Pressure Retarded Osmosis Operation Research Articles

Investigations on membrane performance for coconut water concentration under pressure and by osmotically driven process

AbstractBACKGROUNDCoconut water has grown in popularity among people as a ready‐to‐drink healthy beverage. Nevertheless, its future export expansion is restricted by high operating costs as large volumes of coconut water have to be handled during exporting and for its storage. Besides, large coconut water volumes also consume high energy during the pasteurization/sterilization process. Thus, in this work, we evaluated the potential of employing polymeric membranes for the concentration of coconut water at room temperature via either a pressure‐driven or osmotically driven process.RESULTSOur results indicated that the membrane that was tested under reverse osmosis (RO) mode at 25 bar could significantly concentrate coconut water by reducing its original volume by up to half, with negligible loss of ions/minerals. Upon completion of the RO process, Brix and conductivity of the coconut water were increased by 90% and 61.4%, respectively. As a comparison, the membrane tested under pressure‐retarded osmosis (PRO) mode was only able to reduce the volume of coconut water by ~12%. Although coconut water concentration via PRO operation does not require an external driving force, this process suffers from several significant drawbacks, including reverse draw solute flux and the requirement for draw solution generation.CONCLUSIONBased on the results obtained, it can be concluded that the RO membrane process is still the most reliable and practical method of concentrating fruit juice, and its performance could be further enhanced using a high‐flux polymeric membrane or through process optimization. © 2023 Society of Chemical Industry (SCI).

Rational design of high power density “Blue Energy Harvester” pressure retarded osmosis (PRO) membranes using artificial intelligence-based modeling and optimization

The challenge in harvesting Salinity Gradient Power (SGP) through pressure retarded osmosis (PRO) requires design of high power density (PD) membranes and optimized process for operation. Recent studies show that for a feasible PRO operation the minimum net PD should be around 50 W/m2. In this study, a data-driven approach has been adopted for designing optimum membranes as well as operating conditions. 200 papers, from last decade, were extensively reviewed and 34 experimental research articles were shortlisted for possible data mining, to predict water flux (WF) and PD. Comprehensive screened/pre-processed data related to both membrane and process (16 inputs) was obtained from 18 articles amounting to 339 data points. Two artificial neural network (ANN) models were explored (i) Levenberg-Marquardt (ANN-LM), and (ii) Bayesian Regularization (ANN-BR) along with a combination of three different activation functions i.e., hyperbolic tangent sigmoid transfer function (Tan-Sigmoid), logarithmic sigmoid transfer function (Log-Sigmoid) in the input and output layers. Out of the six resulting combinations, the best performing combination was found to be Tan-Sigmoid activation function in both layers with ANN-BR model having an R2 value of 0.97 for WF and 0.98 for PD. Membrane properties like the type of membrane, thickness, and water permeability coefficient were found to be the major contributing factors for the prediction of WF while for PD, operating conditions such as applied pressure were found to play the major contributing factor (10–16 %). Optimization results yield a maximum WF of 147 LMH and PD of 87 W/m2. These results were compared with the solution diffusion (S-D) model.

Assessing the potential of integrally skinned asymmetric hollow fiber membranes for addressing membrane fouling in pressure retarded osmosis process

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.

Tubular PAN/CNC thin film nanocomposite (TFN) pressure retarded osmosis (PRO) membrane: fabrication and preliminary evaluation in desalination process

