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Thread: Is the formula for "Superchlorination" or "Shock" too high?

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    Is the formula for "Superchlorination" or "Shock" too high?

    Is anyone trying the new formula for shocking (breakpoint chlorination) that Taylor and DuPont are suggesting in this article? Seems that the latest (2009) research suggests that the formula we've been using to determine the shock level of FC is really too high and itself contributes to the creation of excess CC.

    Curious if anyone has adopted these guidelines. According to the article on Taylor's site, the National Swimming Pool Foundation changed to this new formula in the 2009 edition of the "Pool and Spa Operators Handbook".

    The old formula used 10X the level of CC to determine the shock level

    For example, if test results reveal 2 ppm of free chlorine and 0.5 ppm of combined chlorine
    Old Formula: 10 x 0.5 ppm = 5 ppm of FC needed, altogether
    Revised New Formula: 5 ppm – 2 ppm = 3 ppm FC is the targeted increase

    Here is the article on Taylor's website: http://www.taylortechnologies.com/Chemi ... ntentID=76

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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Even the new formula is wrong and I've written about this to numerous people in the industry, but they still don't seem to get it.

    Ammonia

    The original 10x rule came from the relationship of how much chlorine it took to oxidize ammonia. The net equation is as follows for breakpoint chlorination:

    3HOCl + 2NH3 ---> N2(g) + 3H2O + 3H+ + 3Cl-
    Hypochlorous Acid + Ammonia ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion

    So on a molar basis, the ratio of chlorine to ammonia is 3 to 2 or 1.5 to 1. To get to a ppm basis, one must convert to chlorine units which are measured as ppm Cl2 and to ammonia units which are measured as ppm N. The molecular weights for these are 70.9064 and 14.0067, respectively. So on a ppm basis, the ratio is 1.5*70.9064/14.0067 = 7.59. In practice due to some side reactions and needing a little extra to get over the hump of the multiple initial steps for the reaction, it takes 8 to 10x the amount of chlorine as ammonia. This is where the 10x came from.

    However, when it comes to combined chlorine in water, there are two problems with using the 10x rule. First of all, combined chlorine (CC) is measured in chlorine units, not ammonia units, so the factor of 5 difference in these units needs to be taken out. The research referenced in the Taylor article STILL doesn't recognize this flaw of measuring units. Second, combined chlorine already has used up one of the 1.5 chlorine needed in the breakpoint chlorination reaction as seen from the first step of that reaction:

    HOCl + NH3 ---> NH2Cl + H2O
    Hypochlorous Acid + Ammonia ---> Monochloramine + Water

    The research referenced in the Taylor article is only accounting for this latter effect, which is relatively minor compared to the factor of 5 units effect. So in practice, in the worst case where you had monochloramine (combined chlorine), you would have to add a little more than the same amount of chlorine in order to get the breakpoint reaction going (that gets it to dichloramine where any more chlorine starts breaking it down and releasing more chlorine). So that's 1x, not 10x where the actual amount of chlorine used would be 0.5x (i.e. there would be chlorine leftover).

    Urea

    Now what I have described above is for ammonia, but the largest component of sweat and urine is urea and that requires more chlorine to get oxidized as shown below (this model is not definitive, but proposed by Wojtowicz and is roughly consistent with data seen to date; hopefully a more accurate or confirmed model will be created by Blatchley's research):

    (NH2)2CO + 4HOCl ---> (NCl2)2CO + 4H2O
    Urea + Hypochlorous Acid ---> Quadchlorourea + Water

    (NCl2)2CO + HOCl ---> NCl3 + NHCl2 + CO2
    Quadchlorourea + Hypochlorous Acid ---> Nitrogen Trichloride + Dichloramine + Carbon Dioxide

    NCl3 + NHCl2 + 2H2O ---> 2HOCl + N2(g) + 3H+ + 3Cl-
    Nitrogen Trichloride + Dichloramine + Water ---> Hypochlorous Acid + Nitrogen Gas + Hydrogen Ion + Chloride Ion
    --------------------------------------------------------------------------------------------

    3HOCl + (NH2)2CO ---> N2(g) + CO2 + 2H2O + 3H+ + 3Cl-
    Hypochlorous Acid + Urea ---> Nitrogen Gas + Water + Carbon Dioxide + Hydrogen Ion + Chloride Ion

    So on a molar basis it takes 3 times as much chlorine as urea. If we assume the worst case where a monochlorourea was what was getting measured as combined chlorine, then it would take at most 3 more chlorine to get the breakpoint reaction started (this gets it to quadchlorourea where any additional chlorine starts breaking that down and releasing more chlorine). So that's 3x, not 10x where the actual amount of chlorine used would be no more than 2x (i.e. there would be chlorine leftover). In practice, it would take less chlorine than 3x since some of the combined chlorine is probably at least dichlorourea.

