Pool Water Chemistry (Warning: Can Get Technical)

[chem geek]

I've been following this thread for quite a while and am wondering what is the latest date (version) on your spreadsheet. I've downloaded it several times but I'm not sure I have the latest.

Whenever I upload a newer version of the spreadsheet, I update the date near the end of the first post in this thread (near where I have the link to the PoolEquations.zip file) as well as the date in the first line of the spreadsheet and currently it's at 9/12/06 though I've made some minor changes since but haven't uploaded them yet -- mostly relating to this confusion about Trade % vs. % Sodium Hypochlorite for liquid chlorine and bleach. [EDIT] I uploaded a new update 10/30/06, but check the end of the first post to see the date of the latest update. [END-EDIT]

Richard

[chem geek]

Responding to this post.

Brad,

You can first check the first post in this thread on this forum that I created to explore the relationship between CYA and FC (check out the link near the end of the post where I have analysis about ORP as well). There's a lot of info there, but I also list the original source document that determined the equilibrium constants between the chlorinated cyanurates, cyanuric acid (CYA) and hypochlorous acid (HOCl). These constants are also found in this EPA document. There is also this article written by Kent Williams for the Professional Pool Operators Assocation (PPOA), but several people have said that Kent had an agenda as he was promoting ORP sensors, but my independent spreadsheet calculations (the spreadsheet is also found at the end of my post linked above) show that his data was essentially correct. Finally, this research paper indicates that the chlorinated cyanurates do not have disinfecting capability and in fact actually lower the effectiveness of hypochlorous acid -- in other words, not only does CYA significantly lower hypochlorous acid concentration (i.e. chemical equilibrium), but that small remaining hypochlorous acid is less effective in the presence of chlorinated cyanurates (but this is probably something unique to cysts that get some sort of sensitization from the presence of the chlorinated cyanurates). If you look at the table on page 1160 of the paper (click on the button for that page on the link), you can see reductions of "titrable Cl2" which is FC and that for CYA levels that are about the same (in ppm units -- the table shows Cy as moles/liter x10^5) the HOCl level is cut down by over a factor of 5. The situation is far worse when CYA concentrations are much higher than chlorine concentrations. By the way, I ran some of the numbers in the table in my spreadsheet and there is some error -- for example, the table at pH 7, Cy 7.0, Cl 14 shows HOCl as 1.88 while I calculate 2.18, but that's not a terrible error (though I wonder if the older equilibrium constants are off a small amount or if this paper's methodology is off or different...at any rate, it's not as if there is anything terribly wrong).

I am most proud of the fact that my results from theory match what Ben had discovered through observation in creating his Best Guess CYA Chart which I compare with calculated hypochlorous acid concentrations in a link in my post. If the calculations weren't borne out in the real world, they would be pretty worthless. The correlation with Ben's chart is pretty amazing for the Min/Max levels, above about 30 ppm, and the difference below 30 ppm is mostly due to Ben requiring minimum levels of chlorine so you won't run out, not just for disinfection. The shock levels don't track as well, but recent experience with nasty mustard/yellow algae on this forum has borne out the theory as well (some day, Ben's chart may be adjusted, but we'll need more validation before that -- and besides, it's his chart and his forum).

The bottom line is that the chemistry involved has been known for a long time and though a bit complex, it isn't quantum physics. I was able to do an accurate calculation in a spreadsheet. It is also possible to approximate the effects just looking at dominant species and only 2 of the 9 chemical equations (6 of which are independent). From my contact with various manufacturers, I would say that at least some are aware of these effects (one had done a spreadsheet themselves in order to create some of the data for the EPA link above), but that this information does not get out to the industry as a whole, probably for economic reasons. If people truly understood the relationship of CYA and FC to disinfection/oxidation, they would not use Trichlor as their sole source of chlorine (unless they backwash or drain/refill frequently) nor Dichlor as a frequent source in a spa. If people used chlorinating liquid or bleach (or an SWG) and did so properly so as not to get algae, then who would buy the expensive premium products (Trichlor, Dichlor, algaecide, clarifiers, flocculants, degreasers, stain removers)?

