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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]

The fabrication of new membrane that is able to produce stable high power density is essential for the development of pressure retarded osmosis (PRO) technology. In this work, thin film composite (TFC) polyetherimide membranes with three different substrate structures were fabricated and characterized. The PRO performance of the resultant membrane was evaluated through two pressure cycle experiments and stability tests. The primary objective of this work is to systematically study PRO membrane’s mechanical stability and mass transfer properties under high operating pressures, which determine the power density of the membrane in the PRO process. Experiments revealed that water permeability ( A p ) and salt permeability ( B p ) under different pressures varied. The A p and B p are better indicators for examining the variation of PRO membrane structure under pressure. Within the operating pressure range of 17.2 bar, the top polyamide layer of TFC PEI-2# membrane mainly experienced a reversible deformation. In the two pressure cycle tests, the water flux, specific salt flux and power density obtained in the upward and downward measurements in each cycle are close to each other. The first cycle and the second cycle also show excellent reproducibility. The membrane was able to maintain almost unchanged water flux and power density of 12.8 W/m 2 at 17.2 bar over the 10 h testing time, suggesting the membrane’s great potential to be used in practical application in the future. [ABSTRACT FROM AUTHOR]

Pressure-retarded osmosis (PRO) has attracted worldwide attention for its potential applications in renewable osmotic energy harvesting, low energy seawater desalination, and industrial waste brine disposal. However, membrane fouling is one of the major issues limiting PRO performance with the use of a high-salinity draw solution and a low-value impaired feedwater. This study systematically investigated membrane fouling in the PRO process using a real wastewater retentate (a waste byproduct from wastewater reclamation plants) as the PRO feed. Organic fouling (e.g., by humic substances) and inorganic fouling (e.g., by calcium phosphate) were identified to be the major types of fouling, where the latter dominated the overall water-flux decline. The internal PRO membrane fouling was exacerbated by internal concentration polarization (ICP) and intermolecular interactions between the organic macromolecules and the scaling precursor ions (e.g., Ca 2+ ). A limiting flux that was independent of the initial flux (i.e., applied pressure) was observed during the PRO fouling and was explained by a novel PRO limiting flux model. A membrane with a smaller structural parameter could achieve a higher limiting flux in PRO. A simple and effective pressure-assisted osmotic backwashing protocol was developed to clean the internally fouled membrane that could restore over 92% of the water flux within a short cleaning period (15 mins). The PRO performance could also be significantly improved by decreasing the feedwater pH that mitigated alkaline scaling. Finally, performing the PRO membrane fouling test in the active-layer-facing-feed-solution (AL-FS) orientation revealed that the integral cellulose triacetate (CTA) membrane exhibited strong mechanical stability and anti-fouling tendency whereas the thin-film composite (TFC) membrane was not mechanically stable in this orientation. This study for the first time identified that an integral membrane instead of a TFC membrane is an excellent candidate for PRO operation in the AL-FS mode for fouling control. [ABSTRACT FROM AUTHOR]

The performance of pressure retarded osmosis (PRO) processes is significantly limited by membrane fouling. This study systematically investigated PRO scaling by silica, which is one of the most common salts causing membrane scaling. In particular, the effect of silica concentration, the initial water flux, the chemistry of feed solution (FS) /draw solution (DS), and the type of membrane were studied in the PRO mode of operation. It has been observed that a low silica concentration (e.g., 10 mg/L SiO 2 ) could even cause membrane scaling during a PRO process. The rate and extent of the scaling was mainly governed by the internal concentration of silica in the membrane support layer and the solution chemistry of the FS/DS. The results suggested that the scaling could be mitigated by decreasing the internal concentration through the reduction of bulk silica concentration, the optimization of operating water flux, and the decrease in membrane structural parameter. The structural parameter seems to play a dominant role in PRO scaling by silica than the membrane materials and other physico-chemical properties. In addition, silica scaling can be controlled by lowering the pH of the FS and/or DS. For the first time, it has been demonstrated that utilizing an acidic DS was highly effective in controlling PRO scaling, which can be attributed to the low local pH in the membrane support layer as a result of the internal concentration polarization (ICP) of the reversely diffused H + . This method opens a new dimension for PRO scaling/fouling control. [ABSTRACT FROM AUTHOR]

