each header are snugly accommodated in collection pans 15 and 15 39 sized to the base 13 above a permeate collection zone within the pan air is provided through a gas distribution means 19 to maintain beneficial bacteria present in the dirty water permeate withdrawn into the lumens of the fibers preferably under suction collects in the pans and is discharged to a collection point as is described in the 39 424 patent until the flow of permeate is about one half of the flow at initial stable flux at which time the flow of dirty water is shut off so that the lumens of the fibers remain filled with permeate and the cleaning cycle is commenced conduits 21 22 and 23 are provided as shown connecting the lumens of fibers 12 in valved communication with the discharge of a pump 24 via a 3 way valve 25 which in one of its positions allows permeate to be withdrawn from the headers conduit 22 serves as a manifold for the collection pans 15 and an intermediate portion 22 39 of the conduit 22 is provided with a check valve 26 which allows biocidal solution held in cleaning tank 27 to be circulated through the lumens of fibers 12 and returned through conduit 23 to the tank 27 a check valve 28 is provided in conduit 23 to shut off flow of either permeate or biocidal solution to the.