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Thread: CYA Effect on Crypto CT

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    CYA Effect on Crypto CT

    Drafted to be CPO for NC community pool. Code in our state:

    "Pools that use chlorine as the disinfectant must be stabilized with cyanuric acid except at indoor pools or where it can be shown that cyanuric acid is not necessary to maintain a stable free chlorine residual. The cyanuric acid level shall not exceed 100 parts per million."

    It also calls for:

    For accidents involving liquid stools increase the free chlorine residual and closure time to reach a CT inactivation value of 15,300 then backwash the pool filter before reopening the pool. CT refers to concentration (C) of free available chlorine in parts per million multiplied by time (T) in minutes.

    The 15,300 value comes from CDC research that did not include CYA and its reduction of FC disinfection capacity.

    It's not hard to comply with these requirements as they oversimplify to the point of ignoring reality. Reality is that CYA changes everything.

    Now I don't really like CYA and I keep it just under 30, so low in fact that I can't actually measure it with my K-2006, whose lowest CYA value is 30 ppm.

    But even at these low levels I know it has some effect on disinfection and must surely have an effect on AFR response.

    Again, I can comply with the law. My question is more about how to keep people healthy in the presence of the required CYA.

    Note that I've read and recommend these two threads already on the forum:

    Chlorine/CYA Chart

    cdc guidelines for disinfection...

    Thanks for any guidance!!
    50-Yr Old, 50,000-Gal, In-Ground Plaster-Covered Brick

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    JasonLion's Avatar
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    Re: CYA Effect on Crypto CT

    I can't find the reference for the place I got this, but I remember that one approach was to raise FC to your CYA level plus 40 for 24 hours. Hopefully I am remembering that correctly.
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    Re: CYA Effect on Crypto CT

    The code you quote is only for commercial/public pools. Is that what you are managing? If not and it's residential instead, then you can do whatever you want.

    You can use chlorine to super-chlorinate to get rid of Crypto even when CYA is present but it takes a high FC level. Roughly speaking, an FC that is 10 ppm higher than the CYA level is equivalent to 10 ppm FC with no CYA. An FC 20 ppm higher than the CYA level is roughly the same as 20 ppm FC with no CYA, etc. So that's one approach one can take if their CYA level isn't too high, as in your case.

    Possibly a more practical solution after a diarrheal fecal accident is to use 2 ppm chlorine dioxide overnight. You can create that by adding sodium chlorite to the pool and in this case the CYA actually helps reduce the amount of chlorate that is produced. Unfortunately, chlorine dioxide is not officially registered to be used in pools, but the CDC in the upcoming Model Aquatic Health Code (MAHC) is considering its "emergency" use for just this sort of purpose.
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    Re: CYA Effect on Crypto CT

    I saw a poster at the dentist's the other day advertising a chlorine dioxide mouth rinse.
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    Re: CYA Effect on Crypto CT

    Thanks all! Having trouble getting forum notifications, so I missed your replies. My bad!

    FC at X ppm above CYA raises the question of accuracy in CYA tests. I practice regularly with standard solutions, and it only makes me doubt CYA testing more. Especially for CYA < 30 b/c my K-2006 isn't intended for that. Had an enlightening conversation with the health inspector a couple days ago -- totally ignorant of CYA effect on FC -- disturbingly ignorant.

    So, yes, it's a community pool subject to code. I've learned so much on TFP, and one way I can say thanks and also compliment the forum is to say that with the info I've learned here I've been able to nail pool chemistry at a level way beyond what code requires. Our pool is considered exemplary in the county despite it's age and mechanical issues.

    On topic, if I have to have FC at CYA + 10 for crypto, then that implies FC < CYA has no crypto kill? Again, current (old) CDC info was established without CYA, so for all I know FC < CYA really might have no crypto kill.

    Is chlorine dioxide OK for public pools? How do I figure how much is needed for a 50K gal pool? How do I test it to maintain proper concentration?
    50-Yr Old, 50,000-Gal, In-Ground Plaster-Covered Brick

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    JasonLion's Avatar
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    Re: CYA Effect on Crypto CT

    Much lower FC levels (but still fairly high) will kill crypto eventually, just not as quickly as most would prefer.
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    Re: CYA Effect on Crypto CT

    Superchlorinating
    I'll use an example of 30 ppm CYA to give you an idea of what goes on with different FC levels below and above that and the corresponding 99.9% kill times assuming a 15,300 CT value for Crypto. I assume the pH is 7.5 in all cases so when shocking you need to significantly lower the pH prior to adding a hypochlorite source of chlorine or need to adjust with acid afterwards which can be tricky since the pH test becomes invalid at high FC levels (especially as high as needed for faster Crypto inactivation).

