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• TFC membrane with phosphorylated-TiO 2 -modified PVDF substrate for FO application. • P -TiO 2 enhances MPD adsorption and confines MPD migration during IP. • PA layer exhibits smoother surface with p -TiO 2 incorporation in the substrate. • Alleviated ICP and more compact PA layer are achieved. • Modified membranes possess high water flux and low fouling propensity. Forward osmosis (FO) membrane is paramount to the progress and implementation of FO process. However, typical FO membranes are confronted with many unsatisfactory issues, including the low water flux, high fouling propensity and the poor long-term operation stability in most cases, which confines the practical application of FO process to a large extent. This study proposes a feasible way to mitigate the internal concentration polarization (ICP) and optimize the polyamide (PA) layer structure of as-fabricated thin-film composite forward osmosis (TFC-FO) membranes simultaneously by incorporating phosphorylated titanium dioxide (p -TiO 2) in polyvinylidene fluoride (PVDF) substrate. P -TiO 2 nanoparticles with abundant phosphate groups were prepared through a chemical absorption approach followed by the thermal post-treatment. Superior water flux and the better antifouling properties of the resultant PVDF/ p -TiO 2 TFC-FO membrane was achieved, taking advantage of the more compact and smoother PA layer as a result of the confined migration of amine monomer during interfacial polymerization (IP). In addition, effects of p -TiO 2 content on morphological changes and physicochemical properties of modified substrates and the construction of top PA layer are studied and elaborated systemically via many characterizations as well as the permeation tests. As a result, the water flux decline of PVDF/ p -TiO 2 TFC membrane is much lower (16.0%) than that (∼31.3%) of the pristine TFC membrane, in the long-term (20 h) dynamic fouling test against 500 ppm humic acid (HA) solution. Therefore, PVDF/ p -TiO 2 TFC-FO membrane with improved water flux and low fouling tendency is beneficial for wastewater treatment and desalination process, especially for the treatment of brackish water with high fouling tendency or under harsh conditions. [ABSTRACT FROM AUTHOR]

One of the limiting factors in good performance of forward osmosis (FO) membranes is the internal concentration polarization (ICP). To reduce ICP, thin film nanocomposite forward osmosis (TFN-FO) membranes were fabricated by adding different amounts of graphene oxide (GO) nanoplates (0-1 wt. %) to polymer matrix of polysulfone (PSf) substrate. The prepared nanocomposite membranes exhibited both hydrophilicity and porosity higher than that of neat PSf counterpart. An optimum amount of 0.5 wt% was obtained for GO addition into the membranes. The corresponding fabricated thin film nanocomposite (TFN) membrane (TFNG0.5) revealed a water permeability of 2.44 L/m2hbar which is 66% higher compared to an in-house made composite membrane. The FO performance of TFN was assessed by DI water as feed solution and 1 M NaCl as draw solution in AL-DS orientation. The water flux of the synthesized FO membranes increased upon adding of GO nanoplates and reached to a maximum water flux of 37.74 (L/m2h) for TFNG0.5 membrane. This flux is about 3 times higher than TFC membranes without significant changes in their salt rejection. The higher water flux of the TFN membranes can be attributed to ICP decrease originating from reduction of structural parameter of the membranes.

Internal concentration polarization (ICP) in forward osmosis (FO) process is a characteristic problem for asymmetric thin-film composite (TFC) FO membrane which leads to lower water flux. To mitigate the ICP effect, modification of the substrates’ properties has been one of the most effective methods. A new polyethersulfone-based ultrafiltration membrane with increased surface porosity and high water flux was recently produced by incorporating Zn2GeO4 nanowires. The composite membrane was used as a substrate for the fabrication of TFC FO membrane, by coating a thin layer of polyamide on top of the substrate. The substrate and the nanowires were characterized by a range of techniques such as SEM, XRD, and contact angle goniometry. The water permeability and molecular weight cut-offs (MWCO) of the substrate; and the FO performance of the TFC membrane were also determined. The Zn2GeO4-modified membrane showed ~45% increase in water permeability and NaCl salt rejection of 80% under RO mode. In FO mode, the ratio of water flux to reverse solute flux was also improved. However, lower FO flux was obtained which could be due to ICP. The result shows that Zn2GO4 nanowire may be used as a modifier to the substrate to improve the quality of the polyamide layer on the substrate to improve the flux and selectivity, but not as effective in reducing ICP. This work demonstrates that the incorporation of nanomaterials to the membrane substrate may be an alternative approach to improve the formation of polyamide skin layer to achieve better FO performance.

This work provides a facile and effective way of incorporating SiO 2 @MWNTs in polyvinylidene fluoride (PVDF) substrate to alleviate the internal concentration polarization (ICP) impact of the resultant thin film composite forward osmosis (TFC-FO) membranes for the first time. The in-situ decoration of multi-walled carbon nanotubes (MWNTs) using tetraethyl orthosilicate (TEOS) as precursor was conducted, resulting in improved hydrophilicity and the excellent dispersion of synthesized SiO 2 @MWNTs in PVDF membrane substrates accordingly. Effects of the SiO 2 @MWNTs concentration on the morphologies and properties of the modified substrates, and further impact on the polyamide (PA) layer structure are investigated via various characterizations. Besides, the intrinsic separation properties and forward osmosis performance of resultant TFC-FO membranes are also systemically studied. In comparison with the control TFC membrane, SiO 2 @MWNT-modified TFC membranes exhibit higher water fluxes with the enhanced membrane selectivity, as indicated by the decreased J S /J V values (from 1.10 to 0.19 g/L). This work therefore presents an alternative approach to prepare TFC-FO membranes with enhanced water flux as well as increased selectivity. [ABSTRACT FROM AUTHOR]

