•In-situ monitoring techniques for the hollow fiber membrane fouling are reviewed.•In-situ monitoring approaches of local filtration properties are summarized.•The relationship between local filtration and fouling behaviors is discussed.•The application of in-situ techniques to commercial modules is a critical issue.Membrane fouling is the most serious challenge in the hollow fiber microfiltration (MF) and ultrafiltration (UF) processes. A number of in-situ monitoring techniques including optical and non-optical probes have been developed so that membrane fouling is better understood and controlled. This will help advance the membrane technology. In addition, the local filtration hydrodynamics wield a great influence on the membrane fouling formation and system operation stability. State-of-the-art in-situ monitoring techniques for membrane fouling and local filtration characteristics in hollow fiber MF/UF processes are critically reviewed. The principles and applications of these techniques are addressed in order to assess their strengths. This study demonstrated that the real-time observation techniques mainly focus on idealized laboratory apparatus and little on commercial membrane modules. Consequently, more attention should be paid to the development of simple and effective methods or integrated detecting technology so as to satisfy the real status of hollow fiber filtration processes and the optimization of membrane module. On the basis of this review, future analyses considering practical requirements are suggested as R&D priorities.

•PES/SPSf blend membrane was prepared via H2O-induced gelation phase separation.•9 wt% H2O in the PES/SPSf/DMAc casting system led to the gelation formation.•The macrovoids were totally suppressed at 9 wt% H2O in casting solution.•A critical approaching ratio could be used to predict the demixing behavior.•The membrane obtained from the system with 9 wt% H2O exhibited a high performance.Polyethersulphone (PES)/sulfonated polysulphone (SPSf) blend ultrafiltration membrane was prepared via non-solvent induced gelation phase separation (NIGPS) using H2O as the non-solvent additive in the casting solution with N,N-dimethylacetamide (DMAc) as the solvent. An ultrasonic technique was employed to monitor membrane formation so as to provide the quantitative information during the phase separation. The effect of H2O concentration in the PES/SPSf/DMAc casting system on the membrane structure and properties was investigated. Results indicated that the viscosity of the casting solution increased with the increase of H2O content from 3 wt% to 9 wt% owing to the strong interaction between H2O molecules and the –SO3- group of SPSf chains, finally leading to the gelation of the system. Simultaneously, the microporous structure of the resultant membranes changed gradually into sponge-like structure from macrovoids with the increase of H2O content in the casting solution. It was related to the changes of the phase separation behavior from instantaneous demixing to delay demixing. Specifically, the approaching ratio could be used to predict the behaviors of the instantaneous and delay demixings. Moreover, the pure water flux of PES/SPSf blend membrane obtained under the conditions of polymer concentration 18 wt%, PES: SPSf = 84:16 wt/wt and 9 wt% H2O in the casting solution was up to 858 L/m2h. The bovine serum albumin (BSA) rejection was 90.5%.Download high-res image (347KB)Download full-size image

A film material, polyvinyl alcohol-graft-benzo-15-crown-5 ether (PVA-g-B15C5) for lithium isotope separation by liquid–solid extraction was prepared from polyvinyl alcohol (PVA) and 4′-formoxylbenzo-15-crown-5 ether (FB15C5). The effect of immobilization amount of crown ether on film, the counter anion of lithium salt, extraction solvent and temperature on separation factor were explored in detail. The maximum separation factor 1.060 ± 0.002 was obtained by an isopropanol-LiI/PVA-g-B15C5 film system at 20 °C. The heavy isotope, 7Li was enriched in the film phase owning to a stronger bonding environments from the synergistic effect of B15C5 and hydrophilic PVA as well as the linking groups.

