These answers are correct ...

These answers are correct below.  I am agreeing that what you have is best solved via 2 steps: Utilizing Ionic Cationed/Anion Softeners, and then by RO's into Mixed Bed vessels.  I would like to know if 1: Your Hardness of 1500 is true, and exactly what the end result would be - what are the tolerance levels? Many other questions remain, as to best answer you.  What is the water to be used for (Boilers, potable)? What will the flow rates be? Is this a pre-job or existing plant? Especially, what will the budget be like?  Obviously, if money were no option, we could say that 3PPB impurities are easily achievable... I would think to better answer your question Sir, we need a bit more information to do so.  The more information you can provide, the better we may answer your question, tailored to your exact needs.