hardness reduction using cationic exchange

[pbh]

I have an installation of a ion exchange unit using potassium salt for regeneration. Sodium in the untreated water is 155 mg/L, potassium at 15 mg/L, hardness 488 mg/L. Treated water sodium is reduced to 46 mg/L, hardness to 4 mg/L and potassium is 495 mg/L. Ion selectivity would suggest that the potassium would not exchange with the sodium.

I have conducted sample analysis on two sets of samples, the conditions are the same. Any idea what is happening here?

 

[sbush]

After you regenerate your resin bed and return to service, the resin bed continues to regenerate while at the same time softening. There is a zone of ion exchange activity that moves through the bed from top to bottom until excessive hardness leakage occurs and the entire resin bed must be regenerated. In the upper part of this zone, hardness that had been removed is being regenerated off of the original exchange site by sodium ions in the incoming raw water. As this hardness is "bumped" off it migrates further down in the bed. These mini-regenerations happen almost contiuously, until the hardness reaches the bottom of the bed and excessive hardness breakthrough occurs.

If you look at the hardness leakage curves for any cation resin, you will see that it changes in proportion to the total dissolved solids (TDS)content of the raw water. As TDS in the raw water increases so does hardness leakage. The TDS levels are a direct reflection of the amount of sodium ions in the raw water. The higher the sodium levels, the higher the number of mini-regenerations during the service cycle which results in hardness leakage.

So, what is happening to your sodium is that some of it is being taken up on the resin in place of hardness. The reason that all of it hasn't been taken up has to do with regeneration efficiencies. Obviously, the efficency is not 100 percent. This is why excessive amounts of salt (well over the stoichiometric balance between hardness and salt compounds) are used to regenerate the resin bed.

S. Bush

http://www.water-eg.com