Breakpoint Chlorination

This thread is to discuss breakpoint chlorination. Does it work? If so, under what conditions? If not, then how do you get rid of Combined Chlorines?

Let's start by answering a topic Evan (waterbear) had from this thread.

Quote:

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Originally Posted by waterbear

I came across an 'old time pool guy' recipe for reaching breakpoint. Not recommending you try this it and Richard (chem geek) or Ben might want to comment on this.
(Ben, feel free to move this to china shop if it really should be there!)
For each 20000 gallons add 1 gallon of anhydrous ammonia and 8 gallons of 12.5% liquid chloine (this will raise the FC 50 ppm!) with pump off for 24 hours. Turn pump back on and let chorine levels drop to normal.

Seems like overkill to me!

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The first thing to do is to realize the confusion that arises from the 5:1 or 10:1 quantities quoted for the amount of chlorine (FC) needed to breakpoint combined chlorine (CC). It really takes one chlorine to produce monochloramine (NH2Cl) and another to breakpoint this to nitrogen gas (2NH2Cl + HOCl --> N2(g) + 3H+ + 3Cl- + H2O). You can instead produce dichloramine (NHCl2) which would then take two chlorine to go to trichloramine (NCl3) and then nitrogen gas. So for safety, it takes 3 chlorine (moles or molecules) to ensure breakpoint, starting from ammonia with no chlorine.

The molecular weight of hypochlorous acid is 52.46 while that of ammonia is 17.03 so there is a weight difference of 52.46/17.03 = 3.08 so this is where we get 3*3.08 = 9 which is about 10 for the 10:1 relationship in ppm. If you already have monochloramine, then you only need 2*3.08 = 6 so I'm not sure where the 5:1 comes from except it probably assumes that not all of the monochloramine will go to NHCl2 and NCl3 and that some will breakpoint using less chlorine with the other pathway I showed.
[EDIT] CORRECTION: Chlorine is measured in ppm Cl2 (chlorine gas equivalent) with a molecular weight of 70.906 g/mole while ammonia is measured in ppm N (elemental nitrogen) with a molecular weight of 14.0067 g/mole so there is a weight difference of 70.906/14.0067 = 5.06. The normal breakpoint process has 3 chlorine for 2 ammonia (2 of the chlorine convert 2 ammonia to monochloramine and then these two combine with an additional chlorine to produce nitrogen gas) so 5.06 * (3/2) = 7.6 and this is the actual stochiometric ratio. However, because other reactions can occur, the actual ideal ratio is between 8 and 10 so to be safe 10 is what is quoted. [END-EDIT]

One gallon of liquid ammonia (anhydrous, with no water in it) weighs about 5 pounds which in 20000 gallons is (5 lb.) * (453.59237 g/lb.) * (1000 mg/g) / ( (20000 gal) * (3.7854118 l/gal) = 30.0 ppm.

Eight gallons of 12.5% liquid chlorine is (8 gal) * (3785.4118 ml/gal) * (0.125 g NaOCl / g solution) * (1.16 g/ml 12% NaOCl solution) * (52.46 g/mole HOCl) * (1000 mg/g) / (74.4424 g/mole NaOCl) / (20000 gal) * (3.7854118 l/gal) = 40.9 ppm
I believe you got to 50 ppm by neglecting to account for the difference in molecular weights between sodium hypochlorite (NaOCl) and hypochloric acid (HOCl) assuming that the 12.5% refers to the percent weight of sodium hypochlorite and you also neglected the density of the 12% NaOCl solution. You probably just did the simple calculation of (8/20000)*0.125 which is a fraction so ppm multiplies this by 1 million to get 50.

The mixture of 30 ppm ammonia with 40.9 ppm of chlorine will likely result in only some of the ammonia getting converted to monochloramine and nothing will get to breakpoint! Maybe this old-time pool guy was using a diluted ammonia that really was not anhydrous. To achieve breakpoint, you should have 3*3 = 9, so round and say 10 times as much ppm chlorine as ammonia.

