Influence of formulated chemical cleaning reagents on the surface properties and separation efficiency of nanofiltrationmembranes

Removal, Adsorption, and Cleaning of Pharmaceutical on Polyamide RO and NF Membranes

Pharmaceuticals are present in various waters and can be almost completely rejected by membrane separation processes, i.e., nanofiltration (NF) and reverse osmosis (RO). Nevertheless, the adsorption of pharmaceuticals can decrease their rejection, so adsorption can be considered a very important removal mechanism. In order to increase the lifetime of the membranes, the adsorbed pharmaceuticals must be cleaned from the membrane. The used pharmaceutical (albendazole), the most common anthelmintic for threatening worms, has been shown to adsorb to the membrane (solute-membrane adsorption). In this paper, which is a novelty, commercially available cleaning reagents, NaOH/EDTA solution, and methanol (20%, 50%, and ≥99.6%) were used for pharmaceutical cleaning (desorption) of the NF/RO membranes used. The effectiveness of the cleaning was verified by Fourier-transform infrared spectra of the membranes. Of all the chemical cleaning reagents used, pure methanol was the only cleaning reagent that removed albendazole from the membranes.

Establishing a high-performance anti-fouling PEI-ZIF-PAA membrane with improved Lewis acid-base interactions and hydrophilicity

Membrane fouling and the trade-off between membrane permeability and selectivity restrict the potential applications of membrane filtration for water treatment. ZIF-8 was found having great permeability and antibiofouling performance, but with issue on particle aggregation makes it difficult to achieve high ZIFs loading and fabricate a defect-free molecular sieving membrane in previous research. In this study, we formed a scalable antibiofouling surface with improved permeability and fouling resistance on a PEI-ZIF-PAA membrane using a layer-by-layer assembly technique. The synergistic effects of being sandwiched between two different polyelectrolyte layers with opposite charges endowed the ZIF nanoparticles with improved stability and scalability for membrane modification. The PEI-ZIF-PAA membrane exhibited a satisfactory water flux of 120.78 LMH, which was 46.97% higher than that of the pristine PES membrane. The normalized water flux loss was serious in the absence of ZIF-8, and the flux increased with the ZIF-8 concentration. Antifouling tests suggested that the PEI-ZIF-PAA membrane possessed good antifouling performance due to the much higher surface hydrophilicity and positive Lewis acid-base interactions with foulants. The HA rejection increased with the ZIF-8 concentration and reached a maximum of 92.1% in the presence of 1.00% (w/v) ZIF-8. The membrane regeneration was tested under physical and chemical cleaning with flux recovery rates of about 85% and 95%. XDLVO analysis showed that the total interaction energy between HA and the PEI-ZIF-8-PAA membrane was 26.45 mJ/m², and the superior antifouling performance was mainly attributed to Lewis acid-base interactions. This study indicates that ZIF-8 nanocrystals are promising materials for fabricating novel membranes for sewage treatment.

Acid mine drainage and sewage impacted groundwater treatment by membrane distillation: Organic micropollutant and metal removal and membrane fouling

Groundwater is the dominant source of freshwater in many countries around the globe, and the deterioration in its quality by contaminants originating from anthropogenic sources raises serious concern. In this study, a scenario where groundwater is contaminated by acid mine drainage (AMD) from mining activities and/or sewage was envisaged, and the performance of a direct contact membrane distillation (DCMD) system was investigated comprehensively for different compositions of the AMD- and sewage-impacted groundwater. Regardless of the composition, MD membrane achieved 98-100% removal of metals and bulk organics, while the removal of the selected micropollutants ranged between 80 and 100%. Effective retention of contaminants by the MD led to their accumulation over time, which affected the hydraulic performance of the MD membrane by reducing the permeate flux by 29-76%. When persulfate (PS)-mediated oxidation process was integrated with the DCMD, degradation of bulk organics (50-71%) and micropollutants (50-100%) by PS reduced their accumulation. Characterisation of the fouling layer revealed the occurrence of membrane scaling that was mainly due to the deposition of iron oxide or oxyhydroxide precipitates. For an identical composition of the AMD- and sewage-impacted groundwater, flux decline was 10% less in PS-assisted DCMD as compared to that in the standalone DCMD. However, this did not prevent the formation of iron oxide scales on MD membrane during the operation of PS-assisted DCMD. This study demonstrates the long-term performance of a standalone and PS-assisted DCMD operated in continuous-flow mode to treat AMD- and sewage-impacted groundwater for the first time.

