NOVELTY - A header (41b, 41b', 41u, 41u') and permeate collector (46b, 46u) comprises a solid mass of a potting material and several hollow fiber membranes sealed in potting material. The open-ended terminals of hollow fiber membrane protrudes into permeate collection zone formed by potting material that extends and secures inner periphery of the permeate collector.    USE - The collector is useful for a skein useful in wastewater treatment, treatment of activated sludge in bioreactor and microfiltration of liquids.    ADVANTAGE - The fibers in a skein are vertical and do not present an arcuate configuration above a horizontal plane through the horizontal central line of a header. As a result, the path of the rising bubbles is generally parallel to the fibers and is not crossed by the fibers of a vertical skein, yet the bubbles scrub the fibers. The restrictedly swayable fibers, because of their defined length, do not get entangled, and do not abrade each other excessively, as is likely in the prior art. The bubbles of a fiber-cleansing gas flowing parallel to fibers in a vertical skein are more effective than bubbles, which are intercepted by arcuate fibers crossing the path of the rising bubbles. The oxygen-containing gas bubbles promote growth of microbes unexpectedly fails to build-up growth of microbes on the surfaces of the fibers because the surfaces are vertically air-scrubbed. Deposits of animate and/or inanimate particles upon the surfaces of fibers are minimized when the restrictedly swayable fibers are kept awash in co-directionally rising bubbles which rise with sufficient velocity to exert a physical scrubbing force to keep the fibers substantially free of deleterious deposits. Thus, an unexpectedly high flux is maintained over long periods during which permeate is produced by outside-in flow through the fibers. The permeate may be efficiently withdrawn from a substrate for a long period, in a single stage, essentially continuous filtration process, by mounting a pair of headers in vertically spaced apart relationship, one above another, within the substrate which directly contacts a multiplicity of long vertical fibers in a gas-scrubbed assembly comprising a skein and a gas-distributor. The system may be operated to withdraw permeate under gravity alone, so that the cost of any pump to withdraw permeate is avoided.    DESCRIPTION OF DRAWING(S) - The drawing shows a perspective view of the pair of skeins.    Skeins (40, 40')    Lower headers (41b, 41b')    Upper headers (41u, 41u')    Fibers (46b, 46u) permeate collector (42, 42')    Gas distributor. (50)