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Among various pore formers, Pluronic F127 (F127), an amphiphilic nonionic surfactant, is an attractive additive which has often been used to create surface pores on hollow fibers and improve the permeate flux. Even though F127 has been employed in polymer dopes for membrane fabrication in various applications, most studies mainly focused on the enhancements of membrane properties instead of its influences on the phase inversion process and membrane formation. Therefore, the objectives of this study were to investigate its effects on the phase inversion process of a polyacrylonitrile/dimethyl sulfoxide/H2O (PAN/DMSO/H2O) system and the formation of PAN hollow fiber membranes. It was found that the addition of F127 reduced the dope stability and facilitated the solvent-nonsolvent demixing, resulting in a faster phase inversion process. In addition, F127 formed micelles in polymer dopes and induced large pores at membrane surface. Moreover, F127 enhanced the N2 permeance and mechanical properties of the resultant hollow fibers. Finally, the PAN/F127 hollow fibers were tested for air filtration and showed excellent filtration performance of >99.999% against NaCl aerosols with a geometric mean size of 43 nm. The hollow fiber spun from the PAN/DMSO dope containing 3 wt% F127 possesses the most balanced filtration performance in terms of filtration efficiency, mechanical properties, and gas permeance. Its filtration performance is also superior to most hollow-fiber air filters in the literature. This study may provide useful insights of using F127 for membrane fabrication for various applications.

In this study, the application of photoacoustic spectroscopy (FT-IR/PAS) is proposed to investigate membrane fouling phenomena. Ultrafiltration membranes made of polyether sulfone (PES) in modules operated at different flow regimes (dead-end and cross-flow) were used in this paper. Solutions of water-soluble poly(acrylic acid) and suspensions of a natural red clay were used as model media during the filtration tests. The research performed involved the possibility of tracking the in-situ rate of membrane fouling during filtration of these two types of media. Moreover, the influence of the molecular weight of the separated particles on the intensity of the membrane blocking was studied. The results of the experiments showed that fouling phenomena progress for various types of media filtrated is characterized by different particle sizes and can be simply detected using photoacoustic spectroscopy. It was also noted that an increase in the molecular weight of the filtrated medium, e.g., poly(acrylic acid) from 30 to 250 kDa, increased the intensity of membrane blocking of 57 and 87% for membrane installation with dead-end and cross-flow regime, respectively. The performed studies demonstrated that FT-IR/PAS can be also used to assess the efficiency of the methods used for membrane cleaning. The obtained results were confirmed by two other methods: a radiotracer technique and scanning electron microscopy.

The presence of Cr(VI) in the water/wastewater has serious environmental hazards due to its high toxicity. Ultrafiltration membranes are widely used in water/wastewater treatment due to their low cost and simple operation. However, conventional ultrafiltration membrane has low rejection of ions which limits its application for Cr(VI) removal. In terms of this, the CNT/PVDF/stainless steel mesh conductive ultrafiltration membrane was developed via nonsolvent induced phase separation (NIPS) method in this study. The properties of the membranes were characterized and the mechanism for the removal of Cr(VI) under different operational conditions was investigated. The results showed that the new generate membrane has good electrical conductivity and high mechanical strength. The conductive ultrafiltration membrane can realise the efficient removal of Cr(VI) under the application of voltage. It was found that the electrostatic repulsion between CrO42−/HCrO4− and negatively charged membrane plays a main role in Cr(VI) removal in the absence of electrolyte while electrochemical reduction plays an important role in Cr(VI) removal in the presence of electrolyte. Moreover, the removal efficiency of Cr(VI) was positively correlated with the ionic strength and residence time. The acidic condition was favorable for the removal of Cr(VI) when lower cell potentials (1 V, 3 V) were applied to the membrane and the neutral condition was beneficial for the removal of Cr(VI) removal when the applied cell potential increased to 5 V. Increase of the residence time by adjusting membrane operational pressure can further enhance the Cr(VI) removal. The highest removal of Cr(VI) (95.2%) was achieved under the best operation conditions (5 V, pH 7, 100 mM Na2SO4, 3.4 s residence time). It was demonstrated that the conductive membrane offers a promising way of Cr(VI) removal in the electrochemical filtration system.

Photocatalytic membranes (PMs), coupling of membrane filtration and photocatalysis in a single unit, have exhibited a great potential for use in energy-efficient water purification and wastewater treatment. The photocatalytic membranes have presented a trend developed from UV-responsive photocatalytic membranes (UV-PMs) (e.g., TiO2-based and ZnO-based PMs) into visible light-responsive photocatalytic membranes (Vis-PMs) (e.g., modified TiO2/ZnO -based, β-FeOOH-based, and g–C3N4–based PMs). This paper reviews recent development of the PMs with an emphasis on membrane type and fabrication, key criteria, basic performance assessment, application and challenges. The overall evaluation of the PMs in water treatment is presented, and the development direction of the PMs and the significance of developing the PMs and economy are pointed.

