Abstract: An open pore liquid purification apparatus containing semipermeable membranes, is made by: (1) placing a module, made of bonded, resin coated filler particles, having a plurality of spaced circular bores therethrough, in a stationary vertical position, (2) placing a viscous casting composition into each of the bores, the composition containing 10 parts polysulfone resin and 10-40 parts of an organic solvent, the solvent being effective to homogeneously disperse and partly dissolve the polysulfone, (3) gravity dropping a casting bob, having a circular cross-section at its widest point, down through each bore to form a continuous film of casting composition on the inside of each bore, and then (4) immediately exposing the film to a low humidity atmosphere, and (5) contacting the film with a suitable leaching liquid, to form a polysulfone ultrafiltration membrane on the inside of each module bore; the membrane being resistant to solvents and operable at a pH of about 1 to 12 and a temperature of about 5.degree.