A new post-treatment process for attaining Ca 2+, Mg 2+, SO 42− and alkalinity criteria in desalinated water

Abstract

A novel post-treatment approach for desalinated water, aimed at supplying a balanced concentration of alkalinity, Ca 2+, Mg 2+ and SO 4 2−, is introduced. The process is based on replacing excess Ca 2+ ions generated in the common H 2SO 4-based calcite dissolution post-treatment process with Mg 2+ ions originating from seawater. In the first step, Mg 2+ ions are separated from seawater by means of a specific ion exchange resin that has high affinity toward divalent cations (Mg 2+ and Ca 2+) and an extremely low affinity toward monovalent cations (namely Na + and K +). In the second step, the Mg 2+-loaded resin is contacted with the effluent of the calcite dissolution reactor and Mg 2+ and Ca 2+ are exchanged. Consequently, the excess Ca 2+ concentration in the water decreases while the Mg 2+ concentration increases. The process is stopped at a predetermined Ca 2+ to Mg 2+ 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 Mg 2+ ions, while at the same time enables the application of the cheap H 2SO 4-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 Mg 2+ concentration of 12 mg/L using the process is estimated at $0.004/m 3 product water.