Richard

Quote:

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Originally Posted by chem geek

Eight gallons of 12.5% liquid chlorine is (8 gal) * (3785.4118 ml/gal) * (0.125 g NaOCl / g solution) * (1.16 g/ml 12% NaOCl solution) * (52.46 g/mole HOCl) * (1000 mg/g) / (74.4424 g/mole NaOCl) / (20000 gal) * (3.7854118 l/gal) = 40.9 ppm
I believe you got to 50 ppm by neglecting to account for the difference in molecular weights between sodium hypochlorite (NaOCl) and hypochloric acid (HOCl) assuming that the 12.5% refers to the percent weight of sodium hypochlorite and the density of the 12% NaOCl solution.
Actually, I used BleachCalc to figure it out...I'm lazy!
Maybe this old-time pool guy was using a diluted ammonia that really was not anhydrous. To achieve breakpoint, you should have 3*3 = 9, so round and say 10 times as much ppm chlorine as ammonia.
This acually came from the book "The Ultimate Pool Maintenance Manual, second edition by Terry Tamminen. 7th printing McGraw-Hill 2001. The author suggested that monochoramines would form and help kill any algae that might be present. However, he did give it as a recipe for breakpoint chlorination.

Richard

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Just goes to show how much misinformation there is about pools. This book is considered to be one of the "best" referrences for pool maintenance professionals generally available! I have found that much of the info is quite useful but much of it is just a rehash of 'old wives tales'. The problem lies with sorting out the truth from the tales!

Richard,
Are you saying that BleachCalc is wrong? It does indeed return 50ppm when you enter with 8 gallons of 12.5% sodium hypochlorite in 20000 gallons of water.
k

First regarding the book that talks about forming monochloramines to help kill the algae, well this is true that you'd be forming a LOT of monochloramines with that method. But you WON'T achieve breakpoint chlorination without adding a lot more chlorine (unless I screwed up in my calculations).

As for BleachCalc, yes it is "technically" wrong, but as you can tell it's not wrong by much and the error in measuring cups plus the degradation of chlorine over time means that you probably do need to use a little more to end up with what you expect. WHOOPS! This means the error in BleachCalc is in the wrong direction! BleachCalc probably has you add even less than you need rather than more.

Also, the density of the 12.5% liquid chlorine being 1.16 g/ml is not true for bleach which is probably closer to water density, probably around 1.07 g/ml, so BleachCalc would be even a little more "too high" in this case (that is, reporting that the change in ppm is more than it really is). I have verified my calculations with my own pool and the use of 12.5% chlorine to the extent possible, but this isn't easy since I do suspect that the chlorine does degrade somewhat at the pool store.

Mmmmm. I think we need to verify this with some more real-world experience before we get Michael to change BleachCalc. What is your experience?

Richard

My experience with BleachCalc and chlorine is zero. I have never needed to add bleach to my pool with the SWG. Never had any any CC to speak of!

Quote:

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Originally Posted by waterbear

My experience with BleachCalc and chlorine is zero. I have never needed to add bleach to my pool with the SWG. Never had any any CC to speak of!

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Oh yeah, I forgot. Rub it in, why don't you!:D One of these days I'll join the SWG club...one of these days...

Just stating the facts....I had my nephew, his wife and their three kids staying with me this week (they left yesterday) and I checked the pool once during the week (Wednesday) after a day of heavy use and there was still no CC. (I didn't check the water until I got home from work at 1 am and the pump does run for 3 hours in the evening but it seems that was enough to take care of any that might have formed. I also suspect that my 3.5 year old grand nephew might have 'gone number one' in the pool that afternoon from an overheard converstaion between him and his mother in the pool while I was geting ready to go to work....that's why I checked it that evening!

The consensus so far on this thread seems to be that people manually dosing chlorine shock from zero to 3 times per season -- most only once or twice. So assuming some of them also have kids with, well, #1 releases, my guess that a regular manual dosing of chlorine is pretty similar to what an SWG does may be correct. It's obviously not as good since even once or twice a day chlorine addition isn't the same as several hours each day, but it does show that, as CarlD says, "Stay ahead of your water!" really works.

Richard

That small error in the BleachCalc alogorithm may explain why the Best Guess Table seems to sometimes understate the chlorine needs for reaching "shock levels". Some people may not actually ever be getting to the breakpoint if they're dosing based on BleachCalc. Or would that be overstating the effect?

I certainly don't want to sound ungrateful for Michaels fine software. It has made the pool maintenance part of my life much simpler on many occasions.

