Novel integrity monitoring techniques for high-pressure membranes (nanofiltration and reverse osmosis)

Abstract

High-pressure membrane processes such as nanofiltration (NF) and reverse osmosis (RO) are becoming more common in water treatment trains designed to produce high quality effluent from impaired water resources (e.g., potable reuse; desalination) due to their ability to remove dissolved contaminants, salinity, and harmful pathogens. Various drinking water regulations require that water treatment systems achieve at least 4-log removal of pertinent pathogens, dependent on the source water, through a combination of treatment approaches. While previous studies investigating high pressure membrane treatments have demonstrated greater than 5.0-log removal of these pathogens (99.999% removal), only 1- to 2-log pathogen removal credits are granted due to limitations in accepted membrane integrity monitoring techniques. A thorough review of the substantial work that has been dedicated to improving the log removal values granted to high pressure membrane systems revealed that while there are integrity monitoring methods that are able to achieve log removal values above 3-log, only a small subset can be realistically applied by facilities employing high-pressure membranes. From this small subset, two potentially viable integrity monitoring methods were chosen for further investigation. The first method evaluated the calculation of log removal values for full scale membrane systems using statistical analysis of conductivity profiles. The second method evaluated the application of the promising chemical markers identified in a literature review on a closed circuit system. The statistical analysis of conductivity profiling data from two separate facilities resulted in calculated log removal values greater than 3-log, as opposed to values of only 1.5- to 2-log obtained using combined permeate calculations. Sampling and analysis of strontium and two fluorescent dyes during operation of a closed circuit desalination system was able to demonstrate removals of 3.25-log and 2.5-log, respectively. The use of statistical analysis and strontium monitoring both have the ability to demonstrate higher removal capacity of intact membranes than current methods without adding undue cost to the facilities that implement them.