Flux Decline Research Articles

Facile preparation of anti-biofouling nanofiltration membrane using safety inhibitor with high separation performances

Anti-biofouling nanofiltration (NF) membranes can reduce the damage of bacteria to membranes and are typically fabricated by grafting bactericidal chemicals on membrane surface, with potential environmental safety issues as well as health hazards. Vanillin (VA) is a natural non-toxic bacterial inhibitor that has been proven to inhibit a wide range of bacteria. In this study, VA was introduced as an aqueous phase additive to the selective layer by interfacial polymerization reaction thus preparing NF membranes with excellent anti-biofouling properties. The biofilm area on the optimized membrane surface formed by Bacillus subtilis and Escherichia coli were reduced by 66.0 % and 75.1 %, respectively. In addition, the hydrophilicity enhancement and the membrane surface roughness reduction were attributed to the VA introduction, which resulted in an improvement of the anti-adhesion properties of modified membranes, and flux recovery ratio of optimal membranes was 75.7 % while flux decline ratio was 50.4 %. The water permeability for mixed salt solution of membrane with the optimized VA concentration was 1.22 times higher than control membrane and rejection of NaSO4 was 99.0 %, meanwhile the separation factor of NaCl/Na2SO4 stabilized in the range of 27.4–28.0. This facile process provided an non-biotoxic method to prepare environmentally friendly and effective anti-biofouling membranes.

Late-time accretion in neutron star mergers: Implications for short gamma-ray bursts and kilonovae

ABSTRACT We study the long-term ($t\gg 10\rm \, s\!$ ) evolution of the accretion disc after a neutron star(NS)–NS or NS–black hole merger, taking into account the radioactive heating by r-process nuclei formed in the first few seconds. We find that the cumulative heating eventually exceeds the disc’s binding energy at $t\sim 10^2\mathrm{\, s}\, (\alpha /0.1)^{-1.8}(M/2.6\, \mathrm{M}_{\odot })^{1.8}$ after the merger, where α is the viscosity parameter and M is the mass of the remnant object. This causes the disc to evaporate rapidly and the jet power to shut off. We propose that this corresponds to the steep flux decline at the end of the extended emission (EE) or X-ray plateau seen in many short gamma-ray bursts (GRBs). The shallow flux evolution before the steep decline is consistent with a plausible scenario where the jet power scales linearly with the disc mass. We suggest that the jets from NS mergers have two components – a short-duration narrow one producing the prompt gamma-rays and a long-lasting wide component producing the EE. This leads to a prediction that ‘orphan EE’ (without short GRB) may be a promising electromagnetic counterpart for NS mergers observable by future wide-field X-ray surveys. The long-lived disc produces a slow ejecta component that can efficiently thermalize the β-electrons’ energy up to $t\sim 100\rm \, d$ and contributes $\sim \!10~{{\ \rm per\ cent}}$ of the kilonova’s bolometric luminosity at these late epochs. We predict that future ground-based and JWST near-IR spectroscopy of nearby ($\lesssim 100\rm \, Mpc\!$ ) NS mergers will detect narrow (Δv ∼ 0.01c) line features a few weeks after the merger.

Hybrid system coupling ozonation and nanofiltration with functionalized catalytic ceramic membrane for ibuprofen removal

The investigations on the removal of ibuprofen (IBU) in a hybrid system coupling ozonation and nanofiltration with functionalized catalytic ceramic membrane are presented. The gaseous ozone into feed water in concentration of 11 g Nm−3 was supplied. Positive influence of catalytic ozonation on ibuprofen decomposition was observed. The application of catalytic nanofiltration membrane led to the ibuprofen removal of 91% after the first 15 min from the beginning of the O3/NF process, while at the same time, for the pristine membrane, it was equal to 76%. The investigations revealed incomplete degradation of drug under pH 3 after 2 h, i.e., 89%. On the other hand, the addition of inorganic salts did not affect the catalytic ibuprofen removal efficiency. Under acidic pH, the highest permeate flux decline (26%) was noted, whereas no differences between permeate flux measured under natural and alkaline conditions were observed. During the treatment process, three IBU by-products were detected, which significantly affected the permeate toxicity; however, after 2 h of catalytic nanofiltration, the product of treatment process was found as non-toxic.

