No CYA in Indoor Pools & Resulting Air Quality

[chem geek]

This thread is a response to this post some of which is quoted below.

ChemGeek,
The HOCl (as ppm Cl2) is still in the green zone so presumably safe and if maintained little or no algae, assuming a SWG system or gas injection system, or am I wrong?
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The other cultural consideration is that Europeans do not bathe as often as persons living in the US so having a high ORP of 750 might be beneficial to other swimmers in the pool in that it helps to avoid cross infection. Although strangely, generally speaking, Germans bathe much more often than many other Europeans.
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One thing got me concerned and that was your mention of nitrogen trichloride (trichloramine), based on this I would be vary wary of sending young children to an indoor pool, especially one that does not have good modern ventilation, or is public. See here:

http://en.wikipedia.org/wiki/Pool_chlorine_hypothesis

My spreadsheet (here) does not have the HOCl turn red except when it is low. It stays green at high levels as there is no single number indicating safety, unlike the rough limits for certain bacteria kill rate goals or algae inhibition. So you can't go by any color coding and generally just want to use the minimum amount of active chlorine necessary for your purposes, which usually means the lowest amount needed to prevent algae growth since that requires higher active chlorine than killing most pathogens.

The German DIN 19643 standard apparently uses a reference bacteria of Pseudomonas aeruginosa with a required 3-log (99.9%) kill time (one source says 4-log, but I think it's wrong; all the others say 3-log) in 30 seconds. It's hard to find a consistent CT (chlorine concentration in ppm times time in minutes) value, but I've seen it range in studies from 0.05 to 0.16 for a 99.9% kill time. If we use the high number, then this implies a chlorine concentration of 0.16*/0.5 = 0.32 ppm. Other sources refer to a 750 mV ORP level, but to achieve that at a pH of 7.5 requires about 0.6 ppm using a Chemtrol sensor or 1.1 ppm using an Oakton sensor. So something is inconsistent here and I suspect that the ORP level for the required kill rate is closer to 700 mV. The thing is that a 99% kill rate in 1-2 minutes for most bacteria is probably more reasonable to prevent person-to-person transmission and that corresponds more closely to 650 mV (with an Oakton sensor) and to 0.1 ppm FC at a pH of 7.5. This is roughly the target FC of around 10% of the CYA level.

I'm well aware of Dr. Bernard's studies as well as many others all showing respiratory and ocular problems associated with indoor pools and most of these due to nitrogen trichloride (the most irritating and volatile of chloramines). There is also concern for trihalomethanes (THMs) such as chloroform, but that's more of a concern for long-term health effects such as cancer. I've written to Dr. Bernard (and others) about the chlorine/CYA relationship but I usually don't get a response (sometimes I do).

The production of nitrogen trichloride from the breakpoint of ammonia is modeled by the Jafvert & Valentine (1992) model. Earlier models from Wei & Morris and Selleck & Saunier come to similar conclusions, but have different absolute numbers for concentrations and the Jafvert & Valentine model is the newest and matches experimental data the best. I have a spreadsheet with these models and they show the following when there is much more chlorine than ammonia to oxidize (which is the normal situation one wants to have in a pool; otherwise a higher FC level should be used):

1) The rate of formation of monochloramine is proportional to the active chlorine concentration, but it's very fast regardless, in under a minute even at relatively low active chlorine concentrations.
2) The peak concentration of dichloramine and of nitrogen trichloride is roughly proportional to the active chlorine concentration and increases greatly at lower pH.
3) The final concentration of nitrogen trichloride is roughly proportional to the active chlorine concentration and increases greatly at lower pH. Monochloramine and dichloramine oxidize towards zero, but nitrogen trichloride stays around (doesn't further oxidize quickly) and is very volatile so tends to outgas.

On the other hand, high pH favors chloroform production if precursor molecules are present (chloroform does not come from ammonia; it comes from carbon-based organics).

The open question is what happens with urea as no clear model has been developed for chlorine oxidation of urea. Wojtowicz has speculated about some reactions and these produce both nitrogen trichloride directly as well as producing dichloramine that would then follow the breakpoint reaction with the characteristics noted above. Since urea is the primary component of sweat and urine (ammonia is the second largest component), knowing what happens with urea is very important so it's amazing that very little research has been done in this area. As I've noted in other posts, this article shows how Chip Blatchley of Purdue has a grant from NSPF to look at disinfection by-products in pools and part of this work is to develop a model for the chlorine oxidation of urea. I am very much looking forward to the results of that work.

