Can Calcium be removed from water using sequestrants?

[gtemkin]

I thought I'd split this inquiry off from the post linked below so as not to sidetrack that threads focus.

http://www.troublefreepool.com/want-to-add-ca-without-clouding-pool-t63130.html

I've been a member for a while and seen dozens and dozens of posts about high calcium levels and scaling and the responses have always been that the only ways to reduce calcium hardness were to drain and re-fill with less hard water or call in an expensive, not commonly available service to perform RO filtering.

In the post above, it mentions a method of eliminating calcium using sequestrants and filtering out the calcium. Is this real? Why isn't this method talked about more often? Is there a downside?

 

[chem geek]

Well, the claim is that the calcium got backwashed out, but it could just be that it was bound to the HEDP. High doses of metal sequestrant do bind to calcium. I don't know if this will then show up in a Calcium Hardness (CH) test. It might not if the release of calcium is slower than the time of the test.

This long thread talks about how both calcium and phosphate were removed along with iron -- basically precipitating calcium phosphate and iron phosphate. The phosphate level was high because so much HEDP was used over time.

So perhaps at the high HEDP doses in the thread you refer to enough of the HEDP broke down to release phosphate that precipitated calcium phosphate and that got physically removed in the filter.

Of course, one could just intentionally add phosphate directly if the goal is to precipitate calcium phosphate and one can create more of it at higher pH since there is more orthophosphate at higher pH. After removing the precipitate from the filter, one is still left with rather high phosphate levels, though not extraordinary and certainly can be dealt with by properly maintaining FC levels.

 

[gtemkin]

Thanks for the quick reply and insight as to the mechanism involved. I found a similar explanation you wrote back in 2010 which I'm pasting here for reference.

Very high phosphate levels along with high CH levels can precipitate calcium phosphate and some calcium reducers are based on this principle. Also, phosphate buffers added to water that has CH will cloud and reduce CH levels as well and this effect is exacerbated at higher pH. However, the phosphate levels for this effect to occur are usually higher than the phosphate levels in most pools unless a phosphate buffer was intentionally added (this is more common in spas which sometimes use phosphate buffers) or unless the pH gets very high.

After all, if normal CH levels were to precipitate phosphate, then the phosphate levels wouldn't get very high in pools to begin with and the industry of phosphate removers wouldn't have anything to market. I know that for my own pool when it had 2000-3000 ppb phosphates, that it wouldn't cloud up when the pH got to 7.7 and I don't think it would cloud up even at a pH of 8.0 either. So the level of phosphates and CH where cloudiness would occur is probably quite high. Someone else on this forum (JohnT or JasonLion, I believe) has even higher phosphate levels and don't get cloudiness though I don't remember their CH or pH.