טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentNativ Paz
SubjectMembrane based methods for selective separation of
Mg2+ and Ca2+ ions from seawater, for improving
the quality of soft and desalinated
waters
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Professor Lahav Ori
Dr. Birnhack Liat
Full Thesis textFull thesis text - English Version


Abstract

Two new membrane-based methods are introduced for separation of Mg2 (and other multivalent ions) from seawater and seawater reverse osmosis (RO) brine (salinity: twice seawater concentration), for producing a solution characterized by high concentrations of required ions (i.e. Mg2, Ca2 and SO42-) along with minimal concentrations of unwanted species (i.e. Cl-, B and Na). The produced solution can be used within several applications, for example for enriching soft water with Mg2 and Ca2, recovering and reusing NaCl in ion exchange regeneration solutions and any other application which requires separation of monovalent from multivalent ions. The first presented method is pressure-based, and comprises nanofiltration of seawater (or seawater reverse osmosis brine) followed by a DiaFiltration step conducted with a nanofiltration membrane (a process termed DiaNanofiltration). DiaNanofiltration is a nanofiltration procedure that involves dosage of low-Total Dissolved Solids (TDS) water to the feed solution, to improve the “wash out” of unwanted components. The developed method was specifically aimed at replenishing desalinated water with Mg2 ions. The research describes results of experimental and theoretical examination of the process under various operational conditions. A cost assessment (operational and capital expenses) of the examined scenarios is also provided. The results show that the Cl- to Mg2 concentration ratio in the produced solution varies between 1.52 and 3.27 (w:w), depending on the of chosen operational alternative. Improvement suggestions for lowering this ratio are also presented. In the basic manifestation of the process, the cost of all the scenarios in which antiscalants were applied was estimated below 0.4 cent$ to one m3 of desalinated water enriched with 10 g Mg.

The second method that was developed in the work is driven by electrical potential and uses the Flow Capacitive Deionization (FCDI) method, a process similar in its principles to electrical dialysis (ED). In this work a novel set-up of RO and ion exchange membranes (IEM) was suggested for selective separation of multivalent from monovalent ions. Thus far, the so-called "selective IEM" have been suggested for the purpose of separating monovalent from multivalent ions. For example, Astom's CIMS ion exchange membrane is considered monovalent cation selective, since it is ostensibly characterized by higher rejection toward multivalent ions. However, as shown in this work, the extent of the separation of these membranes is relatively poor and strongly dependent on solution characteristics. In contrast, RO membranes are highly selective toward all ions. Nevertheless, the rejection of multivalent ions by RO membranes is significantly more efficient than the rejection of small monovalent ions (particularly Na and Cl-). If a RO membrane could be used in addition to the IEM within the FCDI configuration, separation between monovalent and multivalent ions could be attained.

This work shows, first theoretically and then empirically, that such action is feasible and may result in very effective separation at reasonable kinetics.