Abstract: Hollow fibers of acrylonitrile polymers having water-permeability greater than 0.2 ml/cm.sup.2.min.atm and useful as an inexpensive superior ultrafilter, comprising(1) a porous layer or layers existing on the outer or/and inner sides of said hollow fibers, the average sizes of pores contained in said porous layer or layers and existing on each of assumptive cylindrical surfaces concentric with the central axis of said hollow fibers, having such a gradient that the average sizes of pores contained in the porous layer existing on the outer side become smaller toward the outer surface of said hollow fibers while the average sizes of pores contained in the porous layer existing on the inner side become smaller toward the inner surface of said hollow fibers, and(2) a reticulate porous layer adjacent to said porous layer or layers (1) and containing therein at least one succession of voids having diameters of at least 10-50 .mu.

Abstract: An ultrafiltration apparatus comprising a module in which a number of hollow fibers having semipermeable walls, disposed in approximately parallel and collected orderly to form a bundle and fixed with a cast-molding material at at least one end portion thereof, are used as semipermeable membrane, a back washing means and a control means for controlling said module and said back washing means, is presented. Said apparatus can be used for a long time without accompanying reduction of filtration capacity.