I don't think it is BleachCalc that is the root cause of any underestimation of how much chlorine is needed to shock, though you are correct that if I am right about the error, then it will underestimate how much to add. I believe the much bigger potential problem (that we don't know for certain yet) is that Ben's table tracks reasonably well with the ppm HOCl theory for his "min" and "max" columns, but not well at all for the "shock" column. Look at my FC.htm chart and you can see that (ignoring very low CYA since the required FC is so low as to be impractical to maintain) the 0.02 ppm column roughly correlates to Ben's "min" while the 0.05 ppm roughly corrleates to Ben's "max", but there is no good correlation for his "shock" column. We don't know which way the error is, if any, but if I were to hazard a guess, I would say that the proper "shock" level is at around 0.5 ppm though possibly 0.2 ppm *might* do the trick in some cases. If 0.5 ppm is correct to kill algae, then the high CYA levels in Ben's chart underestimate the amount of chlorine truly needed.

Only real-world experience will tell what we should do to go forward -- if the ppm HOCl is valid for the shock to kill algae and if so, what level is appropriate.

Richard

side bar here:
since you're using metric & imperial for these calculations..
is this imperial gallons or US gallons?

( I always wonder which it is that you guys are actually using... it's so confusing...lol)

Even though beachcals says metric or imperial Michael stated in one of his posts that it is US gallons. He said he will fix that in the next version.

Okay great,
I was specifically refering to the calculations on the previous page of this thread...
( Mls to Gallons & such)

- so when people here say, " my pool is Blah Blah gallons", they mean US Gallons (?) & that is the standard measurement for you USAers?

good to know; Us Canadians get a wee bit confused on the whole gallons issue :rolleyes:

another good reason why we should have all gone metric years ago!

I agree 100%
I can't fathom why the USA wants to keep up this old system,
it makes calculations quite difficult, particularly when comparing to international testings

I was using and assuming U.S. gallons in my calculations. In my spreadsheet I have a switch for metric so instead of U.S. gallons its liters. However, should I also have something for Imperial Gallons? Is that commonly used for pool volumes in the British (shrunken) Empire?

My understanding is that if you are part of the " commonwealth" then you use Metric
...
I was a travel agent for a bit ( one of the many things I've done...lol), & I can't think of anybody else ( off hand) that uses imperial over metric ( other than the USA)...

I believe the scientific community has chosen Metric. period. (lol);)

Actually, when you say no one else uses Imperial over metric except the USA, I think you may not realize that there are THREE systems at work here. The metric system, the Imperial system, and the U.S. system. In the Imperial system, a gallon is 4.54609 liters both for liquid and dry quantities. In the U.S. system, a liquid gallon is 3.785411784 liters while a dry volume measure "equivalent" is called a bushel and is 35.23907016688 liters. What makes the U.S. system even more confusing is that there are liquid quarts which are each 0.946352946 liters AND there are dry quarts that are 1.101220942715 liters (and yes, that implies that there are 4 liquid quarts in a gallon but 32 dry quarts in a bushel.

Of course, there is further insanity with regard to lengths and weights and a decent summary of all of this is found at "this is insane".

We tried to go metric in the U.S. way back when, but too many people's brains hurt too much and being a democracy, well, you get the idea. Politicians would rather get re-elected than to do the right thing.

Richard

you're right of course... i should have said, "no one uses the US imperial system but the USA"...

but do you know of any country that actually uses the old imperial system anymore?
or any other country that uses US imperial?

Hmm, does Puerto Rico use it?... what about the virgin islands ( both british & US...two different systems or all metric)?

I can't think of anyone else that uses imperial systemS ...

maybe it's time to start a converting campaign....
If people could take a moment to look at metric, they might find the math a little easier.
( what if you just stopped teaching US imperial in school?..lol, a new generation could readily accept it...Wake-up USAers & join in with world unity...lol)

- thank goodness this is already in the china shop or I'd be banned...lol

ps- I think there is nothing wrong with USAers btw, & I'm not trying to start any political discussions....

( Please note that I said " USAers" because I personally find the term " American" as offensive,
seeing that we are ALL Americans... Canada, USA, & Mexico... not to mention South American & central American countries too...lol):p

I know of no country using the (UK) Imperial system anymore. And I took no offense in anything you said. Heck, I complain about our country as much as the next guy; I'm just happy I'm able to complain about it without getting locked up (yet)!:eek:

I've done some more research on what the "12.5%" number is supposed to mean and it is used inconsistently in two different ways. Sometimes it appears to be used in the way I wrote, namely as 12.5% by weight of sodium hypochlorite. But other times it is claimed to mean "12.5% available chlorine" where "available chlorine" is normally defined to be relative to chlorine gas (Cl2) at 100% available chlorine.

