![]() Toyobo’s pressure retarded osmosis (PRO) and RO hollow fiber membranes can withstand the maximum pressures of up 29 bar and 69 bar under the out-to-in mode (i.e., from the shell side to the lumen), respectively 21. ![]() Besides, the hollow fiber membrane is easier to scale up and simpler in module fabrication than the flat-sheet one 19, 20.Ĭurrently, the leading hollow fiber membranes suitable for the OARO process are produced by Toyobo (Japan) whose cellulose triacetate (CTA) hollow fibers have been used extensively in various desalination industries. By contrast, the hollow fiber membrane is self-supported and has a higher surface-to-volume ratio than the flat-sheet membrane. Besides, the water transport resistance increases significantly when the special spiral-wound module is operated under high pressures due to the compaction of both the spacer and support layer 16, 17, 18. Although the operating pressure of spiral-wound RO membrane modules could be further increased to 120 bar by employing a special membrane and module design, the effective membrane area inside the special module decreases by about 6 times as compared to a typical similar-sized RO module (e.g., ~ 6 vs. The conventional flat sheet or spiral-wound RO membranes usually are able to withstand a maximum pressure of about 80 bar 3, 15. Thus, the development of strong and efficient membranes is a key in meeting the requirements of the OARO processes. Nevertheless, the OARO process is also limited by the burst pressure of the membrane as the water recovery becomes higher 3, 11, 13, 14. Therefore, OARO makes a high water recovery of >70% via a membrane-based energy-efficient process possible 3, 11, 13, 14. Briefly, a saline stream with a lower or equal salinity is employed in the permeate side as a sweep stream of the OARO process to reduce the difference of osmotic pressure across the membrane, thereby the water transport becomes possible even when the osmotic pressure of the feed is larger than the external applied hydraulic pressure 3, 14. In the OARO process, water is transported across the semi-permeable membrane driven by the hydraulic pressure that overcomes the transmembrane osmotic pressure difference 3, 14. One of the promising technologies to realize high water recovery (e.g., >50%) is the membrane-based osmotically assisted reverse osmosis (OARO) process 3, 10, 11, 12, 13, 14. In order to maximize the RO potential and increase its water recovery, the desalination of the highly saline water has recently received increasing attention from academia and industries 3, 6. For instance, 5–33% of the total cost of the RO desalination process is spent on the disposal of the RO effluent 3, 7. Additionally, the treatment of the concentrated RO effluent stream is not cheap. Thus, the RO process must consume extra energy to overcome the osmotic pressure exerted by the concentrated saline water 2, 9. With continuing water recovery, the saline feed becomes more concentrated. The maximum water recovery of the conventional RO process is about 35–50% 4, 5, 6, mainly due to limitations stemmed from a maximum salinity (e.g., >70 g/L) of the feed stream and practical considerations such as mechanical strength of membranes, economic and environmental concerns 3, 7, 8. Currently, reverse osmosis (RO) desalination is the most energy-efficient process for producing clean drinking water from seawater and/or saline water when compared with the conventional thermal processes 1, 2, 3.
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