A number of treatments for reverse osmosis membranes are known. Most of these treatments fall into two classes; to tighten up the membrane, which reduces salt passage, or to clean a fouled membrane, which increases the permeate flow rate without increasing the salt passage through the membrane. A few methods are also known which increase the salt passage through a membrane. For example, U.S. Pat. No. 4,938,873, teaches for opening polyamide reverse osmosis (RO) membranes by treating the membrane with aqueous, acidic permanganate solution and then stabilizing the membrane with an aqueous solution of alkali metal bisulfite or hydrogen peroxide.
A disadvantage in this process of opening membranes, however, is that it has very little process latitude. That is to say that it is very difficult to control the degree to which the membrane is "opened" by the process, and even more difficult to further increase the degree of opening with further processing. Monitoring the opening process during the permanganate treatment is not entirely possible, because some ions, sulfate for example, don't pass through the membrane until after the stabilizing treatment. In addition, the process time varied considerably from membrane to membrane, thus rendering the process erratic and unpredictable.
Some membranes can also be opened by hydrolysis. Cellulose acetate membranes show increased salt permeability after hydrolysis by either very high or very low pH solutions. Usually these membranes show poor selectivity, i.e., they allow passage of both salts and larger organic molecules.
USPTO Backgrounds
USPTO Backgrounds