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Current IssueAugust 31, 2015
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Ask The Expert: Dialysis: Use of Softener with Mixed Bed DI
Question: I work for a hemodialysis clinic. We have a standard water treatment system, using reverse osmosis (RO) preceded by a softener, among other things. We have backup DI tanks (mixed-bed) that are put on line if the RO fails. I've heard that the softener should be bypassed if we use the DI tanks. Is this correct and could you please explain why?

Guy Tirondola -- New Jersey 

Answer: Yes, you have been told correctly. DI tanks have both cationic and anionic resins that have been regenerated with an acid such as hydrochloric, and a base, such as sodium hydroxide. The cationic resin attaches the hydrogen (H) atom, and the anionic resin attaches the hydroxide (OH) molecule. A water softener uses sodium chloride (NaCl) to regenerate the cationic resin and attaches a sodium ion (Na) to the resin, and one would think that it would be good to precede the DI with a softener. But the softener, when it's in service, releases sodium ions, and the sodium ions are closer in affinity to hydrogen than are calcium, magnesium or other ions. Thus, use of a softener in front of the DI, will introduce sodium ions and interfere with the hydrogen form resin’s ability to attach other cations.

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