I stumbled onto a web page entitled Cyanuric Acid - Debunking the "Chlorine Lock" Myth that was information supplied by Bio-Lab. This is somewhat old as the page was last updated in 2003, but I wanted to comment on this since it is a perfect example of how scientific literature and experiments can be taken out-of-context to make a point. The following quote is from the "Chlorine Lock" - An Historical Perspective section.
There is a clear reduction in kill times seen when CYA is present and it roughly tracks the FC/CYA ratio though chlorine bound to CYA does have around 1/100th the oxidizing power of hypochlorous acid so it's not a perfect tracking. Using 0.5 ppm FC with 25 ppm CYA at 4 minutes as a base, the FC/CYA ratio would predict around 10 seconds for no CYA and around 16 minutes for 100 ppm CYA. The papers measured < 15 seconds (so consistent) and 12 minutes (so lower than 16 minutes). If we account for the 1/100th effect, then the active chlorine level goes roughly from 0.25 to 0.015 to 0.0075 so using 4 minutes as a base this would predict 14 seconds and 8 minutes, respectively. The oxidation effect is not fully applicable, however, because the kill rate requires the molecule to get into the cell or seriously disrupt the cell wall and the primary chlorine bound to CYA is negatively charged so would have similar issues as hypochlorite ion. So the numbers reported are certainly consistent with the known science of chlorine and CYA."Chlorine Lock" is the perceived ability of cyanuric acid to reduce the efficacy of chlorine against bacteria and other microorganisms; in essence, the chlorine is "locked up" in such a way that it can't perform its sanitising function. The origin of the "Chlorine Lock" theory can be traced back to scientific papers by Anderson in 1965,(5) and Fitzgerald and Der Vartanian in 1967.(6) In both, it was reported that cyanuric acid reduced the bactericidal activity of chlorine. For example, Fitzgerald and Der Vartanian6 found that the 99% kill time for Streptococcus faecalis bacteria by 0.5 ppm of chlorine at pH 7.4 and 20°C was less than 15 seconds in the absence of CYA. With 25 ppm of cyanuric acid present, 0.5 ppm of chlorine took 4 minutes to achieve the same kill; with 100 ppm present, the time had increased to 12 minutes.
However, there are two critical issues here. Firstly, the results in both investigations were obtained in a controlled environment using distilled water and pre-washed, artificially-cultured bacteria. Andersen (5) even concluded his paper with the quote, "... these results were obtained under laboratory conditions and caution should be used if extended to actual swimming pool operation", a statement conveniently overlooked by many advocates of "Chlorine Lock". Secondly, the negative effect of cyanuric acid on chlorine efficacy was only observed in the absence of ammonia and other nitrogenous compounds, both of which are known to have a substantial negative effect on the efficacy of chlorine. Indeed, Fitzgerald and Der Vartanian6 found that the 99% kill time for S.faecalis by 0.5 ppm of chlorine increased dramatically from less than 15 seconds in the absence of ammonia, to around 20 minutes in the presence of 0.05 ppm ammonia. More importantly, when 100 ppm of cyanuric acid was added to a solution containing 0.075 ppm of ammonia, the 99% kill time of the chlorine was less than that for the same system in the absence of cyanuric acid. Similar findings were reported by Swatek et al,(7) who compared laboratory tests on distilled water with field tests on swimming pool water.
Field studies have revealed that levels of ammonia nitrogen in public pools are often far higher than the levels examined by Fitzgerald and Der Vartanian; one study in Southern Ontario reported mean concentrations of 0.25 ppm in swimming pools and 0.48 ppm in wading pools.(8) As such, chlorine in pools treated with cyanuric acid should outperform counterparts without stabiliser. This hypothesis was confirmed in field studies carried out by Swatek et al (7) and Vattimo.(9) In summarising the scientific literature supporting or refuting "Chlorine Lock", Mitchell (2) found that there was very little correlation between laboratory studies and actual swimming pool field trials. Reaching the same conclusion as Kowalski and Hilton, (10) he added that cyanuric acid levels in excess of 200 ppm had no impact whatsoever on a pool's sanitation provided that the chlorine residual was maintained between 2 and 3 ppm.
The discussion then turns to the presence of ammonia as can occur in real pools. However, they don't explicitly disclose how the quantities of chlorine and ammonia chosen would essentially wipe out chlorine's effectiveness by having chlorine oxidize the ammonia. The experiment used 0.5 ppm chlorine with 0.05 ppm ammonia but chlorine units are in ppm Cl2 while ammonia units are in ppm N which is a factor of 5.062 difference. So the ammonia in chlorine units is 0.253 ppm and that is how much monochloramine would be almost immediately formed and would be continued to be oxidized by the remaining chlorine. The chlorine demand from the bacterial cells themselves is also a factor. The net effect is that the remaining Free Chlorine (FC) level drops very low to near zero so the kill time of 20 minutes is mostly due to monochloramine itself. If you add Cyanuric Acid (CYA) to this experiment, this slows down the creation of monochloramine somewhat (from seconds to minutes) and greatly slows down the continued oxidation so more Free Chlorine (FC) can remain present longer. With proper timing and experimental setup one can actually have kill times be much faster when CYA is present by slowing down the monochloramine formation enough such that the initial FC kills the bacteria very quickly.
The discussion then turns to field studies of public pools saying they show high levels of ammonia nitrogen. However, they don't say that this isn't ammonia itself so long as there is measurable Free Chlorine (FC), but rather that this is monochloramine which shows up as Combined Chlorine (CC). So long as there is measurable FC, then it is that FC (actually the active chlorine level) that will kill bacteria regardless of the presence of monochloramine. Of course, the kill time depends on the active chlorine level so roughly proportional to the FC/CYA ratio. So in real pools where one manages the FC level, one will see much faster kill times with no CYA and much slower kill times at higher CYA levels. The conclusion that CYA has no impact on sanitation is simply false unless one doesn't have excess (measurable) FC. Now in practice, most bacteria are easy to kill so even with high CYA levels one still kills most bacteria faster than they can reproduce and therefore one does not have uncontrolled bacterial growth. However, part of the purpose for having chlorine in public pools is not just to prevent uncontrolled bacterial growth, but to prevent person-to-person transmission of disease. For this, faster kill times may be needed. The debate/discussion should be, for example, about whether the 99% kill time goal should be 1 minute vs. 10 minutes, but that's not how current standards are set.