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Thread: Effects of high CYA on chlorine demand

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    TimS's Avatar
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    Effects of high CYA on chlorine demand

    Weirdo question time again.

    Let me preface this with the disclaimer that it seems I'm missing something. I'm not advocating high levels of anything. Nor am I planning on increasing my CYA to astronomical levels, I just think that I'm not getting it.

    I understand that the purpose of CYA is to protect chlorine from UV, et.al. I understand that by protecting the chlorine, a lower percentage of the chlorine is available for disinfection, organics control, etc. so that as CYA rises, you must introduce a greater amount of chlorine to provide the same benefit. This is because the CYA binds with some of the chlorine, thus making it less readily available for use. Right so far, even if it is a simplistic description?

    Now, one of the more common statements I've seen here regarding high CYA seems to go something like "when CYA reaches X ppm, the chlorine demand becomes unmanageable, and you won't be able to maintain such high levels of FC." So, accepting that as CYA increases it binds with more and more of the available chlorine, I see how getting to the target level of FC requires higher and higher levels of chlorine, which can become quite expensive to reach.

    (The following numbers are from the Pool Calculator and Chem Geek's CYA chart) In a 10,000 gallon pool, from 0 FC, getting to FC 5 (target for CYA 40) requires 104 ounces of bleach. In the same pool, with a CYA of 100, to get to the target FC 12 requires 249 ounces. Making the assumption that the requirement is somewhat linear (perhaps a bad assumption) if CYA were something like 200, the target FC would be about 24 and require 498 ounces to reach. (Even if my numbers are off at this level, the concept should remain basically the same.)

    Here's where I'm missing something. Shouldn't the actual chlorine demand remain the same? If the above pool uses 1ppm FC/day with a CYA of 40 and a target FC of 5, shouldn't the demand remain 1ppm FC/day regardless of the CYA level? Shouldn't actual maintenance of the appropriate level remain 21 ounces (1ppm) per day, regardless of the actual target level or amount of chlorine bound to CYA? My thinking (probably flawed) is that once the chlorine is bound to the CYA it is no longer available for immediate use, but the CYA is also already bound, so cannot bind with additional chlorine. Therefore it will not bind with the additional chlorine added to the pool to maintain the target. So it will take a large amount of chlorine to initially reach the target level, but once there, should not take any more to maintain it at that level than it would require to maintain it at lower levels.

    What am I missing?

    A side question: Based on other comments that indicate that swimming is safe "up to shock levels" this must mean that with a CYA of 40, an FC level of 16 is safe for swimming. At CYA 40, is 24ppm FC unsafe? I assume that although 24 might be too high with a CYA of 40, that it is perfectly safe at a CYA of 200 since the majority of the chlorine is bound. Right?

    Another side question: Never mind. I'll wait on that one.
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    Re: Effects of high CYA on chlorine demand

    As Jason so kindly taught me yesterday, you'd never be able to get an accurate pH (using phenol red at least) since the FC would always be too high. You'd also go through a fortune in FAS-DPD titrant.

    There are probably other side effects that others will add...
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    Re: Effects of high CYA on chlorine demand

    I'll answer the side question and comment on "safe" levels. YOur assumption is essentially correct. If you mask the FC with enough CYA, much higher levels of FC are considered safe.

    However, we have only chosen shock value as a safe/unsafe criteria because so many people want a number. It's not that (CYA -40) FC-16 is fine and FC-17 will kill you. It is simply there as a guideline for those who are unsure.

    How many folks do you know that stick there hands into the automatic washing machine weekly where the chlorine level is at least 250-300ppm/ Then consider that's without CYA.

    The point is there is no black/white point at which chlorine is safe or not. That said, Shock value is a fine guideleine because it provides all of us a reference point.
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by TimS
    Now, one of the more common statements I've seen here regarding high CYA seems to go something like "when CYA reaches X ppm, the chlorine demand becomes unmanageable, and you won't be able to maintain such high levels of FC."
    I hope you didn't hear that here! That is a myth.

    The higher the CYA level, the higher you need FC to be to keep the pool sanitary, and the less total chlorine you need to add to the pool day to day (because you are losing less to sunlight).

