Impact of membrane ageing on reverse osmosis performance - Implications on validation protocol

Ageing of reverse osmosis (RO) membrane due to irreversible deposition of foulants or by frequent exposure to chemical cleaning agents is expected to result in performance decline or complete membrane failure, leading to noncompliance and loss of process efficiency. Limited studies have related failures as a result of membrane ageing with integrity monitoring and measured virus removal. In the present study, an accelerated ageing method, whereby membranes were subjected to cyclic fouling, cleaning and hypochlorite exposure, was developed and performance benchmarked against passively and industrially aged samples. Membrane performance during accelerated ageing was evaluated with respect to permeability, online conductivity removal (LRVEC), rejection of spiked NaCl salt (LRVNaCl) and reduction of MS2 phage (LRVMS2). Both LRVEC and LRVNaCl correlated well with LRVMS2 (R2=0.8206 and 0.8514 respectively) for the samples tested in this study, LRVEC was 2-3 times lower than LRVMS2 and LRVNaCl was at least 4 times lower than the corresponding LRVMS2. Interestingly, removal of virus sized pathogens (MS2), LRVMS2 was constantly greater than 4 although salt rejection decreased to 80%. Given the difference in the hydrated radius of ions of NaCl (0.36 and 0.33 nm for hydrated Na+ and Cl− ions) compared to MS2 phage (26 nm), spiked salt rejection represents a highly conservative procedure for confirmation of LRVMS2 and was shown to provide an early indication of subtle damage induced by chemical cleaning. In addition, the cyclic cleaning method utilized in this study resulted in a more severe performance decline, at equivalent hypochlorite exposure, when compared to passive ageing procedures.