Pressure Retarded Osmosis Hollow Fiber Membranes Research Articles

Thin-film composite hollow fiber membrane with inorganic salt additives for high mechanical strength and high power density for pressure-retarded osmosis

Using high salinity draw solutions in pressure retarded osmosis (PRO) demands novel membranes with a higher withstanding pressure and a higher power density to take full advantage of the increased osmotic gradient for greater osmotic energy production. This study highlights novel strategies to design an inner-selective thin-film composite (TFC) PRO hollow fiber membrane with a high operating pressure and a high power density by adding an inorganic salt, CaCl2, to the dope solution and fine-tuning the spinning conditions. An optimal CaCl2 dosage can not only effectively increase the dope viscosity but also reduce the mean pore size and narrow the pore size distribution of the polyethersulfone (PES) hollow fiber substrates. A high flowrate ratio of dope solution to bore fluid greatly enhances the mechanical strength of the PES substrates. The combination of these two strategies results in a more permeable, better-supported and less defective polyamide selective layer after interfacial polymerization. The best newly developed TFC hollow fiber has a power density of 38 W/m2 at 30 bar by using 1.2 M NaCl and deionized water as the feed pair. To our best knowledge, these are the highest power density and operation pressure ever achieved by PRO hollow fiber membranes in the literature. The improved performance will advance the PRO technology closer to commercialization and make osmotic energy more competitive with other renewable energies.

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.

Tuning water content in polymer dopes to boost the performance of outer-selective thin-film composite (TFC) hollow fiber membranes for osmotic power generation

The lack of effective membranes greatly hinders the use of pressure retarded osmosis (PRO) to harvest renewable osmotic energy. In comparison to the flat sheet and inner-selective hollow fiber configurations, the development of outer-selective hollow fiber membranes is much slower although it may be more attractive for real PRO applications. This study demonstrates that tuning the water content in polymer dopes can be an effective means to simultaneously enhance the mechanical robustness and water transport properties, boosting the PRO performance of outer-selective thin-film composite (TFC) hollow fiber membranes. Fundamental properties of polymer dopes with different water content and their influence on the hollow fiber supports as well as resultant TFC membranes were systematically investigated. With a low water content of 2wt% in the polymer dope, the newly developed TFC membrane not only has the smallest structural parameter, highest toughness, and largest water permeability among all membranes studied but also displays an impressive peak power density of 10.05W/m2 at 22bar using 1M NaCl and DI water as feeds. To our best knowledge, this is the highest power density of outer-selective TFC PRO hollow fiber membranes reported in the literature. Mathematical models predict that approximately 25% performance increase can be further achieved if the external concentration polarization (ECP) effect is minimized. This study may provide useful insights to design high performance outer-selective TFC hollow fiber membranes for osmotic power generation.

Hollow fiber membrane lumen modified by polyzwitterionic grafting

In this study, we demonstrate an effective way to modify the lumen of polyetherimide hollow fibers by grafting zwitterionic poly(sulfobetaine) to increase the membrane resistance to fouling. Surface-selective grafting of the protective hydrogel layers has been achieved in a facile two-step process. The first step is the adsorption of a macromolecular redox co-initiator on the lumen-side surface of the membrane, which in the second step, after flushing the lumen of the membrane with a solution comprising monomers and a complementary redox initiator, triggers the in situ cross-linking copolymerization at room temperature. The success of grafting reaction has been verified by the surface elemental analyses using X-ray photoelectron spectroscopy (XPS) and the surface charge evaluation using zeta potential measurements. The hydrophilicity of the grafted porous substrate is improved as indicated by the change of contact angle value from 44° to 30°, due to the hydration layer on the surface produced by the zwitterionic poly(sulfobetaine). Compared to the pristine polyetherimide (PEI) substrate, the poly(sulfobetaine) grafted substrates exhibit high fouling resistance against bovine serum albumin (BSA) adsorption, E. coli attachment and cell growth on the surface. Fouling minimization in the lumen is important for the use of hollow fibers in different processes. For instance, it is needed to preserve power density of pressure-retarded osmosis (PRO). In high-pressure PRO tests, a control membrane based on PEI with an external polyamide selective layer was seriously fouled by BSA, leading to a high water flux drop of 37%. In comparison, the analogous membrane, whose lumen was modified with poly(sulfobetaine), not only had a less water flux decline but also had better flux recovery, up to 87% after cleaning and hydraulic pressure impulsion. Clearly, grafting PRO hollow fiber membranes with zwitterionic polymeric hydrogels as a protective layer potentially sustains PRO performance for power generation.