In the present work, a convenient and direct technique which enables to characterize the intrinsic structure and the mechanical properties of the biofilm without altering its chemical and physical properties is proposed. By utilizing the Optical Coherence Tomography (OCT) as a structural imaging tool coupled with an advance mathematical framework, thickness, micro-porosity, normal stress-strain curve, bulk modulus and total permeability of the biofilm structures are determined. The accuracy of this mathematical technique for the in situ characterization is validated by analyzing two different membrane structures for porosity and permeability values against the mercury intrusion porosimetry method. Three-dimensional images of biofouling were obtained with high resolution aided to numerically analyze the intrinsic biofilm structure at microscale. Growth of biofilm in a dead-end filtration experimental setup was investigated by varying the feed flow rate which allowed uniform compression and decompression to compute normal stress-strain relation of the evolving biofilm structure. At early development of biofilm (day 3), the thickness and normal stress/strain curve showed that the biofilm structure behave similar to elastic material. However, hysteresis-like trend starts to appear with the growth of biofilm suggesting the deviation of biofilm properties to viscoelastic nature at day 8. The microstructure porosity increased from 0.214 (day 3) to 0.482 (day 8) at a feed flow rate of 15 mL/min. The total membrane/biofilm permeability decreased with biofilm age to reach 5.19 × 10−15 m2 at day 8 at the same flow rate, leading to a reduction of permeate flux over time. All the structural properties were found to be time dependent as the biofilm continuously evolved.

Nowadays, end-of-life (EoL) reverse osmosis (RO) membranes are considered as waste, hence they are incinerated or discarded in landfills. Due to the environmental problem of disposed membranes, their reuse as nanofiltration (NF) membranes was investigated in this study. At first, the fouling of EoL membrane was cleaned and then the effect of sodium hypochlorite (NaOCl) on removing the polyamide (PA) layer and pure water permeability (PWP) of EoL membranes was investigated. With regard to NaOCl exposure intensities, three groups of EoL membranes with PWP of about 10, 20, and 30 L/(m2h bar) were selected as substrates to convert them to NF membranes by deposition of polyelectrolyte (PE) multilayers. The effects of PE type, substrate permeability, and the charge of outer layer on the performance of the resultant NF membranes were studied. As one of the best results, NF membrane composed of eight bilayers of SC498/KE253 polyelectrolytes on substrates with PWP of 18 L/(m2h bar) had salt water permeability (SWP) of 11 L/(m2h bar) and 94% rejection of MgSO4. Other results are also competitive with commercial NF membranes. The maximum NaCl and MgSO4 rejection of prepared membranes were 92% and 98%, respectively, at a feed pressure of 10 bar. In addition, the stability test showed that the prepared membranes are stable over a long period of filtration.

Ultrafiltration (UF) membrane has recently drawn interest in textile and dyeing wastewater treatment due to its unique advantages of low-cost and large flux. However, its low rejection to dyes and developing appropriate membrane materials remain the major hurdles for its large-scale application. Herein, we developed a novel conductive polyvinylidene fluoride (PVDF)-Ni membrane combined with an external electric field to treat typical dyes. The membrane could be fabricated by a facile and low-cost method. The conductive PVDF-Ni membrane showed favorable rejection rate of Congo red (CR) (100%) and extremely high permeance (137.5 ± 6.8 L m−2 h−1 bar−1). To our knowledge, the permeance has exceeded the maximal literature record for the membranes with similar rejection rates of CR. The novel membrane combined with external electric field also provided a unique aeration mode: in-situ aeration, due to its good electrocatalytic activity for hydrogen evolution reaction (HER) (overpotential of 157 mV at the current density of 10 mA cm−2). Benefit from the in-situ aeration, the PVDF-Ni membrane showed 94% flux recovery after filtration of bovine serum albumin (BSA). Additionally, the PVDF-Ni membrane exhibited evident resistance to dye absorption and antibacterial activity. These versatile proprieties, together with the facile fabrication process, indicated the tremendous application prospect of the fabricated PVDF-Ni membrane for textile and dyeing wastewater treatment.

Comprehensive fouling resistance, high permeability, high rejection, and robustness are indispensable criteria for advanced oil/water separation membranes intended for sustainable operation under harsh conditions. Accordingly, in the present study, a new class of zwitterionic-polyelectrolyte-grafted aliphatic polyketone (PK) membranes was prepared by the highly controllable surface-initiated atom transfer radical polymerization (SI-ATRP) of N-(3-sulfopropyl)-N-methacroyl-oxyethyl-N,N-dimethylammonium betaine (SBMA) on highly porous and hydroxylated PK membranes. The PK-g-PSBMA membrane prepared with 10-min grafting modification exhibited excellent solvent-resistant underoil superhydrophilicity as well as underwater superoleophobicity and ultralow protein adhesion. The low mass-transfer resistance, low fouling potential, and strong solvent resistance of the porous hydroxylated PK substrate along with the ultralow fouling nature of the zwitterionic SMBA layer, endowed the PK-g-PSBMA membranes with self-cleaning properties, sustainably high emulsion fluxes of 1200–1800 L m−2 h−1 bar−1, low oil permeations at 6–33 ppm, and high flux recovery ratios of 82–98%, even for difficult micro- and nanoemulsions containing small oil droplets and complicated pollutants (surfactant, high salinity, and natural organic matters). Thus, PK-g-PSBMA membranes with simple preparation route and comprehensive performances show great potential for treatment of challenging oily wastewater.

Thin-film composite (TFC) polyamide membranes are the most commonly used membrane materials for reverse osmosis (RO) processes; however, the high biofouling propensity is one of major limitations to the widespread application of TFC membranes. In this study, we developed an antibiofouling thin-film nanocomposite (TFN) membrane by incorporating a copper-based water-stable metal-organic framework (MOF), CuBTTri, in the active layer. Results showed that the presence of MOFs in the TFN active layer significantly increased membrane water permeability without compromising selectivity. Furthermore, the incorporation of CuBTTri imparted the TFN membrane with efficient antibacterial properties. After 24 h contact, the colony forming units (CFU) of Pseudomonas aeruginosa on the TFN membrane (with a dosage of 0.1% MOF) was 3.4% of that on the pristine membrane, demonstrating a strong antibiofouling behavior. The cross-flow filtration tests further confirmed the antibiofouling performance of the TFN membrane, with a significantly lower flux decline (∼36%) compared to the control (∼70% flux decline). The TFN membranes also had a thinner biofouling layer and less quantity of biofoulants on their surface compared to the TFC membrane. The antibiofouling properties of the TFN membrane were associated with bacterial membrane potential depolarization and cell damage due to the contact with MOF. These results highlight the potential of water-stable MOFs to be used as functional materials to fabricate antibiofouling TFN membranes for advanced water and wastewater treatment.

