Unravelling the compaction effects and performance of seawater reverse osmosis membranes under high pressure - insights on the design of membranes for hypersaline brine desalination

Abstract

High pressure reverse osmosis (HPRO) is an emerging technology that is being explored for the treatment of hypersaline water with a salinity of more than 70,000 mg/L in order to overcome its high osmotic pressure. Although membrane deformation is expected in almost all pressure-driven processes, such high operating pressure has shown notable decline in the permeability of conventional seawater reverse osmosis (SWRO) membranes. This study demonstrates the deformation effect of four commercial SWRO thin film composite (TFC) membranes LG SW ES, SW30 HR, SWC4-LD and SWC6-MAXX at 200 bar, where the compaction of the selective and support layers has resulted in a decrease of its cross-sectional thicknesses that ranges from 10.1–29.1% and 35.6–62.0% respectively. The decrease in each cross-sectional thickness has different impacts on the transport mechanisms within the membrane, causing a decline in the membrane’s permeability that ranges from 51.5–71.1%, while maintaining a relatively constant salt rejection rate. The constant salt rejection at 55 to 200 bar shows that the selectivity and free volume element (FVE) within the selective layer is not subjected to significant changes after compaction. On the contrary, the support layer that undergoes a significant decrease in thickness along with a decrease in porosity that ranges from 41.7–68.2%, can be identified as the major contributing factor to compaction-induced water permeability decline. This study aims to observe how the compaction induced by the higher operating pressure influences the performance of the four SWRO membranes and provide insights on how SWRO membranes can be modified in order to withstand deformation and improve its permeability under HPRO conditions.