A new post-treatment process for attaining Ca2+, Mg2+, SO42− and alkalinity criteria in desalinated water

A novel post-treatment approach for desalinated water, aimed at supplying a balanced concentration of alkalinity, Ca2+, Mg2+ and SO42−, is introduced.The process is based on replacing excess Ca2+ ions generated in the common H2SO4-based calcite dissolution post-treatment process with Mg2+ ions originating from seawater. In the first step, Mg2+ ions are separated from seawater by means of a specific ion exchange resin that has high affinity toward divalent cations (Mg2+ and Ca2+) and an extremely low affinity toward monovalent cations (namely Na+ and K+). In the second step, the Mg2+-loaded resin is contacted with the effluent of the calcite dissolution reactor and Mg2+ and Ca2+ are exchanged. Consequently, the excess Ca2+ concentration in the water decreases while the Mg2+ concentration increases. The process is stopped at a predetermined Ca2+ to Mg2+ ratio. All water streams used in the process are internal and form a part of the desalination plant sequence, regardless of the additional ion exchange component. The proposed process allows for the supply of cheap Mg2+ ions, while at the same time enables the application of the cheap H2SO4-based calcite dissolution process, thus resulting in higher quality water at a cost-effective price. A case study is presented in which additional cost of supplying a Mg2+ concentration of 12 mg/L using the process is estimated at $0.004/m3 product water.