Effect of crossflow testing conditions, including feed pH and continuous feed filtration, on commercial reverse osmosis membrane performance

Crossflow filtration experiments were performed to characterize the water flux and NaCl rejection of three commercial polyamide reverse osmosis (RO) membranes (LE and XLE from DOW Water Solutions and AG from GE Water and Process Technologies). Thorough cleaning of the crossflow system, combined with following the manufacturer's recommended pretreatment and test conditions (i.e., feed pressure, flowrate, temperature, feed pH, and feed filtration) resulted in measured performance values consistent with manufacturer benchmarks. Correction for the effect of concentration polarization also proved important. The influence of feed pH and continuous feed filtration on water flux and salt rejection was characterized. While rejection was strongly affected by feed pH, water flux was essentially unaffected. Continuous filtration of the feed led to higher water flux and lower salt rejection than that observed in experiments with unfiltered feed, suggesting fouling of the membrane surfaces by unfiltered feed. The flux and rejection of these three membranes obeyed a general tradeoff relation: membranes that exhibited higher flux had lower rejection and vice versa.