Downsides to salt pools

[chem geek]

Sean,

If the SWG manufacturer recommendation for CYA is solely based on retaining chlorine in the water (which is independent of whether you use an SWG or use another source of chlorine), then why did Evan (waterbear) and others see such a difference in efficiency going from 60 to 80 ppm? Yes, more CYA has the chlorine last longer, but it has diminishing returns and the difference between 60 and 80 isn't that high yet he (and others) saw a large difference -- that is, he was able to turn down his SWG a large amount.

The half-life of chlorine is determined by the separate half-lives of unbound chlorine (hypochlorous acid and hypochlorite ion) with a half-life of around 35 minutes AND the half-life of bound chlorine to CYA (i.e. the chlorinated isocyanurates) with a half-life of around 8.4 hours (some sources say 6 hours). The net result is shown in this chart which shows that the bulk of the benefit from CYA occurs at relatively low levels of CYA. This chart is similar to and consistent with the infamous Kent Williams "Cyanurics - Benefactor or Bomb?" article. The half-life of chlorine is about 6 hours at 30 ppm CYA while at 80 ppm this only increases to a little over 7 hours. That's not a huge difference. The reason for the small change is that the vast majority of chlorine, even at 30 ppm CYA, is stored in the form of chlorinated isocyanurates (that is, bound to CYA) so adding more CYA only cuts down the remaining free, unbound and disinfecting chlorine that is already at such a small level that it doesn't contribute much to the total loss. At 30 ppm CYA the disinfecting chlorine level is at 1/30th the level it would be if there was no CYA or put another way, almost 97% of the chlorine is bound to CYA and only 3% is unbound and getting cut in half every half-hour. At 80 ppm CYA, almost 99% is bound to CYA and only 1% is unbound. Though 1% is certainly much smaller than 3%, it is still a small number in an absolute sense -- it's what happens to the 97% or 99% that drives the total chlorine loss more than anything else.

It was interesting that the rate constants for CYA combining with hypochlorous acid turned out to be about the same amount of time that water flows through an SWG cell when the CYA level was somewhere in the neighborhood of 50-60 ppm or so. That's what had me think that this was why the CYA recommendation was higher along with what Evan (waterbear) and others were seeing in terms of efficiency. Also, it seemed quite strange that most non-SWG CYA recommendations are 30-50 ppm while only some of the SWG manufacturers were saying to have it at 70-80 ppm with "80 ppm being ideal". Don't you find that strange?

Richard