The association of chlorine with CYA is governed by reversible reactions, such that as unbound chlorine is used, more is released. As a result of these equilibria, CYA functions as a free chlorine buffer. However, binding of chlorine to CYA reduces the concentration of the most biocidal form of chlorine, hypochlorous acid (HOCl), and increases the time required to inactivate microbial pathogens.
Defined concentrations of chlorine and CYA will result in a well defined concentration of HOCl, that can be calculated with the O'Brien model (or measured with more advanced techniques that are no longer residential pool owner compatible). The involved equilibrium reactions are very fast (which is also the reason why chlorinated cyanurates show up as FC in tests). If you have 0.5ppm HOCl in the water then you have a certain kill time, the kill time is not affected by chlorinated CYA that is also in the water.
Lets take a simple example. Lets assume you have pool without CYA with FC 1ppm at pH 7.5, then you will have an HOCl of 0.5ppm. These 0.5ppm define the pathogen deactivation rate. Lets say you have an amount of pathogens that requires 0.5ppm of HOCl to kill. This means that FC got reduced to 0.5ppm, and while this was happening the equilibrium reactions ensured that we end up with HOCl 0.25 ppm (and OCl- 0.25ppm). From this point on, the deactivation rates will be determined by this value, and your kill times have basically doubled now.
Now we assume a situation where we have the same 0.5ppm of HOCl in the water with CYA of 50. This requires an FC of about 25.3ppm at pH 7.5 (this shows how strong 1ppm without CYA actually is). A nice side effect of CYA is that at pH 8, we'd still have 0.42ppm of HOCl, but in the above case without CYA, we'd only have 0.24ppm HOCl at FC 1 and pH 8. But that's not my point. Lets assume the same amount of pathogens eats up 0.5ppm of HOCl. This happens initially at exactly the same kill rate as above. After FC reduction of 0.5ppm to 24.8ppm, we now still have an HOCl concentration of 0.48ppm. Our HOCl has only been reduced by 4% rather than being halved as in above case. You will hardly notice a change in kill rates from this point forward. That's how the chlorine buffering effect of CYA works.
This has been calculated with the Pool Equations spreadsheet that I have posted earlier.
Exactly!
However, FC can be consumed faster than it is added where CYA remains relatively constant (or increasing if using a trichlor tab feeder).
The same as just explained applies here as well.
The point I wanted to make is that FC and CYI should not be considered independent parameters. Of course, if you keep FC constant and increase CYA, then the pathogen kill time creeps up.
TFP recognises the FC-CYA equilibrium chemistry, and applies it into a method designed for maintenance of residential backyard (outdoor) pools. The FC level needs to be adjusted to the CYA level. In a residential setting, we are not limited to regulated FC and CYA boundaries, and can run a pool with CYA 90 at FC 10 or 12 to ensure that there is sufficient HOCl available. And we discourage usage of chlorinated cyanurates for regular chlorination to avoid escalating CYA levels. The method's aim is to enable simple, affordable pool maintenance. Following the method, there should never be a reason to drain a pool, unless there are other factors like fill water that is high in calcium and/or metals that accumulate over time.
The method is not designed to be compatible with typical methods of companies servicing residential pools. Having trichlor tabs permanently floating in a pool with weekly shock doses, as typically applied to serviced pools without SWGs or chlorine dosing systems, is not compatible with the TFP method.
The aim of the Falk paper was to highlight the importance of the FC/CYA ratio for pool sanitation, and suggest that regulations (applicable for public pools) should recognise that. Rather than regulating FC and CYA independently, they should be regulated together. In many legislations you are currently allowed to run a pool for example at FC 1ppm and CYA 100ppm, which is not great in terms of pathogen kill times.
One solution is to ban CYA altogether (which by the way would not just limit the use of chlorinated Cyanurates, but completely ban them). And this can be a legitimate solution. But in this case FC should be limited to lower values. To swim in FC 4ppm without CYA is not a pleasant experience.
The other option is to allow CYA, but limit the CYA/FC ratio (or, equivalently, specify a minimum FC/CYA ratio). Falk's paper suggests, I'd say, a compromise that would not be too disruptive within the current framework of regulations. One of these framework regulations is that FC is usually limit to values like 4ppm. By suggesting a max CYA/FC of 20 (equivalent to a min FC/CYA of 5%), this effectively limits the max CYA level to 80ppm. This limits (but not eliminates as suggested by you) considerate use of products like Trichlor or Dichlor.
It's not even that TFP recommends running high CYA (and I think that's why I am being met with derogatory comments from admins and members - which is completely uncalled for, disrespectful and unprofessional).
The manufacturer's of SWCGs are recommending higher CYA.
Depends what you consider high. TFP recommends with SWGs CYA up to 80. Some in very hot, sunny climates go even up to 90ppm. With manually chlorinated pools, we recommend more like 40-50ppm.
It's not that I don't believe you or that CYA does what it purported to do.
You shared scholarly articles recently. When reading those, anyone will find a sense of professionalism that is beyond reproach. However, even those studies are scrutinized by peers.
We are all residential pool owners here. We have day time jobs and families to look after. There is not much peer reviewed literature available that shows experiments on CYA concentration effects on FC protection. We have seen in our own pools, confirmed by many members at TFP, that higher CYA provides better protection. But this is more anecdotal evidence, rather than peer reviewed "proof", I get that. Tests like the ones conducted by mas985 have the purpose to quantify this anecdotal evidence better. But it hasn't been through a peer review process, that is correct.
If you have the means to conduct more thorough testing on FC protection by CYA at higher levels and publish it in a peer reviewed journal, then we would be thankful for this.
Other aspects, like the importance of HOCl rather than FC for pathogen deactivation times or the FC/HOCl ratio as a proxy for HOCl, are well covered by peer reviewed literature. We give recommendations on required FC/CYA ratios that work, keep a pool sanitised, algae free and not overchlorinated (contrary to many public pools operating for example at FC 4 with CYA 0). This is the main aspect of TFPC. Which CYA level you chose for a pool, is in the end not that critical, as long as you chose the right FC, and are able to never drop below a critical threshold. TFP gives recommendations that work based on the experience of TFP members, but in the end it is up to the individual to pick what works for them.