It is again NOT true that chlorine is less stable at higher temperatures or that its being unstable is what makes it more effective (it is effective because it is very reactive with certain chemicals; but that is different than stability when such chemicals are not present). Chlorine does NOT break down in the very dilute concentrations in spa water even at hot 104ºF temperatures and without CYA. The only thing that happens is that without CYA the chlorine outgasses faster. It also chemically reacts faster if there is something in the water to react with (some of it oxidizes CYA itself, but at a rate of <= 0.4 ppm FC per day at usual FC/CYA levels). Having CYA in the water properly moderates chlorine's strength so that it no longer outgasses so quickly and reacts more slowly, though still fast enough to kill pathogens quickly. The slower reactions mean it oxidizes skin, swimsuits and hair more slowly. You incorrectly claimed that chlorine was a more effective sanitizer under 75ºF and I listed the correct information that shows that it is in fact the opposite where chlorine kills more quickly at higher temperatures, yet you persist in making these false claims about chlorine's stability.
I suggest you read
Certified Pool Operator (CPO) training -- What is not taught for a small sampling of the incorrect or incomplete information in one of the best courses out there. Hardly any of the standard pools/spa organizations -- NSPF, APSP, IPSSA, EPA, CDC, state regulations -- understand the chlorine/CYA relationship since they all refer to FC and CYA targets (or restrictions) separately. This is ludicrous given the known science since at least 1974.
There are quite a few Dichlor-then-bleach residential spa owners who maintain their spas using chlorine alone (the initial Dichlor is to build up some CYA, usually around 30-40 ppm) and are able to go 2-3 or more times longer between water changes as a result and have minimal smell, oxidation of swimsuits skin or hair, and minimal disinfection by-products (especially nitrogen trichloride). Their spas are perfectly disinfected. If one maintains the appropriate FC/CYA level, then bacteria are killed quickly in spas just as they are in pools. In a residential spa without an ozonator, the FC drops around 25% over 24 hours so one can easily dose every day or two to maintain the spa. It is true that this does not work for those who only use the spa once a week (and don't dose in between times) or for a service person who only visits such a spa once a week, but one should not make generalized statements that bromine is always better for spas since it is not. Also, for someone using a saltwater chlorine generator, they get continuous automated dosing and only have to dose extra right after getting out of the spa to handle their bather load.
Chlorine kills bacteria and inactivates viruses quickly as shown in the table in
this post that shows chlorine kill times when the FC is around 10% of the CYA level in colder water (77ºF). The kill times are even faster at hotter temperatures including those in a spa, not only due to the higher temperature, but that the active chlorine level is higher at higher temperatures at the same FC/CYA ratio. Fecal bacteria are killed with 3-log (99.9%) reductions in less than 1 minute and
Pseudomonas aeruginosa in 1.5 minutes (again, it's even faster at spa temps -- well under 1 minute).
Bromine does not create catalysts to regenerate bromine from bromamine. Again, where are you getting this incorrect information? The way it works is that bromamine (NH
2Br) is still an effective disinfectant because (as described in
this paper) some if it forms bromammonium ion (NH
3Br
+) that breaks up into (or reacts as) ammonia (NH
3) and a highly reactive positively charged bromine ion (Br
+) that is even more reactive than hypobromous acid (HOBr). In fact, this is likely to be the reason why bromine spas can be more irritating to some people -- there are far more reports of skin irritation in bromine spas than in chlorine spas properly managed at appropriate FC/CYA ratios. The bromamines may still be good at disinfecting, but they also cause problems for some.
Bromine is convenient for those who do not use the spa every day since one can use bromine tabs for dosing in between soaks. That is its primary benefit. However, the disinfection by-products from bromine are worse than those from chlorine (brominated THMs cause cancer while chloroform does not at concentrations found in spas). Bromine also costs more (especially when compared to bleach). It also usually requires shocking with chlorine on occasion because bromine doesn't oxidize all the same chemicals as chlorine. It's still a small risk, but unless one really needs the convenience of bromine and can't dose every day or two, then chlorine can be a better choice especially for those who soak every day or two. Both chlorine and bromine have their place in spas.