    The myth comes from a curious interaction that happens at high CYA levels when the FC level is too low and you get algae. The algae is never killed off, because the FC level is too low, but it never really gets fully going because the high CYA level keeps a large amount of chlorine on hand (buffered by CYA) and continues to kill the algae all the time. When this happens your chlorine consumption goes through the roof and yet it doesn't look like you have algae. Instead it looks like there is constantly pollen raining down on the bottom of the pool (except it is actually dead algae).

    If you kill the algae everything goes back to normal. But the people who make these myths never kill all the algae, because they can't believe how high they need to take the FC level to kill it, so they never raise the FC level enough.
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by duraleigh
    It's not that (CYA -40) FC-16 is fine and FC-17 will kill you. It is simply there as a guideline for those who are unsure.


    Quote Originally Posted by duraleigh
    How many folks do you know that stick there hands into the automatic washing machine weekly where the chlorine level is at least 250-300ppm Then consider that's without CYA.
    True. I've done that myself, and I don't think I'm dead yet. Of course I wouldn't want to swim in it
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    Re: Effects of high CYA on chlorine demand

    chem geek is the real expert on the FC/CYA/sunlight relationship, ...
    (edit) so, let's pretend I was smart enough not to say anything in the first place. (end-edit)
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    Re: Effects of high CYA on chlorine demand

    Yes, your explanation of chlorine being bound to CYA is correct, but it's not just "some" of the chlorine that is bound, but "most" of it. With a 3 ppm FC and 30 ppm CYA at a pH of 7.5, 97.2% of the chlorine is bound to CYA, 1.4% is hypochlorous acid and 1.4% is hypochlorite ion.

    The amount of FC for the same disinfection/oxidation level is proportional to the CYA level. So if you double the CYA level, you have to double the FC level.

    You are right that the chlorine demand should mostly be a function of the amount of hypochlorous acid in the water and the amount of contaminants. However, there appears to also be a breakdown by the UV in sunlight of chlorine that is bound to CYA as well, though such breakdown is slower than that of hypochlorous acid and hypochlorite ion. In practice, however, we have found that this effect is outpaced by the CYA protection effect of lower depths so that the chlorine usage from sunlight is lower at higher CYA levels even when having proportionately higher FC levels (at least up to around 80 or 100 ppm CYA).

    Another factor is the chlorine oxidation of CYA itself that I describe here and that should be roughly proportional to the CYA level since the hypochlorous acid and hypochlorite ion levels are assumed to be constant since the FC/CYA ratio is maintained. Such oxidation might be noticeable at very high CYA levels -- at normal CYA levels it's probably not more than 0.2 to 0.3 ppm FC per day at the most (in pools; in hot spas at 104F with 4 ppm FC and 30 ppm CYA there is a daily oxidation of around 0.17 ppm CYA per day resulting in a daily FC consumption for this effect of 0.4 ppm per day).

    As Jason noted, the "unmanageable" is just if you end up having the FC/CYA ratio get too low for too long such that algae starts to grow. At that point, the nascent algae growth results in a chlorine demand even if the algae isn't yet visible and to raise the FC to be able to shock the pool to get ahead of such algae growth takes a heck of a lot of chlorine. It can certainly be done, but just isn't very practical.

    24 ppm FC with 40 ppm CYA is equivalent to 1.5 ppm FC with no CYA so not that much different than a typical indoor pool. This is not unsafe, though it does tend to oxidize swimsuits, skin and hair about 15 times faster than a typical outdoor pool that has an FC that is around 10% of the CYA level. Now, drinking the pool water wouldn't be a good idea since the FC is so high and that would include accidentally gulping it down your windpipe into your lungs, especially if this is done repeatedly.
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by duraleigh
    I'll answer the side question and comment on "safe" levels. YOur assumption is essentially correct. If you mask the FC with enough CYA, much higher levels of FC are considered safe.

