Running anion twice in the 7 stage

[tzabor10]

Just changed out the Cation resin after noticing a color change in my 7 stage. Does anyone else have two cations and a mixed in their 7 stage? Is this advisable? Thanks

 

[Doctorgori]

I thought the anion was the one that usually goes 1st, in most situations, no? ( just haven’t come across someone needing 2 cations)

Anyway, not a direct answer but my setup usually has either 1 anion then 1 mixed bed OR 1ea anion & cation…
FWIW I buy 2 anions for every mixed bed or cation packet

 

[tzabor10]

I thought the anion was the one that usually goes 1st, in most situations, no? ( just haven’t come across someone needing 2 cations)

Anyway, not a direct answer but my setup usually has either 1 anion then 1 mixed bed OR 1ea anion & cation…
FWIW I buy 2 anions for every mixed bed or cation packet

I am currently running cation, anion, and mixed. Since cation is being used most with my source water, I would replace the anion with cation and still keep the mixed.

 

[KStatefan]

I do not know what you would gain. If you run two cation the first one is going to deplete first.

 

[Randy Holmes-Farley]

I cannot think of an ordinary scenario where cation resins are used faster than anion resins, unless the resins have different ion binding capacity. It would require there to be substantial hydroxide, or a bound hydroxide in the water to be treated. Running kalkwasser through an RO/DI would do this, for example.

The reverse is often true, since CO2 is present as carbonic acid, which depletes anion resins (with carbonate) but not cation resins (since H+ does not deplete them).

 

[Sisterlimonpot]

I thought the anion was the one that usually goes 1st, in most situations, no? ( just haven’t come across someone needing 2 cations)

Anyway, not a direct answer but my setup usually has either 1 anion then 1 mixed bed OR 1ea anion & cation…
FWIW I buy 2 anions for every mixed bed or cation packet

I believe you have it backwards. When the ani and cati are separate, the cation goes first then the anion.

 

[Randy Holmes-Farley]

I believe you have it backwards. When the ani and cati are separate, the cation goes first then the anion.

If you mean order in the RO/DI machine, why would order matter?

 

[Hooz]

If you mean order in the RO/DI machine, why would order matter?

According to the BRS video on the subject, they said specifically that the order matters. They gave reasoning for it, but I don't remember what it was. It's been a while.

 

[Hooz]

According to the BRS video on the subject, they said specifically that the order matters. They gave reasoning for it, but I don't remember what it was. It's been a while.

 

[Sisterlimonpot]

If you mean order in the RO/DI machine, why would order matter?

I think your point is by passing water through an ro membrane eliminates the need to have cation 1st.

For me, it's because anion gets spent faster, and because it's usually the one that changes color, it's best to put it last.

 

[brclark82]

The title of the post says running anion twice but the actual post mentions running cation twice.

I can’t imagine a scenario that cation is used quicker than anion but it probably exists somewhere I suppose.

You can absolutely run 2 of one instead of one of each but the only thing it’s going to help is letting you run longer between switching out resins but at the same time you will deplete your mixed bed quicker than if you ran one of each.

I suppose if you’re really burning through one you could make sure the first one is 100% depleted by not switching it out until the second one started changing colors but still would probably be a wash with having to changed the mixed bed more often I’m assuming.

 

[Randy Holmes-Farley]

According to the BRS video on the subject, they said specifically that the order matters. They gave reasoning for it, but I don't remember what it was. It's been a while.

There is a minor reason (IMO) why some companies recommend anion first, but BRS does not have the correct reason.

BRS has stated this in a post at Reef2Reef:

"The reason we want the Cation first is because the Cation resin will exchange all of the positively charged contaminants with a hydrogen. This in turn creates a variety of acids with a very strong electrical charge. This allows the Anion resin in the second canister to better remove those contaminants. Essentially what you're left with after the water passes through both the Cation and Anion resin is Hydrogen (H) and Hydroxide (OH) which combine to create H2O. The third mixed bed canister is more of a final polish for anything that may have made it through the first two single bed cartridges."

IMO, that analysis is nonsensical, and is a reason to not take advice from BRS.

Let's explore what that order actually means for two of the anions that do seem to get through RO/DI to some extent and which causes annoyance, if not actual problems, for reefers.

1. First is phosphate. It can be present in tap water as H3PO4, H2PO4-, HPO4--, and PO4---.

As that passes through a cation resin bed (very acidic, as BRS correctly notes), all of it is converted to uncharged H3PO4.

Then in the high pH of the anion resin bed, it is all converted to PO4---, which is bound.

There is zero benefit to converting it to H3PO4 first. That process does not make it easier to bind in any way.

2. Second ion to consider is silica/silicate. It can come in many slightly different forms, but they all behave similarly and we will summarize then as Si(OH)4 and Si(OH)3O-.

As that passes through a cation resin bed (very acidic), all of it is converted to uncharged Si(OH)4.

Then in the high pH of the anion resin bed, it is all converted to Si(OH)3O-, which is bound.

There is zero benefit to converting it to Si(OH)4 first. That process does not make it easier to bind in any way.

3. So we are left wondering what possible ion is more readily bound to an anion exchange resin after converting it to its most acidic form. Since that protonation is stripped back off in the anion bed, I really cannot imagine any material that is best bound by an anion exchange resin after first protonating it.

Real reason. Now, it is often recommended to put the cation first, but IMO, this is not needed in most cases. The reason is that calcium and magnesium coming into an anion bed first will partly precipitate as calcium and magnesium hydroxide. That is not actually a removal problem since it still removes that calcium and magnesium, though it does shift the burden of capacity depletion to the anion cartridge from the cation cartridge, and is another reason, besides CO2, why an anion bed might deplete first if used first in order.

There may be some scenarios where that might actually clog the DI bed and reduce flow, which is what some water companies suggest although I do not know that such a thing would happen in a home system.

In any case, there is NO reason to think that removal is better putting cation first, but there are some reasons to put it first to preserve the anion bed capacity, and possibly to prevent clogging.

 

[KStatefan]

The response above is why I like the Chemistry Forum

 

[Hooz]

The response above is why I like the Chemistry Forum

Having @Randy Holmes-Farley so involved and readily available is like a reefer cheat code. And Lord knows we can use more of those!

 

[Doctorgori]

I believe you have it backwards. When the ani and cati are separate, the cation goes first then the anion.

I was thinking about what gets depleted faster but regardless that info is useful as I never even considered which order to put them in

According to the BRS video on the subject, they said specifically that the order matters. They gave reasoning for it, but I don't remember what it was. It's been a while.

thanks , I see the vid ….
…and thanks again @Randy Holmes-Farley

 

[tzabor10]

Thanks all. It was anion I ran out of. Thought running it twice would help save time But it seems much better to stay the course. Sorry about the typos