A RO pilot plant operated without antiscalant addition at 85% recovery with
no scaling, although the concentrates were significantly supersaturated wi
th 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 co
ncentrate 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, respecti
vely. Organic matter in 90% RO concentrate did not prolong induction time (
5.5 h), Whereas, commercial humic acid extended induction time in 90% synth
etic concentrate to >200 h. This was most likely due to growth inhibition a
s 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 concentra
te were significant and not likely to limit BaSO4 scaling. Results indicate
that the nucleation rate expressed as induction time is governing the occu
rrence of scaling. (C) 2000 Elsevier Science B.V. All rights reserved.