Pool Water Chemistry (Warning: Can Get Technical)

[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