Inception of CaSO4 scaling on RO membranes at various water recovery levels

This paper describes laboratory techniques for characterizing the permissible water recovery fraction in RO desalination, which is limited by the need to avoid scale deposition on the membrane. The CaSO4 scaling system was adopted for the development of these techniques. An upper water recovery bound is determined by operating an RO laboratory module such that the feed concentration level is gradually increased through continuous withdrawal of permeate while the concentrate is recycled to the feed vessel. The maximum possible water recovery is indicated by the solution concentration at which the supersaturation level triggers an immediate copious precipitation process. The procedure for evaluating a lower scaling threshold limit, at which precipitation is prevented or at least delayed for a sufficiently long period of time, is based on induction time experiments. These are carried out by operating the RO system in a full recycle mode (both permeate and concentrate recycled to the feed vessel) and measuring the time required for the onset of scaling. Results of preliminary tests show that induction time data are well correlated with the supersaturation level according to a relationship based on nucleation theory. Surface energies for CaSO4 nucleation on RO membranes are found to conform with literature data, thus lending support to the proposed induction time technique.