cliff_s said:
CYA is only used to retard the dissipation of Chlorine by UV light from the sun. It is never used in a
indoor pool or a pool that is covered from the sun.
Cliff,
This is the conventional wisdom, but it does not take into account the fact that CYA doesn't JUST protect chlorine from breakdown from sunlight (UV rays), but also combines with chlorine to make the effective disinfecting chlorine (hypochlorous
acid) concentration FAR lower. See
this post where you can see the traditional industry graph of hypochlorous acid (HOCl) and hypochlorite ion (OCl
-) vs. pH, but realize that this is only a correct graph when there is NO CYA present. I then show the correct graph with 30 ppm CYA where you can see that 97% of the measured Free Chlorine (FC) is chlorine combined with CYA (called chlorinated cyanurates) that are not effective sanitizers nor oxidizers. 1.5% is hypochlorite ion and 1.5% is hypochlorous acid (at a pH near 7.5) and only this latter 1.5% is an effective sanitizer and strong oxidizer.
This means that indoor pools that typically run at 1-2 ppm FC with no CYA are way, way, way over-chlorinated. There is plenty of confirmation of that as my wife and friends we've talked to around the country who use indoor pools that don't have CYA in them report swimsuit degradation (mostly elasticity loss, but also some fading even with fade-resistant swimsuits) over just one winter season of use (as well as greater skin flake/dry and hair frizziness). In outdoor pools with CYA, there is minimal such degradation even after 4 years. Furthermore, the very rapid and serious corrosion issues we have seen on pool forums with stainless steel in SWG pools have occurred with indoor pools with no CYA and not with pools with CYA (also see this PDF file that reports how CYA protected stainless steel in a high salt test).
The recommended FC/CYA ratio for manually dosed pools is around 11.5% and this results in a disinfecting chlorine (hypochlorous acid) concentration of 0.05 ppm and is technically equivalent to a pool (at pH 7.5) with 0.1 ppm FC and no CYA. If an indoor pool could maintain a 0.1 ppm FC consistently, then that would be fine, but with typical bather loads this is not possible as the chlorine gets used up locally so one has to have a higher FC to make up for localized demand. This leads to over-chlorination since the FC is raised to try and have enough chlorine to not run out (it's also difficult to accurately manage such a low chlorine level).
A solution to this problem would be to use CYA in indoor pools (and in pools with an opaque cover or that aren't exposed to sunlight) as the CYA isn't just to protect chlorine from sunlight, but also acts as a disinfecting chlorine buffer, keeping most of the FC in reserve and releasing it as needed. The disinfecting chlorine concentration is kept low, via the CYA, and that slows down all the chlorine reactions of disinfection and oxidation, but are still plenty fast enough to kill pathogens and prevent algae growth (as proven in outdoor pools with CYA).
Another positive side effect is that the lower disinfecting chlorine level produces a lower level of disinfection byproducts, such as nitrogen trichloride, and it's roughly proportional to the disinfecting chlorine level. With the latest breakpoint chlorination model I just got today (thank you for the reference, Dr. Valentine), 1 ppm FC with no CYA plus 0.05 ppm Nitrogen amount of ammonia (equivalent to 0.25 ppm Combined Chlorine after combining with chlorine) results in a peak of 25 ppb of Nitrogen trichloride (the chemical that smells the most, is most volatile, and is indicated in asthma, respiratory illness, occular irritation and a possible carcinogen) after 43% breakpoint completion in 9 minutes. If one instead has 4 ppm FC with 20 ppm CYA, then after around 30% breakpoint completion in 23 minutes there is a peak of Nitrogen trichloride of 7.4 ppb or less than one-third.
At the other extreme, having too much CYA without a corresponding increase in FC not only leads to low disinfection with algae (or dull cloudy water) being the most visible initial result, but it also leads to a breakpoint reaction that is too slow. At 3.5 ppm FC with 30 ppm CYA, the breakpoint is half complete after around 80 minutes and is 90% complete after 4 hours. With 2 ppm FC and 100 ppm CYA, the breakpoint is half complete after more than 6 hours and almost 20 hours to get to 90% completion. So monochloramine sticks around too long and can smell and be irritating (though not as much as Nitrogen trichloride).
Richard