Role of pressure-retarded osmosis (PRO) in the mega-ton water project

AbstractReverse osmosis (RO) membranes have been widely applied to seawater desalination and wastewater reclamation, and many large RO plants (>100,000 m3/d) have been constructed since 2000. Energy efficiency is indispensable for large plants, especially for future mega-ton scale seawater reverse osmosis (SWRO) plant (1,000,000 m3/d). In order to reduce energy demand in RO operation, high-efficient low-pressure RO membrane, energy recovery device (ERD), and pressure-retarded osmosis (PRO) which would recover power from salinity gradient between freshwater and concentrated brine were studied in the “Mega-ton Water System” project. A PRO hollow fiber membrane module was newly developed, and a practical continuous operation has been examined at a prototype plant for one year. The maximum 13.5 W/m2 of membrane power density using 10-inch module was established at our prototype PRO plant. As the results of the plant cost estimation and the financial impacts on the energy saving operation at the mega-ton scale...

Analysis of flux reduction behaviors of PRO hollow fiber membranes: Experiments, mechanisms, and implications

Pressure retarded osmosis (PRO) is a promising technology to harvest renewable osmotic energy using a semipermeable membrane. However, a significant flux reduction has been always observed that severely shrinks the harvestable power to a level only marginally higher or even lower than the economically feasible value. This work focuses on the elucidation of various underlying mechanisms responsible for the flux reduction. First, both inner-selective and outer-selective thin film composite (TFC) hollow fiber membranes are employed to examine how the fundamental internal factors (such as the surface salinity of the selective layer at the feed side (CF,m) and its components) interact with one another under the fixed bulk salinity gradient, resulting in various behaviors of external performance indexes such as water flux, reverse salt flux, and power density. Then, the research is extended to investigate the effects of the growing bulk feed salinity due to the accumulated reverse salt flux along PRO modules. Finally, the insights obtained from the prior two stationary conditions are combined with the advanced nucleation theory to elucidate the dynamic scaling process by visualizing how the multiple fundamental factors (such as local supersaturation, nucleation rate and nuclei size) evolve and interplay with one another in various membrane regimes during the whole scaling process. To our best knowledge, it is the first time that the advanced nucleation theory is applied to study the PRO scaling kinetics in order to provide subtle and clear pictures of the events occurring inside the membrane. This study may provide useful insights to design more suitable TFC hollow fiber membranes and to operate them with enhanced water flux so that the PRO process may become more promising in the near future.

Robust outer-selective thin-film composite polyethersulfone hollow fiber membranes with low reverse salt flux for renewable salinity-gradient energy generation

This study reports outer-selective thin-film composite (TFC) hollow fiber membranes with extremely low reverse salt fluxes and robustness for harvesting salinity-gradient energy from pressure retarded osmosis (PRO) processes. Almost defect-free polyamide layers with impressive low salt permeabilities were synthesized on top of robust polyethersulfone porous supports. The newly developed TFC-II membrane shows a maximum power density of 7.81Wm−2 using 1M NaCl and DI water as feeds at 20bar. Reproducible data obtained in the 2nd and 3rd runs confirm its stability under high hydraulic pressure differences. Comparing to other PRO membranes reported in the literature, the newly developed membrane exhibits not only the smallest slope between water flux decline and ΔP increase but also the lowest ratio of reverse salt flux to water flux. Thus, the effective osmotic driving force could be well maintained even under high pressure operations. For the first time, the effect of feed pressure buildup induced by feed flowrate was evaluated towards PRO performance. A slight increment in feed pressure buildup was found to be beneficial to water flux and power density up to 10.06Wm−2 without comprising the reverse salt flux. We believe this study may open up new perspectives on outer-selective PRO hollow fiber membranes and provide useful insights to understand and design next-generation outer-selective TFC hollow fiber membranes for osmotic power generation.