Mixed scaling deconstruction in vacuum membrane distillation for desulfurization wastewater treatment by a cascade strategy

Mineral scaling is one key obstacle to membrane distillation in hypersaline wastewater desalination, but the scaling or fouling mechanism is poorly understood. Addressing this challenge required revealing the foulants layer formation process. In this work, the scaling process was deconstructed with a cascade strategy by stepwise changing the composition of the synthetic desulfurization wastewater. The flux decline curves presented a 3-stage mode in vacuum membrane distillation (VMD). Heterogeneous nucleation of CaMg(CO3)2, CaF2, and CaCO3 was the main incipient scaling mechanism. Mg-Si complex was the leading foulant in 2nd-stage, during which the scaling mechanism shifted from surface to bulk crystallization. The flux decreased sharply for the formation of a thick and compacted scaling layer by the bricklaying of CaSO4 and Mg-Si-BSA complexes in the 3rd-stage. Bulk crystallization was identified as the key scaling mechanism in VMD for the high salinity and concentration multiple. The organic matter had an anti-scaling effect by changing the bulk crystallization. Humic acids (HA) and colloidal silica also contributed to incipient scaling for the high affinity to membrane, bovine serum albumin (BSA) acting as the cement of Mg-Si complexes. Mg altered the Si scaling from polymerization to Mg-Si complex formation, which significantly influence the mixed scaling mechanism. This work deconstructed the mixed scaling process and illuminated the role of main foulants, filling in the knowledge gap on the mixed scaling mechanism in VMD for hypersaline wastewater treatment and recovery.

The influence of multiple fouling and cleaning cycles upon the performance of polyethersulphone membrane filters during coffee extract decaffeination

Filtration processes have been applied widely in food processing industry over the recent decades and could offer a viable alternative to the use of solvents in the decaffeination process, comprising low operational costs, high selectivities and mild processing conditions. This paper reports the development and evaluation of the fouling occurring in multiple filtration cycles during the selective reduction of caffeine from coffee brews comparing the performance of (i) a commercially available synthetic tight ultrafiltration (TUF) polyethersulphone (PES) membrane (GP95PP – Alfa Laval) and (ii) a self-made mixed matrix (MMMs) PES membranes (PSCD) fabricated in-house. The effectiveness and performance of the PES MMMs was benchmarked against the commercial 2 kDa PES membranes showing promising results. A cross-flow rig was operated at transmembrane pressures of 2–9 bar and cross-flow velocities (CFV) of 0.04–0.1 m/s at 25 °C. The flux decline and recovery along with changes in the key component rejection and resistances are reported for multiple fouling and cleaning cycles. PSCD exhibited a higher permeate flux of ca. 10.5 L m-2h-1compared to the GR95PP membranes, which exhibited a permeate flux of 6.1 L m-2h-1 over the 29 h filtration period selected, at 9 bar and a CFV of 0.04 m/s. The rejection ratios of the GR95PP and PSCD membranes were monitored for three consecutive filtration cycles, and showed values ca. 30% and 35% for caffeine,> 90% and ∼90% for both polyphenols & proteins, & ∼ 80% for melanoidins, respectively. An effective cleaning protocol was reported, comprised of 0.5 wt% NaOH at 50 °C, exhibiting cleaning efficiencies (CE)> 99%. FT-IR data indicated the presence of key compounds residuals after membrane cleaning. Modification to the membrane surface occurs due to fouling, altering the hydrophobicity. Differences in filtration performance and cleanability between the two classes of membranes are identified and linked to variations in surface and structure properties.

Understanding Protein and Polysaccharide Fouling with Silicon Dioxide and Aluminum Oxide in Low-Pressure Membranes.

