טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentOren Shaul
SubjectDevelopment of a Post Treatment Process for Magnesium
Addition to Inorganic-Carbon-Rich Desalinated
Brackish Water
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Ori Lahav
Full Thesis textFull thesis text - English Version


Abstract

The worldwide increasing shortage of potable water has lead to a dramatic increase in desalinated water capacity. Desalination plants remove dissolved salts from both brackish water (BW) and seawater (most new plants apply the Reverse Osmosis (RO) membrane technology) in order to increase potable water availability. Desalinated water (DW) differs from most natural waters in the sense that following the RO step it hardly contains dissolved minerals, calcium and magnesium ions being the minerals addressed in this work. While a requirement for minimum concentration of Ca in supplied water has become a consensus and is enforced in many countries, only in 2011 the water quality guidelines of the World Health Organization (WHO) recommended 10 mg/l as the minimal concentration of Mg to be supplied with DW, for human health reasons.

Taking the Israeli water quality standards as benchmark, the current work assessed the feasibility of Mg addition to desalinated BW using a novel ion exchange (IX) - calcite dissolution technology, a process previously developed at the Technion for Mg addition to desalinated seawater. In this process, Ca ions are supplied in excess into RO water via the dissolution of quarry calcite beads. This Ca2-rich water is subsequently passed through a special resin preloaded with magnesium ions, originating from seawater. During this passage a predetermined mass of calcium and magnesium ions exchange places, and the water is enriched with dissolved magnesium.

The current work investigated the practicability of this process for near-shore BW desalination plants whose raw water contains high concentrations of inorganic carbon, and quantified it for an actual desalination plant to enable real-life cost estimation. Since the IX process requires a relatively high concentration of Ca in the IX inlet, the work addressed the option of enhancing calcite dissolution by dosing low-cost strong-acid to the RO feed stream, thus increasing the concentration of CO2 in the water entering the calcite dissolution reactor. Since the acid addition technique resulted in highly CO2-supresaturated water, issues associated with CO2 stripping from the final product water (in order to meet quality regulations) were addressed.

Engineering and economic aspects of a complete desalination post-treatment process, including acid addition, IX and CO2-stripping steps were quantified.

The main conclusion was that attaining the Israeli regulation quality requirements for desalinated water, along with the WHO recommendation for Mg concentration, is possible and would increase production cost by about 0.16 NIS, as compared to the current no-magnesium-addition scenario.