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Thread: How long for CYA to degrade?

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    How long for CYA to degrade?

    Only about a month and a half ago, I had a CYA level of about 50, but now is significantly less than 30 (I can still clearly see the dot after filling the tube when testing using the Taylor K-2006). I backwashed a few times during that time and did not experience much splash-out or carry-out. I do not add CYA regularly as I use bleach for chlorination. I'm left to ponder what the chances are of the CYA degrading on its own in that amount of time. So how long does it usually take for CYA to disappear naturally?
    10k gal. IG vinyl, 1 hp pump, 200 lb sand filter, solar panels & cover

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    JasonLion's Avatar
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    Re: How long for CYA to degrade?

    During the swim season, CYA normally only goes away through water replacement, splash out, leaks, backwashing, overflows, etc. CYA can also sometimes go away over the winter.

    Some drop is to be expected, but falling from 50 to 30 is rather fast for the typical rate of water replacement. On the other hand the test is somewhat subjective and it might not really have fallen as much as it appears to have fallen.
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    Re: How long for CYA to degrade?

    While I would agree that the test is somewhat subjective, there is definitely a significant change in CYA level. Sure, if the difference was between, say, 50 and 40, the change could have been merely observer error. But to go from the dot disappearing around 50 to still being clearly visible with a full tube, there is more than just observer error.
    10k gal. IG vinyl, 1 hp pump, 200 lb sand filter, solar panels & cover

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    Re: How long for CYA to degrade?

    There is a slow decomposition of Cyanuric Acid by hypochlorite ion of around 0.24 ppm CYA per day, according to some experiments made by Wojtowicz. That's a loss of around 7 ppm per month. Since the degradation is dependent on the concentration of hypochlorite ion, it is very pH dependent. The rate just given is for a pH of 7.5. At a pH of 8.0, the hypochlorite ion concentration is 2.7 times higher.

    In any event, the drop you see in just a month and a half is higher than expected unless you were shocking the pool during that time since shocking has both higher chlorine levels and higher pH.

    I'm going to be watching the CYA level in my pool more carefully to see if the Wojtowicz numbers are reproducible in my pool. I don't ever recall seeing a drop as rapid as he has reported, though I have seen the level drop over an entire season -- more like 0.05 ppm CYA per day.

    Richard
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