A new post-treatment process for attaining Ca~(2+), Mg~(2+),SO_4~(2-) and alkalinity criteria in desalinated water

摘要

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.rnThe 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.