A new post-treatment process for attaining Ca2+, Mg2+, SO42− 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(2)SO(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(2)SO(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 12mg/L using the process is estimated at $0.004/m(3) product water.