Humic, protein, and polysaccharide substances have been recognized as significant types of foulants in membrane systems. Despite the remarkable amount of research that has been performed on the interaction of these foulants, particularly humic and polysaccharide substances, with inorganic colloids in RO systems, little attention has been paid to the fouling and cleaning behavior of proteins with inorganic colloids in UF membranes. This research examined the fouling and cleaning behavior of bovine serum albumin (BSA) and sodium alginate (SA) with silicon dioxide (SiO2) and α-aluminum oxide (Al2O3) in individual and combined solutions during dead-end UF filtration. The results showed that the presence of SiO2 or Al2O3 in water alone did not cause significant fouling or a flux decline in the UF system. However, the combination of BSA and SA with inorganics was observed to have a synergistic effect on membrane fouling, in which the combined foulants caused higher irreversibility than individual foulants. Analysis of blocking laws demonstrated that the fouling mechanism shifted from cake filtration to complete pore blocking when the combined organics and inorganics were present in water, which resulted in higher BSA and SA fouling irreversibility. The results suggest that membrane backwash needs to be carefully designed and adjusted for better control of BSA and SA fouling with SiO2 and Al2O3.

Bioinspired N-Oxide-Based Zwitterionic Polymer Brushes for Robust Fouling-Resistant Surfaces

Fouling-resistant surfaces are needed for various environmental applications. Inspired by superhydrophilic N-oxide-based osmolytes in saltwater fish, we demonstrate the use of a trimethylamine N-oxide (TMAO) analogue for constructing fouling-resistant surfaces. The readily synthesized N-oxide monomer of methacrylamide is grafted to filtration membrane surfaces by surface-initiated atom transfer radical polymerization (SI-ATRP). Successful grafting of the amine N-oxide brush layer as confirmed by material characterization endows the surface with increased hydrophilicity, reduced charge, and decreased roughness. Notably, the introduction of the N-oxide layer does not compromise transport properties, i.e., water permeability and water-salt selectivity. Moreover, the modified membrane exhibits improved antifouling properties with a lower flux decline (32.1%) and greater fouling reversibility (18.55%) than the control sample (45.4% flux decline and 3.26% fouling reversibility). We further evaluate foulant-membrane interaction using surface plasmon resonance (SPR) to relate the reduced fouling tendency to the synergic effects of surface characteristic changes after amine N-oxide modification. Our results demonstrate the promise and potential of the N-oxide-based polymer brushes for the design of fouling resistance surfaces for a variety of emerging environmental applications.

Relating critical and limiting fluxes to metastable and long-term stable fluxes in colloidal membrane filtration through collision-attachment theory

In membrane technology for water/wastewater treatment, the concepts of critical flux (JC) and limiting flux (JL) suggest the existence of a threshold flux below which no fouling occurs. However, their important roles on stable flux duration have not been sufficiently understood. This work adopts a collision-attachment approach to clarify the relationship of JC, JL to metastable (i.e., short-term stable) and long-term stable fluxes based on their dependence on initial flux (J0), foulant-clean-membrane energy barrier (Ef-m), and foulant-fouled-membrane energy barrier (Ef-f). When J0 is below JL, water flux remains stable over a long time even for the case of J0 over JC, thanks to the strongly repulsive Ef-f. At J0 > JL and J0 > JC, the water flux is unstable at the beginning of filtration, and the flux ultimately decreases to JL as the long-term stable flux. Under the condition of JL < J0 ≤ JC, an initial metastable flux appears owing to the high Ef-m, with longer metastable period observed at lower J0 and for more hydrophilic/charged membrane or colloids. Nevertheless, rapid flux decline occurs subsequently due to the energy barrier shifting to weak Ef-f, and the water flux eventually degenerates to JL in long-term fouling duration. Our results provide significant guidelines for fouling control strategies with respect to membrane design, feedwater pretreatment, and operational optimization.

Evaluating membrane cleaning for organic fouling in direct contact membrane distillation

To find an effective cleaning method for organic fouling in membrane distillation (MD), this work focused on the fouling characteristics of humic acid (HA), sodium alginate (SA), and bovine serum albumin (BSA), and the cleaning efficiency of deionized (DI) water flushing, NaCl flushing, NaOH flushing and DI water backwashing. The results showed that when Ca2+ was absent from the feed solution, severe fouling was observed for HA and BSA, while SA led to slight fouling due to its hydrophilicity. The addition of Ca2+ alleviated the fouling for HA, but intensified the fouling for SA and BSA. There was a transition from cake filtration to complete blocking in membrane fouling mechanism for most MD tests. NaOH flushing resulted in irreversible fouling ratio of 48–60% after fouling by HA and BSA, while DI water flushing can recover 70% of flux decline caused by SA without Ca2+ present in the feed. For the three organics, only 8–15% of fouling was removed after DI water backwashing. The differences in the cleaning efficiency of the four cleaning methods were further verified by the positive correlation between irreversible fouling ratio and surface energy parameter, as well as Scanning electron microscopy and Fourier transform Infrared analysis of the membranes used in this study.

