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Active Chlorine Level and Disinfection By-Products (DBPs)

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Key Takeaways:

  • Cyanuric Acid (CYA), aka stabilizer or conditioner, significantly reduces the active chlorine (hypochlorous acid) concentration by orders of magnitude.
  • Higher active chlorine levels are associated with faster reactions rates with chlorine including those producing disinfection by-products, oxidizing skin, hair, swimsuits and bather waste.
  • Most pools without CYA (such as most indoor pools) have at least 10 times the active chlorine level as pools with CYA.
  • There is controversy over whether this huge difference in active chlorine levels is appropriate and where the tradeoff should be with respect to oxidation rates vs. rates of creation of disinfection by-products.
  • If a lower active chlorine level is used in higher bather-load situations or for pools not exposed to sunlight, then supplemental oxidation is often required to prevent a buildup of organics (especially urea) to a higher steady-state level.


As described in the Chlorine/ Cyanuric Acid (CYA) Relationship discussion, the active chlorine (hypochlorous acid) level in a pool with CYA in the water is very low.  This has implications for the rate of creation as well as the specific composition of disinfection by-products (DBPs).

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Replies to This Discussion

I made an error in the last post. I tried to correct it, but I must have ran out of edit time. Replace this paragragh.

So how many pools do you know that have maintenance staff available after pool closings to do the MPS shocks almost everyday? Using 2ppm’s of FC under HCF as an example, what happens with the chlorine in our pools at night? You mention that it would create more volatile chloramines for a variety of reasons, which I don’t really dispute... This is due to slower oxidation rates. On the other hand, faster oxidation rates from relatively low realistic chlorine levels of 2ppm's, will create more volatile chloramines than from 0.2 ppm's of FC, (but far less than what is expected from our typical shock levels of chlorine). But I question if it is as bad as what you would like to portray it as being. If we use night time as an example, that’s not necessarily a bad thing is it? Let see, little volatilization due less water movement, so is it more likely that more reactions (like NCL3and others), will be able to go to the next stage and not be as volatile anyway.


I thought part of the purpose of these discussions wasn't just to cover what was allowed or done currently, but also what was possible by understanding how things work.  If we want to just stop with the fact that CYA isn't allowed by most states for commercial/public indoor pools or in spas then there's no point in discussing it, but that wasn't my purpose for this discussion.  The trade-off with choosing an active chlorine level (not just FC level) is between faster oxidation rates vs. lower disinfection by-product amounts.  I completely agree with you that it is far better to have HCF be able to consistently provide 2 ppm with no CYA vs. having pools get up to 6 ppm with no CYA.  However, I am simply discussing how CYA would let the active chlorine level be lower to reduce disinfection by-products, but that doing so with high bather loads will require supplemental oxidation (or coagulation/filtration).  I'm just discussing an approach that could deliver lower DBPs.  With current laws, one could test these things out in outdoor pools where CYA is (mostly) still allowed in a commercial/public setting.


You'll need to show me the specific Law of Reactants you are talking about (when you find it) because doubling concentration doesn't result in 4 times of a product with standard two-reactant equations with a 1-to-1 proportion.  It's only when it takes two molecules of one of the reactants for an elementary equation where doubling that reactant concentration quadruples the rate.  None of the rate-limiting steps we are talking about for the DBPs seem to have that.  DBP production overall is roughly at a rate directly proportional to the active chlorine concentration.  Now what does happen that is very interesting is with the chloramines where there is not only this overall effect on rates, but an additional effect regarding the relative proportion of which intermediate chemical species are present and for how long.  That's where a lower active chlorine level has an end result of producing less nitrogen trichloride, but where the intermediates of monochloramine and dichloramine persist longer.


As for bather waste and equilibrium, the answer is sort-of.  It's not technically an equilibrium, but a steady-state.  An equilibrium is only when you stop introducing more reactants (bather-load) and get to a final stable result.  A steady-state is what happens in the long-run when the amount of new reactants (bather-load) is constant.  You essentially get to a certain level of intermediate chemicals and a certain rate of creation of final end-products.  This is what I meant by saying that these intermediate chemicals don't accumulate forever.  Monochloramine, dichloramine, urea, chlorourea, etc. do not keep climbing higher and higher.  They all get to a point where their rate of creation, starting originally from bather waste, equals their rate of destruction from oxidation by chlorine or reacting with the other intermediates.


