1. Have to ask how a ~102 ...

1. Have to ask how a ~102 membrane SWRO system could get off the ground without any means to manage the permeate LSI unless the stated 1500 m3/d is incorrect.

2. Calcium is not necessary to attain a potable water quality, it is necessary to manage corrosion  (although there are also other theories) or more specifically the LSI which ideally should be > -0.05 but typically difficult to achieve post a SWRO system with a lack of C02 in the permeate.

3. Plausible means to increase the LSI have been mentioned such as chemical dosing or calcite filters however there are limitations with the former affecting the final pH (can hit >pH 10 easily and still have a corrosive LSI) and the latter needing large refillable tanks and C02 injection to effectively dissolve and yet it can still be corrosive (albeit not as bad). On top of this, the companies that sell these consumables do not typically provide system design to dispense, you have to design this yourself or engage a company to design.

In all honesty Ahmadi this is out of your depth to manage as there is no quick fix by simply adding a consumable (especially on a potable water consumed by people) without an in-depth review of the process in full.