In present study, we explored the fabrication of a high performance thin-film composite (TFC) forward osmosis (FO) membrane by minimizing the structural parameter of hollow fiber substrates and enhancing the water permeability of selective layer. A theoretical analysis specified that a minimized structural parameter of substrate combined with an enhanced water permeability of selective layer could generate a significantly high water flux in FO process. The experimental results showed that the addition of LiCl into polyetherimide (PEI) polymer dope made the membrane less tortuous and thereby significantly reduced the structural parameter. The elevated take-up speed reduced the substrate thickness but also made the substrate more tortuous. Further study showed that a robust hollow fiber with low structural parameter can be obtained by reducing the substrate wall thickness and dimension simultaneously. After optimization, the structural parameter of the substrate declined from 308 µm to 172 µm. After interfacial polymerization, the thin-film composite membrane constructed on the optimized substrate showed an improved water permeability (from 2.85 L m −2 h −1 bar −1 to 3.66 L m −2 h −1 bar −1 ) and a comparably low salt permeability (0.36 L m −2 h −1 versus 0.31 L m −2 h −1) . The low structural parameter, in combination with the high water permeability and low salt permeability, contributed to significantly increased water flux ( J v ) from 25.4 L m −2 h −1 to 38.5 L m −2 h −1 when 1 M NaCl was used as draw and deionised water as feed in a configuration of active layer facing feed solution (AL-FS). Furthermore, aquaporins were incorporated into the polyamide selective layer to enhance the water permeability, which remarkably reached up to 7.6 L m −2 h −1 bar −1 . The aquaporin-incorporated FO membrane demonstrated a J v of 49.1 L m −2 h −1 and a J s /J v ( J s indicates the salt flux) of 0.10 g/L with 1 M NaCl as draw and deionised water as feed in the AL-FS configuration, which is favourable to mitigate membrane fouling in practical application. The AQP-FO membrane reported in this study outperforms most of other reported FO membranes. [ABSTRACT FROM AUTHOR]

The use of porous membranes for forward osmosis (FO) applications has been recently reported in the literature. In the current study, a series of porous-FO membranes were fabricated to systematically study the role of membrane separation properties on the FO performance of porous membranes. In addition, two types of draw solutes, a polyelectrolye poly(sodium 4-styrene-sulfonate) (PSS) and a neutral linear polymer polyethylene glycols (PEGs), were evaluated as the draw solutes. The effect of draw solution properties (type of draw solutes, their molecular weight, and concentration) on the FO performance was investigated. Membrane post-treatment at higher annealing temperature was found to increase solute rejection and decrease water permeability. As a result, reverse solute diffusion (RDS) became less severe at higher annealing temperature. The FO water flux of the porous membranes was affected by RDS in addition to the water permeability of the porous FO membranes, both of which had a strong dependence on the annealing temperature. Compared to neutral PEG, the polyelectrolyte PSS was a much better choice for draw solutes with higher FO water flux and lower RDS. The better performance of PSS can be attributed to its higher rejection by the porous FO membranes and its high osmotic pressure compared to PEG at same weight concentration. Implications for the design of porous FO membranes and draw solution are discussed. [ABSTRACT FROM AUTHOR]

Abstract Forward osmosis (FO) membranes have received attention as an energy-efficient and low-cost technique in stream concentrating processes. In this work, a novel double-skinned hollow fiber thin film composite (TFC) FO membrane has been successfully fabricated, which consists of a one-step prepared dual layer substrate and a thin inner selective layer formed via interfacial polymerization. The substrate consists of a relatively dense ultrafiltration (UF) outer layer and a porous UF inner layer, both of which were constructed from polyethersulfone (PES) as the substrate material by using dual-layer co-extrusion technique. Compared to the commercial and reported double-skinned FO membranes, the FO membrane developed in this work exhibited a higher permeate flux with humic acid solution as feed. Furthermore, the double-skinned FO membrane was applied in concentrating activated sludge using 0.5 M NaCl as draw solution, and a permeate flux at 5.4 L/m2h was achieved after 5 h operation, which was higher than or comparable to those of the reported FO membranes. Membrane autopsies and foulant analysis suggested that the dense UF skin layer helped to reject greater-sized organic foulants (> 300 Da). This study shed light on the important fabrication features and promising application of the double-skinned hollow fiber TFC FO membrane in sludge concentration. Graphical abstract fx1 Highlights • A dual-layer UF substrate was prepared via a single-step fabrication process. • TFC FO membrane was fabricated via interfacial polymerization in the lumen of the substrate. • FO performance in sludge thickening process was investigated. • Tight UF layer helped rejecting greater-sized organics in sludge. • A permeate flux of 5.4 L/m2h was achieved after 5 h operation using 0.5 M NaCl as draw solution. [ABSTRACT FROM AUTHOR]