    FC . HOCl . FCequiv . Time for 99.9% kill
    2 ... 0.027 . 0.055 ... 193 days ... this is FC/CYA of 6.7% so a little below the regular minimum
    3 ... 0.042 . 0.085 ... 125 days ... this is FC/CYA of 10%
    12 . 0.294 . 0.595 ... 17.9 days ... this is regular shock level
    18 . 0.687 . 1.390 ... 7.6 days ... this is yellow/mustard algae shock level
    25 . 1.597 . 3.230 ... 3.3 days
    30 . 2.661 . 5.383 ... 47 hours (2 days)
    35 . 4.097 . 8.288 ... 30.8 hours
    40 . 5.851 . 11.837 ... 21.5 hours
    45 . 7.832 . 15.843 ... 16.1 hours
    50 . 9.962 . 20.152 ... 12.7 hours

    So if your CYA test was off by 10 ppm then 40 ppm FC with 40 ppm CYA would be 41 hours instead of 22 hours so figure roughly double for that sort of error. So if you didn't go twice as long, then you would get a 96.8% reduction instead of a 99.9% reduction. Remember that the 15,300 is conservative for the worst-case strain.

    Chlorine Dioxide
    From this paper and using the most conservative 1000 mg•min/liter for the CT for 2-log reduction which corresponds with 1500 for a 3-log reduction, 2 ppm chlorine dioxide would take 1500/2 = 750 minutes (12.5 hours) for inactivation of Crypto. Chlorine dioxide breaks down in sunlight so would be used overnight for outdoor pools. You can create chlorine dioxide by adding sodium chlorite to a pool that already has chlorine in it and the CYA in the pool is a good thing since the lower active chlorine level reduces the formation of chlorate. The product Katadyn Micropur MP1 Purification Tablets has 6.4% sodium chlorite (the image of the package says chloride, but that's a misprint) and 1.0% Dichlor (dihydrate) to make chlorine dioxide for drinking water and is EPA approved for that purpose. On a molar basis, it's (6.4*90.442)/(1.0*237.95) = 2.4 times more chlorite than dichlor, but dichlor produces two HOCl in water so it's really 1.2 times more chlorite than chlorine for just a small excess. You could roughly use equal amounts so if you have 3 ppm FC in your pool then add roughly 3 ppm equivalent of sodium chlorite. 1 ppm sodium chlorite anhydrous in 10,000 gallons is (1 mg/L ClO2)*(37854.1 liters)*(90.442 g/mole NaClO2)/(67.45 g/mole ClO2)/(1000 mg/g) = 50.758 grams or 1.79 ounces weight while for the trihydrate form of sodium chlorite it's (1 mg/L ClO2)*(37854.1 liters)*(144.487 g/mole NaClO2•3H2O)/(67.45 g/mole ClO2)/(1000 mg/g) = 81.089 grams or 2.86 ounces weight. So for 50,000 gallons and for 2 ppm chlorine dioxide (where the pool has roughly 2 ppm FC or slightly less), you need 10x this amount so 1.1 pounds of sodium chlorite anhydrous or 1.8 pounds of sodium chlorite trihydrate. It would be slowly added to the pool water as you would with any other concentrated chemical -- fortunately adding chlorite to chlorinated water minimizes by-products while the other way around would be more of a problem.

    The chlorine dioxide manufacturers did not pay the money for efficacy and especially the expensive safety tests for use in swimming pools so it is not approved by the EPA for that purpose. However, for remediation purposes there is probably nothing against that in your state regs. The proposed CDC MAHC Code and Annex do allow for chlorine dioxide to be used for remediation when the aquatic venue is closed and no bathers are present and is allowed by the EPA under Section 18 (emergency exemptions) of FIFRA regulations.

    The issue with an indoor pool is how to get rid of the chlorine dioxide. Since it breaks down from the UV in sunlight, you obviously want to turn off any UV system that you may be using, but if you have one then you can use it to get rid of the chlorine dioxide by turning the system on and circulating.

    You can measure the chlorine dioxide level in a DPD (or FAS-DPD) test, but it looks very much like chlorine so you can't well distinguish between them. You can remove the chlorine dioxide using the same reducing agents you would use to dechlorinate.

    Filtration
    Also note in the Annex that it says that the use of a sand filter by itself may remove only 25% of oocysts, but when a coagulant is used it may remove 99% of oocysts. If this is the case per pass, then it would not take too long to reduce Crypto to safe levels. The MAHC specifies the following formula for determining the flow rate (Q, in gallons per minute) to achieve this is the following:

    Q = V x (14.8 - ln(V)) / (60 x T)

    or equivalently

    turnovers = 14.8 - ln(V)

    where V is volume in gallons, T is time in hours. So for a 50,000 gallon pool that would be 4 turnovers. This is because the large pool volume dilutes the oocysts from a diarrheal incident already. The standard isn't a 99.9% reduction, but rather an absolute reduction from an assumed 100 million oocysts from a single incident down to a concentration of 1 oocyst/100ml which is considered to be low enough to prevent infection.
    Last edited by chem geek; 05-22-2014 at 02:12 PM. Reason: Correcting quantity

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