Identification of safe and stable operation conditions for pressure retarded osmosis with high performance hollow fiber membrane

Pressure retarded osmosis (PRO) is a promising energy harvesting technique. However, when polymeric hollow fiber membrane is used for the PRO process, the mechanical strength of the membrane is a big concern. As hollow fiber membrane is self-supported and due to its polymeric nature, it may gradually deform over time under high pressure loading, or membrane “creeping” will occur. Current work is the first attempt to analyze the membrane creeping phenomenon of a novel thin film composite (TFC) hollow fiber. The membrane creeping was evaluated via nanoindentation by using atomic force microscope (AFM). A non-stop 200-hour PRO test and integrity check, which have not been reported previously, were carried out to investigate the membrane performance under various operating pressures. The results show that the membrane is able to produce a stable power density output of 19.2W/m2 at 15.0bar, using 1.0M NaCl as the draw solution and DI water as the feed water. Membrane creeping was observed when the applied pressure exceeded the safe operation limit (or the flux turning point, where the membrane flux started to increase with increasing applied pressure in the PRO mode), which caused an irreversible damage to the membranes. This study identified safe and optimum operation conditions of the laboratory-made PRO hollow fiber membrane to achieve the most favorable PRO performance. It provides guidance for practical applications of polymeric hollow fiber membranes in PRO process.

Negatively charged hyperbranched polyglycerol grafted membranes for osmotic power generation from municipal wastewater

Osmotic power holds great promise as a clean, sustainable and largely unexploited energy resource. Recent membrane development for pressure-retarded osmosis (PRO) is making the osmotic power generation more and more realistic. However, severe performance declines have been observed because the porous layer of PRO membranes is fouled by the feed stream. To overcome it, a negatively charged antifouling PRO hollow fiber membrane has been designed and studied in this work. An antifouling polymer, derived from hyperbranched polyglycerol and functionalized by α-lipoic acid and succinic anhydride, was synthesized and grafted onto the polydopamine (PDA) modified poly(ether sulfone) (PES) hollow fiber membranes. In comparison to unmodified membranes, the charged hyperbranched polyglycerol (CHPG) grafted membrane is much less affected by organic deposition, such as bovine serum albumin (BSA) adsorption, and highly resistant to microbial growths, demonstrated by Escherichia coli adhesion and Staphylococcus aureus attachment. CHPG-g-TFC was also examined in PRO tests using a concentrated wastewater as the feed. Comparing to the plain PES-TFC and non-charged HPG-g-TFC, the newly developed membrane exhibits not only the smallest decline in water flux but also the highest recovery rate. When using 0.81 M NaCl and wastewater as the feed pair in PRO tests at 15 bar, the average power density remains at 5.6 W/m2 in comparison to an average value of 3.6 W/m2 for unmodified membranes after four PRO runs. In summary, osmotic power generation may be sustained by properly designing and anchoring the functional polymers to PRO membranes.