As for your 42 ppm FC example that was without CYA, that's a whole heck of a lot of chlorine so there is no wonder that corrosion occurred. By the way, for your indoor pool installations with no CYA, what is the typical FC level? Also, how do you shock the pool to rid combined chlorine (CC) and do you find that CC buildup to be a problem? Or do you use a weekly maintenance dose of non-chlorine shock (potassium monopersulfate, KMPS)? Since the only reports of people getting sick from pools is from indoor pools (presumably without CYA) with asthma and other repiratory ills from competitive swimmers and small children, I propose actually using some CYA (10-20 ppm) in indoor pools to significantly cut down HOCl levels since that would cut down the rate of production of disinfection by-products (DPBs). Also, using KMPS would eliminate such DPBs by avoiding chlorinated organics altogether. Radical ideas, but I don't think anyone has tried them to prevent the reported problems. (I know I'm getting off the topic of corrosion and if we get into this further, we should start a separate thread or go offline).

Richard

[Waterworks]

Once I'm not so busy I'm going to read through those articles, thank you. I had only read "Benefactor or Bomb" before.

Residential indoor pools around here use a FC reading of 1-3 ppm normally. When using a SWC they normally do not ever 'shock' the pool, either by means of the SWC, liquid chlorine of monopersulfates. We test the water for them monthly and always have 0.0 ppm CC using FAS-DPD test kits. Customers that have used their indoor pools for years without a SWC almost always comment that the "chlorine smell" is gone for good once the SWC is running. New pools built with SWC's never mention anything about the smell, leading me to believe that it isn't there at all, and they just don't realize the difference the SWC is making.

Commercial indoor pools use a FC of 4-8-ish when using a SWC. We don't have inspectors around here to check, and our city (120,000 people) does not require anyone to have CPO training. We try to offer training as much as possible for our commercial customers, but we can't teach everyone everything. When we set up a new commercial pool with a salt system, we include pH and ORP controls so that they are never out of the proper range. As far as I know, these customers also do not 'shock' their pools.

For non-SWC customers, I would guess they have some CYA in their pools; from pucks. There are at least a few places that have used pucks, liquid shock, cal hypo, algaecide, flocs, coagulants, and monopersulfate - ALL in the past year. They will often let the chlorine drop to 0, the pool goes cloudy, they add a bunch of algaecide, clarifier and monopersulfate and when that doesn't work, they call us. We pour in some liquid chlorine and it clears up. They hardly listen to what I have to say, and do not feel that a SWC system would be worth the money. They are just content to have their pools shut down for a day or so every few weeks. These pools always have a strong chlorine odour; they just do not understand what they are missing.

I believe that most of the indoor pool related health problems, such as asthma, are related to combined chlorines. I also think that most corrosion of indoor pool areas is from CC's. I remember Sean saying once that they tested the chlorine inside their cell at above 80 PPM. This would suggest a constant shocking happening inside the cell, reducing combined chlorines. I think that when combined chlorines are gone, most of the health problems and corrosion of pool rooms dissappears.

Brad

[chem geek]

Brad,

Thanks for the info. On this forum, feedback from users suggests that for outdoor pools, which have CYA and are exposed to sunlight, that even without an SWG the CC level is almost always at 0. After heavy bather load it may go up briefly, but will then go down on its own in a day (or two with really heavy bather load). For users that don't want to wait, they shock their pools with chlorine, but this only occurs 1-3 times in a swim season. Last year, I didn't have to shock my pool at all and never had CC above the one drop 0.2 ppm minimum on the test. Our bather load is not heavy with my wife swimming 2-3 times during the week and both of us using the pool 1-2 days over the weekend. So the combination of maintining chlorine levels and (possibly) having sunlight seems to keep CC in check. The chlorine levels suggested on this forum are higher than the 1-3 ppm range you describe. It's usually a minimum of 3 ppm at 30 ppm CYA and higher for higher CYA levels. That not only disinfects, but is enough to keep algae from growing (though some users reported needing 5-6 ppm at 30 ppm CYA to keep away mustard/yellow algae).

I would agree that the smell is from combined chlorines, with monochloramine smelling bad, but trichloramine really smelling awful. Health problems generally don't come from monochloramine, but they can from trichloramine and especially from certain chlorinated organics including the trihalomethanes that include chloroform.

