You may consider using a ...

You may consider using a side-stream process whereby about 2 - 5% of the final product is diverted and saturated with CO2, and then contacted with solid CaCO3. The CO2 acidified sidestream is able to take up considerable amount of CaCO3, increasing the alkalinity and the calcium concentration.  The CO2 is then stripped (and captured for re-use if required), and the side stream is blended with the mainstream at the proportion to achieve the desired hardness and pHs of the product.   Do a Google search for the paper entitled "Water stabilisation througha limestone mediated sidestream process" by Ciciyela Tsotsi.