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Thread: Debunking the "Chlorine Lock" Myth

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    Debunking the "Chlorine Lock" Myth

    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.

    "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.
    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.

    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.
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    Re: Debunking the "Chlorine Lock" Myth

    i'd like to hear someone comment on this one quote:

    "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."

    this goes against 99% of what everyone says on this forum, but i know for a fact my mother in law uses only 2-3 trichlor pucks in the skimmer and has never drained her pool and the water is crystal clear.

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    Re: Debunking the "Chlorine Lock" Myth

    Read the final paragraph of chem geek's post very carefully. That paragraph deals with that sentence. In brief, even extremely low active FC levels are capable of killing bacteria over time, which is what that study was testing. What the study did not consider was person to person transmission of illness and growth of algae, both of which are very problematic at the active chlorine levels they are talking about.
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    Re: Debunking the "Chlorine Lock" Myth

    Quote Originally Posted by brg88tx
    i know for a fact my mother in law uses only 2-3 trichlor pucks in the skimmer and has never drained her pool and the water is crystal clear.
    She could have the CYA kept in check by rain overflow and filter backwashing or letting the pool go over the winter and having the CYA go away (for whatever reason) or she could have high CYA but be lucky in having the pool poor in algae nutrients. I'm also assuming she doesn't use any algaecide, copper ions, phosphate removers, borates, etc. Having a low FC/CYA ratio does not guarantee algae growth. It just makes it far more likely. Just remember what Clint Eastwood said in Dirty Harry, "... you've got to ask yourself one question: 'Do I feel lucky?'".
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    Re: Debunking the "Chlorine Lock" Myth

    Quote Originally Posted by chem geek
    Just remember what Clint Eastwood said in Dirty Harry, "... you’ve got to ask yourself one question: 'Do I feel lucky?'".
    lol
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    Re: Debunking the "Chlorine Lock" Myth

    Trichlor in the skimmer? So, has that damaged the skimmer or the pump?
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    Re: Debunking the "Chlorine Lock" Myth

    interesting what this says about CHLORINE LOCK:

    "CHLORINE LOCK: an imaginary condition which, legend claims, that if you have a residual level of CYA of over 100 ppm the chlorine will stop working. This myth has never been proven, and all scientists claim that this conditon can not occur on the planet Earth. Levels of CYA as high as 400 ppm actually increase the disinfection of pool water because of the residual levels of ammonia as proven in the 1990 Pinellas county study by the CDC."

    http://www.thepoolpros.com/library/definitions

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    Re: Debunking the "Chlorine Lock" Myth

    There is something a little garbled about the last sentence. I wonder what they were trying to say.
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    Re: Debunking the "Chlorine Lock" Myth

    What they are referring to in the last sentence is that very high CYA levels slow down chlorine oxidation such that the ammonia in sweat and urine forms monochloramine that persists for longer. This is effective at killing algae and can control bacterial growth though is not a fast kill. The Pinellas County study did not "prove" this or much of anything else. I discuss the Pinellas County, Florida pool study in this thread. Note that 25 of the 49 pools that had ZERO chlorine in them nevertheless had "good" bacterial counts (HPC<=500, TCOLI,FCOLI=0, NCOLI<=200). Also note that sorting by calculated hypochlorous acid (HOCl) concentration gives at least as good a correlation than sorting by FC -- basically, bacteria are so easy to kill in terms of preventing uncontrolled growth that one cannot conclude the determining component from the Pinellas study, though one can from numerous other experiments and data (see the "Chlorine/CYA Relationship" section in this post).

    Residential pools are low bather load and do not have very much introduced ammonia into the pool so counting on monochloramine being formed to kill off algae is not wise. On this forum we've never said that chlorine stops working at any given CYA level, only that for the same FC a higher CYA level makes chlorine less effective. It is the FC/CYA ratio that should be looked at for roughly determining disinfection, oxidation and algae prevention rates.
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    Re: Debunking the "Chlorine Lock" Myth

    So what is taking place when someone adds chlorine to their water and not getting a reading?