My assumption is that the chlorine input is increased as needed when bather load increases, basically keeping the FC at a consistent level regardless of bather load.  This is where HCF is very useful since you don't need excessively high buffers of FC (with no CYA, so the active chlorine level is high) in order to not let FC get too low from being overwhelmed.  Of course, if one is using CYA, then one can have a higher FC to use as a buffer and also have a lower active chlorine level.  HCF is still very useful even in the CYA case, but the FC level (target) that is maintained is higher.  So we are in complete agreement that a proper oxidizing dosing system should be able to modify its output quickly enough to maintain the target FC level, regardless of whether CYA is present or not.  It's just that without CYA, the need for HCF is much higher since there's less of an FC buffer, but I'd still rather see HCF in a CYA pool to ensure that disinfection rates (and chlorine oxidation rates, though likely supplemented) are still reasonable.


As for the end-points of oxidation, for ammonia this is mostly nitrogen gas, hydrochloric acid (which is partly why chlorine consumption/usage is acidic), and water though there is also a substantial amount of nitrate produced as well.  As for organics, they vary in their amount of oxidation, but many of them do not get fully oxidized.   Those that do get fully oxidized produce carbon dioxide in addition to what I've already mentioned.  The somewhat insoluble ones get filtered out (some dissolved ones can also get filtered out with added coagulation chemicals).  The disinfection by-product end-points are small in concentration, but a health concern at such low levels nevertheless.  This includes nitrogen trichloride, trihalomethanes, and many, many organic chemicals though most are not known whether they are health concerns.  Some of these chemicals are volatile (including nitrogen trichloride and trihalomethanes), but they don't escape the water immediately while others build up in the water and are not volatile.  However, there is a very large amount of (essentially non-volatile) urea that builds up and some of this is in the form of chlorourea that measures as CC though is eventually oxidized to chloramines (that get further oxidized) and carbon dioxide.  Note that there is no "next stage" for nitrogen trichloride.  It is produced in the water and outgasses into the air, but not necessarily that quickly.  Same for the trihalomethanes.  Just because a chemical is volatile that doesn't mean that it outgasses quickly in a pool -- it depends on the air/water interface so during the day with splashing it probably happens more (unfortunately, just when you don't really want it) whereas at night with a calm surface it happens less (unfortunately, when it would be better to do so).


As for having faster oxidation that produces volatile disinfection by-products at night, that could be an alternative approach where the chlorine levels are raised substantially at night to accelerate oxidation, but the resulting disinfection by-products need to be removed from the pool so aeration and air exchange would be needed (though non-volatile by-products would remain).  It essentially creates a bad air environment when people are not around for exposure, but helps to remove the disinfection by-products from the pool.  The key is to remove enough of these DBPs from the water since the higher active chlorine levels are creating more overall.  Even if one didn't increase the chlorine level, but had a period of strong water aeration with significant air exchange, that would be very helpful to reduce the volatile disinfection by-products.  My concern wasn't so much what was happening at night, but during the day with bather load where 2 ppm FC with no CYA is oxidizing skin to produce THMs 10 times faster (so 10 times as much in a given time) and where nitrogen trichloride is likewise at 10 times higher levels compared to 4 ppm FC with 20 ppm CYA.


As for the CC limits, they need to be revisited in light of the active chlorine level.  Again, this is a point of discussion, not something that can be avoided now because the regs are what they are.  Given what happens with urea (including Blatchley's results), I would be surprised if in high bather-load situations on a sustained basis over days or weeks that 2 ppm FC with no CYA would be able to keep chlorourea levels below 0.2 ppm and therefore CC below 0.2 ppm, at least not without something else dealing with the urea, be it supplemental oxidation, coagulation/filtration, or water dilution.  At the NEHA conference, there were several examples where even maintaining FC levels (with no CYA) during high bather loads over extended periods of time were not able to keep CC's at proper levels nor prevent air quality from deteriorating so I dispute that 2 ppm FC with no CYA would always work to keep up with sustained bather load (during the day).  In all of these cases, various methods of supplemental oxidation were used to address this issue successfully.  The advantage with supplemental oxidation being that it did not contribute to disinfection by-products (at least not when using MPS; perhaps to some extent with ozone; and we all know the mixed results for UV).  I'm not at all tepid about using these other techniques or using coagulation/filtration/backwashing.


So at this point we should agree-to-disagree on whether maintaining 2 ppm FC with no CYA even using HCF is appropriate in pools.  For indoor commercial/public pools, one really has no choice due to regs, but that doesn't mean I'm not going to talk about other possibilities to reduce DBPs since ultimately that is going to be a long-term goal that regs will need to address.  And in the meantime, I'll have to continue to watch my wife buy new swimsuits every year and complain about how much worse that pool is compared to our own because the indoor community center pool is essentially over-chlorinated even at 2 ppm FC with no CYA.


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