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

Membrane fouling is an intractable issue in wastewater treatment by nanofiltration (NF) membrane, and alkaline cleaning is the most effective approach to remove organic fouling on NF membrane. However, it was found that pore swelling of NF membrane induced by alkaline cleaning might reduce cleaning efficiency, and it is never quantified and its effect on membrane fouling behavior is still mysterious. In this work, membrane pore swelling effect (~9.7%, increment of effective pore size) induced by alkaline cleaning (pH 11) is confirmed and its effect on fouling behavior of the polyamide NF membrane is investigated based on experimental and modelling results. It is found that the alkali-induced pore swelling phenomenon would disappear after water filtration at neutral pH for 30 min, and if such cleaned membrane is faced by the small foulants during this pore shrinkage period, the concentration polarization and membrane fouling would be severer, and the subsequent alkaline cleaning is less effective because more foulants enter the enlarged pores and are tightly embedded in the membrane. Thus, the irreversible fouling of the NF membrane increases from 20% to 40% while its permeability recovery declines from 100% to 67% after six fouling/cleaning cycles. When an anionic surfactant sodium dodecyl sulfate (SDS, 10 mM) is added in the alkaline cleaning solution, the adsorption of SDS in/on the membrane can not only improve its hydrophilicity and negative charge, but also quickly eliminate the alkali-induced pore swelling effect and avoid the accumulation of foulants in the pores, thereby enhancing the antifouling performance of the NF membrane. Using the alkaline SDS cleaning, the irreversible fouling of the NF membrane maintains below 10% while its permeability recovery keeps above 100% in six continuous fouling/cleaning cycles.

Review on characteristics of biomaterial and nanomaterials based polymeric nanocomposite membranes for seawater treatment application

Membrane technology, especially nanofiltration (NF) has great attention to provide an imperative solution for water issues. The membrane is considered to be the heart in the separation plant. Understanding the membrane characteristics could allow predicting and optimizing the membrane performance namely flux, rejection and reduced fouling. The membrane development using biomaterials and nanomaterials provides a remarkable opportunity in the water application. This review focuses on the membrane characteristics of biomaterials and nanomaterials based nanofiltration. In this review, recent researches based on biomaterials and nanomaterials loaded membrane for salt rejection have been analyzed. Membrane fouling depends on the membrane characteristics and this review defined fouling as a ubiquitous bottleneck challenge that hampers the NF blooming applications. Fouling mitigation strategies via membrane modification using biomaterial (chitosan, curcumin and vanillin) and various other nanomaterials are critically reviewed. This review also highlights the membrane cleaning and focuses on concentrates disposal methods with zero liquid discharge system for resource recovery. Finally, the conclusion and future prospects of membrane technology are discussed. From this current review, it is apparent that the biomaterial and various other nanomaterials acquire exclusive properties that facilitate membrane advancement with improved capability for water treatment. Regardless of membrane material developments, still exist considerable difficulties in membrane commercialization. Thus, additional studies related to this field are needed to produce membranes with better performance for large‒scale applications.

