Journal of Membrane Science 179 (2000) 53–68 Stable barium sulphate supersaturation in reverse osmosis ´ Siobhàn F.E. Boerlage a , Maria D. Kennedy a , Ingrida Bremere 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), P.O. Box 3015, 2601 DA Delft, The Netherlands b Laboratory for Process Equipment, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The Netherlands c Amsterdam Water Supply, P.O. Box 8169, 1005 AD Amsterdam, The Netherlands d Kiwa N.V. Research and Consultancy, P.O. Box 1072, 3430 BB Nieuwegein, The Netherlands Received 15 July 1999; accepted 5 June 2000 Abstract A RO pilot plant operated without antiscalant addition at 85% recovery with no scaling, although the concentrates were significantly supersaturated with barium sulphate. Stable supersaturation may be due to slow precipitation kinetics which may be retarded or enhanced by organic matter present in RO concentrate. BaSO 4 precipitation kinetics; crystal nucleation, measured as induction time, and growth were investigated in batch experiments in RO concentrate and in synthetic concentrate containing (i) no organic matter and (ii) commercial humic acid. Supersaturation appeared to control induction time. Induction time decreased more than 36 times with a recovery increase from 80 to 90%, corresponding to a supersaturation of 3.1 and 4.9, respectively. Organic matter in 90% RO concentrate did not prolong induction time (5.5 h). Whereas, commercial humic acid extended induction time in 90% synthetic concentrate to >200 h. This was most likely due to growth inhibition as growth rates determined by seeded growth were reduced by a factor six. In comparison, growth rates were retarded only 2.5 times by organic matter in RO concentrate. However, growth rates measured for 80 and 90% RO concentrate were significant and not likely to limit BaSO 4 scaling. Results indicate that the nucleation rate expressed as induction time is governing the occurrence of scaling. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Barium sulphate scaling prediction; Metastability; Reverse osmosis; Induction time; Growth rate Abbreviations: BET, Brunauer Emmet Teller model; DOC, dissolved organic carbon; LSI, Langelier saturation index 1. Introduction Reverse osmosis (RO) has become a cost effective and viable technology in drinking water production for surface, brackish and seawater sources. However, the precipitation of sparingly soluble salts on the membrane, referred to as scaling, is widely recog- nised as a serious problem in reverse osmosis and ∗ Corresponding author. E-mail address: boe@ihe.nl ( ´ S.F.E. Boerlage). nanofiltration applications. Scaling may result in a decline in membrane water production and poten- tially membrane failure. Barium sulphate (barite) is particularly troublesome in reverse osmosis due to its low solubility (1×10 −5 mol/l in pure water) [1]. Furthermore, if it is not detected at an early stage and has aged into a hard deposit, barite scale is resistant to removal by conventional cleaning chemicals. The Du Pont method is the most widely applied method in RO systems to predict barium sulphate scaling. This method predicts scaling if the solubility 0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0376-7388(00)00504-4