For the hardness increasing ...
Published by Gunther Johne, Consultant - Environment Engineering (Water Treatment / Water Management / Resource Recovery)
For the hardness increasing you can use two ways
1 – blending of the RO permeate with raw water
2 – using of calcium carbonate (granulated or lime milk) and carbon dioxide.
For many industrial applications is the first way more than sufficing. But when your raw water has some contamination inside which your costumer will not have it´s better to use the second way.
For online monitoring of the hardness and the regulation of my blending valve I use an ion selective sensor system – more information to this monitoring units you´ll find here http://water-monitoring.com/produktubersicht/?lang=en
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We also will utilize granular calcite post RO (this will usually add about 30-60 mg/L of Hardness) to protect metallic plumbing and fixtures, or in some cases a Polyphosphate filter which protects the pipe walls as well.