    By the way, the chloroureas may not be as volatile nor irritating so they may just be wasteful readings of Combined Chlorine (CC) that just indicate intermediate compounds that aren't a serious problem (until they get rather high). If you've got a situation with CC where there isn't any obnoxious or noticeable pool smell, then the CC may be something that can be ignored, though it could indicate too slow an oxidation of organics in your pool. Not every chlorine compound is a problem, even though some are. Unfortunately, the current tests don't distinguish between these very well, though an ammonia test kit could potentially be used to measure monochloramine specifically separate from other combined chlorines (I need to confirm this).

    Disinfection By-Products

    As for higher chlorine levels producing more disinfection by-products, this is theoretically true for nitrogen trichloride as I describe in this post, but it is not necessarily true for other disinfection by-products and in fact lower active chlorine levels result in higher monochloramine and dichloramine levels though these are temporary unless there is continual bather load.

    Getting "Stuck" is a Fallacy

    There is also this incorrect information in the industry that somehow if you add insufficient levels of chlorine you then get "stuck" with intermediate by-products you cannot get rid of. This is simply not true. If you do not use sufficient chlorine, then you do get intermediate by-products such as monochloramine or a chlorourea, but you can simply add more chlorine and continue the reactions from that point. Nothing gets "stuck". In practice, if one always maintains some level of Free Chlorine (FC) in the pool at all times, then the breakpoint reactions are occurring continuously, though not necessarily as quickly as one may want.

    Reaction Rates

    Note that the above 1x or 3x rules simply refer to the minimum amount of Free Chlorine (FC) target needed to get the breakpoint reaction going in the worst case. It says nothing about the speed of that reaction. With CYA in the water, the active chlorine level is quite low so the speed of the breakpoint reaction can be slow as well. For ammonia, this time can be readily predicted. At 77F and with the FC at around 10% of the CYA level, it takes around 3-1/2 hours for the oxidation of ammonia to be 90% complete. If there were 2 ppm FC with no CYA in the water, then 90% completion would take around 15 minutes (but with nearly 20 times the amount of nitrogen trichloride produced). The oxidation of urea can take far longer, even days, though it is apparently helped by the UV in sunlight which partly explains why there is usually less Combined Chlorine (CC) in residential outdoor pools than indoor pools. The oxidation of urea is also apparently very temperature dependent, which explains why the CC doesn't usually build up in residential spas that are even used daily.

    The good news with the slower oxidation speed when there is CYA in the water is that there is less nitrogen trichloride produced and that is the most irritating and volatile of the disinfection by-products (though not necessarily the most harmful -- chloroform could be worse, but comes from other organics).

    Commercial/public pools and spas with high bather loads have a bigger problem with CC and disinfection by-products, but this is very much a function of the bather load and there isn't much that can be done about this without supplemental oxidation (ozone, UV, enzymes, non-chlorine shock such as MPS, etc.) or significant water replacement.

    So this entire idea of needing some factor of the CC as chlorine to be added is ridiculous from the point of view of "needing" that much chlorine. Since there is usually measurable FC in the water at the same time there is CC, the only purpose for increasing the FC is to increase the reaction rates to oxidize things faster. It isn't because it is "needed". There is no discussion in the industry about the FC/CYA ratio which is what determines the active chlorine concentration and therefore the reaction rates. Again, the industry focuses solely on FC alone which is relevant for total amount of chlorine needed to complete the reaction, but is irrelevant in regularly dosed pools maintaining a certain FC level.