Conceptual demonstration of novel closed-loop pressure retarded osmosis process for sustainable osmotic energy generation

For the first time, one novel closed-loop pressure retarded osmosis (PRO) process promoted by an effective hydroacid complex draw solution has been demonstrated for harvesting the renewable salinity-gradient energy. The complex draw solute was molecularly constructed to possess unique characteristics of high osmotic pressure, large molecular size and relative low viscosity, and easy regeneration. Compared to conventional PRO processes, the newly developed closed-loop PRO process exhibits promising advantages of sustainable high power output, negligible internal concentration polarization and low membrane fouling, as well as no problems of feed water pretreatment and brackish water discharge. Employing a highly permeable (A=4.30 LMH/bar) and selective (B=0.47 LMH) thin film composite PRO hollow fiber membrane, a power density of 16.2W/m2 can be achieved with an ultralow reverse solute flux (Js/Jw<0.062gL−1) at 12bar when using 1M complex draw solution and deionized water as feeds. The diluted complex draw solution can be regenerated via a solvent precipitation process, and the outstanding PRO performance could be almost fully recovered. We believe the newly developed closed-loop PRO process shows great potential for salinity-gradient energy capture, although the specific benefits have to be fully defined through energy or cost analysis.

Anti-fouling behavior of hyperbranched polyglycerol-grafted poly(ether sulfone) hollow fiber membranes for osmotic power generation.

To sustain high performance of osmotic power generation by pressure-retarded osmosis (PRO) processes, fouling on PRO membranes must be mitigated. This is especially true for the porous support of PRO membranes because its porous structure is very prone to fouling by feeding river water. For the first time, we have successfully designed antifouling PRO thin-film composite (TFC) membranes by synthesizing a dendritic hydrophilic polymer with well-controlled grafting sites, hyperbranched polyglycerol (HPG), and then grafting it on poly(ether sulfone) (PES) hollow fiber membrane supports. Compared to the pristine PES membranes, polydopamine modified membranes, and conventional poly(ethylene glycol) (PEG)-grafted membranes, the HPG grafted membranes show much superior fouling resistance against bovine serum albumin (BSA) adsorption, E. coli adhesion, and S. aureus attachment. In high-pressure PRO tests, the PES TFC membranes are badly fouled by model protein foulants, causing a water flux decline of 31%. In comparison, the PES TFC membrane grafted by HPG not only has an inherently higher water flux and a higher power density but also exhibits better flux recovery up to 94% after cleaning and hydraulic pressure impulsion. Clearly, by grafting the properly designed dendritic polymers to the membrane support, one may substantially sustain PRO hollow fiber membranes for power generation.

Robust and high performance pressure retarded osmosis hollow fiber membranes for osmotic power generation

Novel fabrication perspectives have been demonstrated to molecularly construct robust hollow fiber membrane supports for high performance thin‐film composite (TFC) pressure retarded osmosis (PRO) membranes. For the first time, we found that the desirable hollow fiber supports should possess high stretch resistance and acceptable ductility. The microstructure strength of the hollow fiber support may have more weights on overall robustness of the TFC PRO membranes than the apparent cross‐section morphology. Effectively manipulating the kinetics of phase inversion during spinning by maneuvering bore fluid chemistry, and polymer solution composition is a promising method to tailor the strength of hollow fiber supports. Prestabilization of the TFC membranes at elevated lumen pressures can significantly improve their PRO performance. The newly developed TFC PRO hollow fiber membranes exhibit a power density as high as 16.5 W/m2 and a very low specific reverse salt flux (Js/Jw) of 0.015 mol/L at a hydraulic pressure of 15 bar using synthetic seawater brine (1.0 M NaCl) as the draw solution. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1107–1119, 2014

Outer-Selective Pressure-Retarded Osmosis Hollow Fiber Membranes from Vacuum-Assisted Interfacial Polymerization for Osmotic Power Generation

In this paper, we report the technical breakthroughs to synthesize outer-selective thin-film composite (TFC) hollow fiber membranes, which is in an urgent need for osmotic power generation with the pressure-retarded osmosis (PRO) process. In the first step, a defect-free thin-film composite membrane module is achieved by vacuum-assisted interfacial polymerization. The PRO performance is further enhanced by optimizing the support in terms of pore size and mechanical strength and the TFC layer with polydopamine coating and molecular engineering of the interfacial polymerization solution. The newly developed membranes can stand over 20 bar with a peak power density of 7.63 W/m(2), which is equivalent to 13.72 W/m(2) of its inner-selective hollow fiber counterpart with the same module size, packing density, and fiber dimensions. The study may provide insightful guidelines for optimizing the interfacial polymerization procedures and scaling up of the outer-selective TFC hollow fiber membrane modules for PRO power generation.

