrometry (FFFF-ICP-MS) analysis. Cr(III)-HA complexation was significant at HA concentrations as low as 1 mgC/L. The chosen NF membranes were able to achieve the EU guideline (25 μg/L) for Cr in drinking water for a feed concentration of 250 μg/L.

Advanced drinking water production by 1 kDa hollow fiber nanofiltration - Biological activated carbon filtration (HFNF - BACF) enhances biological stability and reduces micropollutant levels compared with conventional surface water treatment

This study comprehensively investigates the quality of drinking water produced by novel advanced treatment encompassing 1 kDa hollow fiber nanofiltration (HFNF) - Biological Activated Carbon Filtration (BACF) from (reservoir) surface water, and compares this with drinking water after conventional 'CSF' pretreatment (coagulation - flocculation - sedimentation - media filtration - UV-disinfection) - BACF. The objective of HFNF - BACF treatment is to enhance the drinking water's quality in increased biological stability, reduced concentrations of organic micropollutants (OMP), and improvement in other chemical-physical parameters, whilst maintaining sufficient hardness to avoid subsequent remineralization. For this study a large suite of quality parameters was extensively monitored in pilot plants during nearly two years, enabling the incorporation of seasonal effects. HFNF - BACF treatment accomplished a similarly high level of biological stability as regrowth-free drinking waters (total organic carbon (DOC) 0.6 mg/L, assimilable organic carbon (AOC) 4 μg/L Ac-C and <1 μg/L biopolymer-C, total microbial growth potential (MGP) as BPC₁₄ 50 ng d/L and as BGP 170 × 10³ cells/mL), unlike the conventional treatment (1.9 mg/L, 10 μg/L, 9 μg/L, 130 ng d/L and 170 × 10³ cells/mL, respectively) where regrowth occurred in its distribution network. Average OMP removal, including perfluoro-alkyl substances (PFAS), by HFNF - BACF (54%) was higher than conventional treatment (37%). This improvement was mainly attributable to rejection in the HFNF membrane step, indicating that the DOC concentration after HFNF pretreatment was not yet sufficiently low to eliminate competitive adsorption and/or preloading in the BACF (confirmed by laboratory experiments). The advanced treatment also performed better in turbidity, particulates and most trace metals. Importantly, hardness retention by HFNF was only moderate, rendering remineralization unnecessary. Overall, this study demonstrates the superior performance in water quality of advanced HFNF - BACF treatment compared to conventional treatment.

Fouling of the Nanofiltration Membrane NF270 Used for Separation of Fermentation Broths: Impact of Feed Pretreatment Process

Recent findings regarding nanofiltration (NF) have led to indications that it can be successfully used for separation of various biological solutions. As a novelty, this paper is the first to investigate the impact of the feed pretreatment process on the NF membrane performance used for separation of 1,3-propanediol (1,3-PD) fermentation broths. For this purpose, prior to the NF process, the feed was purified by microfiltration (MF) and ultrafiltration (UF) processes. Subsequently, the long-term NF process was carried out with the use of a commercial, flat-sheet, thin-film, polyamide NF270 nanofiltration membrane. Thereinto, to determine the dominant fouling mechanism, Hermia’s model was used. With regards to the pretreatment processes performed, it has been determined that the MF membrane (0.14 µm) provided the reduction in the number of bacteria cells present in the permeate, while the UF membrane (450 Da) allowed obtaining the sterile permeate. Consequently, the NF permeate flux for the UF permeate was significantly higher. Analysis of the fouling mechanisms showed that during the separation of the MF permeate, formation of a cake layer on the NF membrane surface was dominant. In turn, with regards to the UF permeate, membrane blocking occurred in two separate phases involving standard blocking and then cake layer formation. Finally, a strategy of NF membrane cleaning with the use of sodium hydroxide (NaOH) solution has been proposed.

The Application of the Nanofiltration Membrane NF270 for Separation of Fermentation Broths

The potential for nanofiltration (NF) in removing both relatively low molecular weight (MW) organic species and charged solutes from complex media is noteworthy. The main aim of the current work was to improve understanding of the separation mechanisms of fermentation broths components in the NF process. For this purpose, the experimental investigations were performed using the commercial polyamide NF270 membrane. The feed solution was ultrafiltered 1,3-propanediol (1,3-PD) broths. The separation results were analyzed and discussed in light of the detailed characteristics of both the membrane and the broth components. It has been noted that the membrane ensured the complete 1,3-PD permeability and significant rejection of some feed components. A thorough analysis showed that the retention of carboxylic acids was based on both the Donnan effect and sieve mechanism, according to the following order: succinic acid > lactic acid > acetic acid > formic acid. Indeed, acids retention increased with increasing charged acids ions valency, Stokes radius (rS) as well as MW, and decreasing diffusion coefficient (D). In turn, for ions, the following orders retention was determined: SO42− = PO43− > Cl− and Ca2+ > Na+ > NH4+ ~ K+. It indicated that the ions retention increased with increasing ions charge density, hydrated radius (rH), and hydration energy (Eh). It showed that the separation of the ions was based on the Donnan exclusion, sieving effect, and dielectric exclusion.

Fabrication of Loose Nanofiltration Membranes with High Rejection Selectivity between Natural Organic Matter and Salts for Drinking Water Treatment

Loose nanofiltration (LNF) membranes with a molecular weight cut-off (MWCO) of about 1000 Da and high surface negative charge density have great application potential for drinking water treatment pursuing high rejection selectivity between natural organic matter (NOM) and mineral salts. This study was conducted to exploit the novel method coupling non-solvent induced phase separation (NIPS) and interfacial polymerization (IP) for the preparation of high-performance LNF membranes. A number of LNF membranes were synthesized by varying the polyethersulfone (PES) and piperazine (PIP) concentrations in the cast solution for the PES support layer preparation. Results showed that these two conditions could greatly affect the membrane water permeance, MWCO and surface charge. One LNF membrane, with a water permeance as high as 23.0 ± 1.8 L/m²/h/bar, when used for the filtration of conventional process-treated natural water, demonstrated a rejection of NOM higher than 70% and a low rejection of mineral salts at about 20%. Both the mineral salts/NOM selectivity and permselectivity were superior to the currently available LNF membranes as far as the authors know. This study demonstrated the great advantage of the NIPS-IP method for the fabrication of LNF membranes, particularly for the advanced treatment of drinking water.