    Richard
    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
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by J24
    The old formula used 10X the level of CC to determine the shock level

    For example, if test results reveal 2 ppm of free chlorine and 0.5 ppm of combined chlorine
    Old Formula: 10 x 0.5 ppm = 5 ppm of FC needed, altogether
    Revised New Formula: 5 ppm – 2 ppm = 3 ppm FC is the targeted increase
    Thanks. I've been reading about this impending change for at least a year but just recently read the announcement on Taylor's website.

    Is it just me or does the example they provide not make any sense? At best, it's unclear--as is the Testing and Treatment Guide included with their residential test kits. The confusion for me starts when trying to intrepret "5 ppm of FC needed, altogether." Is this the level of chlorine to be ADDED to reach breakpoint (it's "needed", after all)? Or is this a TARGET level ("altogether" or a sum total) to be reached by adding a lesser amount of chlorine? The Treatment guide (p. 24, Oct 2005 rev.) has it both ways, declaring that "FC is raised to 10 times the measured CC" but in the following paragraph admonishes the reader to avoid "adding less than the Breakpoint dosage." Nebulous phrasing, to say the least.

    Quote Originally Posted by Taylortechnologies.com
    Old Formula: 10 x 0.5 ppm = 5 ppm of FC needed, altogether
    Revised New Formula: 5 ppm – 2 ppm = 3 ppm FC is the targeted increase
    Starting with 2 ppm FC, the Old Formula states that altogether we need 5 ppm FC.
    Action: ADD 3 ppm chlorine.

    Starting with 2 ppm FC, the New Formula urges us to target an increase of 3 ppm FC.
    Action: ADD 3 ppm chlorine.

    Go figure! Here's a better idea:
    Quote Originally Posted by polyvue
    Improved TFP Formula: Read Shock Your Pool for 5 Minutes, altogether
    • Speaking of misdirection, how is it that this old chestnut survived all that roasting over an open fire?

    Quote Originally Posted by Taylor Technologies Inc.
    To maximize total lowering of alkalinity and minimize pH drop, slug the acid, i.e. pour the acid in columns ... To maximize pH drop and minimize the total alkalinity drop, walk the acid ...
    • Source: Pool & Spa Water Chemistry - A Testing & Treatment Guide. Taylor Technologies, Inc. 2005, p. 16
    14,555 gal in-ground 16'x29' white plaster Pool w/spa (2007); Goldline Aqua Logic AQL-PS-8 control w/Aqua Cell 15 Salt Water Chlorination (SWCG); Hayward TriStar 1HP (1.85 SF) main / 1.5HP (1.60 SF) spa pumps; Hayward Swimclear cart filter C4025, ColorLogic LED lights; Tankless SP-18-4 electric heater; Polaris 280 cleaner.
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    If you google "breakpoint chlorination formula" you'll find various sets of instructions (e.g. this one) where it is clear that the old formula (10 x CC) calculated the amount to add, not the target level; so 10 x 0.5ppm CC => add 5ppm FC. The new theory is to use this number as the target level, not the amount to add; so 10 x 0.5ppm CC => target FC level 5ppm, so add 5 - (current FC).
    --paulr
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by PaulR
    If you google "breakpoint chlorination formula" you'll find various sets of instructions (e.g. this one) where it is clear that the old formula (10 x CC) calculated the amount to add, not the target level; so 10 x 0.5ppm CC => add 5ppm FC. The new theory is to use this number as the target level, not the amount to add; so 10 x 0.5ppm CC => target FC level 5ppm, so add 5 - (current FC).
    --paulr
    Which theory do you support?

    My point was only that as these formulas are expressed in the Taylor literature they are open to intrepretation; and that this premiere water testing kit manufacturer is not especially precise in their description of either formula. I actually think Taylor's Testing & Treatment guide is an excellent introduction to pool water testing, but one gets the impression that the guide represents a compiled synthesis of industry concepts and conventional practice, and is not the result of original research or hypothetical reasoning.

