Retention of Water-Borne Bacteria by Membrane Filters Part I: Bacterial Challenge Tests on 0.2 and 0.22 Micron Rated Filters

Abstract

The results of bacterial challenge tests conducted on several 0.2 and 0.22 μm rated “sterilizing grade” filter cartridge types with bacteria from a natural water source are presented. Eight different 0.2/0.22 μm rated “sterilizing grade” filter types from four different filter manufacturers, claimed to be capable of retaining Brevundimonas diminuta at a challenge level of 10⁷ CFU/cm², were tested. The filters tested included nylon 6,6 and polyamide filters from two manufacturers, modified or hydrophilic PVDF filters from two manufacturers, modified or asymmetric PES filters from three manufacturers, and cellulose acetate filters from a single manufacturer. Consistent bacterial penetration was observed, over the 18–24 h challenge period, for all twenty-five integral 0.2 and 0.22 μm rated filter cartridges tested, at challenge levels of about 10¹-10⁴ CFU/cm², indicating that natural waterborne bacteria were more penetrative than B. diminuta. The observed penetration was thus qualitatively independent of filter media type or manufacturer. These results add to the growing body of evidence that shows 0.2 and 0.22 μm rated filters may not remove all microorganisms under all conditions. These results further establish that bacterial penetration of 0.2/0.22 μm rated filters is not limited just to (1) specific membrane types, or (2) extended duration challenges (≫ 24 h), or (3) extremely high challenge levels, or (4) bacteria that can only exist in a penetrative state in an artificial laboratory setting.