So, while the first weight definition would divide the weight of chlorine you add by 74.4424 g/mole NaOCl (and multiply by 52.46 g/mole HOCl), the second definition would divide the weight of chlorine by 70.9064 g/mole Cl2 (and still multiply by 52.46 g/mole HOCl). This at least explains how 12.5% NaOCl becomes 12.5% * 70.9064 / 74.4424 = 11.9% or about 12% available chlorine which I have also sometimes seen.

The fact is that most of the Material Safety Data Sheets (MSDS) that I saw for chlorinating liquid show a range of 11-15% so I don't think anyone expects the actual amounts to be precise. The 12.5% is somewhat of a "target" goal and at least I know that the MSDS means that this is the weight percent of the actual component listed which in this case is sodium hypochlorite (NaOCl).

The Clorox liquid bleach MSDS lists 5.25% sodium hypochlorite for most bleach products. Again, this would represent a weight percentage of sodium hypochlorite so following the BleachCalc program for volumes that you put into your pool would lead to lower ppm results than desired. Likewise, if one entered in a volume into BleachCalc then the reported ppm would be higher than what would actually be acheived. [EDIT] An update: Ultra Clorox Regular bleach says 6-7.35% and Clorox Regular Bleach lists 6.15% so the 5.25% that I saw was probably for older bleach -- the latest numbers come from the Clorox website. The density (specific gravity) is listed as approximately 1.1 for both products and the pH is listed as 11.4. [END-EDIT]

Interestingly, in this thread CarlD equates 1.88ml in 5 gallons of water as producing a ppm value that should be the same as in the original bottle. (5 gal) * (3785.4118 ml/gal) / (1.88 ml) = 10067 or about 10000 which is 1/100th of one million so the measured ppm would indeed measure the % HOCl in the original source if the densities were the same, which they are not (the chlorinating liquid is about 1.16 g/ml while pool water is closer to 1.00 g/ml). He found that it was closer to 14% for his 12.5% chlorinating liquid which is the opposite of what I would have expected. If we divide by 1.16 then his 14ppm becomes 12ppm which is a little more like it and implies an original 14.7% which is quite possible given the nominal 11-15% range.

Obviously, more work is needed to get to the bottom of this.

Richard

Wow! And all I was trying to do was determine if the chlorine in the jug had degraded significantly!

The measure of 1.88ml/5gal can also be done in metric: 1ml/10liters--5 2 liter soda jugs.

Metric vs English (Imperial implies Imperial gallons)?

It's a mistake to say we abandoned the Metric system as too hard. It's creeping up on us in many ways. If you work on cars or motorcycles, almost all the wrenches you will use will be metric. 20 years ago I had no idea what a 12 mm wrench was, vs a 17mm. Today an image pops into my head automatically. Soda is sold in 2 liter jugs, 1 liter jugs and even 3 liter jugs. ALL medications are administered in metric measures and only sometimes, for liquids (like cough medicine) is there a teaspoon equiv. You can even use the bleach calculator formulae in metric: Just measure your pool in liters and your bleach in liters and it works out the same.

In the UK, they thankfully STILL keep one imperial measure--the British Pint!:p They also, curiously measure body weight in Stone, rather than pounds or kilos.

Yet another update in this continuing saga. This thread seems to indicate that perhaps the "ppm Chlorine" definition is based on the Chlorine gas (Cl2) equivalent where chlorine gas is 100%. I know that this is what is used for what is termed "available chlorine" when comparing different chlorine sources, but I did not know if that is what is also used for measuring chlorine from the test kits. This seems doubful and strange, but this wouldn't be the first time an unusual convention is used in the pool industry. I'll keep y'all posted. [EDIT] So far, most every source I can find says that ppm chlorine has the "part" in part per million be the weight of chlorine and the context has always been HOCl + OCl- (CYA is not talked about so Cl-CYA is included as if it were HOCl/OCl-). [END-EDIT]