    Furthermore, it appears that Taylor, along with other industry entities, have erred in their calculation of breakpoint chlorination or in their acceptance of it. This is bluntly asserted by Richard. If there is a published countervailing opinion on this subject that uses chemistry or reasoning to support its position, I haven't seen it. Taylor is selling a new version of their treatment guide; it will be interesting to see if they will continue to to promote the notion that pH and Total Alkalinity levels can be selectively and proportionately adjusted in an inverse ratio merely by changing how one pours a single application (dose) of muriatic acid into the pool.
    14,555 gal in-ground 16'x29' white plaster Pool w/spa (2007); Goldline Aqua Logic AQL-PS-8 control w/Aqua Cell 15 Salt Water Chlorination (SWCG); Hayward TriStar 1HP (1.85 SF) main / 1.5HP (1.60 SF) spa pumps; Hayward Swimclear cart filter C4025, ColorLogic LED lights; Tankless SP-18-4 electric heater; Polaris 280 cleaner.
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by polyvue
    Quote Originally Posted by PaulR
    If you google "breakpoint chlorination formula" you'll find various sets of instructions (e.g. this one) where it is clear that the old formula (10 x CC) calculated the amount to add, not the target level; so 10 x 0.5ppm CC => add 5ppm FC. The new theory is to use this number as the target level, not the amount to add; so 10 x 0.5ppm CC => target FC level 5ppm, so add 5 - (current FC).
    --paulr
    Which theory do you support?
    I don't have the chemistry chops to say one way or the other. I use the levels from the Chlorine/CYA chart rather than any computation. However, the consensus on the forum is that 0.5ppm CC is not worth doing anything about, so the example on Taylor's website is lame.
    Quote Originally Posted by polyvue
    My point was only that as these formulas are expressed in the Taylor literature they are open to intrepretation; and that this premiere water testing kit manufacturer is not especially precise in their description of either formula.
    Agreed.
    Quote Originally Posted by polyvue
    I actually think Taylor's Testing & Treatment guide is an excellent introduction to pool water testing, but one gets the impression that the guide represents a compiled synthesis of industry concepts and conventional practice, and is not the result of original research or hypothetical reasoning.
    Yes. My takeaway is that Taylor people are water chemistry nerds but not pool-water-management nerds. If you look at Taylor's home page, "Pool/Spa" is only one of about 10 or so areas they cover.

    Quote Originally Posted by polyvue
    Furthermore, it appears that Taylor, along with other industry entities, have erred in their calculation of breakpoint chlorination or in their acceptance of it. This is bluntly asserted by Richard. If there is a published countervailing opinion on this subject that uses chemistry or reasoning to support its position, I haven't seen it. Taylor is selling a new version of their treatment guide; it will be interesting to see if they will continue to to promote the notion that pH and Total Alkalinity levels can be selectively and proportionately adjusted in an inverse ratio merely by changing how one pours a single application (dose) of muriatic acid into the pool.
    Perhaps you should call and ask them to explain the chemical mechanism for this difference. If they are unable to explain it then perhaps they would be willing to stop recommending it. Offhand I think it's kind of unlikely they would make that change spontaneously; somebody has to prod them about it.
    --paulr
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    IG plaster pool 18.5K gal, Hayward Pro-Grid DE filter, 3/4 HP Hydramax II; Polaris 380, 3/4 HP booster
    AG spa 325 gal, probably Sundance of some kind
    Water testing instructions on one page

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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    I was just going to recommend this. Regarding the TA/pH thing, I suggest someone contact Taylor and get their "side of the story". You might be surprised on hearing the history behind Hales' "Muriatic Acid" article. The 2009 edition of their Treatment Guide includes the "slug" method for lowering TA.

    Don't forget, there's a whole world outside of TFP (residential, non-regulated, anything-goes market) , I'm talking indoor pools and strict government regulations, health inspectors ,for example. Taylor has to address these markets as well, so in that sense I don't think their example of 0,5 ppm of CC is lame, maybe for outdoor pools, but for indoor pools it's certainly not. In that same vein I don't think they'll recommend using borax to raise pH any time soon.

    Taylor has to adopt a much broader vision of the pool/spa industry than what the residential user here sees.