The pressure retarded osmosis (PRO) process requires high performance, high flux, high rejection, and resistant membranes under harsh conditions. Since conventional phase-inversion membranes are insufficient to permit the required water flux, alternative membrane fabrication methods need to be developed. Many studies have recently been carried out to fabricate strong enough nanofiber PRO membranes resistant to higher pressure while providing high flux and high rejection rates. This work aims to fabricate tubular nanofiber PRO membranes by the electrospinning technique. In the study, cellulose nanocrystals (CNCs) were added to polyacrylonitrile (PAN) polymer solution to fabricate nanocomposite nanofiber PRO membranes. According to the scanning electron microscopy (SEM), FT-IR, dynamic mechanical analysis, porometer, and contact angle analysis results, it is concluded that PAN and CNCs provided a complete mixture, and the addition of CNCs increased the mechanical strength in the PAN membranes, which is the crucial phenomena in PRO applications. In this study, the newly fabricated membrane achieves a higher PRO water flux of 405.38 LMH using 1 M NaCl and a DI as feed water. The corresponding salt flux is found as 2.10 gMH, which is higher than our previous study. The selectivity of the reversed flux represented by the ratio of the water flux to the reversed salt flux (Jw/Js) was able to be kept as high as 193.03 L/g for PRO operation. As far as we know, the performance of the work-developed membrane in this study has shown better performance than all PRO membranes reported in the literature previously.Graphic abstract

Salinity gradient energy generation by pressure retarded osmosis: A review

Pressure retarded osmosis (PRO) has gained attention due to its use as a salinity gradient energy-generating membrane process. This process can convert difference in salinity between two streams into energy as it allows water transport through a semi-permeable membrane against the application of hydraulic pressure. This review provides a comprehensive look at the history and latest developments in preparation of membranes and modules for the PRO process, as well as the various applications of PRO. This review also explored the influence of feed characteristics and pretreatment strategies on water permeation and power generation during PRO operation. The current status and technological advancements of PRO as a process were reviewed, revealing how PRO can be operated as a stand-alone process or in integration with other hybrid processes. Despite the recent advancements in material and process development for PRO, membrane performance, wide-scale implementation, and commercialization efforts still leave much to be desired. Recognizing the current challenges facing the PRO technology, the advancements in PRO membrane and module development, and the various applications of the process, this review also draws out the future direction of PRO research and generation of osmotic salinity gradient energy as a viable energy source.

High-performance and durable pressure retarded osmosis membranes fabricated using hydrophilized polyethylene separators

A high-performance and durable thin-film composite (TFC) pressure retarded osmosis (PRO) membrane was fabricated using a polyvinyl alcohol (PVA)-coated polyethylene (PAPE) support via toluene-assisted interfacial polymerization (TIP). The PVA coating uniformly hydrophilized the extremely thin (~8 μm) polyethylene (PE) support with a highly porous structure while marginally deforming the support structure, resulting in a very low structural parameter (~235 μm). The TIP process produced a polyamide selective layer with remarkably higher water permeability (~8.78 L m−2 h−1 bar−1) than those of commercial HTI membranes (0.56–1.40 L m−2 h−1 bar−1). Furthermore, despite its extreme thinness, the PAPE-supported TFC (PAPE-TFC) membrane had higher mechanical properties than the commercial membranes owing to the superior mechanical strength of its PE support. Hence, the PAPE-TFC membrane achieved an unprecedentedly high power density of ~35.7 W m−2 at an applied pressure of 20 bar using a deionized water feed solution and a 1.0 M NaCl draw solution, which significantly outperformed commercial and any other reported lab-made PRO membranes. The mechanically robust PAPE-TFC membrane also enabled stable long-term PRO operation under high pressure conditions.

Melamine-based covalent organic framework-incorporated thin film nanocomposite membrane for enhanced osmotic power generation

A melamine-based covalent organic framework (COF) nanomaterial, Schiff base network-1 (SNW-1), was incorporated into the polyamide layer of a novel thin film nanocomposite (TFN) pressure retarded osmosis (PRO) membrane. The deposition of SNW-1 was made on an open mesh fabric-reinforced polyamide-imide (PAI) support substrate through interfacial polymerization (IP). SNW-1 loading influence on the water permeability and osmotic power density during PRO operation was investigated. The porous and highly hydrophilic SNW-1 nanomaterial facilitated the flow of water molecules across the membranes, while maintaining satisfactory salt rejection ability of the polyamide selective layer. The membranes exhibited significantly enhanced surface hydrophilicity, water permeability, and power density. The mode of incorporation of SNW-1 during IP was also investigated and it was observed that the secondary amine groups of SNW-1 react with the carbonyl groups of 1,3,5-benzenetricarbonyl trichloride, the acyl halide precursor in polyamide formation; thus, SNW-1 was incorporated through the amine precursor, 1,3-phenylenediamine. Testing with 1.0 M NaCl as the draw solution, the TFN membrane with a loading of 0.02 wt% SNW-1 exhibited the highest water flux of 42.5 Lm−2 h−1 and power density of 12.1 Wm−2, while withstanding hydraulic pressure over 24 bar. This study suggests that COF-incorporation can be a promising method in PRO membrane fabrication to improve both osmotic performance and energy harvesting capability for the PRO process.

