RO unit 3 stage DI resin question

[n240sxle91]

I have a 3 stage DI on my RODI system. It seems like most people recommend running Cation - Anion - Mixed. This is what I've done for several years. I, like a lot of people, use up anion pretty quick, and my cation hardly every gets used up.

Here's my question.... why run Mixed on the 3rd stage? Why not just run a second anion? Unless there is a real reason to run Mixed on the 3rd stage I feel like Cation - Anion - Anion would make more sense. Thoughts?

 

[KrisReef]

I think the idea is for you to replace the anion as it is depleted (first) and for the mixed to capture anything that gets by if/when the Cat or Anion deplete.

 

[n240sxle91]

I think the idea is for you to replace the anion as it is depleted (first) and for the mixed to capture anything that gets by if/when the Cat or Anion deplete.

Thanks for your reply! Yeah, I get that the third stage is there to capture anything that gets by, but it seems to me that only the anion in the 3rd mixed stage is getting depleted. If that's the case, wouldn't it make more sense to just do 1st stage Cation - 2nd stage Anion - 3rd stage Anion. That way you're not wasting space in the third stage with any cation.

It probably doesn't make a huge difference either way, but it just feels like it makes more sense to me. Just wondering if anyone runs anion in their third stage and skips the mixed, or if there is a real reason to run mixed in the third stage.

 

[Reef.]

The reason for the mixed at the end is because the resin can create positive or negative reactions, the single resin is more effective at removing either positive or negative, the mixed would be more effective at removing either.