I found the attached article. A city in Michigan is trying to get its residents and business to switch from using Sodium Chloride salt in their softener to using Potassium Chloride salt. Although this is acceptable for most residents and businesses, it is not acceptable if you are doing dialysis. When a softener exchanges its Sodium ion for a Calcium or Magnesium ion, you get more Sodium on the effluent of the softener. Feeding this elevated Sodium to the RO is acceptable since the AAMI limit for Sodium is 70 ppm. If you used Potassium Chloride salt, you would get an elevation of Potassium on the effluent of the Softener. The RO would not be able to reduce this elevated Potassium to below the AAMI limit of 8 ppm. This would increase your dialysate Potassium level.

Dialysis clinics and people doing home hemo must use Sodium Chloride salt. Potassium Chloride salt is readily available in the stores. Patients doing home hemo are probably more proned to picking up the wrong salt at the store than a clinic who gets its salt delivered. Once, I receive a phone call from a dialysis patient who accidentally picked up Potassium Chloride salt at the store. The patient wanted to know if it was acceptable to use.

Please don't use Potassium Chloride salt. If you see anything like this in your area, please let me know.

MI_20061207_Middleville_-_Village_wants_water_softener_switch_to_Potassium_Chloride_salt.txt (2 Kb, 4 downloads) Potassium Chloride salt

Thanks for that info, Larry! I wonder how many hospitals with Acute units are softening or treating their water, and with which salt or chemical. Do you have any idea how much added potassium this might cause? Would it be enough to cause cardiac disturbances from the higher potassium level in the dialysate?

Good question Ktech. The AAMI limit for K is 8 ppm which is 0.2 meq/L. Therefore, 40 ppm of K on the effluent of the RO would raise a 2K acid to 3K. How much K is added to the water by the softener is dependant on how many cations (Ca/Mg) there are in the water to push (exchange) the K off the resin.

For a Softener with resin in the normal Sodium form, the sodium on the effluent of the softener can be calculated. Sodium on the effluent = Sodium on the inlet + (Inlet K x 0.59) + (Inlet Mg x 1.89) + (Inlet Ca x 1.15). If the inlet Na is 10 ppm, K is 5 ppm, Mg is 15, and Ca is 25, the effluent Na is 70 ppm.

For a Softener with resin in the K form, the K on the effluent of the softener can also be calculated. K on the effluent = K on the Inlet + (Inlet Mg x 3.22) + (Inlet Ca x 1.95). For the same inlet water used above, the K on the effluent of the Softener would be 102 ppm. The amount of K on the effluent of the RO depends on the RO's % rejection. Basically, the harder your water is; the more K you will get.

What most people don't realize is that when a softener in the normal sodium form exhausts, Potassium is released from the Softener before hardness. Therefore, currently, if all of the sodium is exchanged off the softener, potassium will start to be released. This K release will not show up on your hardness test. Only after all of the Potassium is exchanged off the softener will you start seeing hardness. The potassium accumulates on the resin from the raw water.

Does the potassium that is bound to the resin only displace when the resin loses all of its sodium ions?...or is/can the bound potassium be purged during a regeneration cycle?

The potassium on the resin will be removed during regeneration. During the service mode, the potassium will only be exchanged off after all of the sodium ions are exchanged off.

Larry, why is the potassium exchannged only after the sodium depleted? I'm guessing because of the weight of each atom. Is any other element exchanged beside the potassium after the sodium is depleted? Just asking.One of our clinics is having softener issues and I want to know what we are up against.

I may be totally off base here but here is my 2 cents....

Potassium and Sodium each have a single + charge, if your resin is charged with sodium, potassium shouldn't appreciably displace it so potassium being released after the sodium is depleted shouldn't be a concern.

Remember, Calcium and Magnessium both have a ++ charge, which is why it is able to displace the sodium from the resin.

Chuck

Chuck,Don't take this wrong, I do know how a softener works. I am seeking knowledge based on facts. I'm really not concerned about the K being there-just want to know why the K waits until after the Na.

Chuck is partially right. If softeners used a Weak Acid Cation (WAC) resin, the accumulation of potassium would be very small if any at all. But, almost every softener uses a Strong Acid Cation (SAC) resin. This is the same resin that is used in a DI system (mixed bed or cation DI). A DI system will remove Sodium and Potassium. Potassium will accumulate on the softener resin. SAC resins will remove all cations with an affinity greater than what it is charged with. The order of affinity from smallest to highest is Sodium, Potassium, Magnesium, Calcium. I could also list other cations such as iron, copper, barrium, chromium, etc. Therefore, the Calcium will displace any Magnesium; the magnesium will displace any Potassium, and the Potassium will displace the Sodium. If you took a core sample of the resin in the softener, you would actually see the different layers (calcium at the top, then a layer of magnesium, then potassium, then sodium). The monovalent ions (sodium and potassium) are the weakest held. Divalent ions (Ca, Mg) have a stonger affinity. And, trivalent ions (Fe3+) have an even stronger affinity than divalent.

For a DI system with the cation resin in the H+ form, sodium has a higher affinity than the H+ ion, therefore it pushes off the H+. This affinity order is why Fluoride is very dangerous. Fluoride is one of the weakest held ions on the anion resin. After the anion resin has exchanged off all of its OH- ions, it starts to release F- ions. The F- ions have a low affinity therefore they are at the bottom of the DI tank. The Cl- ions and SO4-- ions push the F- ions off first. There is a large spike in the F- concentration in the water causing dialysis patients to become ill.

I just ordered a Potassium Kit and can demonstrate this if you would like. The ion exchange process is something dialysis technicians need to understand. I will put this information in a training class for you.

Larry,

Thank you for the explaination.

I understand the difference between mono, di and trivalent ions but, what determines an ions affinity?

Chuck

A modern TFC RO system should be giving 98+% rejection. When the feedwater TDS is very low the apparent rejection as measured by conductivity reduction is less because of the CO2. At 98% rejection the post softener RO feed water would have to contain 400 ppm of K for the product water to exceed the 8 ppm K AAMI limit.

It makes me wonder how low a rejection some dialysis providers are accepting as normal.

The affinity is determined by the resin. Resins can be made to be more selective to certain impurities. The resin manufacturer should be able to provide the affinity order. In general, H+ is held the weakest, followed by Sodium, Potassium, Magnesium, and Calcium. For anion resin, OH- is held the weakest followed by Silica, Fluoride, Chloride, and Sulfate.

You can mathematically model the performance of an ion exchange system using a formula called the Harries Equation. This equation deals with the mass transfer coefficient, the size of the resin bead, the depth of the bed, the cross sectional area of the bed, how much resin is already exhausted, and the flowrate. All of these things can affect the performance of the ion exchange bed.

If you are curious, email me a water analysis and the specs on your softener (cu ft of resin, tank diameter and height, how much salt you use per regeneration). I will put the data into the Harries Equation and send you back a graph of when your softener will start releasing impurities other than sodium.