A researcher named Samples in 1959 proposed the following mechanism to explain the products that he observed.

Cl₂NCONCl₂ + HOCl ---> NCl₃ + HCl + CO₂ + NCl
Quadchloro Urea + Hypochlorous Acid ---> Nitrogen Trichloride + Hydrochloric Acid + Carbon Dioxide + intermediate
NCl₃ + HOCl + 2H₂O ---> HNO₃ + 4HCl
Nitrogen Trichloride + Hypochlorous Acid + Water ---> Nitric Acid + Hydrochloric Acid
NCl + OH^- ---> NOH + Cl^-
intermediate + Hydroxyl Ion ---> intermediate + Chloride Ion
2NOH ---> H₂N₂O₂ ---> N₂O + H₂O
intermediate ---> intermediate ---> Nitrous Oxide + Water

Wojtowicz proposed another mechanism as follows:

Cl₂NCONCl₂ + HOCl ---> NCl₃ + NHCl₂ + CO₂
Quadchloro Urea + Hypochlorous Acid ---> Nitrogen Trichloride + Dichloramine + Carbon Dioxide

He then describes further reactions that are consistent with the Jafvert & Valentine model. He notes that chlorine and urea do not form (or measure as) Combined Chlorine (CC) and he also shows that the oxidation of urea by chlorine is fairly slow taking many hours to days depending on concentrations. This implies that the reaction rate of urea combining with chlorine to form quadchloro urea may be slow but that the above reaction may be fast or that the quadchloro urea (and other intermediate chlorourea) don't measure in the CC test. In any event, you can see that dichloramine is produced so would then follow the breakpoint chlorination model, but nitrogen trichloride is also directly produced so that part is an unknown as to reaction rates and concentrations. We don't yet know enough about which reaction steps are rate-limiting so we don't know if urea will show the same sort of chlorine dependence on nitrogen trichloride production. So half of the nitrogen trichloride will show such dependence (due to the dichloramine) but the other half is unknown. However, with a linear dependence on active chlorine concentration, this would be a linear result of amount of nitrogen trichloride per time period though the total cumulative concentration would be dependent on the amount of urea (again, for half of the nitrogen trichloride concentration). This is why I think that even with urea, lower active chlorine levels would be better and hence CYA in indoor pools would lower the rate of production and total amounts of nitrogen trichloride.

Richard

 

[smallpooldad]

ChemGeek,

This is beginning to go a little above my head. But if I reading your comment correctly my little "Swinehunde" ancestors, as my father use to say, are not giving us the whole truth. They are actually operating at 700 ORP not 750 like they are suppose to.

Well heck I get 700 ORP in my pool in Hawaii, with 84 F and high UV, and at a CyA of between 25-30, and FC of 4.4, giving an HOCl (as ppm Cl2) of around 0.077 . What is wrong with their thinking, why are they doing this and thank goodness I'm only half-German, from now on I'm swimming in dirty old highly chlorinated English pools, no wonder we (my English half) won the war in Europe, albeit with a little help from the US (my adopted home), it has to be high chlorine. That and the fact that in England, when I was in high school, we had to swim outside in 45 F pools, in Germany in the winter we swam in heated pools, not very manly. The English are so much more honest about their pools and swimming, I should have realized this. Oh, I'm so confused.

To think of all the bad things I said about my sister's pool in England. I will have to write and aplogise.

And no wonder the German economy is not as strong as the British economy, they are getting diseases from under-chlorinated, over heated swimming pools. "Ãœber alles", my little Wienerscnitzel, "Ãœnter, alles", more like it, or roughly translated, "Under, all". I have been led astray.

Maybe my pool genes are not German. Maybe Darwin was right, natural selection is faster than we think, maybe my pool genes have become a true red, white, and blue.

I get the outgassing of nitrogen trichloride; but in some older, or poorly designed, indoor pool buildings could it not be a problem? So would it still be better to steer away from them? What would be a good exchange of air on an hourly basis? Although I would only need to worry about this when I travel.

