A process comprising a duplex type impregnating apparatus providing the steps of: (i) providing an elongated porous web, said elongated porous web comprising two outermost surfaces; (ii) transporting said elongated porous web downwards between two impregnating heads comprising two slots each with substantially vertical upper and lower slot faces substantially parallel to said elongated porous web providing simultaneously to both surfaces of said elongated porous web metered substantially identical quantities of a dope, comprising at least one membrane polymer and at least one solvent therefor; (iii) thereby impregnating said elongated porous web completely with said dope and providing substantially equally thick dope layers on each surface of said outermost surfaces of said elongated porous web with a thickness independent of the gap between one of said lower slot faces and the surface of said elongated porous web nearest thereto; (iv) subjecting said dope associated with said elongated porous web to symmetric phase inversion with at least one non-solvent thereby forming a membrane, and (v) removing residues of said at least one solvent for said at least one membrane.