Ph.D Thesis

Ph.D StudentZaslavschi Irina
SubjectStudy of Scaling Propensity of Desalination Concentrates
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


The main objectives of the present work were to advance desalination and waste water purification processes by development of novel scale control techniques. The research involved extension of an innovative electrochemical technique for removal of scale forming species and development of a modified rotating cylinder technique for the evaluating a relative effectiveness of various anti-scalants.

The process path of a sea water thermal desalination plant is confined to temperature-concentration conditions (water recovery of about 50% and the top temperature of 120 °C) which do not exceed the solubility limits of the CaSO4. Higher top temperatures and water recoveries would improve significantly the process economics. The concept developed in this study is to remove calcium from sea water by electrochemical cationic membrane (ECM) system, recently developed in our laboratory, with the injection of CO2 gas to supply the carbon required for carbonate precipitation. The feasibility of the proposed process was demonstrated by experimental data showing precipitation rates as high as 600-700 g/h/m2 cathode area obtained and a relatively low energy consumption of about 6 kWh/kg CaCO3 precipitated.

The potential of the novel ECM technique for heavy metals removal from industrial waste waters was investigated. Removal of over 99% of the ions at a precipitation rates of about 0.85 mol/h/m2 was readily achieved from feed solutions containing 50-60 mg/L of zinc and/or copper. The energy consumption was in the range of 3-12 kWh per kg precipitated metal.

The ECM was further improved by the incorporation of bipolar membranes and additional cation-exchange membranes into the electrochemical cell that led to improved hardness removal, high current efficiency around 100% and lowered energy consumption. Moreover, the electrochemical bipolar membrane (EBM) system enabled complete chlorine free operation by using a 0.1M Na2SO4 electrode rinse solution.

Anti-scalants (AS) are widely used to extend the precipitation threshold of a scaling species. Against this background of dearth of guiding theory in choosing a suitable AS for a particular application, practical needs are prompting development of laboratory techniques. A rotating cylinder system was used to determine the scaling rate of a supersaturated solution dosed with various AS feed concentrations. In the absence of an anti-scalant, scale deposition is mass transfer controlled and with sufficient anti-scalant dosage, precipitation is modified to surface reaction control. Determination of the critical anti-scalant dosage enabling surface controlled precipitation is shown to provide a sensitive technique for comparing the relative effectiveness of different anti-scalants.