I read the paper you referenced by Rubin ...
It seems to me that pH was just as important as concentrations during this study.
:
The author even states "At pH values below 7 free chlorine exists primaily in the form of hypochlorus acid (HOCL) which is 80 times more effective as a bactericide than hypochlorite (OCL-)." Seems to me that keeping pH low would be just as important as concentrations. Maybe even more important.
I usually refer to the paper by the lead author, O'Brien, and not by the editor of the book, Rubin, that has a collection of papers (presentations at a conference) from various authors.
Though there is a dependency on pH, one can easily overcome that and tune the active chlorine level via the FC/CYA ratio. Also note that the hypochlorous acid (HOCl) concentration varies a LOT less from pH when CYA is present. The statement you quote is referring to free chlorine in their definition which is unbound chlorine, so the sum of hypochlorous acid (HOCl) and hypochlorite ion (OCl
-) where their Figure 14.7 shows free chlorine rising somewhat slowly going to pH 7 and somewhat faster going to pH 8, but at pH 7 it's HOCl that is rising while at pH 8 it is OCl
- that is rising (also that Figure looks at a much broader range of pH than just 7-8). Look instead at Figure 14.5 and look at the dotted line labeled "HOCl". Notice how relatively flat it is from pH 7 to 8. This is the same as I show in
this post where you can see that while the HOCl drops by 50% when going from pH 7.5 to 8.0 when no CYA is present, it only drops by around 15% when CYA is present. Similarly, going from pH 7.5 to 7.0 with no CYA increases HOCl by around 50% but with CYA it only increases by around 35%. This is because CYA is a hypochlorous acid buffer.
As Joel noted, there are other reasons you wouldn't want the pH at 7.0 so instead it's much easier just to have a somewhat higher FC/CYA ratio target at pH 7.5 and we pretty much have this set to prevent algae growth since providing adequate disinfection is much easier. So the entire discussion about "germicidal responsibility" in the paper isn't relevant to us since we are already beyond fast kill times (though not a high as the
EPA DIS/TSS-12 which is overkill) and are instead set at a level to prevent algae growth. A list of kill times when the FC/CYA ratio is roughly 10% is shown in
this post. The roughly 5% FC/CYA ratio in SWG pools would have kill times roughly double listed in that post (that shows the times for 3-log reductions or 99.9% kill).