NOVELTY - Withdrawing filtered permeate from activated sludge involves providing an assembly having hollow fiber filtering membranes (12) disposed vertically between two headers (11), an enclosure sealed to upper header to define a cavity, a permeate port in communication with the cavity, and lumen of each membrane in fluid communication with the port. A suction is applied to lumens via the port to withdraw permeate from non-pressurized substrate in reservoir; and blowing air into the sludge to provide bubbles in sludge, where bubbles contact the membranes as bubbles rise through sludge.    USE - For withdrawing filtered permeate from activated sludge useful for e.g. wastewater containing particulate matter, for recovery of water from groundwater containing micron and submicron particles of siliceous materials, preferably gas scrubbing with carbon dioxide, and recovery of solvent from paint-contaminated solvent.    ADVANTAGE - The method avoids problem faced by using conventional membrane module to selectively separate one fluid from another, particularly using module in combination of bioreactor, and attendant costs of operating system. The method is performed without any pumps. The process efficiently withdraws permeate for a long period, in a single stage and preforms continuous filtration process. The fibers are maintained sufficiently free from particulate deposits with little cleansing gas, so that the specific flux at equilibrium is maintained over a long period, e.g. from 50 - 1500 hours.    DETAILED DESCRIPTION - Withdrawing filtered permeate from activated sludge involves: providing an assembly in a tank and having upper and lower headers (11), several hollow fiber filtering membranes (12) disposed vertically between the headers, an enclosure sealed to the upper solid body to define a cavity and to prevent non-pressurized substrate from entering the cavity, a permeate port in communication with the cavity, and lumen of each membrane in fluid communication with the port via the cavity; feeding activated sludge into the tank to immerse the membranes in sludge at ambient pressure; applying a suction to lumens via the permeate port to withdraw permeate from the non-pressurized substrate in the reservoir; blowing air into the sludge to provide bubbles in the sludge, where bubbles contact the membranes as the bubbles rise through the sludge; and withdrawing retentate sludge from the tank. The steps of blowing air and withdrawing the retentate sludge occur during the step of applying the suction. The membranes are sealed in the solid bodies to prevents non-pressurized substrate from contaminating permeate, and each having a length of 1.001 - 1.05 times the vertical membrane. The air is blown intermittently while applying suction.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of membrane device comprising the skein of fibers with the ends of fibers potted in a lower header, along with a permeate collection pan, and a permeate withdrawal conduit.    vertical skein (10)    lower header (11)    fibers (12)    intermediate portions of fibers (12')    collection pan (20)    permeate withdrawal conduit. (30)