an apparatus for treating a multi component liquid substrate while leaving particulate matter therein as a skein of hollow fiber filtering membranes immersed in the substrate which is contained in a non - pressurized reservoir . a pumping fluid communication with the lumens of the membranes draws a component of the substrate as permeate through the membranes by applying a section to the lumens of the membranes . in various embodiments , an aeration system as a gas distributor for discharging air directly into the substrate within the skein , upper and lower headers of the skein are spaced apart by a gas tube , and a gas distribution system has through passages through the lower header to discharge bubbles into the substrate above the lower header .