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An important issue concerning performance of forward osmosis (FO) systems is salt accumulation in the retentate. This occurs due to the highly retentive FO membrane and reverse diffusion of draw solutes. In this study, experimental data from an osmotic membrane bioreactor (OMBR), which epitomizes a challenging application for FO, was analyzed to investigate longer term effects of the above issue. It was found that salt accumulation is controlled by three factors: membrane, influent, and process. The role of the membrane is application-dependent and significant only when influent osmotic pressure is smaller or in the same order of magnitude as the salt to water permeability ratio (B/A). The study also shows that an experimental duration of 3 × solids retention time (SRT) is necessary for adequate study of salt accumulation in FO systems. Analysis of the B/A ratio provided fundamental information into system behavior. A reducing B/A could be associated with the formation of a mild secondary foulant layer, whereas an increasing B/A was observed for more severe fouling cases and indicated further flux reducing mechanisms. The study makes clear that knowledge of factors affecting salt accumulation is essential for optimization of FO systems. [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]

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 emerging technology of pressure retarded osmosis (PRO) has been intensely studied over the past decades due to its potential in harvesting salinity gradient energy, but membrane fouling can deteriorate the PRO performance. To mitigate membrane fouling, the outer surface of the inner-selective thin-film composite (TFC) polyetherimide (PEI) hollow fiber membrane was modified by depositing poly-(allylamine hydrochloride) (PAH) and poly acrylic acid (PAA) polyelectrolytes. The results show that the outer surface of the modified membrane had less pores exposed and was more negatively charged, which was beneficial for preventing the entrance and adsorption of the negatively charged foulants into the membrane substrate. It was further proved that the modified membrane was effective in reducing PRO fouling by organic macromolecules such as alginate and bovine serum albumin, while maintaining the mechanical properties and intrinsic separation properties well. Interestingly, after 200 ppm alginate was introduced to the feed water, the power density of the modified membrane could be maintained constant at 16.2 W·m − 2 at 15 bar while that of the non-modified membrane reduced around 17% at the end of a 300-min testing period. The results suggest that the polyelectrolytes deposition is a feasible strategy for adding anti-fouling property to PRO membranes. [ABSTRACT FROM AUTHOR]

Abstract: This research focused on combined organic–inorganic fouling and cleaning studies of forward osmosis (FO) membranes. Various organic/inorganic model foulants such as sodium alginate, bovine serum albumin (BSA) and silica nanoparticles were applied to polyamide-polyethersulfone FO hollow fiber membranes fabricated in our laboratory. In order to understand all possible interactions, experiments were performed with a single foulant as well as combinations of foulants. Experimental results suggested that the degree of FO membrane fouling could be promoted by synergistic effect of organic foulants, the presence of divalent cations, low cross-flow velocity and high permeation drag force. The water flux of fouled FO hollow fibers could be fully restored by simple physical cleaning. It was also found that hydrodynamic regime played an important role in combined organic–inorganic fouling of FO membranes. [Copyright &y& Elsevier]

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]

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]

Thin-film composite (TFC) membranes are generally preferred over integrally skinned asymmetric (ISA) membranes in pressure retarded osmosis (PRO) process due to their much higher water permeability, but they cannot operate in the active layer facing feed solution (AL-FS) orientation because of potential delamination of the rejection layer under the high applied pressure. However, operating under AL-FS orientation is preferred as a strategy for better fouling control. Hence, in this study, three ISA hollow fiber membranes were fabricated using a dry-jet wet-spinning method, followed by chemical cross-linking to obtain membranes for PRO application. The best-performing membrane was subjected to fouling test in two orientations. Our results demonstrate that the AL-FS orientation is better for a sustainable long-term PRO operation, given the stable performance throughout the two-day testing. Also, the current use of power density per membrane area as a performance benchmark may not be practical for large-scale PRO operation, and the volumetric power density per module is more relevant. In terms of volumetric power density for a hypothetical 5-in. module, our best-performing membrane is predicted to generate a 3.6 to 10 times better power density than commercial modules, rendering our ISA hollow fiber membranes promising for scaling-up to bigger modules. [Display omitted] • Operating under AL-FS orientation is preferred as a strategy for better fouling control. • Integrally skinned asymmetric hollow fiber membranes were developed for AL-FS orientation in PRO. • The membranes achieved sustainable power production when wastewater was employed. • The volumetric power density was predicted to yield promising solution for practical application. [ABSTRACT FROM AUTHOR]