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The focus of whey processing has been changed from waste treatment to the production of valuable products owing to the advancement of membrane technology. Producing high quality whey products at low energy consumption entails novel techniques for whey concentration. This study proposes forward osmosis (FO) as a potential approach to concentrate whey protein solutions, which is superior to the concentration via reverse osmosis (RO), mainly owing to the low hydraulic pressure during FO process. In particular, this work validated the feasibility of FO-based whey concentration using high-performance hollow fiber membranes that were fabricated in-house. The investigation focused on the effects of various operating conditions on the concentration performance. The experimental results reveal that optimal concentration efficiency would be achieved by appropriately choosing the cross-flow velocity, draw solution concentration and the maximum attainable concentration of whey protein solution. Especially, the flux decline behaviour as a function of the whey protein concentration in the feed solution indicated rapid formation of a polarized or gel foulant layer at the membrane surface. The proposed mechanism was supported by the investigation of physical cleaning with water, showing that the post-process cleaning after the concentration cycle was able to recover the membrane performance, whereas the intermediate cleaning during one concentration cycle had little impact on the concentration efficiency enhancement. This study has practical significance for the application of FO in the concentration of dairy streams and other liquid foods. [ABSTRACT FROM AUTHOR]

Abstract: Pressure retarded osmosis (PRO) is a promising membrane process to harvest the vast amount of free energy from the mixing of fresh and salty waters without adverse environmental impacts. A competent membrane for PRO should possess the features of high water permeability to allow water permeation, excellent salt rejection to maintain the osmotic driving force, low structure parameter to minimize the internal concentration polarization (ICP), and robust strength to withstand the required hydraulic pressure. In this work, we have developed a novel hollow fiber membrane by balancing these competing factors based on our prior experience in the thin-film composite (TFC) membranes. The newly developed TFC hollow fiber membrane can achieve a power density of 20.9W/m2 at a pressure of 15bar, using synthetic seawater brine (1.0M NaCl) as the draw solution and synthetic river water (1mM NaCl) as the feed water, respectively. The simultaneous specific reverse salt flux was found to be 0.03mol/L, which is much lower than reported flat-sheet membranes, as the self-supported hollow fiber membrane can eliminate the deformation-enhanced reverse salt diffusion that is typical for flat-sheet membranes. However it was observed that the water permeability and structure parameter of the TFC hollow fiber membrane varied noticeably corresponding to the change in the hydraulic pressure imposed in the fiber lumen. Over a certain range (ΔP<15bar), such variations of increasing A value and decreasing S value proved to have positive impacts, as they facilitate the water permeation and reduce the ICP in the PRO process. [Copyright &y& Elsevier]

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: Hollow fiber membranes with a positively charged nanofiltration (NF)-like selective layer have been successfully fabricated by utilizing asymmetric microporous hollow fibers made of Torlon® polyamide–imide (PAI) material as the porous substrate followed by polyelectrolyte post-treatment using polyethyleneimine (PEI), which has not been reported previously. The reaction between PAI and PEI has been confirmed by experiments. The membrane structure and surface property are carefully tailored by adjusting polymer dope composition, spinning conditions and the post-treatment parameters. These membranes are, subsequently, characterized by a series of standard protocols in terms of membrane morphology, structure, permeability and salt rejection, and are applied in forward osmosis (FO) processes. The newly developed PAI hollow fiber membranes show a pure water permeability of 2.19–2.25L/m2 hbar and reasonable NaCl and MgCl2 rejections of 49% and 94% at 1bar, respectively. It is also found that in FO processes, when using 0.5M MgCl2 as a draw solution and DI water as the feed in the active layer facing feed water (AL-facing-FW) configuration at 23°C, the water fluxes of two PAI FO hollow fiber membranes are 8.36 and 9.74L/m2 h, respectively, and the J S /J V of the two membranes is smaller than 0.4g/L, which is lower than the data of 0.85g/L for commercial FO membrane from Hydration Technologies Inc. (HTI). Different from a neutral membrane, the positively charged FO membrane provides double electric repulsions to the salt transfer through the membrane in the AL-facing-FW configuration, leading to a reduction of salt penetration, while in the active layer facing draw solution (AL-facing-DS) configuration, the positive charges facilitate salt transportation. This study provides a simple and effective approach to make hollow fibers suitable for FO applications. [Copyright &y& Elsevier]

Abstract: A novel functionalized multi-walled carbon nanotube (MWCNT) immobilized polyethyleneimine–poly(amide–imide) (PEI–PAI) hollow fiber membrane was designed and fabricated using an easily scalable method. PAI hollow fiber was spun via phase inversion, followed by functionalized MWCNTs immobilization via vacuum filtration before a chemical post-treatment using PEI was applied to obtain a positively charged selective layer. The resulting membranes were evaluated in the forward osmosis (FO) process. The membrane prepared using a 0.62mg/L of MWCNTs solution shows a pure water permeability of 4.48L/m2hbar with a MgCl2 rejection of 87.8% at 1bar. This is a 44% enhancement in water permeability without significant compromise on the salt permeability when compared to membranes without MWCNTs. In addition, FO water flux of membranes prepared using 0.31 and 1.25mg/L of MWCNTs solutions have also been enhanced up to 29% without compromising the solute flux in the active layer facing feed water orientation using 0.5M MgCl2 solution as the draw solution and deionized water as the feed water. These encouraging results suggest that hollow fiber membranes immobilized with commercial MWCNTs exhibit good potential to be further developed for applications in the FO process. [Copyright &y& Elsevier]