BaSO 4 solubility prediction in reverse osmosis membrane systems S Â iobha Án F.E. Boerlage a,* , Maria D. Kennedy a , Geert Jan Witkamp b , Jan Peter van der Hoek c , Jan C. Schippers a,d a International Institute for Infrastructural, Hydraulic and Environmental Engineering ± IHE, PO Box 3015, 2601, DA Delft, Netherlands b Laboratory for Process Equipment, Delft University of Technology, Leeghwaterstraat 44, 2628, CA Delft, Netherlands c Amsterdam Water Supply, PO Box 8169, 1005, AD Amsterdam, Netherlands d Kiwa Research and Consultancy, PO Box 1072, 3430, BB Nieuwegein, Netherlands Received 26 October 1998; received in revised form 5 January 1999; accepted 5 January 1999 Abstract Barium sulphate scaling in reverse osmosis (RO) causes ¯ux decline and potentially severe membrane damage. Du Pont's method to predict BaSO 4 scale based on predicting barium solubility in RO concentrates is unreliable and limited to 258C. This method predicted barium solubility was exceeded 14 times at 80% recovery and yet no scaling occurred at the pilot plant. Possible explanations are; inaccurate solubility prediction, low rate of BaSO 4 precipitation and/or organic matter effects on solubility or precipitation. This study investigated barium solubility in RO and the effect of, e.g., ionic strength using more theoretical approaches to solubility prediction i.e. Bromley and Pitzer models. Seeded growth determination of barium and synthetic concentrates (no organic matter) con®rmed supersaturation and proved organics had no effect on solubility. Du Pont's method under predicted solubility by ca. 30%. The Pitzer and Bromley methods when calibrated for RO concentrates by applying an experimental K sp gave accurate prediction at 5±258C for the ionic strength range of 0.01±0.1 M. For higher ionic strengths, the Pitzer model was more accurate. The observed stable supersaturation (27 times the solubility at 58C) in the pilot plant is most likely due to the low rate of precipitation and is under investigation. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Barium sulphate scaling; Solubility prediction; Reverse osmosis; Organic complexation; Pitzer model; Bromley correlation 1. Introduction Traditionally, reverse osmosis (RO) was applied for the desalination of brackish water and seawater. Nowadays, RO is increasingly being considered for the treatment of surface water sources. Reverse osmo- sis is under investigation at Amsterdam Water Supply (AWS) as an integrated part of two treatment systems. In both systems, River Rhine water is pretreated by coagulation, sedimentation, and rapid sand ®ltration. Subsequently, in scheme I, the pretreated water is treated by ozonation ± biologically activated carbon ®ltration ± slow sand ®ltration and reverse osmosis (RO1). Whereas, scheme II comprises slow sand ®ltration prior to reverse osmosis (RO2) [1]. In these schemes RO is applied to achieve multiple objectives including the removal of salts, hardness, pesticides and other organic micropollutants, and to ensure adequate disinfection. Journal of Membrane Science 159 (1999) 47±59 *Corresponding author. Tel.: +31-15-215-1715; fax: +31-15- 212-2921; e-mail: boe@ihe.nl 0376-7388/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII:S0376-7388(99)00052-6