    How many folks do you know that stick there hands into the automatic washing machine weekly where the chlorine level is at least 250-300ppm/ Then consider that's without CYA.
    That may sound ok duraleigh but skin absorbtion and lung absorbtion are poles apart. the dangerous area in the pool is at and just above the waterline right where your nose and mouth are. During shock levels chlorine by produts are at their highest levels too and that cannot be good for you and more importantly children who have not yet developed enough to be more resilient to this problem.
    Most of the test are from indoor pools but there must also be a risk from elevated levels of chlorine in outdoor pools.
    There is reseach into adolescent idiopathic scoliosis from chlorinated pools.

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    Re: Effects of high CYA on chlorine demand

    Chlorine byproducts in the air right above the surface of the pool are orders of magnitude higher in indoor pools with no CYA then they are in outdoor pools with CYA, despite the higher FC levels in the outdoor pools. There are three primary effects that cause indoor pools to be much worse. First, indoor pools have much worse ventilation. Any chlorine byproducts that form will tend to stay in place, while in an outdoor pool they will tend to blow away. More importantly, the CYA in the water actually suppresses the creation of the more dangerous volatile chlorine byproducts and favors the production of other relatively harmless byproducts. And finally, sunlight will tend to break down the more harmful byproducts before they buildup to significant levels.

    You can verify this quite simply. The dangerous chlorine byproducts aren't just dangerous, they have an obvious smell. That "indoor pool smell" is almost completely absent in properly maintained outdoor pools.
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by JasonLion
    Quote Originally Posted by TimS
    Now, one of the more common statements I've seen here regarding high CYA seems to go something like "when CYA reaches X ppm, the chlorine demand becomes unmanageable, and you won't be able to maintain such high levels of FC."
    I hope you didn't hear that here! That is a myth.
    Uh, yeah, I did hear that here. "X" seems to vary quite a bit, but there are a number of posts making such statements. Actually, looking back, most of them do not directly state that "chlorine demand becomes unmanageable" but that CYA becomes unmanageable or that water becomes unmanageable, which I interpreted to mean basically the same thing. (Also, having read a huge number of the posts on this board over the past few months, I tend to forget who said what, so may well be putting credence in statements that don't deserve it. )

    Quote Originally Posted by JasonLion
    The higher the CYA level, the higher you need FC to be to keep the pool sanitary, and the less total chlorine you need to add to the pool day to day (because you are losing less to sunlight).
    OK, that fits with what I was thinking.

    Quote Originally Posted by JasonLion
    The myth comes from a curious interaction that happens at high CYA levels when the FC level is too low and you get algae. The algae is never killed off, because the FC level is too low, but it never really gets fully going because the high CYA level keeps a large amount of chlorine on hand (buffered by CYA) and continues to kill the algae all the time. When this happens your chlorine consumption goes through the roof and yet it doesn't look like you have algae. Instead it looks like there is constantly pollen raining down on the bottom of the pool (except it is actually dead algae).

    If you kill the algae everything goes back to normal. But the people who make these myths never kill all the algae, because they can't believe how high they need to take the FC level to kill it, so they never raise the FC level enough.
    Quote Originally Posted by chem geek
    As Jason noted, the "unmanageable" is just if you end up having the FC/CYA ratio get too low for too long such that algae starts to grow. At that point, the nascent algae growth results in a chlorine demand even if the algae isn't yet visible and to raise the FC to be able to shock the pool to get ahead of such algae growth takes a heck of a lot of chlorine. It can certainly be done, but just isn't very practical.
    So my initial assumption was basically correct? Assuming that you started with a clean pool, and assuming that you could introduce enough chlorine to initially reach the target level, your daily chlorine use (all other factors remaining equal) then would remain basically the same regardless of CYA level (with the exception of the additional loss incurred by the FC oxidizing the CYA itself, which is fairly low anyway.) The real problem is if you somehow get algae started, the prodigious amounts of chlorine required to reach shock levels is where the "unmanageable" comes in. (I'll take 500 jugs of bleach, please )
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by JasonLion
    Chlorine byproducts in the air right above the surface of the pool are orders of magnitude higher in indoor pools with no CYA then they are in outdoor pools with CYA, despite the higher FC levels in the outdoor pools. There are three primary effects that cause indoor pools to be much worse. First, indoor pools have much worse ventilation. Any chlorine byproducts that form will tend to stay in place, while in an outdoor pool they will tend to blow away. More importantly, the CYA in the water actually suppresses the creation of the more dangerous volatile chlorine byproducts and favors the production of other relatively harmless byproducts. And finally, sunlight will tend to break down the more harmful byproducts before they buildup to significant levels.