A novel polymer polysulfone (PSF)-graft-4′-aminobenzo-15-crown-5-ether (AB15C5) (PSF-g-AB15C5) for lithium isotope separation was prepared from PSF and AB15C5 as starting materials via nucleophilic substitution reaction. The chemical structure and properties of PSF-g-AB15C5 polymers were characterized by FT-IR, 1H NMR, XPS, and TGA. The polymers obtained were used for lithium isotope separation by solid–liquid extraction. The effects of the immobilization amount of crown ether grafting on PSF, the type of counteranion of lithium salt, and the kind of solvent on the single stage separation factor were explored. Results showed that the single stage separation factor was only 1.002 ± 0.002 for AB15C5 in the traditional liquid–liquid extraction system of H2O–LiCl/CHCl3–AB15C5, whereas the single stage separation factor increased from 1.003 ± 0.001 to 1.015 ± 0.002 with the increase of the immobilization amount of crown ether from 0.23 to 0.79 mmol g–1 on PSF-g-AB15C5 polymers in the solid–liquid extraction system of CH3OH–LiCl/PSF-g-AB15C5 polymers. The order of the single stage separation factor obtained in different lithium salts was LiI > LiBr > LiClO4 > LiCl. Further, the maximum value of single stage separation factor was 1.031 ± 0.002 for the extraction system of CH3NO2–LiCl/PSF-g-AB15C5 polymers. Moreover, 6Li and 7Li were enriched in the polymer phase and the solution phase, respectively.

An innovative auto-catalytic method was proposed to synthesize phytosterol esters from phytosterols and long-chain fatty acids without adding any catalyst and solvent. The effects of reaction temperature, molar ratio of oleic acid/phytosterols, reaction time and carbon-chain length of the fatty acids on the reaction were investigated. The results showed that the conversion, the yield and the selectivity of the reaction were increased with the increase in reaction temperature, molar ratio and a decrease in the length of carbon chains of fatty acids. The selectivity was decreased on prolonging the reaction time. A high conversion (99.1%) with a high yield (94.9%) and selectivity (95.8%) was achieved under a molar ratio of oleic acid/phytosterols of 3:1, a reaction temperature of 220 °C and reaction time of 4 h. The properties of the phytosterol oleic esters by autocatalysis conformed to the quality indices of phytosterol esters from the China Ministry of Health and were superior to the commercial product. The kinetics suggested that the reaction order was 2 and the reaction activation energy was 58.75 kJ mol−1. The auto-catalytic process, omitting the separation step for catalysts and solvents, could be considered as a promising process to synthesize phytosterol esters.

This study described the extension of ultrasonic time-domain reflectometry (UTDR) for monitoring the fouling profile in a submerged hollow fiber membrane module under different operation conditions including aeration rate, fiber length and operational flux. Five 10 MHz ultrasonic transducers employed were mounted along the tubular test module with a single hollow fiber membrane evenly. A polyethersulfone hollow fiber membrane with inside and outside diameter of 1.0 and 1.6 mm was employed to treat 5 g/L yeast suspension. The experimental results showed that the fouling could not be completely prevented under the operation of the sub-critical flux, and still deposited at the upper part of the submerged hollow fiber membrane. The progress of foulant deposition onto the membrane surface gradually migrated from top to bottom and reached the plateau finally. Further, the increase of aeration and curtailing fiber length could only slow down fouling and reduce deposition rate to some extent, but could not fully avoid the membrane fouling. Moreover, UTDR technique was successfully employed to measure the relationship between the operational flux and particle deposition on the membrane surface so as to obtain threshold flux, under which could obviously alleviate membrane fouling.Highlights► UTDR technique was used to monitor fouling distribution under sub-critical operation. ► The increase of aeration could not completely avoid membrane fouling. ► Curtailing fiber length is not an effective method to control membrane fouling. ► Critical flux represents relative rather than absolute value of fouling control.

Poly(styrene sulfonic acid) (PSSA)/Poly(vinyl alcohol) (PVA) blend membranes prepared by the solution casting were employed as heterogeneous acid catalysts for biodiesel production from acidic oil obtained from waste cooking oil (WCO). The membranes were annealed at different temperature in order to enhance their stability. The structure and properties of the membranes were investigated by means of Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), X-ray diffraction (XRD). It is found that the crosslinking structure among PVA and PSSA chains formed when the thermal treatment temperature was higher than 80 °C. The retention of PSSA in the blend membranes in the methanol/water solvent was markedly increased from 50% to 85% with the increase of the annealing temperature from room temperature (for the untreated membrane) to 150 °C due to the formation of the crosslinking structure. The results of esterification of acidic oil show that the conversion was slightly improve with the PVA content in the membrane at a fixed PSSA content. The thickness of the catalytic membrane had no significant effect on the conversion in the end. The membrane annealed at 120 °C exhibited the best catalytic performance among the membranes, with a stable conversion of 80% with the runs.

