STRUCTURE OF POROUS CELLULOSE ACETATE MEMBRANES AND A METHOD FOR IMPROVING THEIR PERFORMANCE IN REVERSE OSMOSIS

Experimental data support the hypothesis that the surface layer of the asymmetric Loeb‐Sourirajan type porous cellulose acetate membranes has a heterogeneous microporous structure. A general method is proposed for improving the performance of the above membranes in reverse osmosis, by which product rates are increased without decreasing solute separation. The method consists in pumping pure water past the back side of the membrane under just enough pressure for a sufficiently prolonged period of time; after such pretreatment, the membrane is used in the reverse osmosis experiments in the normal manner with the surface layer facing the feed solution. Back‐pressure treatment at 400 psig for 85 hr on preshrunk and normally pressure‐treated membranes increases the product rate by over 20% without decreasing solute separation in reverse osmosis experiments at 600 psig with the use of 0.5 wt‐% NaCl–H2O feed solutions; with a different sequence of back‐pressure treatment, similar results have been obtained in reverse osmosis experiments at 1500 psig also. The compaction effect of a normal membrane and that of a back pressure treated membrane are the same during continuous reverse osmosis operation under 600 psig; the effects of back‐pressure treatment on a normal membrane and a compacted membrane are also the same. The pure water permeability data obtained in cyclic experiments show that the smaller pores on the surface layer are opened more than the bigger ones during the back side operation. The probable structural changes taking place in the film during back‐pressure treatment are discussed. Copyright © 1969 John Wiley & Sons, Inc.