In a further preferred embodiment of the process according to the invention a membrane is treated with a solution comprising a hydroxyalkylcellulose and/or a polyvinylalcohol in the presence of an inorganic acid, preferably sulphuric acid, as catalyst, optionally a cross-linking agent selected among polyols, such as sugars. e.g. gluconic acid or sorbitol, inositol, pentaerythrite, aromatic hydroxy