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Abstract: One of the major barriers that hinder the application of forward osmosis (FO) processes is the lack of optimized membranes that possess high water flux, low salt leakage and good anti-internal concentration polarization (ICP) and anti-fouling/anti-scaling properties during practical FO operations. In this study, novel composite FO hollow fiber membranes with an RO-like selective skin and an NF-like secondary selective skin were successfully developed for the first time. The fabrication procedures involved making ultrafiltration hollow fiber substrate using poly(amide–imide) (PAI) polymer material via phase inversion method, followed by interfacial polymerization (IP) and chemical modification to yield a polyamide RO-like inner skin and a positively charged NF-like outer skin, respectively. It was found that the sequence of conducting IP on the substrate inner surface prior to the chemical modification of the outer skin was preferred in order to achieve better performance for the resultant double-skinned hollow fibers. The newly developed FO hollow fibers exhibited high water permeability of 2.05L/m2 hbar and 85% rejection to NaCl at 1bar pressure as well as superior FO water flux of 41.3L/m2 h and low ratio of salt flux over water flux of 0.126g/L when using DI water and 2.0M NaCl as feed and draw solutions, respectively, in the active layer facing draw solution (AL-DS) orientation. Furthermore, the experimental results showed that the composite FO hollow fiber membrane was able to outperform single-skinned membrane when the feed contains divalent ions or the feed exhibits high scaling tendency to membrane. This suggested that the integration of RO- and NF-like two selective skins in FO membrane is an effective way to minimize the ICP effect, mitigate the membrane scaling and thus enhance the feasibility of FO processes for practical applications. [Copyright &y& Elsevier]

Abstract: For the first time, a specially designed pressure retarded osmosis (PRO) hollow fiber membrane has been successfully developed and applied in the PRO process to demonstrate its potential for power generation. The membrane fabrication method is similar to that used for making thin-film composite (TFC) forward osmosis hollow fiber membranes, but further optimization and improvement have led to a new type of TFC hollow fiber membranes with much greater mechanical strength in addition to its excellent separation property and high water flux. The TFC PRO hollow fiber membranes have a water permeability (A) of 9.22×10−12 m/(sPa), salt permeability (B) of 3.86×10−8 m/s and structural parameter (S) of 4.6×10−4 m. It can withstand hydrostatic pressure as high as 9bar with its relatively large dimension of 0.98mm lumen diameter. This PRO hollow fiber membrane is superior to all other PRO membranes reported in the open literature in terms of power density. A power density as high as 10.6W/m2 can be achieved using seawater brine (1.0M NaCl) and wastewater brine (40mM NaCl), which suggests that the newly developed PRO hollow fiber membrane has great potential to be applied in PRO processes to harvest salinity gradient energy. A higher pressure is preferred as it allows generation of higher power density (pressures of 12bar may be optimal for seawater as the high salinity stream), and this can be realized by reduced fiber dimension. Further optimization of the membrane structure will be performed. [Copyright &y& Elsevier]

Abstract: Nanofiltration (NF) membrane process has become increasingly attractive due to their unique characteristics to selectively remove specific compounds or ions. The most commonly NF membranes are negatively charged which is unsuitable for hardness removal. Therefore, the development of novel NF membranes with a positively charged skin has become a key issue for low pressure water softening. In this study, dual layer microporous hollow fiber membranes were fabricated using Torlon poly(amide-imide) (PAI) as the material of the selective outer layer and polyethersulfone (PES) as the material of the porous support inner layer. A positively charged NF-like selective layer was developed by a simple polyelectrolyte cross-linking using polyallylamine (PAAm). The newly developed (PAI–PAAm)–PES dual layer hollow fiber membranes show a salt water permeability of 15.8 L/m2 h bar and high Mg2+ and Ca2+ rejections of 94.2% and 92.3%, respectively, at operating pressure of 2 bar using a 3000 ppm TDS feed solution. This study provides a simple and effective approach to produce positively charged NF hollow fiber membranes for water softening applications with low energy consumption. [Copyright &y& Elsevier]