Antifouling performance of membranes is the key obstacle limiting their practical applications for oil/water separation. In this study, a sustaining antifouling membrane was fabricated by constructing polydopamine (PDA) micro-/nano-spheres on a polyacrylonitrile (PAN) nanofibrous membrane. The secondary PDA nano-spheres not only strengthened the bonding of primary micro-spheres with the substrate, but also diversified the hierarchical structure and chemistry. The composite showed enhanced superhydrophilicity and underwater superoleophobicity. Permeability of PAN-PDAc membrane was maintained as high as 11666 ± 978 Lm−2h−1bar−1 with separation efficiency of higher than 99.9% over a 2-h continuous filtration. This permeability was about 2.7 times of pristine PAN membrane (4260 ± 430 Lm−2h−1bar−1). The extrusion and cutting demulsification on the confined space of PAN-PDA surface was proposed. Antifouling mechanism of the superhydrophilic membrane was first theoretically elucidated based on hydration ability and adhesion free energy with recourse to thermal analysis and Derjaguin-Landau-Verwey-Overbeek theory respectively. It was found that PDA micro-/nano-spheres mediated membrane showed strong hydration ability (higher fraction of non-freezable water) and weak adhesion towards toluene (low free energy of adhesion) compared to pristine PAN membrane. These findings would lead to a better understanding of antifouling demulsification mechanism and improved design of sustaining antifouling membranes for oil/water separation.

Phosphorus (P) is a limited natural resource and essential for global food supply, particularly given our ever-growing world population. However, natural P deposits are restricted to just a few countries and the quality of exploited primary sources is declining. Phosphorus recovery from sewage sludge or sewage sludge ash leached by acids is hence gaining importance. During P recovery the removal of impurities is a challenge that can be addressed by nanofiltration retaining e. g. multivalent metals and allowing phosphoric acid to pass. However, currently available membranes show low permeability and limit this process option economically. Layer-by-layer (LbL) membrane synthesis is a technology that allows membrane performance to be tailored to individual filtration tasks. Little is known for such membranes with respect to acid resistance, acid permeability and impurity rejection. We show that LBL membranes based on PDADMAC/PSS show the desired passage of phosphoric acid with retentions values below 10%, aluminum retention was always above 95%. Permeabilities up to 4 L/(m2 h bar) were reached even up to a phosphorous recovery of 75%. Overall permeabilities were 16 times higher than a commercial benchmark membrane. Initial stability tests and upscaling into a larger module show the viability of the proposed modification approach.

A novel kind of Cu-based catalytic membrane reactor (CMR) has been developed for the efficient reduction of p-nitrophenol (p-NP) to p-aminophenol (p-AP) in a continuous flow-through system. The CMR is prepared by the composite of MOF-derived nanoporous Cu/Cu2O networks and a porous matrix of nylon film. In this process, the pre-synthesized Cu-MOF-nanosheets are deposited onto a porous nylon film via filtration, and then the composite membrane is in-situ reduced by NaBH4 in the liquid phase, producing nanoporous Cu/Cu2O networks/nylon composite membrane. Remarkably, the nanoporous Cu/Cu2O networks exhibit fluffy structure with multidimensional porosity, which shows relatively high mass transportation ability and excellent catalytic activity. Therefore, the nanoporous Cu/Cu2O networks/nylon composite membrane can work as highly efficient and stable CMR, which can continuously convert over 95% of p-NP to p-AP in an 8 h test (8 mL/min) without obvious structure change and deactivation. This new type two-dimensional MOF-derived CMR offers an effective and convenient continuous catalytic process from p-NP to p-AP, which would have a potential application in this industrial reduction reaction.

The core of this study is utilization of the facile one-side self-polymerization of dopamine (DA) on surface of the non-recoverable polyethersulfone (PES) membrane and its antifouling ability assessment during oil-in-water (O/W) emulsion separation. The pristine PES membrane was coated by DA solution for 2 and 4 h. The membranes were characterized using FESEM, AFM, FTIR, water contact angle (WCA) and underwater oil contact angle (UOCA) analysis. The results showed that hydrophilicity and underwater oleophobicity can be achieved successfully. Antifouling properties of the modified membranes were improved so that water flux of the 4 h polydopamine (PDA) coated membrane increased about 793% in comparison to the pristine PES membrane. Also, implementation of classical models showed that PDA coating changes the membrane fouling mechanism from standard blockage model to cake filtration model. The 4 h PDA modified membrane shows antifouling property and easy recovery capability during 30 h of operation. Also, the dependences of steady-state water flux were investigated via changing initial oil concentration, feed salinity, trans-membrane pressure and self-polymerization time of DA. Self-polymerization time of DA was found as the most influential factor on water flux. Also, during O/W emulsion separation, all the membranes exhibited high removal efficiency (above 98%) introducing a promising candidate for practical applications.

