From the Chlorine-CYA chart one can see that 0.11 FC with 0 CYA has the same effect like 1.2 Chlorine with 10ppm CYA and like 2.4 ppm Chlorine with 20 ppm CYA, etc.
Agreed
It can be concluded that if one has 20ppm CYA only 0.11ppm is actually active chlorine and the rest is bound to CYA making just a reserve.
Yes, most of the chlorine is bound to the CYA. BUT, as soon as the free available active chlorine (hypochlorous acid) reacts with and kills a pathogen or oxidizes an organic, the chlorine bound to the CYA is released. The reaction rates for chlorine-CYA equilibrium are very fast, less then 1/4 of second. So even though the chlorine is "bound" to the CYA, it is still very much available for disinfection and oxidation.
It is also stated that at low CYA levels, it is recommended to have 2ppm chlorine just to have a reserve.
This is necessary because, as I stated previously, it is next to impossible to maintain low levels of FC evenly throughout the water volume of a residential pool. There are physical flow rate and mixing limitations that make it impossible to completely homogenize a pool water volume. Therefore, if the residual sanitizer levels are low, "dead" spots with little to no chlorine can form and allow for pathogen growth.
The german norm is 0.3ppm chlorine (0.2 if ozone is used).
This is because the German standard does not allow for the use of CYA in commercial/public swimming pools. As well, filtration and delivery systems are optimized for precise, rapid mixing and homogenization of the pool water volume with multiple stages of carbon filtration, coagulants, secondary sanitizer sources (UV/Ozone system), etc being deployed and used. Finally, chlorine is mixed into the water flow just prior to return of the water to the pool.
This level of filtration and sanitation would be impossible and/or prohibitively costly to replicate in a residential pool.
So, for a residential pool 2ppm bromine is an overkill since even in case some bromine is used, the resulting bromamines are still efficient sanitizers while the chloramines are not. It seems to me that 0.5ppm bromine should the targeted level.
In residential pools, bromine, with no buffering, will exists mostly as hyprobromous acid at a pH of 7.5 (94% HOBr / 6% OBr
-). One would have to add enough sodium bromide to create a "bromide bank" (~30-50ppm bromide) and then tailor oxidation additions (chlorine, MPS or ozone) to create the necessary amount of bromine. Also, one would likely use some DBDMH (1,3-Dibromo-5,5-dimethylhydantoin) as a source of bromine and dimethylhydantoin (DMH) as the DMH plays a similar buffering role for Br as CYA does for Cl. However, while CYA is very good at buffering Cl and modulating it's strength, DMH is a less effective buffer for Br. Another problem is, DMH has no UV shielding effect like CYA does for chlorine. Therefore, bromine in an outdoor swimming pool will not be as effectively stabilized as chlorine is.
I don't have good data on this, but I am willing to bet the bromine loss rate in an outdoor pool is harder to control than the chlorine loss rate. It's hard to make a complete comparison because one almost always uses CYA with chlorine in an outdoor pool and it is not clear how much DMH is needed to achieve a similar result in an outdoor bromine pool or even if a similar effect can be achieved given that DMH has no UV protective quality.
So with that said, one probably needs to maintain the higher residual Br level of 2ppm (with DMH buffering) simply to ensure that there is sufficient sanitizer (HOBr) in the water.
This article by Wojtowicz has a review of standard pool sanitation methods including bromine.
the resulting bromamines are still efficient sanitizers while the chloramines are not.
While this true that brominated THM's have, for some pathogens, faster kill rates (CT inactivation, compare Table 11.21 on pg. 521 with Table 11.24 on pg. 523 in
this link) than their chlorinated counterparts, brominated THMs are much more toxic as well as carcinogenic and mutagenic than chlorinated THM's. The brominated THM's also tend to remain in pool water longer as chlorinated THM's are more volatile.