[SECOND-EDIT] I found the answer! It's at this website where it says "Although Cl2 does not exist in potable waters, for historical reasons FAC, CAC and total chlorine are still expressed as mg/L as Cl2" so I need to correct my spreadsheet. This will somewhat effect all of my graphs, tables, etc. calculating ppm HOCl from FC (the FC numbers will go up by 35% holding everything else constant). The points and principles won't change, but some of the specific numbers will. I'm sure Michael will be very relieved to know that the proper adjustment to his algorithm in his program is quite minor and can possibly be ignored. [END-SECOND-EDIT]

Richard

This is a continuation from this post where the comments in bold are from waterbear (Evan)

Quote:

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Originally Posted by chem geek

I don't have a salt cell, but even at full power and running 24/7 you are right that the increase in chlorine levels attained in the pool will be much slower than dumping liquid chlorine into the pool. However, if one is starting from an existing free chlorine level then whatever chloramines that would have been formed have already done so.
If the organic load is higher than the chlorine available then more chloramine would form as the chlorine was increased, correct? From my understanding it is only after all the ammonia and organics present have combined that breakpoint can be reached.
Free chlorine combines with ammonia very, very quickly to form chloramines. It is the next steps to breakpoint that are slow (and free chlorine oxidizing organics can also be slow). So from a chemistry point of view, I don't think the slow increase in FC from a salt cell would be an issue unless the FC were at or near zero for some reason -- perhaps if there were a major ammonia (i.e. urine) accident that overwhelmed much of the free chlorine in the pool, remembering that you need about 10 times as much FC to achieve breakpoint as there is ammonia.

My understanding of the chemistry is that incomplete breakpoint (from a slow rise in chlorine) would favor the formation nitrogen trichloride (even at higher pH) and if the organics are from complex sources such as algae and not just ammonia (which is often the real world case when we 'shock') then it would cause the formation of other disinfection by products such as organochloramines, which are difficult to break down. Also reaction would not go to endpoint, which would be the release of nitrogen gas as the ammonia is broken down but cause the formation of nitrates in the water, which are a souce of food for algae.

Urine is about 2.5% urea by weight and urea is about 50% nitrogen (ammonia) so 2 cups of urine produces around 5 grams of ammonia and that requires about 50 grams of chlorine for breakpoint. A 10,000 gallon pool is about 38,000 liters fo 50 grams of chlorine in 38,000 liters is 1.3 ppm. So depending on size of pool, FC level, and size and number of accidents, one could use up all the FC in a pool. And certainly one uses up the FC in a local area rather quickly.

So if you need to superchlorinate because of an "accident", then I agree that quickly administering a large dose of chlorine is wise so that breakpoint is more easily achieved. If instead it's just a small amount of measured combined chlorine that has accumulated, possibly from slow combining with organics (not ammonia), then a slow rise in chlorine would probably be fine. Of course, the issue of the life of the salt cell is real and adding liquid chlorine is easy and relatively inexpensive.

[EDIT] In the presence of CYA, the breakpoint chlorination process is slowed down considerably since CYA reduces the disinfecting chlorine concentration. A normal breakpoint at an FC of 2.0 ppm with no CYA takes around 30 minutes to effectively complete so with 30 ppm CYA this would take about 11 hours if there was no sunlight. I don't know how much faster breakpoint goes with sunlight. [END-EDIT]
Once again this seems to indicate that the slow rise in FC caused by superchlorinating with the cell is not an effective way to reach breakpoint.

Richard

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It is true that since the formation of monochloramine (chlorine plus ammonia) is a very fast reaction that this will occur before any formation of dichloramine or trichloramine and that these occur roughly in sequence because each one is created from the previous one. You are also correct that if the chlorine demand from ammonia and organics is larger than the amount of chlorine, then all of the chlorine would get used up and a combination of monochloramines (and chlorinated organics) and leftover ammonia (and organics) would result.

It is not at all necessary to form anything beyond monochloramine to get to breakpoint, assuming you have enough chlorine available. The following reaction can "break" monochloramine directly:

2NH2Cl + HOCl --> N2(g) + 3H+ + 3Cl- + H2O

Monochloramine and dichloramine are the dominant species between a pH of 4.5 to 8.5 though monochloramine is most dominant above a pH of 8.0 while trichloramine is dominant below a pH of 4.5. So, the ideal pH for breakpoint chlorination would be between 8.0 and 8.5. However, normal pool pH above 7.0 and especially around 7.5 will mostly form monochloramines with some much smaller amount of dichloramines.