Recent studies have proved the advantages of novel composite membranes in the extractive membrane bioreactor (EMBR) for recalcitrant organic wastewater treatment. However, more systematic research into membrane properties and mass transfer effects are required. This study investigates the role of internal concentration polarization (ICP), including concentrated and diluted ICP, in an aqueous-aqueous extractive process, and the coupled effects of membrane roughness and hydrodynamic conditions on membrane performance in both cross-flow EMBR (CF-EMBR) and submerged EMBR (S-EMBR) processes. To study these factors, a nanofibrous composite membrane with a four-tiered structure, consisting of a dense polydimethylsiloxane (PDMS) selective layer, a polyvinylidene fluoride (PVDF) nanofibrous sublayer, a non-woven mechanical support and a rough micro/nano-beaded layer, has been developed for the EMBR by electrospinning and spray-coating. A diluted ICP, which occurred when the selective layer faces receiving water (SL-R w ), was less severe than a concentrated ICP in the selective layer facing feed (SL-F) orientation. It resulted in higher overall membrane mass transfer coefficients ( k 0 's) in the SL-R w mode during the aqueous-aqueous extractive tests. In addition, less biofilm was observed on the smooth PDMS surface after 14-days CF-EMBR and S-EMBR operations in the selective layer facing biomedium (SL-R bio ) mode. In contrast, in the SL-F mode during EMBR operations, the ridges and valleys on the membrane surface were able to provide a more favourable micro-environment for microorganisms and protect them from external stresses. The mature micro-colonies that developed in the SL-F mode clogged the membrane support and enhanced concentrated ICP, resulting in a significant reduction of k 0 's by more than 58%. Moreover, as the shear force on the membrane surface due to up-flow of air bubbles in the S-EMBR could mitigate the biofilm attachment, the most effective biofilm control strategy found in this study was to operate the nanofibrous composite membrane in the S-EMBR configuration with a smooth selective layer facing the biomedium. This effectively decreased the amounts of protein by 86%, polysaccharides by 88% and total suspended solids (TSS) by 89% in the biofilms on membrane surface, and achieved the highest stable k 0 of 7.0 × 10 −7 m/s. [ABSTRACT FROM AUTHOR]

This work investigated the influence of a new polymeric blend of polysulfone/polyacrylonitrile (PSf/PAN) nanofibers prepared via the electrospinning process as substrate to produce thin film composite forward osmosis (TFC-FO) membrane. The solvents in the electrospinning process were optimized. A polyamide (PA) thin layer was successfully fabricated on the electrospun nanofibrous substrate via interfacial polymerization. The performance of the nanofiber-based thin film composite (NTFC) membranes was compared with the in-house-made (PSf/PAN) TFC membrane, in which its substrate was fabricated by phase inversion. The NTFC membrane demonstrated significant improvement in hydrophilicity and water permeability, and the reverse salt flux (RSF) was reduced. In addition, the structural parameter (S) value of the fabricated NTFC decreased considerably which represented the reduction of internal concentration polarization (ICP) during the FO process. These achieved results were due to nanofiber structural characteristics such as high porosity and interconnected open pore structure. The effects of different salts as draw solutions (NaCl, KCl, MgCl 2 , MgSO 4 ) on the osmotic performance of the NTFC and TFC membranes were evaluated. Among the tested draw solutions with the same osmotic pressure, the NTFC membrane exhibited higher water flux (38.3 LMH) than that of the TFC membrane (14.3 LMH) for KCl draw solution. [ABSTRACT FROM AUTHOR]

A thermally induced phase separation followed by non-solvent induced phase separation (TIPS-co-NIPS) was used to prepare polyethersulfone (PES) substrate with reduced internal concentration polarization (ICP) for thin film composite forward osmosis (TFC-FO) membrane. Using polyethylene glycol (PEG) as the additive, impacts of TIPS process on substrate properties, TFC layers and FO performances were investigated. The results revealed that the substrate prepared by TIPS-co-NIPS method with 10 s cooling time showed a fully finger-like structure, enhanced permeation performance and better surface hydrophilicity. As a result, the corresponding TFC-FO membrane exhibited 44% lower structural parameter than the TFC-FO membrane based on NIPS substrate (684 µm versus 380 µm), high water fluxes up to 32.9/20.4 LMH and reverse salt fluxes of 14.1/8.2 gMH in the PRO/FO mode using 1 M NaCl as the draw against DI-water feed. Therefore, the proposed TIPS-co-NIPS method utilizing PEG as the additive may be considered as a feasible approach to regulate substrate structure and property for TFC-FO membrane. [ABSTRACT FROM AUTHOR]

Internal concentration polarization (ICP) severely limits water flux performance in forward osmosis (FO). We investigated the use of ultrasound to mitigate ICP. Various parameters affecting the performance of the novel ultrasonically-assisted FO were studied, such as ultrasonic frequency and constant versus pulsed operation. With either deionized water or polyphenolic tannin solution as the feed and sodium sulphate as the draw solution, the water flux was nearly doubled for a thin film composite FO membrane upon the application of a 20 kHz ultrasound, with stronger enhancement achieved when the ultrasound was applied to the support layer of the membrane. High frequencies of 573 and 1136 kHz had much weaker effects. Pulsed application of ultrasound can significantly reduce the energy consumption of sonication. For the first time, the current study provides compelling evidence that ultrasonic vibrations applied to porous support structure of an FO membrane is highly effective in mitigating ICP. [ABSTRACT FROM AUTHOR]

