An apparatus is described for withdrawing filtered permeate from a
substrate contained in a reservoir at ambient pressure. The apparatus
includes a plurality of membrane assemblies. Each assembly has a
plurality of hollow fiber filtering membranes, immersed in the reservoir,
at least one permeating header with the membranes sealingly secured
therein, and a permeate collector to collect the permeate sealingly
connected to the at least one permeating header and in fluid
communication with lumens of the membranes. The membranes of each
assembly extend generally vertically upwards from a first header during
permeation. One or more sources of suction are provided in fluid
communication with the lumens of the membranes of each assembly through
the permeate collectors and apply sufficient suction to withdraw permeate
from the lumens of the membranes. An aeration system for discharging
bubbles assists in keeping the membranes clean. In other aspects, a
method of removing fouling materials from the surface of a plurality of
porous membranes includes providing, from within a membrane module, gas
bubbles in a uniform distribution relative to the membranes. The bubbles
move past the surfaces of the membranes to dislodge fouling materials
from them. The membranes are arranged in close proximity to one another
and mounted to prevent excessive movement.