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Thread: CYA and SWG

  1. Back To Top    #1

    CYA and SWG

    While I have perused the forum and have seen some reference to the fact that CYA level needs to be 60 to 80 with a SWG, I guess I can't convince myself of its use. I have an outdoor pool with a swg/ozone system called a claripure. This system is patented on the basis that the synergies of ozone and chlorine together outweigh the swg alone. They also claim novel chemistry in the cell whereas chlorite and chlorates are also formed which have superior properties killing some organisms. I have a link to the patent somewhere if that would be of interest.

    That said, my pool looks great and after two plus years the only thing I have ever added to the pool (aside from some spilt beer ) is acid, about 1.5 cups per day to maintain pH. I have almost no detectable CYA in the water and my generator is set on 50%. I run the pool pump 10 hours per day and the water temp is just over 90 degF. FC is 4, pH is 7.6, TA is 60 and CH is 550. My CH has stayed on the high side but our makeup water in florida has lots of Ca.

    Anyways, the local pool store, along with my manual, suggest I add a bunch of CYA. With such clear water, as the saying goes, if it's not broke, don't fix it. No knock against pool store folks, but the ones at my local store just say add it because that's the guidelines, but they don't have the chemistry to back it up.

    After reading posts regarding the unnecessary sale of pool chemicals I'm not sure if I'm being hard headed and should just go with the flow. That said, I have also been told I need a metal sequestrant because I have a SWG due to iron and copper though their testing shows the levels are 0 ppm


    Tampa, Florida

    17,000 gallon IG pool, DE FIlter, pebble finish, 1 1/2 hp, 8 4x10 solar panels, screen cage, combined ozone/SWG.

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD
    Most everyone here believes in having some level of CYA. If you are generating chlorine continuously while the sun is out there is no absolute need to have CYA. Your chlorine will fall very quickly without any CYA unless you are constantly adding more. With a SWG you can be constantly adding more, so that doesn't have to be a problem. CYA will dramatically slow the rate at which chlorine is used up by sunlight, which can have significant advantages.

    Some people run with CYA levels around 5 to 10 in a situation like yours. That dramatically helps hold chlorine levels, reducing the amount of time that you need to run your SWG, while minimizing the theoretical problems with CYA.

    Obviously you don't need sequestrant if your levels are zero.
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  3. Back To Top    #3

    Join Date
    Mar 2007
    San Rafael, CA USA
    Ozone is very effective at breaking down organics better than chlorine and at killing certain pathogens better than chlorine (cysts such as Giardia and Cryptosporidium and heartier bacteria such as Pseudomonas aeruginosa). The biggest problem with using an ozonator is that it does not provide a residual of disinfection in the general body of pool water so any pathogens growing on biofilms will not be affected and transmission of pathogens from person-to-person remains possible. This is why ozone systems are required by law to have an additional EPA-registered disinfectant such as chlorine (or bromine or biguanide). In the Claripure system, the residual chlorine is provided through a saltwater chlorine generation system (SWG). The problem is that ozone breaks down chlorine (ironically, the patents describe the reverse process -- more on that below). Their claims about any significant amounts of formation of chlorine dioxide are an exaggeration -- chlorate is the primary end product from ozone combining with chlorine. [EDIT] Chloride is also formed and is dominant with 77% while chlorate is the remaining 23% (see page 11 of the PDF, page 35 in the paper, in this link or PDF page 3, page 104 in the paper, in this link). Your pool is free of algae because you have a residual FC level and the ozonator kills any free-floating algae as well.

    By not having any Cyanuric Acid (CYA) in your pool, the generated chlorine gets broken down by sunlight quickly so you wouldn't have the residual of chlorine in your pool that you should. However, the fact that you measure FC indicates that you may actually have a small amount of CYA since it doesn't take much to give chlorine a much extended life -- also, your SWG is set at 50% 10 hours a day so is on the high side. Adding a small amount of CYA in your pool to get the level to 30 ppm or even just 20 ppm (or even less, as JasonLion points out) should allow you to turn down your SWG to less than 50%, thus extending its life. That would be the main reason to add CYA in your pool. Another reason is that you actually have too much disinfecting chlorine in your pool at 4 ppm FC with very little (say less than 10 ppm) CYA so will cause faster degradation of swimsuits and corrosion (by a factor of 4 to 10 or more depending on actual CYA level). With your ozonator, you should be able to lower your SWG output and add CYA to run your pool at an FC target of 2 ppm with a CYA level of 20-30 ppm.


