A method for cleaning and maintaining reverse osmosis membrane filters by injecting sulfurous acid into water to form sulfurous acid (H.sub.2SO.sub.3), and then sequentially filtering the acidified water through membrane filters to reduce alkalinity and mineral scaling, add sufficient SO.sub.2 as a biocide to attack bacteria and other micro organisms to prevent membrane fouling, reduce iron to prevent iron deposit build-up, scavenge and remove dissolved oxygen prior to filtration to prevent membrane oxidation, and prevent concentrated salts within the retentate from precipitating out of solution during transport for land application.