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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by CaOCl2
    Regarding the TA/pH thing, I suggest someone contact Taylor and get their "side of the story". You might be surprised on hearing the history behind Hales' "Muriatic Acid" article. The 2009 edition of their Treatment Guide includes the "slug" method for lowering TA.
    I've heard that history and both sides of this argument having met Que Hales personally in Arizona and having talked to the people at Taylor via E-mail and over the phone. It's not that the slug method doesn't have any differential effect whatsoever, but (from Que's experiments) that it is not efficient and can be potentially dangerous to pool surfaces. Regardless of how the acid is added, the TA drops by a fixed amount determined by the chemistry and there is no way around that. The only thing that you can control, to some degree, is the amount of carbon dioxide outgassing that will determine whether the pH drops as much when the acid is added. A much more controlled way of doing this is to lower the pH of the pool overall and to increase aeration -- it is simply a more efficient way and as shown by many people on this and other forums, it works faster and, by the way, isn't something that either Que nor Taylor suggests (i.e. their debate was only about slug vs. even distribution -- neither looked at the approach we recommend on this forum). If we were more aggressive on the overall pH lowering for the process, it would work even faster, but most pH test kits don't measure below 6.8 so 7.0 is generally the lowest target we use. As shown in this chart the possible rate of carbon dioxide outgassing increases rapidly as the pH is lowered (I say possible since the rate could be even higher since the chart only shows out-of-equilibrium ratios while rates may be more than linear as appears to be the case with TA levels).

    There is a similar controversy regarding the addition of baking soda, especially in spas. There are those who use baking soda to raise the pH in spas even though theoretically it mostly increases the TA. In practice, with the hot water and significant aeration in most spas as well as concentrated higher TA near the surface when broadcast, adding baking soda results in a lot of carbon dioxide outgassing upon addition so not only does the TA get increased, but the pH rises as well.

    One thing I have learned since I first started investigating pool water chemistry is that you can't assume that even those with PhD's at companies really get this stuff right, at least all the time. This 10x rule is just one example of someone misapplying chlorine oxidation of ammonia rules to combined chlorine (CC) measurements that are in entirely different units and that this was done decades ago and yet no one has fixed this yet. There is a lot of resistance to change since it is just assumed that something that has been thought to be true for such a long time must be true.

    Richard
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by chem geek
    One thing I have learned since I've first started investigating pool water chemistry is that you can't assume that even those with PhD's at companies really get this stuff right, at least all the time. This 10x rule is just one example of someone misapplying chlorine oxidation of ammonia rules to combined chlorine (CC) measurements that are in entirely different units and that this was done decades ago and yet no one has fixed this yet. There is a lot of resistance to change since it is just assumed that something that has been thought to be true for such a long time must be true.
    Understood. So, it is even more surprising (or disheartening) to have witnessed a heralded change in the formula used for breakpoint chlorination that per your analysis manages to get it wrong both times. And I didn't think it was just Hale that determined the slug vs. walk application of acid was more myth than not... I have a collection of articles on my other computer I can check later; several of them were obtained from PDF files or links you provided here or on Pool Forum, but don't recall if the older study I read was from you.
    14,555 gal in-ground 16'x29' white plaster Pool w/spa (2007); Goldline Aqua Logic AQL-PS-8 control w/Aqua Cell 15 Salt Water Chlorination (SWCG); Hayward TriStar 1HP (1.85 SF) main / 1.5HP (1.60 SF) spa pumps; Hayward Swimclear cart filter C4025, ColorLogic LED lights; Tankless SP-18-4 electric heater; Polaris 280 cleaner.
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by polyvue
    So, it is even more surprising (or disheartening) to have witnessed a heralded change in the formula used for breakpoint chlorination that per your analysis manages to get it wrong both times.
    The analysis that the Taylor article refers to is also described here. The good news is that the proposals lower the chlorine levels when trying to get rid of CC. The bad news is that they just adjusted it from an incremental increase to a target, still based on a 10x rule that is fundamentally flawed when starting from CC (as opposed to starting from ammonia levels in ppm N). The Taylor article does say
    ... what Ed and Roy believe is that a breakpoint dosage of chlorine, as calculated according to current teaching (i.e., 10 times the measured level of combined chlorine), results in an over-application of chlorine because the formula doesn’t account for the chlorine content of the combined chlorine compounds themselves. These compounds, our visitors explained, are already more than halfway to breakpoint.
    but then unfortunately they don't subtract a needed FC amount based on the chlorine already in CC, let alone the conversion rate problem (i.e. it's not 10x to begin with), but rather based on the FC that is already present. And as I pointed out, this is all pretty irrelevant anyway since it's the active chlorine level that is important and to know that you have to know the CYA level (and pH, though one can assume it's near 7.5). All of Blatchley's research (here and here) are without any CYA in the water. In spite of my telling APSP, MAHC, NSPF, NSF and individuals about the predicted relationship of chlorine vs. nitrogen trichloride and the effect of CYA as described here, these effects seem to be treated as if they are new discoveries when in fact they are predicted from the fundamental chemistry worked out years ago where I just put two pieces of fundamental work together (O'Brien 1974 with Jafvert & Valentine 1992). I am still waiting for someone to do similar tests with CYA in the water to see if the predicted significant reduction of nitrogen trichloride (at least from ammonia and urea) occurs, though with temporarily higher monochloramine and dichloramine due to their slower oxidation.