This study is to establish computational fluid dynamics (CFD) simulations of disordered fibrous filters under unfavorable conditions to investigate the effects of chemical and physical factors on filtration performance. We employed a discrete phase model (DPM) to track individual particles, and user-defined functions were implemented to consider the particle deposition to the collector surface via interception and interaction energy. During the DPM process, calculations based on interactional and fluid hydrodynamic effects were proceeded for all injected individual colloids. The results of filtration and collision efficiencies of the single collectors obtained by our CFD simulations showed a very good agreement with the existing expressions and data, accurately predicting the trends of particle deposition and reentrainment. After the validation of the developed particle tracking method, the disordered fibrous filters with randomly distributed monodisperse fibers were studied. The results showed that the reentrainment of deposited particles was enhanced with increasing fluid velocity and solid volume fraction and decreasing fiber size due to the hydrodynamic effects. Moreover, the combined effect with ionic strength, Hamaker constant and zeta potential determined particle deposition behavior by altering particle/filter interactions. The developed simulation method for predicting filtration performance does not contain any free parameters or empirical factors.

Biofilms have permeable structures, which can be used as membranes for solids separation, e.g., dynamic membrane bioreactors (DMBRs) that relied on the biofilm developed on the support material with large apertures for filtration. This study found that the biofilm formed on the support material in DMBRs under different solids retention time (SRTs) had diverse structures, which in turn determined the hydraulic resistance and filtration performance. At 5-day SRT, a thin but compact gel-like biofilm layer (porosity = 27.5%) was formed on the support materials, which had a higher hydraulic resistance of approximately 4.9 × 1011 m−1. As a result, the transmembrane pressure (TMP) rose dramatically every 20–40 days' operation. A thick but porous biofilm layer (porosity > 60% and hydraulic resistance

Membrane-based processes have gained prominence for environmental and industrial separations, especially in water purification and desalination. In spite of these developments, a fundamental knowledge gap exists with regards to fully understanding the processes that occur at or near the membrane surface. Concentration polarization is a phenomenon that may have a significant impact on mass transfer and fouling of the membrane. While attempts have been made to experimentally characterize this phenomenon for pressure-driven filtration processes, most studies employ semi-empirical methods to estimate the extent of concentration polarization. This study presents a novel non-intrusive spectrophotometric method developed to measure the concentration profile of solute near the membrane surface in a direct contact membrane distillation system. A custom membrane cell was designed and fabricated for this study to allow accurate in-situ characterization of the concentration polarization layer with spatial resolution of 4.5 μm. The spectrophotometric method was developed and validated using transition metal salt (i.e., nickel chloride) in the feed solution to probe the concentration profile near the membrane surface. The method developed in this study can be extended to other membrane-based separation processes and other solutes with known absorption spectra.

Membrane-based filtration is a promising technique to treat the enormous amounts of oily wastewater, specifically those with micron-sized oil droplets. However, the understanding on the effect of the surfactants, that are inevitably present to stabilize the oil emulsion, on the filtration performance remains poor. This study aimed to investigate the effect of surfactant type (namely, non-ionic Tween 20, positively charged CTAB, and negatively charged SDS) on filtration flux and membrane fouling during the microfiltration of the surfactant-stabilized oil emulsion with mean droplet sizes of approximately 20 μm. The Optical Coherence Tomography (OCT) was employed to quantify the evolution of fouling, and both the DLVO and XDLVO models were used to quantify the oil droplet-membrane and oil droplet-deposited layer interaction energies. Two key understanding on the correlation between the DLVO and XDLVO predictions with flux and fouling were obtained. Firstly, the steep flux enhancement vis-à-vis a DI water feed by the feed containing CTAB-stabilized oil emulsion was tied to the attractive interaction of the surfactant with the membrane, as evident from the DLVO model. This attraction was not related to the extent of membrane fouling, which was relatively lesser for the CTAB-stabilized oil emulsion. Secondly, the extent of membrane fouling was tied to the repulsive energy magnitudes rather than attractive ones, specifically in that the least repulsive energy values of the Tween 20 – stabilized oil emulsion was linked to the most extensive fouling. Economic Development Board (EDB) Ministry of Education (MOE) We acknowledge funding from the Singapore Ministry of Education Academic Research Funds Tier 2 (MOE2014-T2-2-074; ARC16/15) and Tier 1 (2015-T1-001-023; RG7/15), the GSK (GlaxoSmithKline) – EDB (Economic Development Board) Trust Fund, and the Joint Singapore Germany Research Project Fund (SGP-PROG3-019).

Porous SiOC ceramic membranes (PSCM), with a narrow pore size distribution, were prepared by a casting method, using a polymer liquid (polydimethylsiloxane, PDMS), as a pore-forming agent, and a pre-ceramic liquid (polysiloxane, PSO), followed by pyrolysis at 1200 °C in flowing Ar gas. Pore size, porosity, and N2 and water permeation increased with increasing PDMS content in the mixture of the precursors. An oil-in-water emulsion, with an average oil diameter of 0.83 μm, was effectively separated (95% rejection rate) with membranes which had an average pore size of 0.59 μm. PDMS was also used for membrane surface modification and the surfaces became super-hydrophobic. After modification, membranes with an average pore size of 0.95 μm were tested in sweeping gas membrane distillation. High salt rejection of 99.9% was achieved with NaCl solution (at various concentrations, 4–16 wt%) at several temperatures (55–85 °C). A stable performance was recorded for over 100 h, using 4 wt% NaCl in the feed at 75 °C. The produced membranes displayed clear separation and high efficiency in separation rates, which was reflected in the high permeation flux in both oil-in-water emulsion filtration and membrane distillation, attributed to the narrow pore size distribution.

