Monitoring reverse osmosis performance: Conductivity versus fluorescence excitation–emission matrix (EEM)

This paper evaluated dissolved organic matter (DOM) rejection by reverse osmosis (RO) membranes employed in full-scale water reclamation plants with two techniques based on fluorescence to assess its suitability as a novel method for verification of membrane process integrity. Excitation–emission matrices (EEM) of feed and permeate samples from individual pressure vessels, complete stages and RO trains of two full scale plants were analysed with a fluorescence regional integration technique. Depending on the excitation–emission region quantified, DOM rejection up to around 99.5% was regularly measured and fluorescence measurements could be used as more sensitive tool compared to conductivity profiling when assessing membrane installations. A blue-shift in the fluorescence of the humic substances peak was observed and could be explained by determining size distribution of organic matter by size exclusion chromatography (SEC) with fluorescence detection. The results demonstrated that the size distribution of fluorescent DOM changed towards lower molecular weight from feed to permeate due to increased rejection of high molecular weight compounds. Preliminary trials showed rejection of high molecular weight substances and consequentially membrane integrity beyond 99.9%. We conclude that fluorescence coupled with regional integration techniques and potentially SEC is a promising sensitive technique to assess RO membrane integrity.