Effects of chemical preservation on flux and solute rejection by reverse osmosis membranes

• Impact of chemical preservation depends on solution pH and redox potential.
• A reducing condition was necessary to minimise the impact of chemical preservation.
• Severe impact membrane performance from formaldehyde and SMBS was observed at pH 3.
• The impact was minimised by maintaining the preservative solution at pH 7.

This study investigates the impacts of chemical preservation on the performance of polyamide reverse osmosis membranes with respect to water permeability and solute rejection. Three preservative chemicals, namely formaldehyde, sodium metabisulfite, and 2,2-Dibromo-3-Nitrilopropionamide, were evaluated for membrane preservation at pH 3 and 7. Experimental data show that chemical preservation may change the membrane surface properties, and consequently water permeability and solute rejection efficiency of the membrane are negatively impacted. The impacts of preservation on boron rejection and sodium rejection are similar in magnitude and more significant than those on water permeability. The results indicate that the impact of chemical preservation on the membrane depends on both the preserving chemicals used and the solution pH value. More importantly, the undesirable impacts of chemical preservation can be minimised by appropriate selection of the preservatives and by preserving the membrane in a reducing condition. A near-neutral pH (i.e., pH 7) is necessary to avoid any significant negative impacts on membrane performance due to chemical preservation using either formaldehyde or sodium metabisulfite. Results reported here suggest that the previously recommended minimum pH value of 3 of the preservative solution may be inadequate.