Pressure retarded osmosis (PRO) is an innovative technique to yield osmotic energy. Nevertheless, the PRO performance is exacerbated by the severe membrane fouling occurring within the porous support. It is a potential alternative for sustainable power harvesting to change the membrane orientation from active layer facing draw solution (AL−DS) or PRO mode to active layer facing feed solution (AL−FS) or FO mode for a low-fouling propensity. A novel integral PRO hollow fiber membrane was developed in this work by phase inversion and followed with a facile cross-linking post-treatment. The structure and properties of the resultant membrane were systematically investigated using a series of characterization protocols. The PRO performance of the membrane was studied under various hydraulic pressures at the AL−FS orientation. The results reveal that the substrate with sponge-like structure and relatively dense inner surface can better withstand high pressure in PRO process. Two steps of cross-linking endowed the membrane with a smaller pore size on the outer surface that exhibits high rejection against various inorganic salts. The novel integral membrane can achieve a stable power density output around 4.3 W m −2 at 12 ~ 13 bar hydraulic pressure, using a real wastewater reverse osmosis (RO) retentate and 1.0 M sodium chloride as the feed and draw stream, correspondingly. This result demonstrates that the PRO operation in the AL−FS orientation (FO mode) offers a significant advantage over the AL−DS orientation (PRO mode) by eliminating the pretreatment of the feed water or/and other measures associated with membrane fouling control in the PRO mode. To implement the PRO operation with the AL−FS orientation, developing a robust membrane with an integral structure is critical. The newly developed integral hollow fiber membrane via chemical cross-linking presents the potential for PRO operation in the AL−FS orientation. [ABSTRACT FROM AUTHOR]

The salinity gradient is a renewable and sustainable energy source that can be utilized by a pressure-retarded osmosis (PRO) process. Development of the PRO technology was impeded by shortage of efficient membranes until recent surge on PRO research. In this work, flat-sheet thin film composite (TFC) PRO membranes with reinforced support layer were developed. The membrane properties were optimized by blending high molecular weight polyvinylpyrrolidone (PVP) into the polysulfone (PSf) casting solution. Influences of PVP addition on the kinetic and thermodynamic properties of the casting solution, resultant membrane properties as well as PRO performance were systematically investigated. It is showed that PVP can effectively improve the membrane morphology and pore structure. By adjusting the PSf/PVP composition in the substrate recipe, the reinforced TFC membrane is able to achieve a power density of 12.9 W/m 2 at 22 bar using 1 M NaCl draw solution and pure water feed. Long-term PRO test at the peak power density and multi-pressure cycle tests were performed on the membranes. Consistent power density and low salt flux/water flux ratio were observed in the whole tests. The results demonstrated that the reinforced TFC membrane in this study had excellent mechanical strength and stability. It would be promising for future engineering application. [ABSTRACT FROM AUTHOR]

Pressure retarded osmosis (PRO) is a promising energy harvesting technique. However, when polymeric hollow fiber membrane is used for the PRO process, the mechanical strength of the membrane is a big concern. As hollow fiber membrane is self-supported and due to its polymeric nature, it may gradually deform over time under high pressure loading, or membrane “creeping” will occur. Current work is the first attempt to analyze the membrane creeping phenomenon of a novel thin film composite (TFC) hollow fiber. The membrane creeping was evaluated via nanoindentation by using atomic force microscope (AFM). A non-stop 200-hour PRO test and integrity check, which have not been reported previously, were carried out to investigate the membrane performance under various operating pressures. The results show that the membrane is able to produce a stable power density output of 19.2 W/m 2 at 15.0 bar, using 1.0 M NaCl as the draw solution and DI water as the feed water. Membrane creeping was observed when the applied pressure exceeded the safe operation limit (or the flux turning point, where the membrane flux started to increase with increasing applied pressure in the PRO mode), which caused an irreversible damage to the membranes. This study identified safe and optimum operation conditions of the laboratory-made PRO hollow fiber membrane to achieve the most favorable PRO performance. It provides guidance for practical applications of polymeric hollow fiber membranes in PRO process. [ABSTRACT FROM AUTHOR]

