Cellulose derivative reverse osmosis membrane

A reverse osmosis membrane comprising a cellulose derivative selected from the group consisting of cellulose ether or cellulose ester prepared from a casting solution containing a tetracarboxylic acid having by the formula:

• R--(CO.sub.2 H).sub.4
• wherein R is a member of the group consisting of a tetravalent aliphatic or alicyclic organic radical having from 2 to 10 carbon atoms, said membrane having not less than about 3.times.10.sup.-10 g.sup.3 /cm.sup.7.sec.sup.2.atm.sup.3 of A.sup.3 /B and not more than about 0.2 voids per 0.844 mm.sup.2, wherein A is the water permeation coefficient (g/cm.sup.2.sec.atm) and equal to F.sub.1 /(.DELTA.P-.DELTA..pi.), where F.sub.1 is the product water flow through the membrane (g/cm.sup.2.sec), .DELTA. P is the applied pressure differential(atm) and .DELTA. .pi. is the osmotic pressure differential(atm);
• and B is the salt permeation coefficient (cm/sec) and is equal to F.sub.2 /(C.sub.1 -C.sub.2), wherein F.sub.2 is the salt permeation through the membrane (g/cm.sup.2.sec), C.sub.1 is the salt concentration in the feed water (g/cm.sup.3) and C.sub.2 is the salt concentration in the product water (g/cm.sup.3).