M.Sc Thesis

M.Sc StudentRachamim Sarit
SubjectInvestigation of Scale Inhibition on RO Membranes by Trace
Amounts of Zinc Ions
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


Raw feed waters usually contain ions of sparingly soluble salts, notably CaCO3. The process of desalination, commonly performed using RO membranes separates pure water leaving a concentrate waste stream which is prone to precipitate sparingly soluble salts. Scale precipitation is usually suppressed to acceptable limits by dosing trace amounts of inhibiting agents, consisting of polymeric organic compounds.

Recent studies indicate that Zn+2 ions, which are environmentally friendly and can be conveniently released to the water by contact with Cu-Zn alloy redox media, exert scale suppression effects. The first objective of this research was to study the effectiveness of CaCO3 scale inhibition by Zn+2 in RO membrane systems and to compare the performance of  conventional  organic antiscalants (Permatreat PC-191 and Genesys CAS) with that of Zn+2 inhibition.

The inhibitory capability of the tested antiscalants was assessed using a continuous-flow laboratory system, equipped with a tubular RO membrane. Feed solution of controlled composition, dosed with an antiscalant, was continuously passed through the membrane. Antiscalant effectiveness was evaluated from the rate of membrane permeability decay.

 Experiments were conducted in a three modes of operation:

  • Full recycle mode, maintaining constant composition of the solution in the feed vessel.
  • Permeate withdrawal mode, simulating the concentration effect occurring in RO desalination module.
  • Once through mode, a solution of controlled composition was continuously fed to the membrane and both permeate and concentrate streams were withdrawn from the system.

The issue of possible deviation in antiscalant performance measured by different modes of operation is controversial. Another objective of this study was to compare results measured by the convenient full recycle technique with those obtained by the more elaborate once through technique.

The Zn+2 inhibition experiments performed by both the full recycle mode and the permeate withdrawal mode, displayed a significant scale suppression effect at the test conditions.

The comparison between inhibition results observed in full recycle tests and once-through tests was carried out from flux decline data measured in runs performed with both Zn+2 and commonly used organic antiscalants. The results showed that when using the same inhibitor the permeability data measured in full recycle and once through mode are in substantial agreement within the experimental accuracy of ±5%.

A comparison between the effectiveness of scale suppression by Zn+2 ions and by commonly used organic antiscalants showed substantially similar results under the tested conditions.