As the material of the hollow fibers to be used in this invention, selections may be made from among such polyesters as polyethylene terephthalate, polyvinyl chloride, and polyvinyl acetate; such polyolefins as polyethylene and polypropylene; such polyamides as nylon 6 and nylon 66; polyurethane, cellulose acetate, and polyacrylonitrile.