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Shocking With High Cya


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I would characterize a CYA of 400 ppm as unadvisable and with typical FC levels it is slow to kill pathogens. The easy-to-kill bacteria typically have a CT (chlorine concentration in ppm times time in minutes) value of around 0.08 for a 2-log (99%) kill. This translates roughly into a 50% CT of 0.01 so with a 15 minute generation time (being conservative; it's generally 15-60 minutes depending on conditions) this implies a required chlorine level of 0.00067 ppm. 2 ppm FC with 400 ppm CYA would have an effective chlorine level of around 0.005 ppm with no CYA.

So easy-to-kill bacteria that weren't in biofilms would still be killed faster than they could reproduce, but harder-to-kill pathogens (see this link) wouldn't and person-to-person transmission would not be inhibited very much at all and, of course, algae could grow since that generally requires even more chlorine to prevent its growth. So "safe" is a relative term. It's certainly less safe than more normal levels of chlorine with an FC/CYA ratio closer to 5-10% (compared to 1%), but it's more safe than not having any chlorine at all. Even with no chlorine, it's not like people immediately get ill if they enter the water; it's just unwise and in some infrequent cases it can be quite dangerous and even deadly (see this link, for example, as well as this link demonstrating that it is the hypochlorous acid and not the chlorine combined with CYA that does the killing).

Basically, having a pool with a CYA of 400 ppm is essentially having a pool with about 1/6th the amount of disinfection and oxidation capability as the more normal CYA of 50 ppm, all else equal.

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