Mitigation of inorganic fouling on pressure retarded osmosis (PRO) membranes by coagulation pretreatment of the wastewater concentrate feed

Utilization of wastewater or wastewater concentrate as the feed solution in pressure retarded osmosis (PRO) via thin film composite (TFC) membranes causes rapid and significant fouling within the porous substrate and onto the polyamide selective layer facing the substrate. Scaling of calcium phosphate salts has been identified as the predominant cause for the severe PRO membrane fouling. In this study, an acidic coagulant, AlCl3, and a caustic coagulant, NaAlO2, are employed to remove the phosphate from the wastewater concentrate. Their optimal dosages are investigated in order to most effectively remove phosphate and maintain a mild pH environment. 1000 ml of AlCl3 and NaAlO2 pretreated and untreated wastewater concentrates are then used as the feed solutions in the PRO fouling studies. The untreated wastewater concentrate that is saturated with calcium phosphate causes a dramatic flux reduction of 45.2% during the first 360 min of the tests and further reduces to 30.7% after a 25% recovery of the wastewater concentrate. Results show that both AlCl3 and NaAlO2 pretreatments can increase the initial water flux to 25.5 LMH and 24.8 LMH at 20 bar respectively, and increase the normalized water flux to 66% and 64% respectively upon a total permeation volume of 250 ml. However, the AlCl3 pretreatment shows a lower water flux recovery after a hydraulic backwash and a faster flux reduction in the repeated fouling tests. Elemental analyses reveal that AlCl3 is less effective in silica removal from the wastewater concentrate, which causes silica condensation underneath the selective layer of the TFC membrane. NaAlO2 appears to be the more effective coagulant in both phosphate and silica removal for a prolonged PRO operation. Moreover, a critical water flux framework is developed to analyze the PRO calcium phosphate fouling, in order to determine the stable water flux at a given phosphate concentration or the maximum allowable phosphate concentration to achieve the target PRO flux and power density.

Advanced Anti-Fouling Membranes for Osmotic Power Generation from Wastewater via Pressure Retarded Osmosis (PRO).

A facile and versatile approach was demonstrated for the fabrication of low-fouling pressure retarded osmosis (PRO) membranes for osmotic power generation from highly polluted wastewater. A water-soluble zwitterionic random copolymer with superior hydrophilicity and unique chemistry was molecularly designed and synthesized via a single-step free-radical polymerization between 2-methacryloyloxyethyl phosphorylcholine (MPC) and 2-aminoethyl methacrylate hydrochloride (AEMA). The P[MPC- co-AEMA] copolymer was then chemically grafted onto the surface of PES/Torlon hollow fibers via amino groups coupling of poly(AEMA) with the polyimide structures of Torlon, leaving the zwitterions of poly(MPC) in the feed solution. Because of the outstanding hydrophilicity, unique cationic and anionic groups, and electrical neutrality of the zwitterionic brush, the newly developed membrane showed great resistances to both inorganic scaling and organic fouling in PRO operations. When using a real wastewater brine comprising multifoulants as the feed, the P[MPC- co-AEMA] modified membrane exhibits a much lower flux decline of 37% at Δ P = 0 bar after 24-h tests and a smaller power density decrease of 28% at Δ P = 15 bar within 12-h tests, compared to 61% and 42% respectively for the unmodified one. In addition to the low fouling tendency, the modified membrane shows outstanding performance stability and fouling reversibility, where the flux is almost fully recovered by physical backwash of water at 15 bar for 0.5 h. This study provides valuable insights and strategies for the design and fabrication of effective antifouling materials and membranes for PRO osmotic power generation.