A novel thin-film composite (TFC) forward osmosis (FO) membrane with dual-layered substrate membrane was fabricated by a double-blade casting technique using different polysulfone (PSf) concentrations for top (15 wt%) and bottom (7 wt%) substrate layers. Graphene oxide (GO) was incorporated in the substrate layer, and the dual casting approach resulted in a membrane support with a highly porous bottom structure and a dense top skin layer on which the polyamide active layer was effectively formed. The dual-layered TFC PSf/GO membrane (TFC-PSf d GO) exhibited high water permeability, and ion selectivity was enhanced by the presence of well dispersed hydrophilic GO in the PSf substrate. The TFC-PSf d GO also exhibited the lowest specific reverse salt flux ( J s /J v = 0.19 g L -1 ) and a more favorable structural parameter (S = 130 μm) compared to GO-free membranes. Using deionized water as feed solution and 1 M NaCl as draw solution (DS), TFC-PSf d GO had J v = 33.8 L m −2 h −1 and J s = 6.9 g −2 h −1 under AL-FS mode, and J v = 61.5 L m −2 h −1 and J s = 14.0 g −2 h −1 under AL-DS mode. The potential of TFC-PSf d GO for commercial application was further evaluated by fabricating it with a fabric backing support (denoted as TFC-PSf d GO f ). Compared to TFC-PSf d GO, TFC-PSf d GO f exhibited only 14% decline in its water flux. The overall results reveal that, fabrication of TFC substrate membrane via dual-blade casting approach along with GO incorporation produced high-performance TFC FO membranes which likely reduced the internal concentration polarization effects. [ABSTRACT FROM AUTHOR]

The ever-growing global energy crisis has been pushing industries to lean towards renewable resources as an alternative for power generation. Recent research is focusing mainly towards achieving water sustainability and producing green energy without creating any liquid discharge. In this review paper, a complete analysis of the latest developments in membrane modification for pressure retarded osmosis (PRO) processes has been discussed. Moreover, the use of different nanomaterials like zeolite, graphene oxide, carbon nanotubes, halloysite nanotubes, TiO 2 , SiO 2 , silica, carbon quantum dots, covalent organic frameworks, and metal-based nanoparticles for membrane modification to maximize PRO performance have been presented. Besides, the hybrids of PRO in combination with reverse osmosis (PRO-RO), forward osmosis (PRO-FO) and PRO thermal have also been reviewed. Additionally, the recent test results on the incorporation of novel nanofibers like polyetherimide and polyacrylonitrile into different thin film composite and hollow fiber membranes have been presented here. Also, this review presents the future direction of PRO research, economic analysis on cost reduction and its commercialization potentials. Due to its immense benefits offered, the Middle East is beginning to consider PRO process to enhance the shrinking fresh water source in the region, while producing salinity gradient energy. [Display omitted] • Different membranes for PRO process reviewed. • Utilization of nanomaterials in PRO membranes analyzed. • Nanofiber-based membranes for PRO process examined. • Performance limiting factors for PRO membranes studied. • PRO full-scale, pilot-scale demonstrations and commercialization discussed. [ABSTRACT FROM AUTHOR]

The characteristic parameters of pressure retarded osmosis (PRO) membranes (water permeability, A , solute permeability, B , and the structural parameter of the membrane support layer, S ) were estimated by a single PRO test combined with a statistical approach. The single PRO test measures water and reverse solute flux ( J w and J s ) as functions of the hydraulic pressure in the draw solution (DS) using sodium chloride solutions as feed and draw solutions. The PRO test data were obtained from literatures. A statistical approach developed in this work finds the most appropriate parameters ( A , B , and S ) to predict J s / J w and power density of the tested PRO membrane using the internal concentration polarization (ICP) model. The model prediction results of J s / J w and power density using the PRO membrane parameters obtained from the single PRO test matched the experimental data better than those using the membrane parameters obtained from the conventional reverse osmosis (RO) and forward osmosis (FO) tests. The A and B parameters from the single PRO test were consistently higher than those from the RO and FO tests, which could be related to the membrane deformation under the high-pressure PRO operation with the poor support by the spacer. [ABSTRACT FROM AUTHOR]

The filtration performance of combined organic foulants by forward osmosis (FO) in active-layer-facing-the-draw-solution (AL-facing-DS) orientation was investigated systematically. Tannic acid and alginate were used as model organic foulants for polysaccharides and humic dissolved organic matters, respectively. The FO could reject combined and single tannic acid and alginate foulants effectively. The more severe fouling flux decline, accompanied with lower combined foulants' retention, was observed with increasing proportions of tannic acid in the combined foulants-containing feed, which was ascribed mainly to the more severe fouling resulting from tannic acid adsorption within the porous support layer of the FO membrane compared to minor alginate deposition on the membrane surface. It was found that the higher the initial flux level and cross flow velocity, the faster the flux decline with lower mixed foulants retention. It was also revealed that the calcium ions in a basic solution enhanced the combined fouling flux reduction and combined foulants retention. As the major constituent of the combined fouling layer, the adsorption of tannic acid might play a more significant role in the mixed fouling of the FO membrane, which was probably influenced by permeation drag caused by water flux and chemical interactions induced by feed solution pH and calcium ion concentration. [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]

Internal concentration polarization (ICP) in substrate layer is one of the most critical bottlenecks of the forward osmosis (FO) process. In this study, we explored the use of metal–organic frameworks (MOFs) as a removable filler to prepare MOF-based porous matrix membranes (PMMs) for improving the mass transfer in the FO substrates and hence controlling the ICP. MOF-based porous matrix substrates (PMSs) with three different types of MOFs were prepared via phase inversion by adding MOF particles into the polyacrylonitrile (PAN) dope solution. A thin selective layer was prepared using a layer-by-layer (LbL) deposition method on top of the porous matrix FO substrate. The bond dissociation energy (BDE) between metal ions and organic linker of MOF particles played an important role for the selection of fillers of PMMs. For MOF particles with lower BDE (<~200 kJ/mol), the corresponding MOF-based porous matrix FO membranes had higher membrane bulk porosity. This study shows the effect of different types of MOF particles in MOF-based porous matrix FO substrate for controlling the ICP in FO application for the first time, which provides an additional dimension for ICP control in osmotically-driven membrane processes. [ABSTRACT FROM AUTHOR]

