Characterization of the effectiveness of silica anti-scalants

The presence of silica in brackish water greatly complicates the reverse osmosis desalting process. Threshold limits of silica scale precipitation are difficult to predict and there are no reliable techniques for evaluating the performance of silica anti-scalants (AS) used to extend water recovery limits. The aim of the present work is to illustrate a fundamentally oriented procedure for assessing the relative effectiveness of various anti-scalants. Rigorous analysis of inhibition data obtained in comparative tests of four commercial anti-scalants, dosed at the recommended dosage levels, revealed that there was no significant difference in their inhibitory effectiveness. The mitigating effect provided by all AS tested was increase of the threshold limit for the onset of scaling from a membrane surface supersaturation (s/s) level of 1.7-1.9 in the absence of an AS, to the level of 2.1-2.3, in the presence of the AS.