Integral hollow fiber membrane with chemical cross-linking for pressure retarded osmosis operated in the orientation of active layer facing feed solution

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 4.3Wm−2 at 12 ~ 13bar hydraulic pressure, using a real wastewater reverse osmosis (RO) retentate and 1.0M 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.

Carbon Quantum Dots Grafted Antifouling Membranes for Osmotic Power Generation via Pressure-Retarded Osmosis Process.

Osmotic power generated by pressure-retarded osmosis (PRO) has attracted global attention as a clean, abundant and renewable energy resource. However, the substrates of PRO membranes are particularly prone to fouling because of their direct contact with various foulants in raw water. This leads to a significant decline in power density and impedes the commercialization of PRO technology. In this work, a facile surface modification method has been developed to obtain a new type of nanoparticle functionalized antifouling PRO membranes. Carbon quantum dots (CQDs), with an average size around 3.2 nm, are fabricated from citric acid via a simple method. Subsequently, they are immobilized onto the polydopamine (PDA) layer grafted on the substrate surface of poly(ether sulfone) (PES) membranes via covalent bonding. The bacteria diffusion tests show that the CQD modified PRO membranes possess much enhanced antibacterial activity and antibiofouling propensity. The continuous PRO operations at 15 bar also confirm that the CQD modified membranes exhibit a much higher power density (11.0 vs 8.8 W/m2) and water recovery after backwash (94 vs 89%) than the unmodified ones. This study may open up a new avenue in the fabrication of nanostructure functionalized polymeric membranes for wastewater treatment and osmotic power generation.

Pressure-retarded osmosis with wastewater concentrate feed: Fouling process considerations

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., Ca2+). 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 (15mins). 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.

A pilot study on pressure retarded osmosis operation and effective cleaning strategies

A SWRO-PRO pilot was built to systematically investigate the osmotic power generation from state-of-the-art thin-film composite (TFC) hollow fiber membrane modules. For performance benchmarking, three feed pairs were examined. They were (1) synthetic brine and tap water, (2) real seawater reverse osmosis brine (referred to as SWBr) and tap water, and (3) real SWBr and wastewater retentate (referred to as WWBr) feed pairs. All feed pairs had the capabilities to produce power densities ranging from 5W/m2 to 5.7W/m2, which were obtained based on draw solution pressure and NaCl concentration of 15bar and 0.8M, respectively. These results were equal or more than the required power density of 5W/m2 proposed by Stafkraft. Various cleaning strategies at pilot level were adopted and proven to be effective and feasible to sustain module performance. Online cleaning approaches; namely, backwash, clean-in-place (CIP) and maintenance cleaning (MC) were performed to evaluate the cleaning efficiencies of the fouled membrane modules. The effectiveness of various cleaning agents was also evaluated using (1) tap water, (2) 200ppm HCl solutions at pH2.3 and (3) 200ppm NaOCl solutions at pH10.5. The results showed that for the case of SWBr and WWBr feed pair, acid maintenance cleaning was able to fully recover the water flux or power density, whereas caustic cleaning was only able to recover the water flux up to 80%. Chemical-free cleanings by backwash and CIP could only recover water flux from 42% to 51%. The introduction of intermittent CIP cleanings at a fixed interval was found to be effective in slowing down the water flux decline, thereby prolonging the operational duration.