In general, the twisted fiber bundle with good fiber cohesion could be regarded as a new kind of supporting layer. Herein, a new type of poly(p-phenyleneterephthalamide) (PPTA) twisted fiber bundle-reinforced (FR) cellulose triacetate (CTA) hollow fiber reverse osmosis (RO) membrane was fabricated using a twin-screw spinning machine by the melt spinning method. Moreover, the morphologies, surface analysis and permeability of FR CTA hollow fiber RO membrane were investigated by scanning electron microscopy, confocal scanning microscopy and filtration performance. The CTA separation layer exhibited the favorable interfacial bonding state between CTA separation layer and PPTA twisted fiber bundle supporting layer. Especially, the advantages of FR CTA hollow fiber RO membrane could overcome the negative effect of permeability flux loss, due to the easy vulnerable to the compaction of CTA membrane when operating at moderate or high pressure process. Besides, the tensile strength increased from 9.2 to 96.8 MPa because of the higher interfacial stress transfer coefficient, which was achieved due to the interfacial bonding of FR CTA hollow fiber RO membrane. Besides, based on the filtration performance and long-term running test, FR CTA hollow fiber RO membrane with thin separation layer not only produced higher and more stable permeability flux while their salt rejection were basically the same, but also came into being smaller amount of deformation without defects than pristine CTA hollow fiber RO membranes after 200 h operation.

The evolution of the cake during dead-end microfiltration of different compositions of bidisperse feeds was investigated using network modeling and three-dimensional (3D) optical coherence tomography (OCT) imaging. The foulant particles were latex of two different sizes (namely, 3 and 5 µm) and the membrane was polycarbonate track-etched with nominal diameter of 2 µm. The results from the network model indicate that (i) the deposition of the smaller particles on the non-porous area was reduced by the presence of larger particles, whereas that of the larger particles was not affected by the concentration of the smaller ones; and (ii) monodisperse small particles gave a greater specific cake resistance than monodisperse large particles, and the specific cake resistance of the bidisperse mixture can only be reduced with a sufficient proportion of larger particles. OCT results substantiated the model by revealing that (i) bidisperse feeds tended to form homogeneous cakes, but the presence of a higher concentration of large particles led to heterogeneous cakes; (ii) the presence of a lower concentration of larger particles enhanced clustering, while a higher concentration reduced that; and (iii) in the longer-term filtration, the monodisperse feed of small particles dynamically deposited and detach continuously, while the bidisperse feed with the highest concentration of larger particles exhibited negligible deposition and detachment. Economic Development Board (EDB) Ministry of Education (MOE) We acknowledge funding from the Ministry of Education - Singapore Academic Research Funds Tier 2 (MOE2014-T2-2-074; ARC16/15) and Tier 1 (2015-T1-001-023; RG7/15), the GSK (GlaxoSmithKline) – EDB (Economic Development Board - Singapore) Trust Fund, and the Joint Singapore-Germany Research Project Fund (SGP-PROG3-019).

Hydrogel-composite membranes fabricated using poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS) hydrogel and a polyethersulfone (PES) support present a promising avenue toward developing self-healing membranes—membranes that autonomously heal from physical damage without any external intervention. The previously developed pore-filled structure, however, suffers from low water permeability, since the hydrogel spans the depth of the PES substrate. This study explores a new asymmetric architecture in which the hydrogel layer is asymmetrically confined to the top several tens of micrometers of a single side of the substrate. We present an approach to achieve this configuration based on controlling the viscosity of the acrylamido-2-methylpropanesulfonic acid (AMPS) monomer solution used during membrane fabrication. The asymmetric PAMPS composite membranes exhibited improved water permeability compared to their pore-filled counterparts, as well as nearly equal self-healing performance following the same mechanism of self-healing. The results of this study collectively suggest that asymmetric hydrogel composite membranes are a more promising route to developing practical water filtration membranes with self-healing capability.

We develop a mathematical model of direct-flow filtration operating at constant flux to understand the pressure–time signature. We combine the effects of fluid flow with those of membrane fouling and caking to explain the gradual increase in driving pressure that is often reported. We model the periodic backflushes used to clean such membranes and show that the elasticity of the membrane may explain the limited effectiveness of these backflushes. We also consider strategies for the operation of direct-flow filtration and show that tuning the flux and rate of backflushing, as well as the membrane material structure, may allow for improvements in membrane performance.

Despite the vast number of studies on the understanding and estimation of the permeate flux in ultrafiltration, most of them base their estimations on either one or another mechanism, without pointing out a clear ‘bridge’ between them. The aim of this paper is to assess these mechanisms on the determination of the permeate flux, using as feed a multicomponent mixture of BSA, NaCl and H2O. Maxwell-Stefan Equations expressed as function of the components' volume fractions were used for an easier consideration of the non-idealities of the system. These non-idealities (hydration, adsorption, electrical interactions and volume exclusion) were critical in the local fluxes calculation, for which an increase in the thickness of the boundary layer along the filtration channel was considered. The developed model proved to be suitable for the estimation of fluxes lower than the limiting flux. Since the non-idealities of the system can be calculated along the concentration polarization layer, no extra information on the protein diffusivity was needed. Additionally, the fact that the model includes all the components from the solution offers the possibility of including the rejection of the accompanying ions in the calculations.

