aapetyo,
I think you should take a look at the "Chlorine/CYA Relationship" section in
this thread on my comments on the CPO manual for the CPO course (you can read the other sections as well, if you want). The chlorine/CYA relationship is known science since at least 1974 and there is no
real controversy or debate. The only "controversy" is from the chlorinated cyanurate (stabilized chlorine) manufacturers who come up with arguments like "CYA doesn't matter" or that it doesn't matter up to 200 ppm or that they understand the chemistry but don't believe the lab tests apply to real pools or that they disagree with the conclusions or that CYA only protects chlorine breakdown from sunlight so shouldn't be used in indoor pools, etc. Even their own studies (see the Pinellas link in that section) didn't really prove what they wanted in spite of their claims in that study.
The CDC, on the other hand, is looking mostly at killing pathogens including Crypto without regard for other aspects of human health such as the rate of production and quantity of disinfection by-products including nitrogen trichloride. I've sent the CDC links to the source science as well as detailed calculations showing how their own results on Crypto kill times at various CYA levels are predicted from the theoretical chemistry, but I don't think they fully understand that. I just came back from the NEHA conference (that I pay for myself since I am just a residential pool homeowner and do not work in the industry and am doing this completely on my own) and there is still incorrect information being discussed. No one has yet acknowledged that the 10x super-chlorination rule for breakpoint chlorination is wrong when applied to Combined Chlorine (CC) measurements -- the 10x comes from chlorine oxidation of ammonia where ammonia is in different units of measurement than chlorine so that there is a factor of 5 times the 3/2 ratio of chlorine to ammonia on a molar basis. CC, on the other hand, is in the same units as FC and CC already has 1 of the 1.5 chlorine (per ammonia) used for oxidation. This mistake was made decades ago and everyone is repeating it like a bunch of lemmings.
At the conference, there was a woman and her son presenting info about her son's swim team that had many respiratory symptoms (cough, etc.) using an indoor swimming pool with no CYA but that was "in spec". They looked at papers written in Europe ranking improved systems such as using UV to help reduce chloramines and referred to Ernest "Chip" Blatchley's work in this area as well showing UV lowering nitrogen trichloride. When UV was later used in the pool, symptoms dropped by around 2/3rds, yet no one is talking about possibly using a small amount of CYA in the water to significantly lower the active chlorine (hypochlorous acid) concentration to theoretically lower nitrogen trichloride concentrations by orders of magnitude -- that's factors of 10 or more.
As for Crypto, I talked to one presenter who won an award for having the best maintained pool and he uses a multi-faceted approach, mostly having better air circulation, use of non-chlorine shock to supplement oxidation, and significant water replacement. For Crypto, he uses both coagulation with filtration and super-chlorination if needed due to an incident, but when I asked what about person-to-person transmission he said that it really comes down to prevention by not having people with diarrhea enter the pool in the first place. So in practice Cyrpto is a problem whether you have CYA in the water or not. As for being able to super-chlorinate, you can get to your 10 ppm FC for 25 hours by simply having the FC about 10 ppm higher than the CYA level, so 40 ppm FC with 30 ppm CYA or 30 ppm FC with 20 ppm CYA would work (assuming the pH is adjusted to 7.5). There are other methods such as using just 1 ppm of chlorine dioxide overnight, but no one talks about that as a possibility and no manufacturer has ponied up the $1-2 million to get EPA approval for chlorine dioxide in swimming pools, even for this purpose. UV can be used against Crypto, but only through multiple turnovers so it isn't fast (but is still faster than chlorine at normal levels, with or without CYA).
Crypto is not normally an issue in residential pools since they are mostly used by single families who aren't generally going to infect themselves and who tend to be careful about going into a pool when sick. It's an issue for commercial/public pools because one sick person can infect hundreds of people, but again, not having CYA in the water isn't going to significantly affect person-to-person transmission in the short-run (even 4 ppm FC with no CYA takes over 2-1/2 days for a 99% kill) and only makes it easier to super-chlorinate the pool AFTER an outbreak has already been detected (or a diarrhea incident). Yes, chlorine without CYA will kill off Crypto in days compared to over a month, but at the price of higher disinfection by-products, faster oxidation of swimsuits, skin and hair.
In the CPO thread I linked to, take a look at the link on ORP which uses real pool measurements from a public health inspector on hundreds of pools and shows very clearly how ORP is not correlated to FC, but rather to active chlorine (hypochlorous acid, HOCl). This, of course, is not a surprise, but based on additional bacterial sample data he had plus other data I've seen we came to the conclusion that a reasonable "sweet spot" for commercial/public pools would be something like 4 ppm FC with 20 ppm CYA which is roughly equivalent to 0.2 ppm FC with no CYA in terms of sanitation and oxidation rates, but has a chlorine capacity or reserve of 4 ppm. Note that the German DIN 19643 standard is 0.3 to 0.6 ppm FC with no CYA and 0.2 to 0.5 ppm FC if ozone is also used (in the circulation system, since it does not form a residual). They do this to keep disinfection by-products to a minimum, but their system typically uses activated carbon that strips not only all chloramines, but also chlorine itself that must later be re-injected.
Instead of looking at pools with a small amount of CYA being too low in disinfection/oxidation, one should look at pools with no CYA (and 1 ppm FC or higher chlorine levels) as being over-chlorinated and producing too many disinfection by-products (especially nitrogen trichloride) as a result. As Jason points out, different camps are taking completely opposite extremes with no one seemingly taking a rational balanced viewpoint. Instead, it's all or nothing, mostly because there is not a clear understanding of the fundamental chemistry that could be used to guide a rationally informed decision. I have asked multiple researchers to add CYA as a parameter of investigation into their disinfection by-product investigations, but none has done so. If $50,000 fell from a tree, I'd send it to NSPF to add CYA as a parameter to Dr. Blatchley's work (NSPF has been funding much of that work).
Richard
We all appreciate everything you do here!