Hyperfiltration studies x. Hyperfiltration with dynamically-formed membranes

Abstract

The usual configuration projected for the hyperfiltration (reverse osmosis) desalination process utilizes specially cast cellulose acetate membranes placed on suitable supports. We have found that salt-rejecting interfacial layers or membranes with frequently remarkably high permeation rates can be formed dynamically on porous bodies when certain additives are present in the pressurized feed solutions. Materials with nominal pore diameters near one micron may be used; rejecting layers have been formed dynamically on substrates with pore diameters as high as 5 microns. After initial formation of the membranes, very low concentrations of additives, of the order of 1 ppm are frequently sufficient to maintain their properties. The additives which so far have given the most interesting results would be expected to form ion-exchange membranes. Examples are: organic polyelectrolytes, colloidal dispersions of hydrous oxides, solutions of hydrolyzable ions, ground-up low-cross-linked ion-exchange beads, and certain natural products such as clays and humic acid. For these, dependence of rejection on feed concentration and on the charge type of the salt is generally consistent with an ion-exchange mechanism for rejection. Membranes have also been formed from neutral (uncharged) additives. The chemical nature of the porous body on which the membranes are dynamically deposited does not seem of primary importance. Membranes have been formed on porous silver and other porous metals, porcelain, porous carbons, sintered glass, and “Millipore” filters. The principal potential advantages of this class of membranes are their high production rates, frequently several hundred gallons ft -2 day -1, the simplicity of their formation, and the possibility of making them “self-healing”. In addition, some solutes which are to be removed from feeds are capable of forming dynamic membranes, which in a sense are then “self-rejecting”. With the dynamic membranes so far developed, salt rejection is less than with cellulose acetate and, for most, rejection decreases with increasing salt concentration. In addition, performance is often deleteriously affected by presence of polyvalent counter-ions in the feed solutions. At the present stage of development these dynamic membranes thus seem likely to be more applicable to the treatment of low concentration saline feeds, wastes and polluted waters, than to high concentration feeds.