Removal of Sulfadiazine by Polyamide Nanofiltration Membranes: Measurement, Modeling, and Mechanisms

In this study, a complete steric, electrostatic, and dielectric mass transfer model is applied to investigate the separation mechanism of typical antibiotic sulfadiazine by NF90, NF270, VNF-8040 and TMN20H-400 nanofiltration membranes. FTIR and XPS analysis clearly indicate that the membranes we used possess skin layers containing both amine and carboxylic acid groups that can be distributed in an inhomogeneous fashion, leading to a bipolar fixed charge distribution. We compare the theoretical and experimental rejection rate of the sulfadiazine as a function of the pressure difference across the nanopore for the four polyamide membranes of inhomogeneously charged nanopores. It is shown that the rejection rate of sulfadiazine obtained by the solute transport model has similar qualitative results with that of experiments and follows the sequence: RNF90>RVNF2−8040>RNF270>RTMN20H−400. The physical explanation can be attributed to the influence of the inhomogeneous charge distribution on the electric field that arises spontaneously so as to maintain the electroneutrality within the nanopore.

Stress Tolerance of Yeasts Dominating Reverse Osmosis Membranes for Whey Water Treatment

Filamentous yeast species belonging to the closely related Saprochaete clavata and Magnusiomyces spicifer were recently found to dominate biofilm communities on the retentate and permeate surface of Reverse Osmosis (RO) membranes used in a whey water treatment system after CIP (Cleaning-In-Place). Microscopy revealed that the two filamentous yeast species can cover extensive areas due to their large cell size and long hyphae formation. Representative strains from these species were here further characterized and displayed similar physiological and biochemical characteristics. Both strains tested were able to grow in twice RO-filtrated permeate water and metabolize the urea present. Little is known about the survival characteristics of these strains. Here, their tolerance toward heat (60, 70, and 80°C) and Ultraviolet light (UV-C) treatment at 255 nm using UV-LED was assessed as well as their ability to form biofilm and withstand cleaning associated stress. According to the heat tolerance experiments, the D60°C of S. clavata and M. spicifer is 16.37 min and 7.24 min, respectively, while a reduction of 3.5 to >4.5 log (CFU/mL) was ensured within 5 min at 70°C. UV-C light at a dose level 10 mJ/cm2 had little effect, while doses of 40 mJ/cm2 and upward ensured a ≥4log reduction in a static laboratory scale set-up. The biofilm forming potential of one filamentous yeast and one budding yeast, Sporopachydermia lactativora, both isolated from the same biofilm, was compared in assays employing flat-bottomed polystyrene microwells and peg lids, respectively. In these systems, employing both nutrient rich as well as nutrient poor media, only the filamentous yeast was able to create biofilm. However, on RO membrane coupons in static systems, both the budding yeast and a filamentous yeast were capable of forming single strain biofilms and when these coupons were exposed to different simulations of CIP treatments both the filamentous and budding yeast survived these. The dominance of these yeasts in some filter systems tested, their capacity to adhere and their tolerance toward relevant stresses as demonstrated here, suggest that these slow growing yeasts are well suited to initiate microbial biofouling on surfaces in low nutrient environments.

Influence of reverse osmosis membrane age on rejection of NDMA precursors and formation of NDMA in finished water after full advanced treatment for potable reuse

The influence of reverse osmosis (RO) membrane age on rejection of N-nitrosodimethylamine (NDMA) precursors was evaluated for a full-scale potable water reuse facility. The rejection of NDMA precursors decreased slightly with increased membrane age in most RO membrane products evaluated, but remained high overall (91% average). Chloride rejection was well-correlated with rejection of NDMA precursors. Precursor removal varied (75-98%) by membrane product, with certain membrane products maintaining better precursor rejection over time. NDMA rejection, however, did not decline significantly over time, while passage of other low molecular weight organics (LMWOs) increased with membrane age. Thus, rejection of NDMA was not highly correlated with rejection of these LMWOs, suggesting that NDMA is not a good surrogate for these compounds. Incomplete removal of NDMA precursors by RO and a UV/advanced oxidation process (UV/AOP) led to NDMA formation in the finished water and miles downstream in the transmission pipelines. An average NDMA formation rate of 0.7 ng/L/hr in the transmission lines was observed, despite typical removal of NDMA by UV/AOP to non-detect levels. The study indicates that RO membranes throughout their lifetime are not an absolute barrier to NDMA precursors, and that while older membranes continue to sufficiently remove NDMA precursors to a high degree, NDMA precursor rejection may decrease slightly as membranes age. Thus, the potential exists for NDMA to form from these precursors in purified, potable reuse water after treatment despite the effective removal of NDMA by UV/AOP.