Internal concentration polarization (ICP) in forward osmosis (FO) process is a characteristic problem for asymmetric thin-film composite (TFC) FO membrane which leads to lower water flux. To mitigate the ICP effect, modification of the substrates' properties has been one of the most effective methods. A new polyethersulfone-based ultrafiltration membrane with increased surface porosity and high water flux was recently produced by incorporating Zn2GeO4 nanowires. The composite membrane was used as a substrate for the fabrication of TFC FO membrane, by coating a thin layer of polyamide on top of the substrate. The substrate and the nanowires were characterized by a range of techniques such as SEM, XRD, and contact angle goniometry. The water permeability and molecular weight cut-offs (MWCO) of the substrate; and the FO performance of the TFC membrane were also determined. The Zn2GeO4-modified membrane showed ∼45% increase in water permeability and NaCl salt rejection of 80% under RO mode. In FO mode, the ratio of water flux to reverse solute flux was also improved. However, lower FO flux was obtained which could be due to ICP. The result shows that Zn2GO4 nanowire may be used as a modifier to the substrate to improve the quality of the polyamide layer on the substrate to improve the flux and selectivity, but not as effective in reducing ICP. This work demonstrates that the incorporation of nanomaterials to the membrane substrate may be an alternative approach to improve the formation of polyamide skin layer to achieve better FO performance. [ABSTRACT FROM AUTHOR]

A novel method to determine reverse draw and forward feed solute fluxes in forward osmosis (FO) membrane was developed to analyze FO performance more accurately. Specifically, apparent draw solute permeability (B d ) and feed solute permeability (B f ) were proposed, instead of relying on single solute permeability (B). Our results clearly demonstrated that both draw and feed fluxes were not well predicted with the solute permeability (B) measured by RO mode experiment, typically employed in FO membrane characterization. In this study, the draw and feed solute permeabilities were evaluated independently by the experimental protocols which simulated actual FO operation more closely. Much better agreement between experimental observations and theoretical predictions was obtained when both B d and B f were applied for the analysis of draw and feed solute fluxes, respectively. Thus, the utilization of apparent draw and feed solute permeabilities provides more precise assessment of draw solute loss and permeate water quality, which are very important for FO membrane process design and operation. [ABSTRACT FROM AUTHOR]

Abstract: Forward osmosis (FO) is an emerging technology, however the internal concentration polarization (ICP) problem needs to be resolved in order to improve the water flux. Hydrophobic/hydrophilic interpenetrating network composite nanofibers (HH-IPN-CNF) was used for FO membrane support layer, which was successfully designed and fabricated by electrostatic spinning technology. Hydrophobic polymer is polyethylene terephthalate (PET) and hydrophilic polymer is polyvinyl alcohol (PVA). The FO membrane flux has a significant increase due to the HH-IPN-CNF support layer. The membrane flux improved with the increase of PVA nanofiber content in the HH-IPN-CNF support layer. When the ratio of PET/PVA composite nanofibers was 1/4, the FO membrane exhibited the highest water flux (47.2 LMH) and low salt leakage (9.5gMH). In the test, deionized (DI) water was used as feed solution (FS), and 0.5M NaCl solution was used as draw solution (DS) in the mode of active layer facing DS. The improvement of FO flux was attributed to the increase of wetting performance of support layer and the water-transferring function. The existence of the HH-IPN-CNF structure formed between PET and PVA nanofibers leads to the reduction of the ICP. The FO membranes with HH-IPN-CNF support layer achieved a significant increase in water flux. [Copyright &y& Elsevier]

Abstract: A major barrier for forward osmosis (FO) process is concentration polarization (CP) which dramatically reduces the performance of FO. This study focused on providing a simple method of mitigating CP, especially dilutive internal CP (DICP), in the absence of additional energy consumption and investigate the impact of spacer location on CP quantitatively in both FO (AL-FS) and PRO (AL-DS) mode; finally we tried to provide a spacer location that could best decrease the adverse effect of CP on FO performance. The findings of the research led to the conclusion that in FO mode, placing the spacer (1mm×1mm) in the draw channel with one end connecting the membrane could well mitigate DICP. Besides, placing the same spacer in both feed and draw channels could be an acceptable method of decreasing concentrative external CP (CECP) and DICP simultaneously. If locating the spacer thus, the average water flux could increase by 7.57%, 14.1% and 18.7% when the concentration of the draw solution was 1M, 2M, and 4M separately. Similarly, in PRO mode we found that spacer can also mitigate concentrative ICP and dilutive ECP. Besides, the pore size in the spacer should also be considered when it was used to mitigate CP. [Copyright &y& Elsevier]

Abstract: The purification performance of tannic acid and dissolved organic matter (DOM) contained real surface water by forward osmosis (FO) was investigated systematically. Since representing the main characteristics of natural DOM fractions, tannic acid was first used as a surrogate for natural DOM analysis. The FO could reject tannic acid effectively with removal efficiencies of approximately 99.0%, as well as for natural DOM removal. The more severe fouling flux decline accompanied with lower tannic acid retention was found at greater draw solution concentrations and in the active-layer-facing-the-draw-solution (AL-facing-DS) orientation, which was ascribed mainly to the more severe fouling resulting from greater tannic acid adsorption in the porous support layer of the FO membrane under these conditions. In the AL-facing-DS mode, it was revealed that the higher the concentrations of initial substances (i.e., tannic acid and calcium ion) in the feed water, the more notable fouling flux decline and tannic acid retention occurred. In natural DOM purification experiments, the main factors investigated had the similar influences on FO performance as those mentioned above. Further, the membrane orientation has a minor impact on the natural DOM removal by FO membrane. The permeation drag influenced by the water flux probably played a critical role in the retention of tannic acid and natural DOM contained surface water by the FO membrane in AL-facing-DS mode. [Copyright &y& Elsevier]