In this study, we designed three turbulence promoters using three-dimensional (3D) printing technology for cross-flow microfiltration. Designing turbulence promoters with optimal geometry for microfiltration systems remains a key challenge. The effects of the operating conditions, such as cross-flow velocity, transmembrane pressure and geometry of the turbulence promoter on the filtration flux and cake properties were investigated. The distribution of the fluid velocity was examined using computational fluid dynamics (CFD). For normal microfiltration tests without promoters, the filtration flux merely increased by 10% when the cross-flow velocity increased from 0.1 to 0.5 m/s under a transmembrane pressure of 20 kPa. Therefore, the turbulence promoters were individually added into the channel of the microfiltration device to induce vortices in the flow stream and increase cross-flow velocity. Three turbulence promoters with different configurations (circular, diamond and elliptic) were made with a 3D printer. The elliptic type of promoter with a hydraulic angle of 90° displays the flux enhancement by approximately 30–64% under a transmembrane pressure of 20 kPa compared to the normal type of microfiltration, whereas the diamond type of promoter with a hydraulic angle of 60° shows a lower flux enhancement by approximately 7–16% under the same transmembrane pressure. Overall, adding the turbulence promoter to the microfiltration module shows a great potential to significantly mitigate membrane fouling and enhance filtration flux in cross-flow microfiltration.

A novel submerged rotational vibration membrane system was investigated using high concentration yeast suspensions to demonstrate the capability of this system for the separation of high concentration biomass. The critical flux and shear rate for this system can be correlated by the expression, J critical = 1.2 γ 0.55 , for the filtration of 200 g/L yeast suspension. Good fouling control performance as indicated by low and stable TMP at approximately 13 kPa as well as high protein transmission of 84% was maintained over 26 h of filtration using 200 g/L yeast suspension at a constant permeate flux of 15 L/m2 h before a rapid TMP rise was observed mainly due to the increase of feed concentration. It was determined that there is a limiting concentration, defined here as the critical concentration point, below which low and stable TMP operation can be maintained and above which rapid TMP rise was observed. Fractionation analysis of the foulant layer at the end of long term filtration experiments showed that majority of fouling was loose and reversible, which can be easily recovered by simple rinsing. The implementation of intermittent shear enhancement and periodical relaxation were found to be effective in fouling limitation, which extended the critical concentration point. However, applying periodical backwash using the permeate is not advantageous as the permeate contains protein and carbohydrate which can cause internal fouling of the pores.

This study investigated the removal of natural organic matter (NOM) from water, and its effect on membrane fouling, by mixtures of heated aluminum oxide particles (HAOPs) and powdered activated carbon (PAC) in both batch and microgranular adsorptive filtration (µGAF) reactors. In batch reactors, increasing the portion of HAOPs in the adsorbent mixture led to a steady increase in membrane fouling. However, this trend was reversed when the water was pretreated with the same adsorbent mixtures in µGAF reactors. The opposing trends tracked the trend for removal of the biopolymer fraction of the NOM. The results are consistent with a hypothesis that the fouling was caused primarily by formation of a condensed, biopolymer-rich gel layer on the membrane surface. According to this hypothesis, when the feed was pretreated in batch reactors, some foulants were collected on each adsorbent, with PAC collecting more foulant than HAOPs did, but the accumulated material did not condense into a gel. However, when μGAF reactors were used instead of batch reactors for pretreatment, the tight packing of the adsorbent particles promoted the formation of a gel, leading to increased NOM removal in general and increased foulant removal in particular. The fact that fouling mitigation increased with increasing amounts of HAOPs and decreasing amounts of PAC in the μGAF adsorbent layer suggests that HAOPs are much more effective than PAC at promoting formation of the gel layer. Generalizing this result, it appears that pretreatment by mixtures of adsorbents in μGAF reactors could have significant advantages over conventional approaches in which a single adsorbent is applied in a well-mixed reactor.

Growth Media Filtration Using Nanocellulose-based Virus Removal Filter for Upstream Biopharmaceutical Processing

Membrane fouling is indispensable in all membrane systems and remains one major problem to large scale application. In view of this, an in-situ method of fouling control is desperately needed to sustain membrane technologies. The objective of this study is to develop electrically conductive PANI membranes with stimuli-response properties and to solve fouling under applied potential. Electrically conductive polyaniline membranes were prepared by non-solvent induced phase separation method. Exfoliated graphite and DBSA were incorporated into membrane to form PANI membranes with different electrical conductivities. The results showed that the conductivity of Memb-PAMPSA was improved by two orders of magnitude by the incorporation of EG, and further enhanced by twenty times using a long chain acid DBSA as a secondary dopant. Compared to Memb-PAMPSA whose effective contact angle did not change between 0 and 30 V measurements, the significant dynamic contact angle change of Memb-PAMPSA-EG and Memb-PAMPSA-EG/DBSA gave a quantitative indication of the electrical tuneability of these membranes. All the membranes showed fouling removal ability under applied electrical potential and the fouling removal behaviour was closely related to the membrane conductivity. The results of this study open the pathway for the development of electrically responsive membranes for in situ control of fouling in membrane filtration systems.