    My pool companies believe this to be chlorine demand or chlorine lock and the cure being to throw a s**tload of chlorine at it until it starts to read. I always found this hard to swallow. Is it just a matter of TDS? Maybe partial draining and refilling can solve it?
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    Re: Debunking the "Chlorine Lock" Myth

    Quote Originally Posted by swimu
    So what is taking place when someone adds chlorine to their water and not getting a reading?

    My pool companies believe this to be chlorine demand or chlorine lock and the cure being to throw a s**tload of chlorine at it until it starts to read. I always found this hard to swallow. Is it just a matter of TDS? Maybe partial draining and refilling can solve it?
    If there's no FC reading, it means there are organics - algae - consuming it extremely fast. Even if CYA is astronomical, the FC will read - it just won't do anything. And the cure is to overwhelm the organics - the shock process.

    People here who have cleared pools have reported losing 6-7 ppm in an hour when starting at 15 FC or better... If one goes by the industry standard of 3 is good, well.....it will disappear in minutes.

    What are TDS? I've never had mine tested in 3 years. What do they affect besides the bottom line. ka-ching. You're in the profession - you tell me.
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    Re: Debunking the "Chlorine Lock" Myth

    Could also be ammonia in the water. Call "it" what you wish, but the simple answer is to continue adding enough chlorine to raise the water to shock FC levels, which is dependent on CYA levels. The pool industry also likes to call some types of chlorine "shock", but we know shocking your pool water is a process, not a product.
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    Re: Debunking the "Chlorine Lock" Myth

    If a pool gets to zero chlorine, then bacteria can start converting CYA into ammonia. Also, with too low a chlorine level algae can grow. So in an algae filled pool one may have unusually high chlorine demand from the very high amounts of ammonia. This would be seen as getting little or no FC, but a high CC. If instead you add chlorine and get some FC at first but it goes away quickly within an hour, then that's more likely to be algae alone where the chlorine is oxidizing lots of it and getting used up, sometimes resulting in CC but not always.
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    Re: Debunking the "Chlorine Lock" Myth

    Ok, this makes sense.

    So am I safe to say that chlorine demand and chlorine lock are two different things and chlorine lock really isn't a thing?

    When I use to work for a pool retailer, we use to sell Bioguard. They actually had a complete chlorine demand test station that cost an extreme amount of money, which I always found a bit odd. Chlorine demand meant you had to "feed the beast." There is something in your water that demands chlorine, and in order to break that demand, you had to give it what it wants - it would eventually break. This Bioguard test station would supposedly tell you how much chlorine it would take to meet that demand. We all know this is just a way to sell chemicals.

    So to conclude, chlorine demand is real, and chlorine lock...not so much?
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    Re: Debunking the "Chlorine Lock" Myth

    Basically yes. Chlorine demand is a real phenomenon. Chlorine lock isn't, but is what is often attributed to high CYA levels because most of the chlorine does get bound to CYA so in an overly simplistic sense it is "locked up" but not in a way that it is not released. Increasing the FC level will still result in a higher active chlorine level even if the CYA level is high and most chlorine is bound to the CYA. It is the FC/CYA ratio that is relevant and a higher CYA just means you need a higher FC to get the same active chlorine level. In practice, a very high CYA could be a challenge for shocking a pool that already has algae since the FC level would have to be very high -- 40% of 200 ppm CYA is 40 ppm FC, for example -- so one usually just dilutes the water first since one should get the CYA lower long-term anyway.
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    Re: Debunking the "Chlorine Lock" Myth

    Awesome! Thanks for clearing that up for me. I knew there was a confusion between the two. I also thought they were the same thing. Mainly, I knew chlorine demand was a thing, but I thought chlorine lock was just another name for it. Now I know and I appreciate it
    Matt Giovanisci

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