    I have been patient with all of this and continue to do so. I just E-mailed those involved with this again and we'll see if the unit conversion problem and the CYA effects are understood or at least questioned enough to do some more relevant experiments.

    Richard
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    [/quote]Perhaps you should call and ask them to explain the chemical mechanism for this difference. If they are unable to explain it then perhaps they would be willing to stop recommending it. Offhand I think it's kind of unlikely they would make that change spontaneously; somebody has to prod them about it.
    --paulr[/quote]

    I don't think Taylor is actually recommending the change. They mention in the web article that they are still recommending the status quo and watching to see how the real-world experience develops. Seems they may agree with all of you that this issue is far from settled.

    Great discussion...

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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Quote Originally Posted by J24
    Perhaps you should call and ask them to explain the chemical mechanism for this difference. If they are unable to explain it then perhaps they would be willing to stop recommending it. Offhand I think it's kind of unlikely they would make that change spontaneously; somebody has to prod them about it.
    --paulr
    I don't think Taylor is actually recommending the change. They mention in the web article that they are still recommending the status quo and watching to see how the real-world experience develops. Seems they may agree with all of you that this issue is far from settled.

    Great discussion...
    That particular remark of mine was directed at the "slug" acid method, not the breakpoint formula. I hold no opinion on the breakpoint formula.
    --paulr
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    Question - what is the risk for over-shooting the breakpoint as it relates to cc reduction or elimination?
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    Re: Is the formula for "Superchlorination" or "Shock" too high?

    There is no risk with overshooting the breakpoint. Having extra chlorine available simply doesn't get used. The risk isn't in the Free Chlorine (FC) total capacity, but rather in the active chlorine (hypochlorous acid) level during the process. A higher active chlorine level results in more irritating and volatile nitrogen trichloride being produced, at least from ammonia and urea and possibly some other organics. The tradeoff, however, is that the monochloramine and dichloramine levels are higher in the interim and take longer to get broken down. They do not get "stuck", however, so one can tune the active chlorine level with a reasonable balance between these various chloramines.

    Current Experiments vs. Real Pools
    The graphs coming from Blatchley's work are for adding chlorine to a fixed amount of organics and having no CYA in the water. This is not how it works in a real pool (some of Blatchley's studies do look at real pool measurements, but don't see if there is any CYA in the water). One already has Free Chlorine (FC) and often also has CYA if the pool is outdoors. Ammonia, urea and other organics get introduced via sweat, urine, leaves, etc. and these begin to get oxidized by chlorine. The concept of having a fixed amount of FC added all at once doesn't even make sense since real pools are maintained to keep a certain FC level so more chlorine is added, either manually or automatically, to maintain the FC. So in practice, breakpoint oxidation is occurring continuously and the only question or issue is how quickly it occurs. More realistic experiments would add precursor chemicals to chlorinated water and would then add more chlorine to roughly maintain an FC level and would get results at different FC levels including different FC/CYA ratios (even adding more precursors to simulate more continual bather load as an alternative test scenario). If there is CYA in the water, then the active chlorine level is very low -- with an FC around 10% of the CYA level, this is roughly equivalent to having only 0.1 ppm FC with no CYA in terms of reaction rates. So the nitrogen trichloride production rate should be fairly low.

