Application of two-stage reverse osmosis system for desalination of high-salinity and high-temperature seawater with improved stability and performance

Owing to the significant increase in seawater desalination, seawater exhibiting extreme conditions of high salinity and high temperature cannot be avoided in the production of fresh water. Although seawater reverse osmosis (SWRO) is one of the most energy-saving desalination technologies, it might not be favorable for treating extreme seawater due to the operational limitations associated with the single-stage SWRO configuration. To overcome these limitations, the applicability of two-stage SWRO was examined. The recovery of single-stage SWRO was limited to 31% for stable operation, whereas two-stage SWRO, utilizing pressure exchangers as energy recovery devices, achieved 34% recovery for the same number of pressure vessels. Furthermore, the maximum water fluxes in the front SWRO elements were reduced under the design constraints of the two-stage configuration than that of the single-stage configuration. The two-stage SWRO system was energy-efficient with high recovery; however, under extreme conditions, the permeate quality was similar to that of single-stage SWRO. To improve the applicability of two-stage SWRO, internally staged design (ISD) and split partial second pass (SPSP) can be adopted. ISD can reduce water fluxes in the front elements, increasing system recovery, whereas SPSP design enables the reverse osmosis system to produce high-quality permeate with low additional energy.