Abstract: Pressure retarded osmosis (PRO) is an osmotically-driven membrane process that can be used to harvest salinity-gradient power. The PRO performance (both water flux and power density) can be severely limited by membrane fouling. The current study, for the first time, investigates PRO scaling in a bench-scale pressurized system using calcium sulfate dihydrate (gypsum) as a model scalant. In addition to the bulk feed solution (FS) saturation index (SIbulk), gypsum scaling was found to be strongly affected by the draw solution (DS) type and concentration, the applied hydraulic pressure, and the membrane orientation. The commonly recommended active layer facing draw solution (AL-DS) orientation was highly prone to internal scaling. In this orientation, severe internal concentration polarization (ICP) of scaling precursors induced gypsum clogging in membrane support layer even when the FS was undersaturated (e.g., SIbulk = 0.8). At higher SIbulk values, external gypsum crystal deposition occurred in addition to internal scaling. More severe scaling was observed when the DS contained scaling precursors such as Ca2+ or , suggesting that the reverse diffusion of these precursors into the FS can significantly enhanced gypsum scaling. Increasing applied hydraulic pressure could enhance reverse solute diffusion and thus result in more severe gypsum scaling when the DS contained scaling precursors. A conceptual model, capturing the two important PRO scaling mechanisms (ICP of scaling precursors from FS and reverse diffusion of scaling precursors from the DS), is presented to rationalize the experimental results. Our results provide significant implications for PRO scaling control. [Copyright &y& Elsevier]

Abstract: Forward osmosis (FO) is an emerging membrane separation technology with many potential applications. The water flux performance of existing FO membranes is largely limited by the internal concentration polarization (ICP) of solutes in their porous substrates. The current study investigated the use of nanocomposite substrates for controlling ICP and thus improving FO water flux. Polysulfone-nanocomposite (PSfN) substrates were prepared via phase inversion by incorporating porous zeolite nanoparticles in polysulfone. The PSfN0.5 (with 0.5wt% zeolite loading) substrate showed improved surface porosity and hydrophilicity. A thin film composite (TFC) polyamide active layer was prepared by interfacial polymerization on the PSfN0.5 substrate, and the resulting TFC-PFSN0.5 membrane showed significantly enhanced water permeability compared to the TFC membrane prepared on a conventional polysulfone substrate. Under identical testing conditions, the FO water flux of the TFC-PFSN0.5 membrane was more than doubled of that of the conventional TFC membrane. Using 2M NaCl as draw solution (DS) and 0–0.01M NaCl as feed solution (FS), PSfN0.5-TFC had an FO water flux as high as ∼80LMH in the active layer facing DS (AL–DS) orientation and ∼40LMH in the AL–FS orientation. Further analysis revealed that the incorporation of zeolite nanoparticles significantly reduced the substrate structural parameter (S=0.34mm for PSfN0.5-TFC compared to 0.96mm for TFC). The current study demonstrates for the first time the use of porous particles and nanocomposite substrates for controlling ICP in FO operation. [Copyright &y& Elsevier]

Abstract: The physical and chemical factors affecting boron solute flux behavior and membrane transport mechanisms in forward osmosis (FO) have been systematically investigated. Boron solute flux behavior in FO was further compared with that in reverse osmosis (RO) by employing identical plate-and-frame cells and membranes under the same filtration conditions. The influence of draw solution pH, draw solution type, and membrane orientation on boron solute flux was examined for FO, and the effects of water flux, cross-flow velocity, feed water boron concentration, and solution pH on boron solute flux were examined for both FO and RO. Results show that reverse salt diffusion, a unique feature of FO, is a key mechanism governing boron solute flux in FO. Boron solute flux through the FO membrane was inversely proportional to the degree of reverse salt diffusion by draw solution. The higher boron rejection observed in FO compared to RO is also attributed to reverse salt diffusion in FO. It is also shown that membrane orientation in FO plays an important role, affecting boron solute flux due to different degrees of internal concentration polarization. In both FO and RO, boron solute flux increased with increasing water flux. However, the influence of water flux on boron solute flux was less significant in FO than RO. Furthermore, boron solute flux decreased with increasing feed water pH due to the conversion of the neutral boric acid to borate anions. The findings provide new insight into the mechanisms and factors controlling boron solute transport in FO. [Copyright &y& Elsevier]

Abstract: Novel crosslinked layer-by-layer (xLbL) forward osmosis (FO) membranes with a double-skinned design have been synthesized and characterized in the present work. Cross flow reverse osmosis test results showed that the membrane top skin generally had higher hydraulic resistance and better solute rejection compared to the bottom skin even when the same number of polyelectrolyte layers were deposited. The overall water permeability of the double-skinned FO membrane can be estimated from the respective values of the top and bottom skins using a resistance-in-series model. For FO tests performed at high feed solution (FS) concentration (0.5M MgCl2), the water flux was not affected by the membrane separation properties nor its orientation. However, much higher FO water flux was obtained in the top-skin-facing-the-draw-solution (Top-DS) orientation compared to the alternative Top-FS orientation when the FS concentration was low. Using DI water as FS and 0.5M MgCl2 as DS, FO water fluxes ranged from 40 to 80L/m2 h in Top-DS and ∼30L/m2 h in Top-FS. Higher DS concentrations resulted in greater FO water fluxes (e.g., 50–110L/m2 h at 2M MgCl2 DS). Fouling tests demonstrated that the double-skinned membrane had much better resistance against FO fouling in the Top-DS orientation. To the best knowledge of the authors, this is the first systematic study on the synthesis and characterization of double-skinned xLbL membranes. In addition, their superior anti-fouling ability was experimentally demonstrated for the first time. [Copyright &y& Elsevier]