A scaled-up hollow fibre thin-film composite (TFC) module was successfully developed for organic solvent forward osmosis (OSFO) application in the pharmaceutical industry. The membrane consists of cross-linked P84 polyimide substrate with a polyamide selective layer, formed via interfacial polymerisation. The performance of the lab-scale module was compared to the scaled-up module in reverse osmosis and forward osmosis modes. The results from both tests revealed that the solvent permeability of the scaled-up module decreased while its selectivity increased slightly. This might be caused by the uneven distribution of liquids during interfacial polymerisation, leading to varied membrane performance. The scaled-up module was further tested for OSFO performance using a range of PEG-400 concentrations (0.5–2 M) in acetone and isopropanol as draw solutions. At 2 M PEG-400 concentration, the module demonstrated high isopropanol and acetone permeability of 0.63 LMH and 13.9 LMH, respectively. Moreover, the reverse solute flux of PEG-400 in acetone and isopropanol were relatively low at 0.188 g/l and 0.153 g/l, respectively. The module was further tested for OSFO application in the pharmaceutical industry by concentrating levofloxacin in acetone, using 2 M PEG-400 as draw solution. The process was able to concentrate levofloxacin from 1000 ppm to 16,000 ppm with very low reverse solute flux of 0.178 g/l. Therefore, this work presents a scalable TFC membrane for OSFO application in the pharmaceutical industry. [Display omitted] • Scaled-up module (0.4 m2) of TFC hollow fibre membrane for organic solvent forward osmosis process was successfully fabricated. • High porosity (79%) and low structural parameter (217 μm) for membrane substrate was achieved. • Membrane exhibited excellent acetone and IPA solvent flux of 13.9 LMH and 0.63 LMH, respectively, using a 2 M PEG-400 draw solution. • A pharmaceutical stream was concentrated to a factor of 16 with low specific reverse solute flux [ABSTRACT FROM AUTHOR]

A scaled-up hollow fibre thin-film composite (TFC) module was successfully developed for organic solvent forward osmosis (OSFO) application in the pharmaceutical industry. The membrane consists of cross-linked P84 polyimide substrate with a polyamide selective layer, formed via interfacial polymerisation. The performance of the lab-scale module was compared to the scaled-up module in reverse osmosis and forward osmosis modes. The results from both tests revealed that the solvent permeability of the scaled-up module decreased while its selectivity increased slightly. This might be caused by the uneven distribution of liquids during interfacial polymerisation, leading to varied membrane performance. The scaled-up module was further tested for OSFO performance using a range of PEG-400 concentrations (0.5–2 M) in acetone and isopropanol as draw solutions. At 2 M PEG-400 concentration, the module demonstrated high isopropanol and acetone permeability of 0.63 LMH and 13.9 LMH, respectively. Moreover, the reverse solute flux of PEG-400 in acetone and isopropanol were relatively low at 0.188 g/l and 0.153 g/l, respectively. The module was further tested for OSFO application in the pharmaceutical industry by concentrating levofloxacin in acetone, using 2 M PEG-400 as draw solution. The process was able to concentrate levofloxacin from 1000 ppm to 16,000 ppm with very low reverse solute flux of 0.178 g/l. Therefore, this work presents a scalable TFC membrane for OSFO application in the pharmaceutical industry. [Display omitted] • Scaled-up module (0.4 m2) of TFC hollow fibre membrane for organic solvent forward osmosis process was successfully fabricated. • High porosity (79%) and low structural parameter (217 μm) for membrane substrate was achieved. • Membrane exhibited excellent acetone and IPA solvent flux of 13.9 LMH and 0.63 LMH, respectively, using a 2 M PEG-400 draw solution. • A pharmaceutical stream was concentrated to a factor of 16 with low specific reverse solute flux [ABSTRACT FROM AUTHOR]

External concentration polarization (ECP) remains a pervasive challenge for submerged forward osmosis (FO) systems, as it critically reduces the water flux of membranes. In this study, we propose to use transverse vibration on hollow fiber membranes to effectively reduce the ECP and increase the FO water flux performance. The effects of different vibrating amplitude and frequency were investigated. Also, the vibration effect was evaluated under different draw solution concentrations, membrane orientations and draw solute types. Our results show increased water flux with increasing vibrating intensity (i.e., amplitude and frequency) while maintaining a constant specific reverse solute flux (J s /J w). At the highest amplitude (1.2 cm) and frequency (3 Hz) used, the mass transfer coefficient is increased from 0.7 × 10−5 m s−1 (at no vibration) to 1.8 × 10−5 m s−1, which falls within the optimal flux range as determined from our FO modelling results. The increment in mass transfer coefficient is found to be closely related to the improved hydrodynamic conditions due to vibration, and less dependent on the membrane orientation and draw solution concentration. In addition, both modelling and experimental results show that maintaining a reasonably high mass transfer coefficient is especially important for FO process with severe ECP conditions, such as when dilutive ECP is present (i.e., active-layer-facing-feed-solution orientation) or when a high water flux is applied (usually in the active-layer-facing-draw-solution orientation). However, the severe ECP arising from large draw solutes (e.g., larger sized polymer with low diffusivity and high viscosity) might not gain greater benefits from the vibration or improved hydrodynamic conditions. Image 1 • Transverse vibration can mitigate ECP and improve flux in submerged FO processes. • Increasing vibration intensity increases mass transfer coefficient at membrane surface. • Maintaining a high mass transfer coefficient is important for severe ECP conditions. • Severe ECP resulting from large draw solutes cannot gain more benefits from vibration. [ABSTRACT FROM AUTHOR]