    You can verify this quite simply. The dangerous chlorine byproducts aren't just dangerous, they have an obvious smell. That "indoor pool smell" is almost completely absent in properly maintained outdoor pools.
    So a poorly maintained indoor pool is more dangerous than a poorly maintained outdoor pool? I'd never considered that before, but it makes a lot of sense. Note to self - avoid smelly indoor pools.
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    Re: Effects of high CYA on chlorine demand

    Higher CYA levels mean less total chlorine used day to day, not "basically the same". With CYA around 50, most people use around 2 ppm of chlorine a day. With CYA around 100, most people would use around 0.5 ppm of chlorine a day. All assuming an outdoor residential pool.

    Quote Originally Posted by TimS
    Uh, yeah, I did hear that here. "X" seems to vary quite a bit, but there are a number of posts making such statements. Actually, looking back, most of them do not directly state that "chlorine demand becomes unmanageable" but that CYA becomes unmanageable or that water becomes unmanageable, which I interpreted to mean basically the same thing.
    Here we are in The Deep End, so I will try to clear this up even more completely.

    The problem with really high CYA levels isn't that chlorine demand that goes up. Chlorine demand actually goes down in a properly maintained pool with higher CYA levels. "The problem" with high CYA levels is really several problems.

    1) At high CYA levels you never really know what your CYA level is. The best available test only measures CYA levels up to 100. Using dilution you can get approximate levels up to perhaps 180, but the precision of the test goes down dramatically. With CYA at 200 or higher you really can't get a useful measurement of the CYA level at all except to say that it is very high.

    2) The high FC levels required by high CYA levels can cause problems for PH measurement. This starts to be a real problem when CYA reaches about 150 and FC needs to be above 10. If you know what you are doing, this can be compensated for up to CYA levels around 200, at which point measuring PH becomes very problematic.

    3) At high CYA levels, it takes extreme amounts of chlorine to reach shock level, should you ever need to shock the pool. With CYA around 100, this isn't so bad, but as CYA goes up, so does the amount of chlorine required, more or less without limit. And once you are done shocking, you will be sitting there will an extremely high FC level that won't come down on it's own for a good long time.

    4) Given the extreme amounts of chlorine required to shock, you tend to get large PH shifts when adding the chlorine during shocking. That can cause some fairly serious secondary problems if not compensated for. But compensating for the PH shift becomes problematic because you can't measure the PH at those FC levels.

    5) The high FC levels required by high CYA levels makes testing the FC level tedious. Not as serious as the other four, but you need to measure the FC level all the time and this gets to be a real drag.

    Maintaining pools with very high CYA levels can be done. I have heard of at least one pool service that runs the CYA level so high that they only need to add chlorine once a week. They use a number of special tricks and lots of specialized knowledge to avoid the various high CYA effects.

    We set an upper limit on CYA of 90 primarily because that is the highest level that can be tested for reliably and because most people can't manage to shock the pool when CYA is over 90, even though it can be done if you know what you are doing.
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    Re: Effects of high CYA on chlorine demand

    With regards to a saltwater chlorine generator, is the high CYA level recommend to reduce the demand on the generator, thus prolonging the life?

    Based on the math I see here, if the pool has a daily chlorine demand of 3 ppm at a CYA level of 30, with a CYA level of 90, that demand drops to 1 ppm (all other factors being the same). Thus, a SWG only has to generate 1 ppm per day.

    Is my math correct, and is that the reason for recommending the higher CYA levels?
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by JasonLion

    You can verify this quite simply. The dangerous chlorine byproducts aren't just dangerous, they have an obvious smell. That "indoor pool smell" is almost completely absent in properly maintained outdoor pools.
    True Jason but the properly maintained ones wouldn't need to shocked, only the badly maintained ones.