Abstract: Pressure retarded osmosis (PRO) is a potential technology to harvest the renewable osmotic power from the salinity-gradient resources. This study systematically investigated the effects of operating conditions (feed and draw solution concentration, membrane type, membrane orientation, and temperature) and reverse solute diffusion on PRO performance using commercially available osmotic membranes. The PRO performance was improved by decreasing the feed solution concentration, increasing the draw solution concentration, orientating the membrane with active layer facing draw solution (AL-DS), and increasing temperature. The membrane with higher water permeability, lower solute permeability and lower structure parameter performed better in PRO process. However, the experimentally obtained power densities for all the membranes used in this study were lower than the predictions from conventional ICP model that assumes membrane separation parameters are constant in PRO process. It was found that this was mainly caused by the severe reverse solute diffusion and thus the enhanced internal concentration polarization (ICP) in PRO. The specific reverse solute flux was found to increase with increasing the applied hydraulic pressure, but the increase of experimental results was much more drastic than the theoretic prediction especially under higher hydraulic pressure, probably due to the increased solute permeability caused by membrane deformation. [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: The potential application of forward osmosis (FO) membranes in water treatment and desalination requires an improved understanding of the factors that govern the rejection of trace contaminants. This study investigated the influence of membrane orientation and organic fouling on the performance of FO membrane in removing boron and arsenic. Results of laboratory-scale crossflow membrane filtration experiments showed that the inorganic contaminants were rejected at a much lower rate when membrane active layer was facing draw solution (AL-DS) compared to the active layer-facing feed water (AL-FW) orientation, as a result of the more severe concentrative internal concentration polarization (ICP) in the latter orientation. The difference in boron rejection between the two membrane orientations was greater due to its higher permeability through the FO membrane. In the AL-FW orientation, the formation of an alginate fouling layer on the membrane surface could enhance the sieving effect and thus improve the rejection of arsenious acid with relatively larger molecular size. In the AL-DS orientation, alginate fouling in the membrane support layer had adverse effect on boron rejection at water flux below 4.2μm/s (15.3L/m2/h), attributed to the foulant enhanced concentrative ICP effect. Findings have important implications in the performance and applicability of FO membrane processes. [Copyright &y& Elsevier]

Abstract: Forward osmosis (FO) has attracted wide interest for its many potential applications. Recent developments suggest that double-skinned FO membranes may perform superiorly over conventional single-skinned ones. In this design, a dense rejection skin facing the draw solution is responsible for the separation, while a second skin facing the feed solution is designed to prevent foulants entering into the porous support layer sandwiched between the two skins. For the first time, an analytical model is developed for double-skinned FO membranes and is verified experimentally. Model simulations suggest that an optimal double-skinned membrane shall have a feed skin similar to that of a low rejection nanofiltration membrane to minimize the overall hydraulic resistant and to reduce internal concentration polarization. On the other hand, the optimization of the draw skin calls for a compromise between its water permeability and solute rejection. FO water flux is greater for support layers with higher mass transfer coefficients. In addition, it decreases with decreasing draw solution concentration and increasing feed solution concentration. While membrane orientation plays an important role for applications with low feed solution concentrations (with the denser skin preferentially facing the draw solution), this effect becomes negligible at high feed solution concentrations. [Copyright &y& Elsevier]

Abstract: Polyamide thin film composite (TFC) membranes with tailored porous substrate and rejection layer are promising for forward osmosis (FO) applications. The current study investigates the effect of rejection layer synthesis conditions on the performance of the resulting TFC polyamide FO membranes. The influence of monomer concentrations (i.e., m-phenylenediamine (MPD) and trimesoyl chloride (TMC) concentrations) on the membrane separation properties as well as FO performance was systematically studied. A strong trade-off between the water permeability and salt rejection was observed, where increasing the TMC concentration or reducing the MPD concentration resulted in higher membrane permeability but lower salt rejection. In FO tests, membranes with poor salt rejection had severe solute reverse diffusion, which enhanced the internal concentration polarization (ICP). It was found that the FO water flux was governed by both the membrane water permeability and its solute rejection. For a membrane with higher water permeability but lower solute rejection, the reduced membrane frictional resistance was compensated by the simultaneously more severe solute-reverse-diffusion-induced ICP. The net effect on the FO water flux depends on the competition of these two opposing mechanisms. Under conditions where solute reverse diffusion may cause severe ICP (e.g., high draw solution concentration and high water flux level), membranes need be optimized to achieve a high salt rejection even if this is at the expense of lower water permeability. [Copyright &y& Elsevier]

Abstract: There has been a resurgence of interest in forward osmosis (FO) as a potential means of desalination, dewatering and in pressure retarded osmosis, which Sidney Loeb was advocating over 3decades ago. This paper describes the characteristics and potential applications of a newly developed FO hollow fiber membrane, which was fabricated by interfacial polymerization on the inner surface of a polyethersulfone (PES) hollow fiber. This FO membrane presents excellent intrinsic separation properties, with a water flux of 42.6L/m2 h using 0.5M NaCl as the draw solution and DI water as the feed with the active layer facing the draw solution orientation at 23°C. The corresponding ratio of salt flux to water flux was only 0.094g/L, which is superior to all other FO membranes reported in the open literature. To evaluate different application scenarios, various NaCl solutions (500ppm (8.6mM), 1wt.% (0.17M) and 3.5wt.% (0.59M)) were used as the feed water to test the performance of the FO membrane. The membrane can achieve a water flux of 12.4L/m2 h with 3.5wt.% NaCl solution as the feed and 2M NaCl as the draw solution, suggesting it has good potential for seawater desalination. [ABSTRACT FROM AUTHOR]

