Reverse Osmosis Membrane Treatment Improves Salt-Rejection Performance

This study systematically investigated applying rejection-enhancing agents (REAs) that can plug or seal defects on flat-sheet polyamide (PA) membrane surfaces to improve salt rejection with minimum flux reduction. Polyvinyl methyl ether (PVME) and tannic acid (TA) were used as REAs on low-pressure reverse osmosis (LPRO) membranes. Treatment of an unused LPRO membrane (membrane A) with REAs in the range of 10–30 ppm PVME followed by 10–30 ppm TA significantly improved membrane salt-rejection performance with a concomitant flux reduction less than 20 percent. However, similar treatment of another new membrane (membrane B1), which exhibited good initial salt-rejection performance, did not produce any further improvements. A used membrane B2 was also treated with PVME and TA and showed significant salt-rejection improvement with modest flux reduction. In addition, when the initial salt rejection of a sample from this membrane was relatively small, possibly due to severe degradation, the coating application resulted in significant salt-rejection improvement. Successful testing with injecting REAs online, without interrupting pilot plant operation, suggests that the process is attractive for practical applications. Recommendations are provided for implementing the process in RO desalination plants.