Multispecies biofilms on reverse osmosis membrane dictate the function and characteristics of the bacterial communities rather than their structure

The main reason for the deterioration of membrane operation during water purification processes is biofouling, which has therefore been extensively studied. Biofouling was shown to reduce membrane performance reflected by permeate flux decline, reduced selectivity, membrane biodegradation, and consequently, an increase in energy consumption. Studies of biofouling focused on the identification of the assembled microbial communities, the excretion of extracellular polymeric substances (EPS), and their combined role in reduced membrane performance and lifetime. However, the link between the structure and function of biofouling communities has not been elucidated to date. Here, we provide a novel insight, suggesting that bacterial functions rather than composition control biofouling traits on reverse osmosis (RO) membranes. We studied the potential activity of RO biofilms at metatranscriptome resolution, accompanied by the morphology and function of the biofouling layer over time, including microscopy and EPS composition, adhesion, and viscoelastic properties. To that end, we cultivated natural multispecies biofilms in RO membranes under treated wastewater flow and extracted RNA to study their taxonomies and gene expression profiles. Concomitantly, the biofilm structure was visualized using both scanning electron microscopy and laser scanning confocal microscopy. We also used quartz crystal microbalance with dissipation to characterize the affinity of EPS to membrane-mimetic sensors and evaluated the viscoelasticity of the Ex-Situ EPS layer formed on the sensor. Our results showed that different active bacterial taxa across five taxonomic classes were assembled on the RO membrane, while the composition shifted between 48 and 96 h. However, regardless of the composition, the maturation of the biofilm resulted in the expression of similar gene families tightly associated with the temporal kinetics of the EPS composition, adhesion, and viscoelasticity. Our findings highlight the temporal selection of specific microbial functions rather than composition, featuring the adhesion kinetics and viscoelastic properties of the RO biofilm.

Antifouling poly(phenylene sulfide) membrane with an amphiphilic surface for efficient oil/water separation

Poly(phenylene sulfide) (PPS)-based membranes hold great promise in the disposal of oily wastewater. However, their inherent lipophilic chemical structure and the pronounced concentration polarization effect will lead to serious fouling during operation. Herein, a PPS membrane with an amphiphilic surface (aPPS) is designed and fabricated by assembling phytic acid (PA) and perfluorohexanoic acid (PFHA) on the PPS membrane to obtain a superior antifouling property. PFHA as the hydrophobic domains are dispersed on a continuous hydrophilic PA layer at a molecular scale, which achieves the decrease of apparent surface energy with preserving the underwater superoleophobicity. Owing to the low intermolecular interaction and small contact area with oil, the amphiphilic surface exhibits nearly zero adhesion force towards oil, and accordingly, the aPPS membrane shows a low flux decline ratio (<15%), a high flux recovery ratio (∼100%) at high permeance (∼4100 L m−2 h−1 bar−1) and desired oil rejection (>98%). Moreover, the aPPS membrane exhibits stability under pH 3–11 and the temperature from ambient temperature to 80 °C, which demonstrates a promising potential for the practical separation of oil/water emulsions.

Application of Artificial Neural Networks for Modelling and Control of Flux Decline in Cross-Flow Whey Ultrafiltration

