You could study and learn ...

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You could study and learn about cold lime process if use cation exchange not admitted. PH control at 9.5 or hydrate alkalinity +/- 5 ppm CaCO3, after it reduce the pH in level 7.5- 8.5 range, first. 

Or You could use RO the permeate can be added from mucron filtered fresh water. To arrange the level Calsium in the product you can calculate ion balance, then define the fresh water rate to the permeate.