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.

Oil fouling during crossflow microfiltration of oily wastewater using a single hollow fiber membrane filtration module as outside-in conformation was analyzed experimentally by ultrasonic reflectometry and wavelet transform in real time. Three 10 MHz ultrasonic sensors mounted along the tubular test module were utilized to monitor the fouling profile of the hollow fiber membrane. Results showed that the instantaneously rapid flux decline at the onset of fouling was caused mainly by concentration polarization and the fast oil adsorption on the lower part of the hollow fiber near the inlet of the membrane module. Further, wavelet analysis of the ultrasonic spectra revealed that the amount of oil deposited on the lower part of the hollow fiber near the inlet of the membrane module was more than those on the other parts of the hollow fiber due to the inertial impaction of oil droplets and local shell-side hydrodynamic effects. Moreover, the oil diffusion behavior (the relaxation and disappearance of the fouling layer) was also visualized by ultrasonic reflectometry in real time after the microfiltration system was shut down. The flux decline data and SEM micrographs corroborated the ultrasonic measurements and wavelet analysis. Overall, this technique will provide a useful and quantitative approach to the on-line assessment of fouling remediation and cleaning strategies of hollow fiber membranes.

A functional electrocatalytic membrane reactor (ECMR) was performed for the electrocatalytic oxidation of 2,2,3,3-tetrafluoro-1-propanol (TFP) into high value-added sodium 2,2,3,3-tetrafluoropropionate (STFP). A computational fluid dynamics (CFD) technique was applied to simulate the hydrodynamic distributions along a tubular ECMR so as to provide guidance for the design and optimization of ECMR. Two-dimensional simulation with porous media model was employed to predict the properties of fluid dynamics in ECMR. The experimental investigation was carried to confirm the CFD simulation. Results showed that a uniform distribution of permeate velocity along the tubular reactor with short length and large diameter could be obtained. TFP conversion of 97.7%, the selectivity to STFP of 99.9% and current efficiency of 40.1% were achieved from the ECMR with a length of 40 mm and an inside diameter of 53 mm. The simulations were in good agreement with the experimental results.Download high-res image (251KB)Download full-size image

•UTDR was used to monitor the combined organic–colloidal fouling for NF membrane.•Flux decline was greater for silica–BSA solution due to exceeding critical flux.•Silica–BSA feed could induce denser fouling layer than silica–NaCl feed.•Absorption of BSA on silica played a key role in formation of fouling layer.Ultrasonic time domain reflectometry (UTDR) was used to monitor the deposition of combined organic–colloidal fouling on a nanofiltration membrane. The fouling experiments were performed with different feed solutions: a mixture of 1000 mg/L silica and 1000 mg/L NaCl, a mixture of 1000 mg/L silica and 250 mg/L BSA, and 250 mg/L BSA alone, respectively. Results showed that the rate of flux decline obtained in the constant-pressure experiment with the mixture of silica + BSA was greater than that with the mixture of silica + NaCl and BSA alone. The acoustic measurements indicated that the fouling layer obtained from the combined organic–colloidal fouling was denser than that obtained from the colloidal fouling layer in the presence of NaCl. Furthermore, the mixed foulants rapidly deposited on the membrane surface in the early fouling phase, and then reached a plateau in the later fouling phase under a constant pressure operation, whereas the mixed foulants gradually deposited on the membrane surface as the fouling progressed under a constant flux operation. The main reason for the rapid foulant deposition during the initial fouling stage under constant pressure operation was because the initial flux was above the critical flux. The off-line AFM analysis and zeta potential measurements corroborated the ultrasonic measurements.Download full-size image