Influence of Chemical Cleaning on Physicochemical Characteristics and Ion Rejection by Thin Film Composite Nanofiltration Membranes

The impact of membrane cleaning with NaOH and HCl on the characteristics and associated changes in ion rejection was investigated in this study. NaOH affected the zeta potential of membranes with a greater concentration of carboxylic groups so that it was negative across the entire pH range investigated. Exposure to NaOH led to swelling of the active layer after each cleaning, especially for poly(piperazineamide) membranes. A 23% increase in the effective pore radii for these membranes after NaOH cleaning for 18 h led to 25, 36, 53 and 62% decrease in the rejection of magnesium, calcium, sodium, and chloride ions, respectively. Sulfate rejection decreased only slightly even for poly(piperazineamide) membranes (i.e., 7%) despite an appreciable increase in pore radii, which can be explained by the impact of charge exclusion on ion rejection that was enhanced by the 16% reduction in zeta potential. On the other hand, cleaning with HCl had a negligible impact on the zeta potential and performance of all membranes evaluated in this study. The increase in permeability after chemical cleaning was in agreement with the decrease in rejection of inorganic ions and correlated well with the effective pore radii measured using the membrane potential technique. The importance of charge exclusion in the rejection of inorganic ions was highlighted by the observed differences in rejection and permeability values when testing membranes after NaOH cleaning.

Efficiencies and mechanisms of chemical cleaning agents for nanofiltration membranes used in produced wastewater desalination

A spiral-wound nanofiltration (NF) membrane module harvested from a full-scale produced wastewater desalination plant was examined and cleaned to explore appropriate chemical cleaning protocols. Foulant identification and cleaning efficiency and mechanisms were investigated. For total foulants, the organic components, including anionic polyacrylamide (APAM) and crude oil, accounted for a weight percentage of 86.3%, while the remaining foulants constituted the inorganic fraction, including Na, Mg, Ca, Ba, Al, Fe and Si. Short-term cleaning experiments were designed to identify effective reagents that could be used for further evaluations of their cleaning efficiencies in long-term cleaning. For citric acid and ethylenediaminetetraacetic acid tetrasodium (EDTA-4Na), the long-term cleaning efficiencies were relatively slight or even negative, while said values varied with different surfactants. Dodecyltrimethylammonium chloride (DTAC) achieved the greatest flux recovery; conversely, cetyltrimethylammonium chloride (CTAC) provided insignificant, even negative effects, on flux recovery, as well as salt rejection, of the fouled NF membranes. FTIR and zeta potential analyses of the fouled membranes indicated that all the tested surfactants were identically effective in removing the foulants from the membrane surface, but their cleaning efficiencies differed. Moreover, a strong correlation between the flux ratio (S_f) and concentration of surfactant in the permeate (C_ps) was observed. Among the tested chemical reagents, DTAC yielded the highest C_ps and the greatest flux recovery, with an S_f of 2.25. Considering this correlation and the characteristics of the fouled membranes and surfactants, it is proposed that DTAC molecules penetrated the membrane pores and removed the foulants that were attached to the pore walls.

Removal of fluoride and natural organic matter from natural tropical brackish waters by nanofiltration/reverse osmosis with varying water chemistry