Highly Robust Thin-Film Composite Pressure Retarded Osmosis (PRO) Hollow Fiber Membranes with High Power Densities for Renewable Salinity-Gradient Energy Generation

The practical application of pressure retarded osmosis (PRO) technology for renewable blue energy (i.e., osmotic power generation) from salinity gradient is being hindered by the absence of effective membranes. Compared to flat-sheet membranes, membranes with a hollow fiber configuration are of great interest due to their high packing density and spacer-free module fabrication. However, the development of PRO hollow fiber membranes is still in its infancy. This study aims to open up new perspectives and design strategies to molecularly construct highly robust thin film composite (TFC) PRO hollow fiber membranes with high power densities. The newly developed TFC PRO membranes consist of a selective polyamide skin formed on the lumen side of well-constructed Matrimid hollow fiber supports via interfacial polymerization. For the first time, laboratory PRO power generation tests demonstrate that the newly developed PRO hollow fiber membranes can withstand trans-membrane pressures up to 16 bar and exhibit a peak power density as high as 14 W/m(2) using seawater brine (1.0 M NaCl) as the draw solution and deionized water as the feed. We believe that the developed TFC PRO hollow fiber membranes have great potential for osmotic power harvesting.

Mega-ton Water System: Japanese national research and development project on seawater desalination and wastewater reclamation

Reverse osmosis (RO) membranes have been widely applied to seawater desalination and wastewater reclamation. Since 2000, large RO plants (100,000m3/day plants) have been constructed. However, much larger plants will be required in the near future because of its worsening situation of water problems. Energy efficiency is indispensable for the larger plants, especially mega plant (1,000,000m3/day plant). “Mega-ton Water System” project which consists of eight themes believes that there is plenty of room for improvement as well as optimum layout of each component used in the mega plant. In order to reduce energy demand in RO operation, high efficient RO membrane is studied in the project. The relationships between RO membrane structure and its performance were investigated by transmission electron microscope–tomography. The utilizing of pressure retarded osmosis (PRO) energy recovery is one of a solution for energy-saving and defusing the environmental impact caused by concentrated brine. A PRO hollow fiber membrane module was newly developed and a practical continuous operation has been examined at a prototype plant for 1year. The concept of the project will be established by combining the results of eight themes as competing with other membrane and its process studies.

Thin-film composite hollow fiber membranes for pressure retarded osmosis (PRO) process with high power density

For the first time, a specially designed pressure retarded osmosis (PRO) hollow fiber membrane has been successfully developed and applied in the PRO process to demonstrate its potential for power generation. The membrane fabrication method is similar to that used for making thin-film composite (TFC) forward osmosis hollow fiber membranes, but further optimization and improvement have led to a new type of TFC hollow fiber membranes with much greater mechanical strength in addition to its excellent separation property and high water flux. The TFC PRO hollow fiber membranes have a water permeability ( A) of 9.22 × 10 −12 m/(s Pa), salt permeability ( B) of 3.86 × 10 −8 m/s and structural parameter ( S) of 4.6 × 10 −4 m. It can withstand hydrostatic pressure as high as 9 bar with its relatively large dimension of 0.98 mm lumen diameter. This PRO hollow fiber membrane is superior to all other PRO membranes reported in the open literature in terms of power density. A power density as high as 10.6 W/m 2 can be achieved using seawater brine (1.0 M NaCl) and wastewater brine (40 mM NaCl), which suggests that the newly developed PRO hollow fiber membrane has great potential to be applied in PRO processes to harvest salinity gradient energy. A higher pressure is preferred as it allows generation of higher power density (pressures of 12 bar may be optimal for seawater as the high salinity stream), and this can be realized by reduced fiber dimension. Further optimization of the membrane structure will be performed.