Pretreatment - Hardness


Hardness can cause scaling in the reverse osmosis and EDI units, If this occurs, it will take place in the concentrate chambers at the high-pH surfaces of the anionic membrane. This formation of scale which in turn increases the electrical resistance of the module, as well as the pressure drop in the concentrate, and current efficiency is reduced. Minimization of inlet hardness will lengthen the time between cleanings.

All EDI devices tolerate a maximum feed hardness of 1 ppm as CaCO3 and some manufacturers recommend as little as 0.1 ppm as CaCO3.  In some instances, feed hardness limits the EDI system recovery.  If the feedwater source has little to no hardness, a softener may or may not be required.  If a softener is required, there are two main methods of removing the hardness.

RULE#1 - Reduce the hardness feeding the EDI module to less that 1 ppm (the lower the  better)

Reduction Methods

1) Pre-RO Softener - This design protect the RO from scaling as well as the EDI system.  The downside is that more salt is used than post-RO softening.


2) Post RO Softener - This design has advantage of being able to use a smaller softener and less salt vs. Pre-RO softening.


3) Antiscalent Injection - This method involved the dosing of chemicals but is not recommended due to higher operator attention and more room for error.


  • All EDI devices sensitive to hardness Most tolerate maximum 1 ppm as CaCO3
  • Some recommend 0.1 ppm as CaCO3
  • Hardness may limit recovery
  • LSI not good predictor of scale potential
  • Hardness scale increases module electrical resistance, reduces product water quality, increased reject pressure drop and cleaning frequency

Suggested Test Kit - Hardness – Hach Model HA-71A (#145201)