In the context of decentralised brackish water treatment in development applications, the influence of water quality on membrane separation was investigated with real waters. High ionic strength (low net driving pressure) on fluoride (F) retention by nanofiltration (NF) and reverse osmosis (RO) was investigated over a wide pH range (2-12). Further, the influence of pH on the permeation of natural organic matter (NOM) fractions, in particular low molecular weight (LMW) neutrals, was elucidated. Natural and semi-natural waters from Tanzania with similar F concentrations of about 50 mg L^-1 but varying NOM and inorganic carbon (IC) concentration were filtered with an NF and RO, namely NF270 and BW30. F retention by NF270 for the feed water with highest ionic strength and IC concentration was lower and attributed to charge screening. This parameter further reduced at high pH due to co-ions (F^- and CO₃^2-) interactions and combined (synergistic) effect of high salt concentration and pH on F. High NOM resulted in higher membrane zeta potential in comparison with low NOM natural water. However, there was no significant difference in F retention due to the fact that F retention enhancement was annulled by deposit formation on the membrane. The fraction of NOM found in NF/RO permeates was dominated by LMW neutrals. This was attributed to their size and uncharged nature, while their higher concentration at low pH remains unexplained. More humic substances (HS) of higher molecularity and aromaticity permeated the NF270 when compared with BW30, which can be explained with the different membrane molecular weight cut off (MWCO). The study highlights the complexity of treating tropical natural waters with elevated F and NOM concentrations. In order to develop appropriate membrane systems that will achieve optimal F and NOM removal, the influence of water quality parameters such as pH, NOM content, ionic strength and IC concentration requires understanding. Seasonal variation of water quality as well as operational fluctuations, which occur in particular when such treatment processes are operated with renewable energy, will require such challenges to be addressed. Further, given the high permeability of low molecular weight (LMW) organics significant permeate side fouling may be expected.

Renewable energy powered membrane technology: Impact of pH and ionic strength on fluoride and natural organic matter removal

Real water pH and ionic strength vary greatly, which influences the performance of membrane processes such as nanofiltration (NF) and reverse osmosis (RO). Systematic variation of pH (3-12) and ionic strength (2-10g/L as total dissolved solids (TDS)) was undertaken with a real Tanzanian water to investigate how water quality affects retention mechanisms of fluoride (F) and natural organic matter (NOM). An autonomous solar powered NF/RO system driven by a solar array simulator was supplied with constant power from a generator. An open NF (NF270) and a brackish water RO (BW30) membrane were used. A surface water with a very high F (59.7mg/L) and NOM (110mgC/L) was used. Retention of F by NF270 was <20% at pH <6, increased to 40% at pH6, and 60-70% at pH7-12, indicating a dominance of charge repulsion while being ineffective in meeting the guideline of 1.5mg/L. Increase in ionic strength led to a significant decline in retention of F (from 70 to 50%) and electrical conductivity (from 60 to 10%) by NF270, presumably due to charge screening. In contrast, BW30 retained about 50% of F at pH3, >80% at pH4, and about 99% at pH >5, due to the smaller pore size and hence a more dominant size exclusion. In consequence, only little impact of ionic strength increase was observed for BW30. The concentration of NOM in permeates of both NF270 and BW30 were typically <2mg/L. This was not affected by pH or ionic strength due to the fact that the bulk of NOM was rejected by both membranes through size exclusion. The research is carried out in the context of providing safe drinking water for rural and remote communities where infrastructure is lacking, and water quality varies significantly. While other studies focus on energy fluctuations, this research emphasises on feed water quality that affects system performance and may alter due to a number of environmental factors.

Electro-Conductive Membranes for Permeation Enhancement and Fouling Mitigation: A Short Review

The research on electro-conductive membranes has expanded in recent years. These membranes have strong prospective as key components in next generation water treatment plants because they are engineered in order to enhance their performance in terms of separation, flux, fouling potential, and permselectivity. The present review summarizes recent developments in the preparation of electro-conductive membranes and the mechanisms of their response to external electric voltages in order to obtain an improvement in permeation and mitigation in the fouling growth. In particular, this paper deals with the properties of electro-conductive polymers and the preparation of electro-conductive polymer membranes with a focus on responsive membranes based on polyaniline, polypyrrole and carbon nanotubes. Then, some examples of electro-conductive membranes for permeation enhancement and fouling mitigation by electrostatic repulsion, hydrogen peroxide generation and electrochemical oxidation will be presented.

Optimization of a membrane cleaning strategy for advanced treatment of polymer flooding produced water by nanofiltration

An optimized cleaning strategy against NF membranes fouled by polymer flooding produced water has been proposed and is proven to be high-performing.