Membrane fouling always decreases the separation efficiency and shortens the membrane life, which severely hinders the practical application of the membrane technology. The photo-Fenton process can degrade various foulants with the generation of hydroxyl radicals, and its integration with membrane filtration may become an efficient way to improve the antifouling property and filtration performance of the membrane. In this study, the CuFe2O4 particles were synthesized and doped in the PVDF@CuFe2O4 membranes with increasing concentration from 0% to 1.0%. The degradation measurement of methylene blue (MB) solution shows the optimal conditions for the photo-Fenton process as CuFe2O4 concentration of 1.0%, pH of 3.0, and H2O2 dosage of 400 μL. With the photo-Fenton cleaning process, the PVDF@CuFe2O4 membrane (1.0%) exhibits versatile antifouling property to different types of foulants, including organic dyes (e.g. MB and rhodamine B (RhB)), nature organic matter (e.g. humic acid (HA)), and protein (e.g. bovine serum albumin (BSA)). With the integration of photo-Fenton and membrane process, the PVDF@CuFe2O4 membrane (1.0% of CuFe2O4) dramatically enhanced the separation efficiency, with the results of 99.77% to MB, 81.02% to RhB, 36.35% to HA, and 82.94% to BSA. The flux and rejection have been increased respectively to threefold and double of the corresponding data from the membrane filtration alone. Moreover, even after fifteen cycles of experiments, the average MB rejection is still higher than 70%, which further indicates the good stability and reusability of the PVDF@CuFe2O4 membrane. Therefore, this study provides a promising methodology for the successful fabrication of high-performance membrane through the integration of photo-Fenton and membrane process, and further proposes a new strategy on the design and application of functional materials for new generation of catalytic membranes.

Fabrication of antibiofouling membrane is a vital but challenging task for the treatment of water and wastewater using membrane-based processes. In this work, quaternary ammonium compounds (QACs) were grafted onto polyvinylidene fluoride (PVDF) membrane via surface-initiated activators regenerated by electron transfer atom-transfer radical-polymerization (ARGET ATRP) method. Membrane surface characterization demonstrated that QAC was successfully grafted onto the PVDF membrane surface. The QAC-modified membrane (MQ) demonstrated prominent antimicrobial capability by destruction of cell integrity, with an inhibition rate ~98.3% of E. coli and ~98.5% of S. aureus, respectively. Batch filtration tests showed that MQ membrane had lower flux decline rate compared to control membrane (M0) under constant-pressure filtration. Confocal laser scanning microscopy analysis revealed that less total cells and more dead cells were present on MQ membrane compared to M0. The quantity of polysaccharides and proteins on MQ membrane was reduced as well. The grafted QAC on PVDF membrane also showed favorable stability during repeated fouling-cleaning tests. These results highlight that the ARGET ATRP method for grafting QAC onto polymeric membrane surface is a robust way to fabricate low-biofouling membrane for water and wastewater treatment.

Exploring high selectivity molecular sieving membranes and the corresponding facile assembly methods are extremely critical for the gas separation. In this work, we demonstrated a high-temperature-tolerance 2D membrane for hydrogen sieving. This membrane was prepared by a vacuum-assisted filtration with a subsequent drying process in which all the manipulations were based on the ambient conditions. The XRD study indicates that the crystal structure is quite stable even at a wide temperatures range from 25 to 500 °C. The interlayer spacing of MXene membrane was shrank from 3.4 A (25 °C) to 2.7 A (500 °C) after such high temperature treatment, reinforcing the molecular sieving properties. The membrane exhibited the moderate H2 permeation of 2.05 × 10−7 mol m−2 s−1 Pa−1 and good selectivity of 41 for H2/N2 mixture at 320 °C. Benefiting from the excellent chemical stability of MXene membranes, no degradation was found for permeation and separation test up to 200 h. The high separation performance and the exceptional high-temperature stability mirror the 2D MXene membranes as a promising candidate for the separation of industrial gas mixtures containing hydrogen.

Information about the sizes of the solute molecules and membrane pores is needed to estimate solute rejection in filtration processes. Molecules are normally regarded as spheres, and the Stokes radius is commonly used to represent their molecular size. However, many molecules used in food and pharma processes are oligomers or polymers which are strongly elongated; therefore, considering them spherical affects the accuracy of the model predictions. We here adapt the so-called Steric Pore Model to a more realistic representation of the transfer of rigid elongated molecules into and through ultrafiltration membrane pores. To do so, sugars with different degree of polymerization were used as model molecules. They were considered to be capsule-shaped to facilitate their size estimation. In order to represent the system as accurately as possible, the effect of hydration on the sugars size was included, and the membrane pore size distribution was estimated based on rejection data. It was demonstrated that considering these molecules to be capsule-shaped instead of spherical generates better predictions over the entire rejection spectrum using a unique pore size distribution. Additionally, this capsular geometry lets us simplify the calculations, making the estimation of the rejection straightforward.

Depth filtration is a commonly-used bioprocessing unit operation for harvest clarification that reduces the levels of process- and product-related impurities such as cell debris, host-cell proteins, nucleic acids and protein aggregates. Since depth filters comprise multiple components, different functionalities may contribute to such retention, making the mechanisms by which different impurities are removed difficult to decouple. Here we probe the mechanisms by which double-stranded DNA (dsDNA) is retained on depth filter media by visualizing the distribution of fluorescently-labeled retained DNA on spent depth filter discs using confocal fluorescence microscopy. The extent of DNA displacement into the depth filter was found to increase with decreasing DNA length with increasing operational parameters such as wash volume and buffer ionic strength. Finally, using 5ethynyl-2'-deoxyuridine (EdU) to label DNA in dividing CHO cells, we showed that Chinese hamster ovary (CHO) cellular DNA in the lysate supernatant migrates deeper into the depth filter than the lysate re-suspended pellet, elucidating the role of the size of the DNA in its form as an impurity. Apart from aiding DNA purification and removal, our experimental approaches and findings can be leveraged in studying the transport and retention of nucleic acids and other impurities on depth filters at a small scale.