Abstract: Forward osmosis (FO) is attracting increasing interest for its potential applications in water and wastewater treatment and desalination. One of the major drawbacks of FO is internal concentration polarization (ICP), which significantly limits the FO flux efficiency. In addition, FO membrane flux can be adversely affected by membrane fouling. The effects of ICP and fouling on FO flux behavior were systematically investigated in the current study. Both theoretical model and experimental results demonstrated that the FO flux was highly non-linear with respect to the apparent driving force (the concentration difference between the draw solution and the feed water) as a result of ICP. ICP played a dominant role on FO flux behavior at greater draw solution concentrations and/or greater membrane fluxes due to the exponential dependence of ICP on flux level. Compared to the active layer facing draw solution (AL-facing-DS) configuration, more severe ICP was observed when the membrane active layer faced the feed water (AL-facing-FW) as a result of dilutive ICP in the FO support layer. Interestingly, the AL-facing-FW configuration showed remarkable flux stability against both dilution of the bulk draw solution and membrane fouling. In this configuration, any attempt to reduce membrane flux was compensated by a reduced level of ICP. The net result was only a marginal flux reduction. In addition, foulant deposition was insignificant in this configuration. Thus, the AL-facing-FW configuration enjoyed inherently stable flux, however, at the expense of severer initial ICP. In contrast, the AL-facing-DS configuration suffered severe flux reduction as porous membrane support faced the humic acid containing feed water. The flux loss in this configuration was likely due to the combined effects of (1) the internal clogging of the FO support structure as well as (2) the resulting enhanced ICP in the support layer. The latter was caused by reduced porosity and reduced mass transfer coefficient of the support. The pore clogging enhanced ICP mechanism probably played a dominant role in FO flux reduction at higher flux levels. To the authors’ best knowledge, this is the first study to systematically demonstrate the coupled effects of ICP and fouling on the FO flux behavior. [Copyright &y& Elsevier]

Abstract: Forward osmosis (FO) and pressure retarded osmosis (PRO) are concentration-driven membrane processes. While they can be potentially used in water, wastewater, and energy applications, these processes suffer from the concentration polarization inside the porous membrane support resulting in severe flux decrease, a phenomenon known as internal concentration polarization (ICP). Researchers have investigated the effect of ICP both in theoretical and experimental studies. The current study extends the existing ICP model to include the effect of draw solution dilution by membrane permeate flow in a spiral wound FO module (SWFO). FO and PRO experiments were performed using a Hydrowell® SWFO under both submerged and cross-flow conditions. The effect of draw solution concentration, draw solution flow rate, feed water flow rate, and membrane orientation on FO and PRO water flux performance was systematically investigated. Permeate flow increased with greater draw solution concentration in both FO and PRO modes. ICP was found to drastically limit the available membrane flux in the concentration-driven membrane processes, and its adverse effect was more severe at greater draw solution concentration. Membrane flux was also affected by the dilution of draw solution when the permeate flow rate was comparable or greater than the draw solution flow rate. The submerged FO configuration performed nearly as good as the cross-flow configuration with feed water circulating outside of the membrane envelope (shorter flow path). In this case, the feed water flow rate only had limited effect on membrane flux likely due to its low mass transfer resistance. In contrary, the membrane flux can be adversely affected at low feed water flow rate when it was circulated inside of the membrane envelope (longer flow path). [Copyright &y& Elsevier]

Sludge thickening from sewage treatment plants is an essential step in reducing the sludge volume and processing cost of the subsequent treatment. Forward osmosis (FO) technology is one of the research endeavors used in the field of membrane separation and feed solution concentration. However, the traditional used single-skinned FO membranes encounter the problems of serious internal concentration polarization (ICP) and fouling phenomenon within porous substrates. A novel double-skinned FO membrane was proposed in this study in order to enhance the anti-fouling performance and mitigate the ICP phenomenon. Single-skinned FO membrane using the same substrate was also fabricated. Results demonstrated that the double-skinned FO membrane significantly reduced ICP and exhibited higher flux when using draw solutions with high concentrations as compared to the single-skinned FO membrane. Furthermore, the double-skinned FO membrane demonstrated reasonably high-water flux (11.6 LMH), whereas zero flux was recorded from the single-skinned FO membrane when using the feed solution that contained 15.0 g/L organic foulants in the fouling tests. At last, the double-skinned FO membrane exhibited superior anti-fouling properties and much higher flux than that of single-skinned FO membrane operated under both modes during the long-term sludge thickening process, indicating the great potential of the double-skinned FO membrane to deal with the high-water-content sewage sludge in the water treatment process. [Display omitted] • Double-skinned FO membranes were fabricated by double sided interfacial polymerization. • The secondary skin was fabricated using lower concentrations of monomers in IP. • The double-skinned membranes showed excellent flux performance and good antifouling property. • Satisfactory results were obtained for the double-skinned membranes in sludge thickening. • The double-skinned membranes have great potential in treating high concentration wastewaters. [ABSTRACT FROM AUTHOR]

Osmotically assisted reverse osmosis (OARO) has shown great potential for low-cost and energy-efficient brine management. However, its performance can be significantly limited by membrane fouling. Here, we performed for the first time a comprehensive study on OARO membrane fouling, explored the associated fouling mechanisms, and evaluated fouling reversibility via simple physical cleaning strategies. First, internal membrane fouling at the draw (permeate) side was shown to be insignificant. Flux behavior in short-term operation was correlated to both the evolution of fouling and the change of internal concentration polarization. In long-term operation, membrane fouling constrained the OARO water flux to a singular, common upper limit, in terms of limiting flux, which was demonstrated to be independent of operating pressures and membrane properties. Generally, once the limiting flux was exceeded, the OARO process performance could not be improved by higher-pressure operation or by utilizing more permeable and selective membranes. Instead, different cyclic cleaning strategies were shown to be more promising alternatives for improving performance. While both surface flushing and osmotic backwashing (OB) were found to be highly effective when using pure water, a full flux recovery could not be achieved when a nonpure solution was used during OB due to severe internal clogging during OB. All in all, the presented findings provided significant implications for OARO operation and fouling control.

Forward osmosis (FO) hybrid systems have the potential to simultaneously recover nutrients and water from wastewater. However, the lack of membranes with high permeability and selectivity has limited the development and scale-up of these hybrid systems. In this study, we fabricated a novel thin-film nanocomposite membrane featuring an interlayer of Ti 3 C 2 T x MXene intercalated with carbon nanotubes (M/C-TFNi). Owing to the enhanced confinement effect on interfacial degassing and increased amine monomer sorption by the interlayer, the resulting M/C-TFNi FO membrane has a greater degree of cross-linking and roughness. In comparison with the thin-film composite (TFC) membrane without an interlayered structure, the M/C-TFNi membrane attained a water flux that was four times higher and a lower specific salt flux. Notably, the M/C-TFNi membrane exhibited excellent concentration efficiency for real municipal wastewater and enhanced rejection of ammonia nitrogen, which breaks the permeability-selectivity upper bound. This study provides a new avenue for the rational design and development of high-performance FO membranes for environmental applications.

