Analysis of the onset of calcium sulfate scaling on RO membranes

Inhibition of the formation of an initial scale layer is the key measure for scale prevention. Major efforts are therefore focused on understanding the complex processes involved in the inception of scaling on reverse osmosis membranes. The objective of this research was to elucidate phenomena involved in the development of the initial scale layer from a supersaturated solution of calcium sulfate by dual measurements of the flux decline and frictional pressure drop. A single parameter model describing surface coverage of the membrane by lateral crystal growth of initial particles was found to give an excellent fit to experimental flux decline data. The source of the initial particles is, as generally accepted, by membrane surface nucleation and/or by deposition from small particles existing in the bulk. Frictional pressure drop measurements clarify some roughness features of deposits. In the absence of anti-scalants scale deposits from solutions of different supersaturations have similar roughness morphologies whereas, morphological changes, induced by the anti-scalants result in higher roughness heights.