Abstract: Composite nanofiltration (NF) hollow fiber membranes desirable for water softening under low operating pressure (<2bar) were successfully developed in the current study. The thin-film selective layer of the composite hollow fiber was formed through interfacial polymerization on the inner surface of a polyethersulfone (PES) ultrafiltration (UF) membrane substrate with branched polyethyleneimine (PEI) and trymesoyl chloride (TMC) employed as the monomers in aqueous and organic phases, respectively. It is found that a proper molecular weight of PEI and the presence of sodium dodecyl sulfate (SDS) in the aqueous phase are important for a successful interfacial polymerization reaction. The resulting membrane prepared with optimized preparation parameters possesses a positively charged thin-film selective layer with pure water permeability (PWP) of about 17l/m2 hbar and a molecular weight cut-off (MWCO) of around 500Da, which translates into the effective pore diameter of about 1.29nm. With combined separation mechanisms of Donnan exclusion and steric hindrance, it was possible to achieve MgCl2 and MgSO4 rejection of 96.7% and 80.6%, respectively, when tested for 1000ppm feed solutions at 2bar operating pressure. In addition, for a 3000ppm TDS feed stream containing salt mixtures, the membrane rejections for Mg2+ and Ca2+ ions were found to be around 90% while the water flux was about 20l/m2 h at 2bar pressure, suggesting the potential of the newly developed composite hollow fibers for effective water softening application. [Copyright &y& Elsevier]

An inorganic-organic fluorinated titania-silica (fTiO 2 -SiO 2 )/polyvinylidene fluoride (PVDF) composite membrane was fabricated, via facile in-situ vapor-induced hydrolyzation method followed by hydrophobic modification. This low mass-transfer-resistance membrane, composing of a mesoporous layer deposited onto macroporous substrate, was designed for biogas upgrading in gas-liquid membrane contactor (GLMC) application. The surface hydroxylation was introduced to facilitate the bridging of TiO 2 -SiO 2 nanoparticles and PVDF substrate, which resulted in a more coherent deposition of the fTiO 2 -SiO 2 layer onto the substrate. The surface microstructure was fine-tuned by controlling the amount of doped Si precursor, forming an integrated mesoporous fTiO 2 -SiO 2 layer. The resultant fTiO 2 -SiO 2 /PVDF composite hollow fiber membrane exhibited a tighter pore size of ~25 nm and a desired water contact angle of ~124°, which effectively prevented membrane wetting. The CO 2 absorption fluxes of 8.0 and 5.6 × 10 −3 mol m −2 s −1 were achieved due to the lower mass transfer resistance, by using 1 M of monoethanolamine (MEA) and sodium taurinate as absorbents with a liquid velocity of 0.25 m s −1 , respectively. The long-term stability test showed a good integrity between the fTiO 2 -SiO 2 layer and the PVDF substrate after 31-days of GLMC operation. The main benefit is the robust fluorinated inorganic layer which exhibited strong chemical resistance and high hydrophobicity, thus preventing membrane damage and pore wetting. Overall, this work provides an insight into the preparation of high–performance inorganic/organic composite hollow fiber membranes for carbon dioxide (CO 2 ) removal in GLMC application. [ABSTRACT FROM AUTHOR]

