Stable barium sulphate supersaturation in reverse osmosis

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. BaSO₄ 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 BaSO₄ scaling. Results indicate that the nucleation rate expressed as induction time is governing the occurrence of scaling. (C) 2000 Elsevier Science B.V.