Lastly did I get this all wrong and will I have to send letters of apology to my German relatives and of course switch sides? Thank goodness I now live in America we even have an enormously popular song that mentions water, "America The Beautiful", with its glorious lyric of "From sea to shining sea", only a nation of swimming pool afficianadoes, even in the early 1900's, could have created such a magnificent tune and lyric. Can you imagine the English or Germans singing about their swimming pools, not likely.

Aloha and from now on I promise to be serious. But who is really telling the truth here?

 

[chem geek]

But if I reading your comment correctly my little "Swinehunde" ancestors, as my father use to say, are not giving us the whole truth. They are actually operating at 700 ORP not 750 like they are suppose to.

Well heck I get 700 ORP in my pool in Hawaii, with 84 F and high UV, and at a CyA of between 25-30, and FC of 4.4, giving an HOCl (as ppm Cl2) of around 0.077 .
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I get the outgassing of nitrogen trichloride; but in some older, or poorly designed, indoor pool buildings could it not be a problem? So would it still be better to steer away from them? What would be a good exchange of air on an hourly basis? Although I would only need to worry about this when I travel.

Since the absolute ORP reading varies so much by sensor manufacturer, I wouldn't give much weight to the 700 vs. 750. I don't have an actual copy of the DIN standard but most references talk about the kill times and not the ORP value so I really don't know what's in the standard with certainty (and it's not cheap to buy, but I may see if a library has a copy). The same thing about the reading on your sensor -- I wouldn't give it much weight in terms of an absolute measurement, though it's useful for automation of chlorine level once you define a setpoint.

You are correct that the traditional way of dealing with the air quality problem is to increase air exchange. It used to be that there weren't so many reported problems, but then energy efficient buildings started to lower the air exchange, get rid of leaks, etc. so exacerbated the problem. UV will breakdown nitrogen trichloride which, along with better air flow and lower active chlorine concentration due to CYA, is why this isn't a problem in outdoor pools exposed to sunlight. The issue I was raising was the possibility of a simple solution for cutting down nitrogen trichloride in indoor pools by using a small amount of CYA to lower the effective chlorine concentration. However, nothing comes for free and the amount of monochloramine would be higher longer similar to outdoor pools.

As for staying away from indoor pools, that's a personal choice. Sometimes we don't have much choice. During the winter, my wife swims in a community center indoor pool and they don't use CYA and they won't add any (I asked them) because they are told not to (or recommended not to) by the industry. So my wife is exposed to that every winter, but the bather load isn't extremely high and the air flow is apparently reasonable as she hasn't complained about respiratory or ocular problems though she can smell the pool a lot more than ours. Mostly what we see is that we have to replace her swimsuits that she uses each winter which we don't have to do with her summer swim suits. I don't like the situation, but there's not much I can do about it right now.

Richard

 

[smallpooldad]

ChemGeek,

As always thank you for your help and clear reply.

From now on I will just use the ORP value for setting the free chlorine level and not worry about kill times. In other words base my level on HOCl (as ppm Cl2), which can be easily defined using the pH, CyA, and FC level.

It was interesting to note your mention of indoor pools and sunlight, if we ever move from Hawaii to the mainland (highly unlikely), or build an indoor pool on the mainland (possible) as our children are on the East Coast, we will take that into consideration and try to incorporate windows and doors which can be opened, such as casement windows, for sunlight. Not only is it more decorative but it seems from your comment helpful in breaking down nitrogen trichloride UV rays will be allowed to react with it. Again, I hope I understand your reply correctly.

In England, when I was young 40 years ago, some people had their swimming pool building on roller tracks so that it could be open to the elements in the summer. I do not know if US building codes allow for such an arrangement, but it would be nice.



Aloha.

 

[Swimgirl]

I swam in a hotel indoor swimming pool in Northern Arizona which had a really nice arrangement. One whole wall of the room was glass. Had two nice large French doors in it, and the rest of it was just stationary glass panes. Anyway, since it was a nice Summer evening, they had both French doors open so that the air circulation was extremely good, and also during the day, the pool got quite a lot of sunlight. I could still smell the chlorine more than an outdoor pool, but it wasn't overpowering. :-D