Multilayer membrane and process of manufacturing the same

A process of manufacturing a multilayer membrane including a carrier layer composed of fibers of a material selected from the group consisting of polyamide, polyvinylidene difluoride, polyester and glass;

• a porous supporting layer composed of a material selected from the group consisting of polysulfone, polyvinylidene difluoride, polyethersulfone, polyimide, polyvinyl alcohol, polyurethane, polyacrylonitrile and polyether imide;
• and a separating layer composed of polyvinyl alcohol, said separating layer being nonporous and having a structure having a high microcrystalline content with crystallites which exhibit in the X-ray diffraction pattern a reflection in a range of 1.2 to 1.5 degrees at diffraction angles H 2 O of 19.6.degree. to 20.degree., the process comprising the steps of making initially a composite material from the materials of said carrier layer and said porous supporting layer;
• subsequently applying to the supporting layer of the composite material a diluted solution which contains 1 to 10% by weight polyvinyl alcohol, balance water, and which before use has been stored in a diluted or undiluted state at temperatures from -10.degree. to +10.degree. C. for 3 to 20 days;
• and then treating the composite material with an aqueous solution of polyvinyl alcohol at a temperature from 100.degree. to 180.degree. C. for 1 to 60 minutes.