I can tell you from my own personal experience using Trichlor tabs over just one and a half seasons at a very low chlorine demand at less than 1 ppm FC per day that high CYA levels are a problem. I got to 150 ppm CYA and couldn't keep up with chlorine demand as algae (not yet visible, but water was a little dull) started to grow faster than chlorine could kill it and I was using PolyQuat 60 though only every other week. I have a cartridge filter so no backwashing. In pools with lots of dilution, that's one thing, but in situations such as mine getting to high CYA is just too easy when using stabilized chlorine products.
I can also attest to algae growing when the FC/CYA ratio gets somewhat below 5% since 2 ppm FC when I had 50 ppm CYA would start to get some algae growth (water turning dull and higher chlorine usage). Also, these levels of FC and CYA were originally determined by Ben Powell who started
The Pool Forum and
Pool Solutions based on his experience of pools he serviced. And as Jason has said and if you look on this and other pool forums, virtually every single case of algae growth is from an FC/CYA ratio that is too low, not just an FC that is too low by itself (though that is also common). There are many SWG pools with 1-2 ppm FC (and a few with 3 ppm FC) and 70-80 ppm CYA that get algae whereas after they shock and then set a target of 4 ppm FC, they do not.
Now as far as bacterial kill times, that's not something you would readily see in your pools and besides, bacteria are incredibly easy to kill for the most part. It takes a lot higher chlorine level to prevent algae growth than to kill bacteria or inactivate viruses. Nevertheless, 1 ppm FC with 100 ppm CYA results in slower 10-20 minute 99% kill times so might be an issue for person-to-person transmission though would not be an issue in terms of uncontrolled bacterial growth. For example, see Figure 4 for a kill time graph in
this document which states in its abstract:
Concentrations of 25, 50, and 100 mg of the chlorine stabilizer cyanuric acid per liter increased the time required for a 99% kill of Streptococcus faecalis by 0.5 mg of chlorine per liter at pH 7.4 and 20 C from less than 0.25 min without cyanuric acid to 4, 6, and 12 min, respectively.
In fact, the 99% kill time with no CYA is 7-10 seconds since the 6-log 99.9999% kill time as specified for 1 ppm FC of chlorine in the
EPA DIS/TSS-12 standard is less than 30 seconds. If you want me to link you to the many other studies measuring bacterial, virus, protozoan kill/inactivation times as a function of chlorine level with and without CYA, I can do that, but again that's not the reason we are focussed on CYA in this forum -- it's to prevent algae growth using chlorine alone. You can look at a large study I analyzed from Pinellas County
here, but that just shows that bacteria are so easy to kill that it's hard to see whether FC alone or the hypochlorous acid concentration is the primary factor. It's mostly lab studies under carefully controlled conditions that show that it is the hypochlorous acid concentration that matters and not FC alone in terms of sanitation (for algae growth, we have thousands of multiple forum users to attest to prevention via paying attention to FC/CYA). For example, see
this link that states the following with regard to inactivation of one species of protozoan cyst:
The results strongly suggest that HOCl is the predominant cysticide with no measurable cysticidal effect of the chlorinated cyanurate species.
The "chlorinated cyanurate species" is chlorine bound to CYA and most of the chlorine is bound to CYA (that comes directly from the chemical equilibrium constants) while "HOCl" is hypochlorous acid. If you believe that ORP measures anything significant, then you can see from the following two graphs that ORP is not correlated with FC alone but rather to hypochlorous acid concentration (and this is reasonably approximated by the ratio of FC to CYA though this isn't shown in the graph, but I linked you to the derivation of the approximation and you can validate that it's the case using my spreadsheet to calculate hypochlorous acid concentraton).
https://www.troublefreepool.com/~richardfalk/pool/FC-ORP.gif
https://www.troublefreepool.com/~richardfalk/pool/HOCl-ORP.gif
The above was taken from 620 samples from 194 pools and spas with the data collected by Jeff Luedeman in Bloomington and Richfield, Minnesota. I didn't have some newer temperature dependence on some of the equations that I found out later, but this won't change the results very much. There's a lot of scatter and outliers (a small amount of variation is from the different pH), but the basic trend is pretty obvious. In the lower graph, the diamonds to the upper right are unstabilized pools, the diamonds to the lower left are stabilized pools, and the squares with the red borders are stabilized pools with CYA < 30 so 15 ppm was used in the calculations. (The first graph says "DPD" for the Free Chlorine axis, but in fact a FAS-DPD test from the Taylor K-2006C kit was used).
[EDIT] In the above study, comparing ORP readings from a portable Oakton ORP measuring device vs. the built-in ORP controllers in many of the pools, 30 out of 130 (23% of those that had built-in ORP controllers) were off by more than 100 mV. [END-EDIT]
If you wanted a more concentrated source of chlorine to use to avoid carrying around heavier bleach or chlorinating liquid, then why not use Cal-Hypo instead? It raises CH, but in percentage terms the effect is far less since for 10 ppm FC it's 7 ppm CH out of perhaps 300 ppm (2%) compared to Dichlor which is 9 ppm CYA out of perhaps 70 ppm (13%). Or if price is no object, then you can use lithium hypochlorite which won't add to either CYA nor CH.
Richard