• A simple method produced high performance TFC-FO membrane with low cost materials. • New FO substrate with blended cellulose acetate butyrate & polyvinyl butyral. • PVB enhanced hydrophilicity & played a critical role in tunning pore structure. • Respective 1.4 & 2.3-fold higher flux compared to commercial CTA & control CAB TFC. • ICP was reduced significantly with up to 98% flux recovery at 2 wt% PVB. A thin-film composite forward osmosis (TFC-FO) membrane was developed by combining a new substrate consisting of cellulose acetate butyrate (CAB) and polyvinyl butyral (PVB) with the conventional polyamide active layer. The incorporation of the PVB additive (0–4 wt%) was to improve the hydrophilicity and alter the pore formation mechanism of the substrate to reduce the internal concentration polarization (ICP) effect in FO. The results showed that promising structural parameters of the CAB/PVB substrate such as A value (1.08 L m−2 h−1 bar−1) and S value (363.5 μm) could be obtained for the M2 substrate with 2 wt% PVB, due to the combined effect of finger-like pores and high porosity with high pore connectivity. The evaluation of the TFC-FO membranes showed that the water flux of the TFC-M2 was up to 27.5 L m−2 h−1 with 1 M NaCl draw solution in active-layer facing draw solution (AL-DS) mode, which was 40% and 130% flux enhancement compared to the reported commercial CTA and pure CAB control TFC membrane, respectively, with lower specific salt flux J s / J w (0.35 g/L). The fouling experiments using model feed containing BSA foulant showed that the TFC-M2 was almost unaffected (98%) by the dilutive ICP effect in the active-layer facing feed solution (AL-FS) mode; while it also exhibited the highest flux recovery of 88% in the AL-DS mode. Overall, the total mass transfer resistance R t of the TFC-M2 was the lowest compared to all as-prepared TFC-FO membranes, indicating the key role of the PVB additive in tuning the CAB substrate structure for designing high performance FO membranes. [ABSTRACT FROM AUTHOR]

Pressure retarded osmosis (PRO) is an emerging technology capable of extracting energy from salinity gradients of wastewater paired with SWRO brine. However, this process's performance is hindered by irreversible biofouling due to bacteria-containing wastewater and the sponge-like support layer of PRO membranes. In this study, chlorhexidine gluconate (CHG), a non-oxidizing biocide, was continuously added to feed solution to investigate its anti-biofouling performance during PRO. CHG showed higher anti-microbial and anti-biofilm activity than did other non-oxidizing biocides. Even at low dosages of CHG, water flux declines were greatly mitigated and benefited from the internal concentration polarization (ICP)-elevated concentrations within the active-support layer interface. CHG plays a critical role by inhibiting bacterial growth, and a 65–88% reduction of extracellular polymeric substances was achieved on the membrane surface and throughout the feed spacers. Membrane characterization demonstrated that the improved performance could be attributed to a consistent structural parameter and alleviation of ICP self-compensation effects. This study thus shows that a combination of biocide dosing and pressure assisted-osmotic backwashing can be a useful strategy for controlling biofouling during the PRO process. Unlabelled Image • Chlorhexidine Gluconate (CHG) had high anti-microbial and anti-biofilm effects. • Biofouling was greatly mitigated by dosing with CHG even at low concentrations. • Internal concentration polarization of CHG reduced its dosage and adverse effects. • CHG dosing + pressure-assisted osmotic backwashing ensures sustainable PRO. [ABSTRACT FROM AUTHOR]

Zirconiumv (IV)-carboxylate metal-organic framework (MOF) UiO-66 nanoparticles were successfully synthesized and incorporated in the polyamide (PA) selective layer to fabricate novel thin-film nanocomposite (TFN) membranes. Compared to unmodified pure polyamide thin-film composite (TFC) membranes, the incorporation of UiO-66 nanoparticles significantly changes the membrane morphology and chemistry, leading to an improvement of intrinsic separation properties due to the molecular sieving and superhydrophilic nature of UiO-66 particles. The best performing TFN-U2 (0.1 wt % particle loading) membrane not only shows a 52% increase of water permeability but also maintains salt rejection levels (∼95%) similar to the benchmark. The effects of UiO-66 loading on the forward osmosis (FO) performance were also investigated. Incorporation of 0.1 wt % UiO-66 produced a maximum water flux increase of 40% and 25% over the TFC control under PRO and FO modes, when 1 M NaCl was used as the draw solution against deionized water feed. Meanwhile, solute reverse flux was maintained at a relatively low level. In addition, TFN-U2 membrane displayed a relatively linear increase in FO water flux with increasing NaCl concentration up to 2.0 M, suggesting a slightly reduced internal concentration polarization effect. To our best knowledge, the current study is the first to consider implementation of Zr-MOFs (UiO-66) onto TFN-FO membranes.

Forward osmosis (FO) is an emerging low-energy technology. Much effort was given on developing a new membrane material and engineering membrane structure to improve the performance of FO membranes. The performances of two newly developed polyamide-based thin-film composite (TFC) FO membranes were tested and compared with the commercially available cellulose triacetate (CTA) FO membrane. The intrinsic properties of the two TFC FO membranes determined in RO experiments indicate superior performance of the membranes. When tested in FO experiments, TFC membranes delivered consistent results, con- firming their outstanding permeability and selectivity properties. The study shows that future studies on membrane fouling will be necessary to have a better understanding of membrane performance and to further optimize membrane properties. [ABSTRACT FROM AUTHOR]