    Most of the relevant patents may be found here. The formation of chlorates and chlorites is not novel nor new and occurs when chlorine (hypochlorous acid) reacts with ozone. Putting an SWG after an ozonator will produce this effect and there is nothing novel about this. The proposed reactions that occur according to the patent are the following:

    1) Cl- +O3 +H+ --> HOCl + O2
    Chloride Ion + Ozone + Hydrogen Ion --> Hypochlorous Acid + Oxygen

    2) HOCl + O3 -->ClO2- +H+ +O2 + e-
    Chlorine + Ozone --> Chlorite Ion + Hydrogen Ion + Oxygen + electron

    3) O3 + 2ClO2- + 2H+ --> 2ClO2 +H2O
    Ozone + Chlorite Ion + Hydrogen Ion --> Chlorine Dioxide + Water

    4) O3 + 3ClO2- + 2H+ --> 3ClO3 +O2 +H2O
    Ozone + Chlorite Ion + Hydrogen Ion --> Chlorine Trioxide + Oxygen + Water

    The second equation (2) from the patent apparently contains a typo since it has an extra "electron" in the reaction -- they probably took two half-reactions and forget to remove the electrons that canceled since the equation balances charge correctly if one removes the "e-" or "electron" at the end. The third equation (3) from the patent also apparently contains a typo since it is missing an "O2" or "Oxygen" on the right hand side. The fourth equation (4) from the patent is just plain wrong as it is not charge balanced (the left-hand-side has a net negative charge while the right-hand-side is uncharged). Obviously, the patent examiner was not very careful with this patent.

    The rest of the reactions come from standard redox half-reactions so one can calculate their likelihood (thermodynamically, not reaction rate) as follows:

    Cl- + H2O --> HOCl + H+ + 2e- ..... Eo = -1.482V
    O3 + 2H+ + 2e- --> O2 + H2O ..... Eo = +2.076V
    1) Cl- + O3 + H+ --> HOCl + O2 ..... Eo = +0.594V

    Though this reaction is less likely at the normal pH in pools, in areas with high concentration of ozone, it is certainly reasonable and just states that some of the ozone does the same thing as electrolysis in terms of generating chlorine. However, other reactions are also possible as follows:

    HOCl + H2O --> HClO2 + 2H+ + 2e- ..... Eo = -1.645V
    O3 + 2H+ + 2e- --> O2 + H2O ..... Eo = +2.076V
    2) HOCl + O3 --> HClO2 + O2 ..... Eo = +0.431V
    and HClO2 --> H+ + ClO2- is also likely at the pH of pools.

    2ClO2- --> 2ClO2(aq) + 2e- ..... Eo = -0.954V
    O3 + 2H+ + 2e- --> O2 + H2O ..... Eo = +2.076V
    3) O3 + 2ClO2- + 2H+ --> 2ClO2(aq) + O2 + H2O ..... Eo = +1.122V

    However, in addition to the above reactions, there is also the following very likely reaction that the patent oh so conveniently neglected to mention:

    ClO2- + 2OH- --> ClO3- + H2O + 2e- ..... Eo = -0.33V
    O3 + 2H+ + 2e- --> O2 + H2O ..... Eo = +2.076V
    O3 + ClO2- --> ClO3- + O2 ..... Eo = +1.91V

    Basically, the above reaction converting chlorite to chlorate is far more likely to occur at the pH of pools than the reaction (3) in the patent where chlorite is converted to chlorine dioxide and requires quite acidic conditions to occur. I'm not saying that there is no chlorine dioxide formed, but only that it is a small fraction of the end products and that the combination of ozone with chlorine mostly forms chlorate which is not an effective algaecide nor disinfectant. The writers of the patent were oh so clever to make the following bold statement:

    The combination of ozonation and electrolytic chlorination produces at least one of chlorite, chlorine dioxide, and chlorate that react synergistically to kill bacteria, spores, viruses, algae, and other microorganisms, and to oxidize sulfides, cyanides, and other organic and inorganic substances.

    The key phrase in the above are the words "at least one of" where in reality it is chlorate that is primarily formed and all of their talk about the superiority of chlorine dioxide while true completely misses the point that hardly any chlorine dioxide is, in fact, produced!

    This is an example of a company using the patent system to generate an essentially invalid (obvious and full of errors) patent possibly for the purposes of touting "patented technology" to drum up more sales.

    [EDIT] As already noted above, this paper (which references this paper describes how the reaction of ozone with hypochlorite ion produces 77% chloride ion and 23% chlorate ion. It does not appear that chlorine dioxide is produced. [END-EDIT]

    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
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  4. Back To Top    #4
    richard, doesn't it hurt to keep all those formulas in your brain?
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  5. Back To Top    #5
    Way back where somewhere, I think maybe poolforum, I remember a graph that showed a lot of benefit and chlorine protection at 20ppm CYA, and not much additional protection above it.

    I run my CYA at 20-30 (close as I can tell with my walmart HTH kit), and haven't had any problems. I don't have an swg, but rather add bleach manually every night.

  6. Back To Top    #6

    Thanks for the reply. I was somewhat skeptical of the claims when we got the system, but it didn't cost much more than the SWG alone. It does sound like I should go ahead and add the CYA. As you mentioned, I was concerned whether I should lower the FC without any CYA present. I suspect if the CYA was up, the cell wouldn't work as hard and I wouldn't need as much acid. As a former ChE, I appreciate the effort you put into your explanations I have read on the forum. I say former as I have not done any real chemical engineering in about ten years (was a six sigma bb) and have been a stay at home dad the past five.

    Thanks gain,

    Tampa, Florida

    17,000 gallon IG pool, DE FIlter, pebble finish, 1 1/2 hp, 8 4x10 solar panels, screen cage, combined ozone/SWG.

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