Flow conditions affecting the induction period of CaSO4 scaling on RO membranes

Abstract

The extent of the induction period preceding the inception of scale formation is of major interest in the design and operation of RO desalination plants. The objective of present work was to investigate induction time phenomena related to CaSO4 scale precipitation from supersaturated solutions without or with the presence of a polycarboxylic acid based anti-scalant (AS). Membrane tests were carried out at different CaSO4 supersaturation levels (0.75–4.20) and crossflow velocities (0.1–2.5m/s) using both tubular and spiral wound membranes. Stirred beakers tests were also conducted.Membrane induction times in the flow systems increased linearly with the increase of flow Reynolds (Re) number from 80 to 6800 both without and with AS. The induction period in the presence of the AS was higher than that observed in AS free solutions. However, the extension of the induction period induced by the AS increased markedly with the Re number. Comparison of induction times measured in membranes and stirred beaker systems revealed a critical Re number below which induction time was shorter in the membrane system and above which the induction time was longer in the membrane system compared to the beaker test.