Investigations of inorganic and organic fouling behaviors, antifouling and cleaning strategies for pressure retarded osmosis (PRO) membrane using seawater desalination brine and wastewater

By employing seawater desalination brine (SWBr) and wastewater brine (WWBr) as the feed pair, membrane fouling behaviors as well as antifouling and cleaning strategies for the state-of-the-art thin-film composite polyethersulfone (TFC-PES) hollow fiber membrane have been systematically investigated under pressure retarded osmosis (PRO) operations. Fouling on the polyamide selective layer induced by the SWBr draw solution is relatively mild because of the outstanding membrane rejection and the hydration antifouling layer formed by the permeating water. However, using WWBr as the feed causes fast and severe internal concentration polarization (ICP) and fouling within the porous PES substrate, which result in dramatic flux and power density declines. In addition, the PRO fouling upon and within the porous substrate is highly irreversible. Experimental data show that both anti-scalant pretreatment and pH adjustment of WWBr could effectively mitigate inorganic fouling, while increasing feed flow velocity along the substrate surface is ineffective for fouling control. To clean the fouled membranes, hydraulic-pressure induced backwash and flushing with alkaline and NaOCl solutions on the fouled surface are effective strategies to remove foulants and regenerate membranes with a flux recovery of 83–90%. However, osmotic backwash shows low cleaning efficiency in PRO. In summary, a proper combination of feed pretreatment and membrane cleaning strategies has been demonstrated in this study to sustain PRO operations with a high water flux and power density.

Concentration polarization effect and preferable membrane configuration at pressure-retarded osmosis operation

The effect of concentration polarization on performances of integrally asymmetric and thin film composite forward osmosis (FO) membranes was systematically investigated in two membrane configurations — active layer facing feed solution (AL-FS) and active layer facing draw solution (AL-DS). Water fluxes of the membranes using pure water as FS in AL-DS were 1.7 to 2.6 times higher than those in AL-FS. However, with increasing FS concentration, the water flux in AL-DS declined rapidly and became lower than that in AL-FS. A modeling study showed that the reversal of water flux in the two configurations were resulted from concentrative ECP, and theoretical FS concentrations at which water fluxes in both configurations were equal were inversely proportional to the initial water flux of the membranes. The turnover of the water flux in AL-DS and AL-FS was also observed within 8h when seawater and river water were used as DS and FS, respectively. Moreover, the AL-DS faced the challenge of high fouling propensity and difficulty in cleaning more than the AL-FS. This study suggests that AL-FS membrane configuration is proper for both PRO and FO processes employing real natural water in terms of high membrane performance and ease of cleaning.

Predicting power density of pressure retarded osmosis (PRO) membranes using a new characterization method based on a single PRO test

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 (Jw and Js) 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 Js/Jw and power density of the tested PRO membrane using the internal concentration polarization (ICP) model. The model prediction results of Js/Jw 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.

Changing membrane orientation in pressure retarded osmosis for sustainable power generation with low fouling

Pressure retarded osmosis (PRO) operated with active layer facing feed solution (AL-FS), in contrary to typical PRO membrane orientation (active layer facing draw solution, AL-DS), has a strong potential for sustainable power generation from salinity gradient energy with low membrane fouling. The feasibility of PRO in AL-FS mode was first examined by verifying less membrane deformation and insignificant shadow effect under pressurized PRO operation. The simulation of such PRO process showed that its low power density due to internal concentration polarization could be improved through reducing structural parameter of the support layer. Fouling experiments using synthetic wastewater clearly demonstrated that severe fouling observed with PRO operation at AL-DS was greatly reduced by switching PRO membrane orientation to AL-FS. Lastly, specific energy consumption of a PRO-RO hybrid process was assessed to be favorable in view of both power density and membrane fouling, suggesting that economically feasible power generation as well as effective water treatment can be achieved by operating PRO in the AL-FS orientation.