This study is part of the re-valorisation of the dairy waste industry through the use of membrane ultrafiltration (UF), in order to recover whey proteins and remove as much water as possible from the permeate. This study aimed to predict and control the permeate flux decline in cross-flow whey UF through a step procedure, and to compare different Artificial Neural Networks (ANNs), followed by a genetic algorithm (GA), as the optimization strategy. Models were developed in Matlab® Neural Network Toolbox. ANNs of one or two hidden layers were trained and simulated. A trial-and-error procedure identified the best network based on its performance values. The networks were trained through a selected set of experimental data obtained for lab-scale hollow-fibre membrane modules used to re-value scotta, the final waste of the dairy industry. The operating conditions considered as the input of the ANN were: operating time (top), sampling time (tsample), cross-flow velocity (CFV) and transmembrane pressure (TMP), while the output of the network was exclusively the normalized permeate flux (Jn). GA optimization was carried out to the following range of operating conditions to reach the best performances and to manage the fouling effect: 225 &lt; top &lt; 300 min, 8.33 &lt; tsample &lt; 15.9 min, 6.25 &lt; CFV &lt; 8.33 L/min, and TMP equal to 1.33 bar, otherwise it can be ignored. In fact, it has been noted that the networks with only three inputs, without TMP, predict and control Jn output better. Moreover, considering the normalized flux, it was possible to ignore some other important operating conditions, such as the membrane geometry. Consequently, the proposed general solution could also be used for other kinds of membrane applications. Finally, a hybrid approach among the ANN networks and a theoretical model was also used to better predict the resistance trend. It also returned more evident correspondence results than the ANN simulation alone, especially in the initial drop of Jn. The use of the theoretical part in the hybrid approach acts as a filter and returned the following order of significance of the operational input conditions on the resistance: top, tsample, CFV and TMP.

Removal of per- and polyfluoroalkyl substances by nanofiltration: Effect of molecular structure and coexisting natural organic matter

This study investigates the removal efficiency of anionic, cationic, and zwitterionic per- and polyfluoroalkyl substances (PFAS) by nanofiltration (NF) in the presence of three representative natural organic matter (NOM) types: bovine serum albumin (BSA), humic acid (HA), and sodium alginate (SA). In particular, effects of PFAS molecular structure and coexisting NOM on the transmission and adsorption efficiency of PFAS during NF treatment were analyzed. The results indicate that NOM types dominate membrane fouling behavior despite the coexistence of PFAS. SA exhibits the most significant fouling propensity, resulting in maximum water flux decline. NF effectively removed both ether and precursor PFAS. The effects of the three typical NOM on the membrane-passing behavior of PFAS were consistent for all PFAS investigated. Generally, PFAS transmission decreased in the order of SA-fouled > pristine > HA-fouled > BSA-fouled, indicating that the presence of HA and BSA enhanced PFAS removal while SA declined. Furthermore, reduced PFAS transmission was observed with increased perfluorocarbon chain length or molecular weight (MW), regardless of the presence or type of the NOM. The impacts of NOM on PFAS filtration diminished when the PFAS van der Waals radius was > 4.0 Å, MW > 500 Da, polarization > 20 Å, or LogKow > 3. These findings suggest that both steric repulsion and hydrophobic interactions, especially the former, play important roles in PFAS rejection by NF. This study provides insights into the specific applicability and performance of membrane-based processes for eliminating PFAS during drinking and wastewater treatments, and highlighting the importance of coexisting NOM.

Control the hydrophilic layer thickness of Janus membranes by manipulating membrane wetting in membrane distillation

Janus membranes with asymmetric wettability have attracted wide attentions for their robust anti-oil-wetting/fouling abilities in membrane distillation (MD). Compared to traditional surface modification approaches, in this study, we provided a new approach which manipulated surfactant-induced wetting to fabricate Janus membrane with a controllable thickness of the hydrophilic layer. The membranes with 10, 20, and 40 μm of wetted layers were obtained by stopping the wetting induced by 40 mg L−1 Triton X-100 (J = 25 L m−2 h−1) at about 15, 40, and 120 s, respectively. Then, the wetted layers were coated using polydopamine (PDA) to fabricate the Janus membranes. The resulting Janus membranes showed no significant change in porosities or pore size distributions compared with the virgin PVDF membrane. These Janus membranes exhibited low in-air water contact angles (< 50°), high underwater oil contact angles (> 145°), and low adhesion with oil droplets. Therefore, they all showed excellent oil-water separation performance with ∼100% rejection and stable flux. The Janus membranes showed no significant decline in flux, but a trade-off existed between the hydrophilic layer thicknesses and the vapor flux. Utilizing membranes with tunable hydrophilic layer thickness, we elucidated the underlying mechanism of such trade-off in mass transfer. Furthermore, the successful modification of membranes with different coatings and in-situ immobilization of silver nanoparticles indicated that this facile modification method is universal and can be further expanded for multifunctional membrane fabrication.