    Have you not noticed how the fog sits on a lake thats outdoors but the gaseous mix prefers to hover over the liquid just the same

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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by baudilus
    Is my math correct, and is that the reason for recommending the higher CYA levels?
    I think the chlorine usage will go down more than you estimated, but not a whole lot more. We haven't done enough testing to be really sure of the exact ratios, and the ratio also depends on the depth of the pool, so I can't give you an exact number, but it is somewhere between what you said and a bit more savings than that.

    With a SWG the odds of needing to shock the pool go down significantly, which is the main disadvantage of higher CYA levels. Meanwhile, the costs of adding chlorine go up a little (both as a capital investment in purchasing the SWG and in the cost of the cell over it's lifetime). The balance point between the advantages and disadvantages therefore shifts to higher CYA levels.

    A similar thing happens in very high sunlight regions even without a SWG. If you live in the desert, with direct sunlight on the pool the entire time the sun is above the horizon and very few clouds to reduce the amount of sunlight hitting the pool, it is often good to run at an intermediate CYA level, say between 50 and 70. In this case, you are losing more chlorine to sunlight than others would be losing, so the increased CYA level savings in chlorine lost to sunlight saves you more total chlorine.
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    Re: Effects of high CYA on chlorine demand

    Well, if one is maintaining an FC that is 5% of the CYA level, then at 30 ppm CYA that's only 1.5 ppm FC while at 80 ppm CYA that's 4 ppm FC. I don't think the difference in FC loss is that large in this case.

    Mark's experiments showed a gain, but it wasn't a factor of 3 at constant FC/CYA ratios. This post describes Mark's experiment where you should note that he had the SAME FC level, not a constant FC/CYA ratio. He basically saw a chlorine loss at 45 ppm CYA of 50% per day while at 80 ppm CYA he saw a chlorine loss of 15% per day. So let's apply this to a constant FC/CYA ratio used in SWG pools where the FC is around 5% of the CYA level. 2.25 ppm FC at 45 ppm CYA would lose 1.13 ppm FC the first day while 4 ppm FC at 80 ppm CYA would lose 0.6 ppm FC the first day. So it's more like a factor of 2 difference. However, when I've looked at daily losses from reports on this forum and from some pool services, the 15% daily loss in hot areas is only achieved at 100 ppm CYA while the losses at 45 ppm CYA aren't as high as 50%.

    Then there's daily losses from organic load that are independent of sunlight. So it's far more complicated than the above analysis. We know that higher CYA is generally better at reducing chlorine usage even at constant FC/CYA ratios, but as for exactly how much, it will depend on the specifics of the pool and its other chlorine demand sources.

    In my own pool which is warm at 88ºF, even keeping the pool covered (with an opaque cover) still has a daily loss of at least 0.8 ppm FC per day. The higher temperature just has the chlorine react with whatever is in the pool more quickly, including CYA itself (though as I've noted that's a smaller effect, but could be 0.2 to 0.3 ppm of the daily demand). This demand is VERY temperature dependent and lower temps have far lower daily chlorine usage rates.
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    Re: Effects of high CYA on chlorine demand

    As for the disinfection by-products (DBPs), I had forgotten about that. Yes, the higher active chlorine levels will likely produce more disinfection by-products, especially nitrogen trichloride, but in a residential pool the bather load and organics in the pool are so much lower than in commercial/public pools that it's probably not a problem even at shock levels. To some extent it depends on the buildup of slow-to-oxidize organics relative to the dilution rate, but at least for the bulk of bather load with ammonia and urea these will produce very little DBPs with the low bather load. Residential spas are another matter (which is why adding chlorine after the soak is better) as are commercial/public pools with higher bather loads.

    I wouldn't recommend regularly swimming in a pool at shock levels, but that's not the same thing as deeming it unsafe. Of course, it's all relative risk.
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    Re: Effects of high CYA on chlorine demand

    Quote Originally Posted by JasonLion
    Here we are in The Deep End, so I will try to clear this up even more completely.
    That's why I like this site - I learn a lot here. I always want to know "why," and I can usually get that from all of you.

    Jason and Richard, you have pretty well answered my original question. (Sometimes takes me a bit to "get it" though )
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