Induction times induced in an RO system by antiscalants delaying CaSO 4 precipitation

Abstract

The aim of the present work is to consolidate laboratory techniques we have developed for determining the scaling threshold limits of RO feed waters and for assessing the relative inhibitory effectiveness of various antiscalants. The maximum permissible water recovery may be determined from induction time measurements of supersaturated solutions recycling through a laboratory RO module with permeate recycling so as to maintain a desired supersaturation level. A systematic series of experiments was carried out so as to characterize the extent of the induction period induced by various antiscalants. Five antiscalants were tested at two dosage levels (6 and 12 ppm) and at three CaSO 4 supersaturation levels in the range of 3.7–5.5. Induction times data, extracted from plots of permeate flux vs. time, were successfully correlated with the supersaturation level according to a relationship based on nucleation theory. The results obtained confirm the capability of the proposed recycle technique to discriminate clearly among various antiscalants with respect to their scale suppression effectiveness. Further consolidation of the proposed techniques was obtained by demonstrating that induction times measured in recycle tests are virtually identical to those in once-through runs and that antiscalant molecules retained their full activity under prolonged exposure to pump shear action emanating from the recycling. Finally, surface energies for CaSO 4 nucleation measured in this study with and without the presence of an antiscalant were found to be in excellent agreement with literature data. The results of this study lend confidence to the techniques we have developed for characterizing scaling threshold limits and for evaluating rationally the performance of anti-scalants.