Polyethersulfone/polyamide thin-film nanocomposite membrane decorated by WS2-Cys-UiO-66-(CO2H)2 nanocomposites for forward osmosis

The fabrication of some new high-performance WS2-Cys-MOF-incorporated thin-film nanocomposite (TFN) forward osmosis (FO) membranes for use in desalination applications is reported. Herein, WS2-Cys-MOF nanosheets are synthesized through UiO-66-(CO2H)2 grafting on WS2-Cys and integrated into the PA selective layer of TFN membranes. The nanofillers content in the MPD aqueous solution was varied from 0 to 0.25 wt% to acquire the optimal loading and the best FO performance. The presence of polymer-matrix compatible WS2-Cys-MOF in the PA selective layer led to the formation of a defect-free smooth surface with additional channels within this layer, which can boost water flux and fouling resistance of TFN membranes. The TFN-0.1 membrane containing 0.1 wt% WS2-Cys-MOF results in the best FO desalination performance. Compared to the virgin TFC membrane, its water fluxes are almost doubled with a slight increase in the selectivity ∼18% evaluated under AL-FS orientation using 1 M NaCl as the draw solution and deionized water as the feed solution. Moreover, in the fouling test, this optimum TFN-0.1 membrane with a flux decline of 7.63% revealed a ∼1.4-fold enhancement in anti-fouling tendency. Hence, these high-performance WS2-Cys-MOF-incorporated TFN membranes show excellent potential for commercial utilization in the FO-desalination application.

Insights into the membrane biofouling behavior of planktonic anammox bacteria: Effect of solution pH and ionic strength

Understanding the effect of solution pH and ionic strength on membrane biofouling of anammox bacteria is essential for the widespread application of anammox MBRs. To provide an original elucidation, this study combined interfacial thermodynamics analysis and filtration experiments with an established planktonic anammox MBR to explore the biofouling behavior of anammox bacteria under varying solution pH and ionic strengths. Preliminary results showed that variation in solution pH and ionic strength has critical impacts on the thermodynamic properties of planktonic anammox bacteria and membrane surfaces. The further interfacial thermodynamics analysis and filtration experiments indicated that an increased pH and a decreased ionic strength could reduce membrane fouling by planktonic anammox bacteria. More specifically, a higher pH or lower ionic strength resulted in a stronger repulsive energy barrier due to the larger interaction distance covered by the dominant electrostatic double layer (EL) component compared to the Lewis acid-base (AB) and Lifshitz–van der Waals (LW) components, which corresponded to a reduction in the normalized flux (J/J0) decline and the accumulation of cake resistance (Rc) during the filtration process. Furthermore, the aforementioned effect mechanism was verified by a correlation analysis of the thermodynamic properties and filtration behavior. These findings have generalized significance for understanding the biofouling or aggregation behavior of anammox bacteria.

Evaluation of Different Reverse Osmosis Membranes for Textile Dyeing and Finishing Wastewater Reuse

Different commercial reverse osmosis (RO) membranes from Vontron and DuPont Filmtec were evaluated for textile dyeing and finishing wastewater (TDFW) reuse in China. All six tested RO membranes produced qualified permeate meeting TDFW reuse standards at a water recovery ratio (WRR) of 70% in single batch tests. The rapid decline of apparent specific flux at WRR over 50% was mainly ascribed to feed osmotic pressure increase caused by concentrating effects. Multiple batch tests using Vontron HOR and DuPont Filmtec BW RO membranes with comparable permeability and selectivity demonstrated the reproducibility and showed low fouling development. The occurrence of carbonate scaling on both RO membranes was identified by scanning electron microscopy and energy disperse spectroscopy. No obvious organic fouling was detected on both RO membranes by attenuated total reflectance Fourier transform infrared spectrometry. From the orthogonal tests, with an integrated RO membrane performance index (i.e., 25% rejection ratio of total organic carbon + 25% rejection ratio of conductivity + 50% flux ratio of final to initial) as a target, the optimal parameters were determined as WRR of 60%, cross-flow velocity (CFV) of 1.0 m/s, temperature (T) of 20 °C for both RO membranes, while trans-membrane pressures (TMP) of 2 and 4 MPa were optimal for Vontron HOR RO membrane and DuPont Filmtec BW RO membrane, respectively. Both RO membranes with the optimal parameters produced good permeate quality for TDFW reuse and kept a high flux ratio of final to initial, demonstrating the effectiveness of the orthogonal tests.