Due to its toxicity to ecosystem, phenol removal from industrial wastewater before discharge is a priority concern. Extractive membrane bioreactor (EMBR), a novel wastewater treatment process combining aqueous–aqueous extractive membrane process and biodegradation, has shown potential in treating phenol in wastewater. In this paper, composite hollow fiber membranes with different levels of poly(dimethylsiloxane) (PDMS) intrusion were prepared by coating a layer of PDMS on a Polyetherimide (PEI) hollow fiber substrate. Their applicability to EMBR for phenol removal was studied. The prepared membranes were characterized by microscopy and gas permeation test, and their performances were evaluated in aqueous–aqueous extractive membrane processes and EMBR process. The overall mass transfer coefficient for phenol, or k 0 , was found to be significantly affected by the level of PDMS intrusion in the composite membranes. This is because the penetration of PDMS into the porous substrate results in a denser membrane structure, which consequently increases the membrane resistance. A slight penetration of PDMS into the substrate was found to be necessary for the composite membranes to achieve high k 0 while maintaining low inorganic flux across the membranes. Wilson-plot analysis suggests that membrane resistance dominated over liquid boundary layer resistances. After more than 250 h of EMBR operation, significant biofilm growth was observed on the composite membranes and the k 0 was dropped but stabilized at around 7.5×10 −7 m/s. This k 0 was 7.5 times higher than commercial PDMS tubular membranes (without biofilm development) reported in previous studies, confirming the superiority of thin film composite membranes prepared in this work. It was also found that process optimization to control biofilm thickness is important in order to enhance phenol removal rate in EMBR. [ABSTRACT FROM AUTHOR]

Novel composite forward osmosis (FO) hollow fiber membranes with two selective skin layers were developed in the present work. The polyethersulfone (PES) hollow fiber substrate was firstly fabricated by a phase inversion method. The RO-like polyamide skin layer was then prepared on the inner surface of the hollow fiber substrate via interfacial polymerization (IP), while the NF-like outer skin layer was prepared via polyelectrolyte layer-by-layer (LBL) assembly. Three double-skinned membranes with negative (DS#1.5), positive (DS#2.0) and minimal (DS#2.5) outer surface charges under neutral pH environment were evaluated for their capabilities against organic fouling in the FO process. The newly developed double-skinned membranes possess water permeability greater than 2 l/m 2 h bar and NaCl rejection higher than 95% under 2 bar pressure, and have superior water flux and less salt leakage than the commercial HTI FO membranes as well as all the reported double-skinned FO membranes with RO-like selective layers. Most importantly, the double-skinned DS#2.5 hollow fiber membrane exhibits exciting organic fouling resistance because of its highly hydrophilic NF-like secondary layer with minimal surface charge and tightened surface structure to mitigate possible substrate pore clogging and fouling layer formation. A stable water flux of around 25 l/m 2 h was attained for DS#2.5 membrane using <0.5 M NaCl as the draw solution and a water containing 200 ppm dextran (DEX) or Aldrich humic acid (AHA), or lysozyme (LYS) and 10 mM NaCl as the model feed in the active layer against the draw solution (AL-DS) configuration, suggesting its great potential for practical FO applications. [ABSTRACT FROM AUTHOR]

Abstract: The performance of a forward osmosis (FO) system may be characterised by the assessment parameters: FO–RO flux ratio (J w/J w(RO)), apparent FO water permeability (J w/(π ds − π ml)), and the newly developed flux efficiency factor (J w,ob/J w,re). The former two parameters offer information on extent of internal concentration polarisation and driving force utilisation, respectively. The J w,ob/J w,re factor has practical relevance, and reveals the inevitable trade-off between flux and recovery (φ) for a FO system. The derived J w,ob/J w,re factors corresponded well to experimental observations. High water permeability, low salt-to-water permeability ratio, and large mass transfer coefficient improve the performance of a FO system, but these may also be influenced by operational and fouling effects, such as draw solute transmission, fouling resistance and cake-enhanced concentration polarisation. It was shown that membrane properties also play a significant role in fouling behaviour. Fouling amelioration factors include aeration and osmotic backwash. A thin-film composite membrane showed potential for FO application with favourable intrinsic transport parameters. It was demonstrated that a FO system could achieve stable water production with both relatively high flux efficiency (J w,ob/J w,re =0.8) and high recovery (φ =95.8%), which attested to the technology potential. [Copyright &y& Elsevier]