Miscellaneous Short Discussions

[chem geek]

(CONTINUATION FROM ABOVE)


As for point 2 (boy, I sure got long-winded about point 1!), there are reasons to use CYA in indoor pools that have nothing to do with the lack of sunlight. Remember that CYA not only protects chlorine from breakdown from sunlight, but it also significantly lowers the disinfecting chlorine concentration. Because one cannot maintain 0.1 ppm FC in a pool realistically, even indoor pools with no CYA tend to have at least 1 ppm and usually 2 ppm FC or even higher. That is a HUGE amount of disinfecting chlorine! Reaction rates are proportional to the concentration of disinfecting chlorine (most reactions are "first-order" so are proportional). The mid-point of Ben's chart is about 0.05 ppm of disinfecting chlorine while even 1 ppm FC with no CYA is about 0.5 ppm disinfecting chlorine so an indoor pool has AT LEAST 10 times the amount of disinfecting chlorine because no CYA is used. Swimsuits will degrade 10 times faster. Skin cells will get chafed 10 times faster. Disinfection by-products will get created 10 times faster. And yes, bacteria and algae will get killed 10 times faster, but so what? They were already killed more than fast enough at far lower disinfecting chlorine levels. All reports of asthma and respiratory problems in competitive swimmers and children have been in indoor pools and though better ventilation has helped a little, there is still a big problem there. Though some of the problem is due to a lack of sunlight to help breakdown combined chlorines including some disinfection by-products and though some of the problem is due to less air circulation, I believe a BIG part of the problem is the 10 times (or higher) faster production of disinfection by-products due to the 10 times (or higher) disinfecting chlorine levels. It is well known in the water treatment industry that high chlorine levels lead to faster production of DPBs and one of the ways this was historically handled was to try and minimize such chlorine levels.

So I am an advocate of using a small amount, say 10-20 ppm, of CYA in indoor pools. Since indoor pools have a very hard time dealing with combined chlorines and since having CYA in the water will slow down that breakpoint process (this happens in outdoor pools, but sunlight compensates for that by helping to break down CCs), then the use of a weekly non-chlorine shock (potassium monopersulfate) makes sense. That would also virtually eliminate the creation of DPBs as well by oxidizing organics before chlorine has a chance to combine with them. The downside is that there will be a buildup of sulfates over time due to the non-chlorine shock, but I don't see a way around that (so periodic regular dilution would be wise).

I know that my wife would be extremely happy if the indoor pool she went to used CYA since every winter (and it's been 4 years now) every single one of her swimsuits she uses in the indoor pool get degraded. They don't all fade because she uses "chlorine-resistant" swimsuits, but the rubber degrades or the fabric gets thin or other effects occur and sometimes there is even some visible fading. I finally checked their water and they did NOT have unusually high FC levels. They were below 3 ppm FC every time I checked (usually around 2-3 ppm FC), but had no CYA. The pH was normal and there was some CCs (it varied each time, but were about 0.5 - 1.5), but that is expected since I don't think they are using non-chlorine shock and the pool was always heavily used.

Richard

[chem geek]

Watermom,

In this thread you mention how pouring bleach down the skimmer with the pump running is not a problem and from a practical point of view I would tend to agree, but do want to be more specific about the potential for harm.

If we assume a GPM rate of 40 when the pump is running, then 6% bleach is around 60,000 ppm FC and if it is poured slowly at a rate of about 5 seconds per cup, then that's 1.33 minutes per gallon (0.75 GPM) so this is a dilution ratio of 53 to 1 so an FC level hitting the pump, filter, etc. of about 1000 ppm FC (it's higher in the PVC line from the skimmer, but I'm assuming that all gets mixed together with the water from the other return lines; the GPM rate at the skimmer will be lower, but it will get diluted later so it nets out to the overall GPM rate of the system). This isn't all disinfecting chlorine since the pH will be high. I calculate that with normal pool conditions (pH 7.5, TA 100, CYA 30) that the pH will rise to about 9.3 and the disinfecting chlorine level will be 13.8 ppm. For comparison, the recommendation for the mid-point of Ben's Min/Max is 0.05 ppm and for shocking is around 0.3 ppm so the disinfecting chlorine level is indeed quite high and is similar to having 27 ppm FC with no CYA.

The time for exposure is probably only around 30 seconds to a minute depending on how much is being added, but this could happen every day.

I am bringing this up not only for the issue of how to pour chlorine into the pool, but to address concerns with using a liquid form of chlorine in vinyl pools. Some pool builders swear that using a sodium hypochlorite will damage vinyl pools and some say to use lithium hypochlorite instead. Though more expensive, it dissolves quickly and can be broadcast around the pool so is not so concentrated in one place. I'm guessing that pouring chlorine quickly in one place may have the chlorine settle briefly near the bottom (it's denser than water so may settle until it mixes). Clearly this can happen with Cal-Hypo which is why it should get mixed in a bucket before adding to a vinyl pool. I was hoping that pouring sodium hypochlorite slowly in front of a return would be the best approach, especially in the deep end where the water flow is strong from return to skimmer and return to floor drain(s). It's almost impossible to calculate what happens, but I can imagine that slowly pouring in front of a return such that very little chlorine avoids hitting the flow would cause the chlorine to rapidly disperse into a larger volume of water and therefore get diluted rather quickly and therefore be safer.

No conclusions here -- just putting out some info.

As for Cyanuric Acid dissolving in the skimmer, that's a whole other matter since it dissolves so slowly. In this case, it may take many hours to dissolve so if it took as long as one turnover of water, then it essentially would not cause any higher concentration of acid or CYA at all. Of course, the pump must be running continuously or else there can be a localized buildup of acidity.

A Trichlor tablet in the skimmer may also be safe for the same reason, so long as the pump is continually running. I don't remember how long it takes for a tablet to dissolve, but I do think it's hours.

Richard