Thanks for the explanation of the faster kill at higher temps. The higher temps make all chemical reactions go faster (due to higher energy so more reactants can get "over the hump" of an activated complex to form products), but even with faster uptake of the "poison" if the bugs were killed twice as fast AND reproduced twice as fast, then there would have been no net change. So it must be that the increased temps increase the uptake faster than the reproducion rate increases. That makes sense since the reproduction rate isn't based on a single simple chemical reaction, but on a complex series of them PLUS physical processes of moving organelles around as part of cell division.

In the formula, the "y%" is the CT value that you are converting FROM, say CT(99%) which is 2-log or CT(99.99%) which is 4-log. As you point out, I was developing a formula for how to get TO CT(50%) (which is "0.3-log" if anyone called it that). The purpose of the formula was just to convert from the "we most definitely have killed this bug since 99% of it is gone" CT value to the "we are on the cusp of just keeping this bug at bay, killing it about as fast as it reproduces" so that we could get an absolute bare minimum "critical" value. The CT values were developed mostly for water treatment where what was important was ensuring that some large percentage of bugs were killed since they were generally only exposing the water to high disinfectant levels for a limited period of time. That is different than our concern, especially in a residential pool, where we have continuous and long-term exposure from disinfectant and our goal is also to kill the bugs, but over a relatively much longer period of time.

At least with E.coli, it is killed very, very easily so we have nothing to worry about. But that Crypto, hey, now that's some hearty protozoa! Fortunately, the really hard-to-kill bugs that are of concern are ones that come from people, which is why they are an issue in public pools. In a residential pool, it's very unlikely to see such bugs unless you've already got the bug and are introducing it (I won't say how) into your own pool! Then, again, a paranoid pool owner can test their pool party invitees for Crypto (not...)!

Richard