module had a pressure drop at a feed flow of 20 gallons per minute gpm of about 10 psi the 33 mil material showed a pressure drop of only 6 psi the 50 mil material a drop of only about 2 psi and there was no measurable pressure drop with respect to the 90 mil material all at the same feed flow of 20 gpm it can be appreciated that this very significantly smaller pressure drop can offer substantial advantages in utilizing two or more membrane cartridges in series flow particularly within a single elongated outer casing which should improve overall performance and furthermore will be even of greater advantage when a feed mixture having a tendency to foul the membrane is being treated moreover this minimal pressure drop when utilizing the 90 mil thick material offers significant advantages from the standpoint of being able to create arrays of membrane cartridges in series and parallel arrangements because the decrease in pressure becomes of minor significance although an even thicker material having ribs of a height of about 150 mils has been tested this further increase in thickness creates a cartridge having only about one third the total surface area of the standard cartridge and although it performs acceptably at the present time it does not seem to have any advantage over the 90 mil material which has generally no measurable pressure loss when operating.