Stable Barium Sulphate Supersaturation in Reverse Osmosis

58 59 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. Barium sulphate 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 hour). Whereas, commercial humic acid extended induction time in 90% synthetic concentrate to >200 hours. This was most likely due to growth inhibition as growth rates determined by seeded growth in synthetic concentrate containing commercial humic acid were reduced by a factor of 6. 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 barium sulphate scaling. Results indicate that the nucleation rate expressed as induction time is governing the occurrence of scaling. 60