    Bather Load and Thresholds of Irritation
    In commercial/public pools with high bather load, it is bather load that is the fundamental problem since it will build up chemicals that need to get oxidized by chlorine. If one doesn't have CYA in the water, then the oxidation will go faster which tends to keep the CC lower, but will produce more nitrogen trichloride. If there is CYA in the water, then the oxidation will go slower, but will measure more Combined Chlorine (CC) since intermediate products will build up, but the nitrogen trichloride should be lower. If one freaks out with the higher CC and increases the FC level substantially, then one would speed up the oxidation, but would also produce more nitrogen trichloride. The CC in and of itself is not necessarily a problem, depending on its level and most importantly its specific chemicals. The odor thresholds are as follows: hypochlorous acid 0.28 mg/L, monochloramine 0.65 mg/L, dichloramine 0.1-0.5 mg/L, trichloramine 0.02 mg/L. According to this study eye irritation with Free Chlorine (FC) at pH 7.5 (so half is hypochlorous acid) started at 16-20 mg/L, monochloramine at 3-4 mg/L, chlorourea at 10 mg/L. Note that in a pool with an FC that is 10% of the CYA level, the hypochlorous acid is at around 0.05 mg/L. The chlorine combined with CYA has no indication of volatility nor irritation that I can find and its skin absorption is minimal (see here).

    Using Cyanuric Acid (CYA) to Control Volatile, Irritating Nitrogen Trichloride
    As I had indicated earlier, I wrote in this thread about the theoretical effects of chlorine on ammonia and urea with and without CYA in the water. When I assumed a constant bather load introduction of 0.1 ppm N per hour consisting of a mixture of 80% urea with 20% ammonia, then in the steady state maintaining 3 ppm FC with no CYA vs. 30 ppm CYA I found that the no CYA case had 0.01 ppm monochloramine, 1.19 ppb (0.00119 ppm) dichloramine, and 70.84 ppb (0.0784 ppm) nitrogen trichloride which is well beyond the irritation threshold (0.02 ppm) for nitrogen trichloride. With 30 ppm CYA, we had 0.28 ppm monochloramine, 34.17 ppb (0.03417 ppm) dichloramine, and 2.35 ppb (0.00234 ppm) nitrogen trichloride which is below irritation thresholds for all three of these chloramines. I can't determine the chlorourea level since there is no rate-based model for the oxidation of urea by chlorine, but it would be higher in the 30 ppm CYA case resulting in a higher CC measurement, but it may not matter (i.e. not be at a level that is irritating).

    How To Handle High Bather Load
    When one measures CC, it is most likely that one is mostly measuring monochlorourea or possibly some dichlorourea. There may also be a smaller amount of monochloramine, but as indicated earlier, this does get oxidized by chlorine much faster than chlorourea. When the bather load is high, one will build up a lot of chemicals to get oxidized and over time with regular bather load a steady-state will be reached with a high amount of intermediate products and a balance between introduction of new chemicals to oxidize vs. the outgassing or removal of final products that are of concern. So the best approach to handling high bather load is to have supplemental oxidation for the higher bather load (e.g. UV, ozone, enzymes, non-chlorine shock, etc.) to get rid of urea, chlorourea, monochloramine, and other organics including organic chloramines. Water dilution is another way to deal with this and in practice a combination of supplemental oxidation with water replacement makes the most sense. To minimize the amount of nitrogen trichloride that is produced, one can use at least a small amount of CYA in the water -- for indoor commercial/public pools I think that 4 ppm FC with 20 ppm CYA is a nice sweet spot roughly corresponding to an equivalent of 0.2 ppm FC with no CYA. Of course, this is all theoretical speculation until SOMEONE does the experiments I've been asking for using CYA in the water to lower the active chlorine concentration. If I had a spare $50-$100,000 to give to Dr. Blatchley to do this, I would. I've asked NSPF to pursue this since they are funding Dr. Blatchley's work, but they won't change the scope of the work (at least not yet). I've also brought this up at NSF since some other people are also doing nitrogen trichloride studies, but again no one has said they would do the work. Such is the curse of having no credibility since I'm just a pool homeowner not working in the industry or academia and without a PhD.

    Summary
    The recommendations in the article by Ed and Roy are quite reasonable in terms of promoting supplemental oxidation (remember that they work for Dupont that makes potassium monopersulfate non-chlorine shock). What is missing is an understanding of how the active chlorine concentration affects nitrogen trichloride levels (at least in theory) and how CYA can be used as a chlorine buffer to lower the active chlorine concentration while still providing a sufficient reserve of chlorine to not run out under higher, even local (i.e. urinating), bather load.

    Richard
    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
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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