The scaling potential of barium sulphate in reverse osmosis systems

Barium sulphate scaling in reverse osmosis (RO) causes flux decline and potentially severe membrane damage. Existing methods, e.g. Du Pont, predict barite scaling when the concentrate is saturated. However, precipitation in supersaturated concentrate may be limited by slow nucleation kinetics, measured as long induction times. Induction time decreases, hence precipitation is more likely, with supersaturation and temperature. Therefore, induction time may be used to express the scaling potential of RO concentrate. This research aims to develop a more realistic method to predict barite scaling based on the assumption that an induction time (unsafe) can be defined, operating below which scaling will most likely occur in an RO system. While operating above a certain induction time (safe) should prevent scaling. Induction times were calculated for various supersaturation (employing Pitzer coefficients) and temperature conditions at an RO pilot plant using a previously derived relationship of measured induction time versus supersaturation at 25°C. Safe (≥10h) and unsafe (≤5h) induction time limits, were derived from periods when scaling did and did not occur in the RO system and translated into safe and unsafe supersaturation limits for 5-25°C. Use of these supersaturation limits allows more flexible operation in optimising RO recovery while avoiding scaling. The general validity of these limits needs to be verified in further pilot studies with feedwater of different quality and using different RO elements.