The SWG does superchlorinate a fraction of the water that goes through the cell. All of the water is not exposed to the high chlorine level -- if it were, then you would have 80 ppm FC emerging from the cell and that's not what happens. Nevertheless, this seems to be enough to keep CC at bay after multiple turnovers. There is a long and interesting discussion about this at this thread where I finally figured out (in the last post in that long thread) that regularly adding chlorine (which is very concentrated -- 60,000 to 125,000 ppm when added to the pool) doesn't superchlorinate the pool the way I would have thought because the pH is so high, so it doesn't get above about 14 ppm of disinfecting and oxidizing chlorine (hypochlorous acid) when no CYA is present and with CYA this amount is even lower. Though hypochlorite ion also has some oxidizing capability and is extremely high in concentration (when adding chlorine to the pool), it apparently is not as effective at eliminating combined chlorine.

As for health problems in indoor pools, the SWG keeping CC near zero may very well prevent these problems. Disinfection byproducts (DPBs) are probably being created faster than in outdoor pools because the CYA levels are zero in indoor pools (making the chlorine 10-30 times more reactive) but because they don't build up with the SWG, perhaps they are kept below dangerous levels that affect swimmers. It would be interesting to see if competitive swimmers report no problems in indoor SWG pools. As for corrosion, that should be related to the hypochlorous acid concentration, so not having CYA in a pool should make it more corrosive.

I will say that from my wife's personal experience, that her swimsuits degrade (the rubber falls apart and they start to fade) over a single winter of use in an indoor pool (presumably with no CYA) at our a local community center. She never has any problem at all with her swimsuits over a long summer in our own outdoor pool that has CYA. So this is part of the reason why I think that using a small amount (10-20 ppm) of CYA in an indoor pool would be beneficial since it minimizes exposure to chlorine in the pool and this would apply to SWG pools as well. This winter, I'm going over to the community center pool and will measure the chemistry to see if there's anything going on besides no CYA (i.e. is the FC super high).

Richard

[chem geek]

I wasn't sure where to put this interesting piece of info, so I'm putting it here in the Pool Water Chemistry thread since it is related to the chlorine/CYA relationship. This link describes a patent where a fluid is initially viscous in order to carry particulate matter and then becomes less viscous (more fluid) over time in order to deposit or settle its particulate matter. The patent uses a thickener, a hypochlorite, and cyanuric acid. Differing ratios of the CYA to chlorine determine how quickly the thickener breaks down (which changes the viscosity of the water). It's interesting that this is a patent that DEPENDS on the chlorine/CYA relationship for its proper functioning in having only disinfecting chlorine (that is, chlorine not bound to CYA and probably only hypochlorous acid) break down the thickener and having that concentration (and therefore reaction rates) be dependent on the chlorine/CYA ratio.

Richard

[chem geek]

I didnt think this post deserved its own thread, but it is some useful info regarding ozone and chlorine. waterbear (Evan) and I worked out how ozone could oxidize chlorine since there were some reports of people using ozonators finding greater chlorine usage (loss). The following is apparently what happens:

O3 + H2O + 2e --> O2 + 2OH- ............ Eo = +1.24V
OCl- + 2OH- ... --> ClO2- + H2O + 2e .. Eo = -0.66V
-------------------------------------------------------------
O3 + OCl- ...... --> O2(g) + ClO2- ........ Eo = +0.58V
Ozone + Hypochlorite ion --> Oxygen + Chlorite ion

O3 + H2O + 2e --> O2 + 2OH- ............ Eo = +1.24V
ClO2- + 2OH- .. --> ClO3- + H2O + 2e .. Eo = -0.33V
-------------------------------------------------------------
O3 + ClO2- .... --> O2(g) + ClO3- ......... Eo = +0.91V
Ozone + Chlorite ion --> Oxygen + Chlorate ion

With the net result being the following:

2O3 + OCl- ... --> 2O2(g) + ClO3- ......... Eo = +1.49V
Ozone + Hypochlorite ion --> Oxygen + Chlorate ion

So ozone will oxidize chlorine to form chlorate. This is a very strong (likely) reaction, but the above info does not determine the rate of the reaction.

Richard