In this work, a lab-scale membrane system was developed to accommodate angular vibrations with a hollow fiber module. A series of filtration experiments were designed and implemented to investigate the effects of angular vibrations on fouling by various algae-derived foulants, including algal cells, debris, extracellular organic matter (EOM), and intracellular organic matter (IOM). The experimental results indicated the effects of angular vibrations on fouling mitigation at the frequency of 2 Hz in the following order: algal cells (~ 97.4%) > debris (~ 93.6%) > IOM (~ 81.8%) > EOM (~ 52.3%). The vibrations mainly targeted the cake layer formed by these foulants, but was ineffective for pore blocking which constituted a large portion of EOM fouling. Although the fouling rates were accelerated during the normal ultrafiltration of the combined foulants, the effects of angular vibrations continued to be impressive (67.8–92.6%). This was because the cake layer formation was the dominant fouling mechanism. Furthermore, the vibrations were found to improve the organic removal by alleviating concentration polarization. In this paper, quantitative analysis is presented to demonstrate the potential of angular vibrations to control algal fouling with the relatively low energy consumption of 0.034 mW.

In this study, we present hydrophobic yttria-stabilized zirconia capillary membranes conditioned for virus filtration. These macroporous ceramic filters (d50 = 150 nm) efficiently extract viruses regardless of their surface charge with high throughput rates. For hydrophobic functionalization of the ceramic membranes we used two different silanes, n-hexyltriethoxysilane (HTS, C6-chain) and n-octyltriethoxysilane (OTS, C8-chain), in three different molarities. The virus retention of the membranes is tested in dead-end mode by intracapillary virus feeding using two small bacteriophages as model species: MS2 and PhiX174. Virus retention increases most strongly for hydrophobic capillaries functionalized with 0.05 M OTS, showing a virtually complete retention with log-reduction values (LRVs) of ~ 9 for both bacteriophages compared to the non-functionalized membrane with LRVs of 0.3 ± 0.1 for MS2 and 3.4 ± 0.2 for PhiX174. The functionalized membranes allow a high membrane flux of ~ 150 L/(m2hbar), with throughput rates up to ~ 400 L/(m2 h) while maintaining high filtration efficiency. Even under varying feed conditions using only mono- or divalent salt ions or pH values ranging from 3 to 9, retention capacities of the capillary membranes are high. Accordingly, such hydrophobic ceramic membranes offer a versatile alternative to conventional polymeric membranes for virus removal with greatly improved membrane flux.

Concentration polarization and surface fouling may be two of the most remarkable features in the pressure-driven membrane filtration process. A new numerical simulation model is proposed in this paper to study the concentration polarization (CP) and the inorganic fouling growth. The numerical study is based on the lattice Boltzmann method (LBM), which allows a simultaneous solution of the Navier–Stokes equations and the convection-diffusion equation. Simulation results are verified by comparisons with published CP and permeate flux data under the same operating conditions. Then the model is extended to predict CP in a spacer filled desalination channel. The prediction result indicates that there is a higher fouling potential near the spacer filaments due to higher CP values in that area. Coupling of the CP prediction model with gypsum growth kinetics provides an approach to study the inorganic fouling growth on the membrane surface at a single crystal level, with respect to a given solution supersaturation near the membrane surface. Predicted equivalent radius and accumulated mass of the growing gypsum crystal, under the effects of growth retardation by bicarbonate, agree with published test data and analytical results. The presented numerical model enables a direct evaluation of the impacts on the surface crystal development in the presence of antiscalants. This numerical model can be applied to identify suitable operating conditions, assist in dose selection of antiscalants when required properties are available, and predict the fouling mitigation effects in the pressure-driven membrane filtration process.

A simple and robust strategy is suggested for fabricating multifunctional hybrid separation membrane loading with catalytic nanoparticles based on the interception effect of porous CNTs network membrane. CNTs functionalized covalently with hydrophilic polyacrylic acid brushes assemble into a superhydrophilic/underwater superoleophobic network structure, which displays thickness dependent porosity. This allows the CNTs membrane capture catalytic nanoparticles by simple filtration process and construct flexibly desired separation membrane. Not only effectively separate oil-in-water emulsion, the prepared CNTs hybrid membranes can also catalytically decompose different organic pollutants in water by conveniently altering catalytic nanoparticles in the membrane. This robust construction provides a convenient means to obtain multifunctional CNTs separation membrane, promoting the practical application of superwettable CNTs membrane in the treatment of complex oily wastewater.

Membranes of anion channels in matrix were constructed through amphiphilic self-assembly of polyelectrolyte, SO3H group containing polyamide 6 (SPA6). SPA6 molecules self-aggregated into 255 nm micelles with negative charges at the surface in casting solution. Micelles switched into nanofibers, which formed pompoms of divergent arrangement of nanofibers from the center after immersed in coagulation bath. They adhered and piled up to construct the membrane. Charged groups were remained at the surface of pompoms during solidification. The gaps between spheres provided channels full of anions, which supplied great repulsion to anions. It endowed membranes the potential to remove anion dyes from aqueous solution. The rejection was up to 99.73% for 500 mg/L Congo red and 85.68% for 100 mg/L Acid blue 93 and 63.06% for 100 mg/L Methyl orange after first filtration. Total removal percentage was 95.48% for Acid blue 93 and 79.51% for Methyl orange after second filtration. Self-aggregation of dye molecules also helped their high removal efficiency. Flux of film 30 M of 100 mg/L CR solution dropped to 67.34% and rejection rate was higher than 95% in 1500 min. Flux could be brought back to more than 90% of the original values after washed by deionized water, suggesting that 30 M had good anti-fouling capacity. SPA6 membranes displayed a potential for effective treatment of textile wastewater.