Microbial removal and integrity monitoring of RO and NF membranes

The successful application of high-pressure membrane processes as effective barriers to pathogens requires the use of online methods for monitoring and controlling membrane integrity during process operation. Currently available integrity monitoring methods involving conductivity and total organic carbon have a resolution of only ∼99% (2 log removal), but intact nanofiltration (NF) and reverse osmosis (RO) membranes and systems can provide much greater pathogen removal (>4 logs). Reliable and cost-effective integrity methods having equivalent resolution are needed in order for regulatory agencies to permit (and utilities to realize) the pathogen-removal credits for which membranes are capable. This research evaluated two new integrity monitoring methods that use nonmicrobial viral surrogates - fluorescent microspheres and fluorescent Rhodamine WT dye (RWT) - to measure NF-RO membrane and system integrity under both intact and compromised conditions. Microsphere dosing to feedwater demonstrated viral removal at greater than the target level of 4 logs, but given the current cost of such microspheres and the required analytical sophistication, such resolution is only cost-effective at pilot scale. In contrast, RWT proved a practical full-scale viral surrogate for RO, and to a lesser extent, NF system integrity. RWT is inexpensive, easy to dose, and can accurately measure ∼4 log removal at a feed concentration of <2 mg/L. The study also found that quality assurance/quality control (QA/QC) measures currently used by manufacturers are insufficient and that membranes passing normal QA/QC tests vary widely in their viral removal ability. However, membrane imperfections that allow significant virus passage can be "healed" through fouling from normal operation. This operational benefit should be considered by regulators when granting log-removal credits for membranes. - MPM.