Removal of pharmaceutically active compounds from water sources using nanofiltration and reverse osmosis membranes: Comparison of removal efficiencies and in-depth analysis of rejection mechanisms

Trace organic compounds from effluent streams are not completely removed by conventional purification techniques and hence, contaminating groundwater sources. Herein, we report the removal efficiency and rejection mechanisms of three common pharmaceutically active compounds (PhACs); caffeine (CFN), omeprazole (OMZ), and sulfamethoxazole (SMX), using commercial nanofiltration (NF) and reverse osmosis (RO) membranes with different surface characteristics. The RO membranes showed near-complete removal of all PhACs with rejection rates >99%. On the other hand, retention capabilities for the NF membranes varied and were influenced by the characteristics of the PhACs, membranes, and the feed solution. In general, during long-term testing, the rejection did not show much variation and followed a trend compatible with the size exclusion (steric hindrance) mechanism. When a real matrix was used, the rejection of CFN by the more tight NF membranes, HL TFC and NFW decreased by ∼10%, whereas the removal of SMX by the loose NF membrane, XN45, increased by the same ratio. In short-term testing, the rejection of negatively charged SMX increased significantly (∼20–40%) at a higher pH of ∼8 and in the presence of salts. Fouling by the PhACs was more severe on the high-flux NF membranes, HL TFC and XN45, as witnessed by the significant change in Contact angle (CA) values (∼25–50°) as well as the flux decline (∼15%) during long-term testing. To summarize, the removal of PhACs by membranes is a complex phenomenon and depends upon a combination of several factors.

Internal Concentration Polarization in the Polyamide Active Layer of Thin-Film Composite Membranes.

A free-standing polyamide (PA) film is fabricated via in situ release from a thin-film composite (TFC) membrane achieved through the removal of the polysulfone support. The structure parameter S of the PA film is measured to be 24.2 ± 12.6 μm, which is about 87-fold of its film thickness. A significant decline in water flux of the PA film from an ideal forward osmosis membrane is observed. We find that the decline is predominantly influenced by the internal concentration polarization (ICP) of the PA film based on our experimental measurements and theoretical calculations. We propose that the asymmetric hollow structures of the PA layer with dense crusts and cavities may be the underlying cause of the occurrence of the ICP. More importantly, the structure parameter of the PA film can be reduced and its ICP effect can be mitigated by tuning its structures with fewer and shorter cavities. Our results for the first time provide experimental evidence to prove that the PA layer of the TFC membrane has the ICP effect, which could potentially provide fundamental insights into the influence of structural properties of PA on the membrane separation performance.

Microfiltration Performance of α-Alumina Membrane for Removal of Glycerol from Biodiesel

In biodiesel industries, the removal of glycerol from biodiesel is very important in the downstream process of the biodiesel production since the presence of glycerol in biodiesel causes diesel engine problems. Glycerol is commonly separated from biodiesel by extraction method using water, however, this method results in a vast amount of wastewater and requires a high energy consumption. In this work, a ceramic microfiltration membrane made of α-alumina was applied to remove glycerol from biodiesel. The microfiltration experiment was carried out using biodiesel containing various glycerol concentrations as the feed. For all investigated glycerol concentrations from 1000 ppm until 10,000 ppm in the feed, the membrane showed an excellent separation performance with rejection values of 91 to 99%. The profile of the permeate flux against the permeation time showed a flux decline because of the fouling phenomenon during the crossflow microfiltration experiment, and stable permeate fluxes were obtained after 2 h of permeation time. The result of this work showed that the separation process using the microfiltration membrane is a promising method to purify biodiesel instead of the conventional water washing method.