This thread is in response to this post.
Yes, you are correct, though the CDC link I gave said 20 ppm FC for at least 12.75 hours, but amounts to the same thing (and the chart they show has 10 ppm FC for 25.5 hours similar to your calculation). The problem is that these are CT values WITHOUT CYA. With CYA in the water, it's not practical to get to the very high FC levels needed for equivalent disinfection rates. At 30 ppm CYA, you'd have to raise the FC to 37.5 ppm FC to be equivalent to 10 ppm FC with no CYA (it just works out that way -- at a pH near 7.5 so you'd better lower the pH before adding that much chlorine).
The CDC link (repeated again here) talks about this CYA limitation as follows:
Crypto CT values are based on the inactivation of 99.9% of oocysts. Laboratory studies indicate that this level of Crypto inactivation cannot be reached in the presence of 50 ppm chlorine stabilizer,â€*** even after 24 hours at 40 ppm free chlorine, pH 6.5 at a temperature of about 77Â°F (25Â°C).
It is pretty obvious that the CDC is not aware of the chlorine/CYA relationship in detail because a straightforward calculation using my spreadsheet shows that 40 ppm FC with 50 ppm CYA at a pH of 6.5 and temperature of 77F is technically equivalent (in hypochlorous acid concentration which is the disinfecting form of chlorine) to 4.8 ppm FC with no CYA at a pH of 7.5. To be equivalent to 10.625 ppm FC for 24 hours (to get the 15,300 CT), you would need 51 ppm FC with 50 ppm CYA at a pH of 6.5 or 57 ppm FC with 50 ppm CYA at a pH of 7.5. This clearly becomes impractical at high CYA levels and even at lower 30 ppm CYA levels raising the FC to 38.4 ppm FC and holding it there for 24 hours isn't practical.
Generally speaking, I wouldn't worry about Crypto in a residential pool. As for what commercial/public pools can do, they are in a tough position. There currently is no method they can use that will clear the pool of Crypto in any reasonable timeframe. UV will kill Crypto, but it takes 4.6 turnovers (with perfect circulation) to have 99% of the water go through the UV system. Chlorine Dioxide is at least 10 times as effective as chlorine and does not get reduced in effectiveness by CYA and could potentially be created in the pool by addition of sodium chlorite to produce just 2 ppm over 12 hours for 99.9% inactivation of Crypto, but the EPA won't allow this without studies that would cost $4+ million showing that the chlorate and chlorite byproduct concentrations would be safe (this is part of what I learned talking to people at the NEHA conference this past week).