Fabrication and characterization of fabric-reinforced pressure retarded osmosis membranes for osmotic power harvesting

In recent years, pressure retarded osmosis (PRO) has attracted increasing interest in the harvesting of the renewable osmotic power. However, its performance can be significantly influenced by the membrane deformation in the operation when the PRO membrane is lack of sufficient mechanical strength. In this study, we fabricated three different fabric-reinforced thin-film composite (TFC) flat-sheet PRO membranes for osmotic power harvesting. These membranes were prepared through integrating three different types of fabric reinforcement (i.e., tricot fabric, woven fabric and nonwoven fabric) in the membrane substrate layer. It was found that the fabric reinforcement plays an important role in the membrane structural property and mechanical property, both of which can significantly influence the PRO performance. The nonwoven-fabric-reinforced membrane had the greatest structural parameter and thus exhibited the lowest performance. Although the tricot-fabric-reinforced membrane and the woven-fabric-reinforced membrane had similar performance in the forward osmosis (FO) condition (∆P=0), the former showed superior performance in the PRO condition (∆P>0). This is mainly because the tricot-fabric-reinforced membrane had better mechanical resistance to the multi-directional tensile stretching, which rendered it less prone to changes in structural and separation properties in the PRO operation. This further suggests that the tricot fabric has high potential for future PRO membrane fabrication. The current study also elaborates the coupled effects of compression and stretching on PRO membrane deformation and performance. The results obtained in this study may provide important insights into reinforced PRO membrane design.

Fouling behaviors of polybenzimidazole (PBI)–polyhedral oligomeric silsesquioxane (POSS)/polyacrylonitrile (PAN) hollow fiber membranes for engineering osmosis processes

This paper investigated the individual effects of reverse salt flux and permeate flux on fouling behaviors of as-spun and annealed polybenzimidazole (PBI)–polyhedral oligomeric silsesquioxane (POSS)/polyacrylonitrile (PAN) hollow fiber membranes under forward osmosis (FO) and pressure retarded osmosis (PRO) processes. Two types of membrane fouling had been studied; namely, inorganic fouling (CaSO4·2H2O gypsum scaling) during FO operations and organic fouling (sodium alginate fouling) during PRO operations. It is found that gypsum scaling on the membrane surface may be inhibited and even eliminated with an increase in reverse MgCl2 flux due to competitive formations of MgSO4° and CaSO4·2H2O. In contrast, the increase of reverse NaCl flux exhibits a slight enhancement on alginate fouling in both FO and PRO processes. Comparing to the reverse salt flux, the permeate flux always plays a dominant role in fouling. Therefore, lesser fouling has been observed on the membrane surface under the pressurized PRO process than FO process because the reduced initial flux mitigates the fouling phenomena more significantly than the enhancement caused by an increase in reverse NaCl flux.

Thin-film composite P84 co-polyimide hollow fiber membranes for osmotic power generation

A series of well-designed thin-film composite (TFC) hollow fiber membranes via dual-layer co-extrusion technology for pressure retarded osmosis (PRO) applications is reported in this work. By controlling the phase inversion process during spinning, we have molecularly engineered hollow fiber membranes with various structures, dimensions, pore characteristics, and mechanical properties as supports for the synthesis of TFC membranes. Under hydraulic tests, these hollow fiber membrane supports possess high burst pressures from 13 to 24bar. The TFC membranes fabricated by interfacial polymerization on the inner surface of the hollow fiber supports not only exhibit relatively high power densities of 5–12Wm−2 but also display a superior tolerance to high pressures up to 21bar. The TFC membrane synthesized on a small dimensional hollow fiber support, which was spun from a P84 co-polyimide/ethylene glycol (EG)/N-methyl-2-pyrrolidinone (NMP) dope solution with a bore fluid of a water/EG/NMP mixture, shows the most impressive PRO performance (i.e., 12Wm−2 at 21bar using water and 1M NaCl as feeds). Experimental results also suggest that inner-selective TFC hollow fiber membranes made from small dimensional fiber supports by means of delayed demixing during the fiber spinning are preferential for high pressure PRO processes. In addition, it was found that the flow rate of brine solutions plays a crucial effect on TFC membrane performance for osmotic power generation. By investigating the pressure drop as a function of flow rate, one may be able to choose appropriate PRO operation conditions to further ensure the sustainability of hollow fiber membranes for power generation.