At lower pH, there is more HOCl present (relative to OCl-) though there is less NH3 present (relative to NH4+). Since the reaction time for the conversion of free chlorine to monochloramine is slower at lower pH, it would appear that the NH3 concentration is the dominant factor. At a pH of 7 and 25C temperature with 2x10^(-4) moles/liter HOCl (actually 14.2 ppm Cl2) and 1x10^(-3) moles/liter NH3, it takes 0.2 seconds to form monochloramine.

What is not clear to me is whether the formation of di- and tri-chloramines at lower pH is due to the slower reaction rate for the formation of monochloramine or specifically to the higher hydrogen ion concentration. I'll continue to research this to see if I can figure this out since clearly the presence of CYA significantly reduces HOCl concentration and would slow down reactions involving that species, so could significantly affect how breakpoint chlorination works (or doesn't). The CYA would not affect the stoichiometry, so there can still be sufficient chlorine to achieve breakpoint, but the slowdown in the reactions could produce different products. I have also not found literature on the photolytic breakpoint to explain whether sunlight can also breakdown combined chlorine, though the reference I give later in this post does refer to monochloramine being prone to decomposition by light and heat.

So the bottom line at this point is that we don't know what's going on. It's one thing to have a pure chlorine and ammonia mix, but quite another when CYA is present, or when sunlight is present, etc.

[EDIT] Here's an interesting link that summarizes both existing literature on breakpoint chlorination and gives some real-world measurements, albeit with no CYA (since it's for sewage treatment). I'll see if I can work with the rate and equilibrium constants to come up with some meaningful conclusions. [END-EDIT]

Richard

Quote:

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Originally Posted by chem geek

[EDIT] Here's an interesting link that summarizes both existing literature on breakpoint chlorination and gives some real-world measurements, albeit with no CYA (since it's for sewage treatment). I'll see if I can work with the rate and equilibrium constants to come up with some meaningful conclusions. [END-EDIT]

Richard

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Richard, Funny you should post that link. That is one of the sources that I was looking over before posting my comments!;) I am continuing to research this also. There really does not seem to be any definative answers from what I have been able to find and there are a lot of 'real world' variables that need to be taken into consideration. My main interstest would be if a slow rise in chlorine over a, say 24 hour period, would be as effective as a fast addtion of chlorine all at once to reach breakpoint since this the main difference between shocking with a SWG and shocking with the manual addition of chlorine. It might be interesting to also try and determine what difference shocking at night vs. shocking during the day might produce since from what I have been able to research does indicate that sunlight is effective in breaking down the chlorine-ammonia bond (at least for the lower, non-organic chloramines). It does seem that most of the research on the subject is from the field of sewage treatment and not specifially pools.

Edit: I was able to find some info that indicates that incomplete oxidation of organic ammonia compounds by chlorine will lead to the formation of trihalomethanes (chloroform seems to be the most commen one formed in pools) which are very difficult to break down by normal means. Not sure what effect sunlight would have on them.

Evan,

Here is another interesting source of information giving rate constants for a variety of reactions. I verified that these constants are consistent with those in the earlier link (note the constants are in "per hour" rather than "per second"). Unfortunately, some of the data in this table is incomplete and some seems just plain wrong as there is no path to breakpoint that occurs in a reasonable time. The original paper I read on CYA equilibria was in a book that also had breakpoint data that pretty much had breakpoint occurring in around 30 minutes or so for 0.1 - 1.0 ppm ranges of chlorine and ammonia. If you find anything that makes sense, let me know. At least I've got logical reaction times even with CYA for monochloramine (seconds) and dichloramine (minutes) formation. I just need the rest of the breakpoint times so I can figure out dominant species.

As for the THMs, these appear to occur when organics are present and happen more when the pH is [EDIT] higher [END-EDIT]. It isn't so much an incomplete breakpoint that makes them happen as just having chlorine available to combine with such organics -- if there's enough chlorine, then THMs can form. The newer approach to water treatment using chloramines avoids the THMs because the chlorine is all consumed with the fast reaction to form chloramines and doesn't combine with organics. That's not what we do in pools as we usually have enough chlorine for continual breakpoint (hopefully).

At higher pH, including those of typical pools, there are [EDIT] more [END-EDIT] THMs formed, but [EDIT] fewer [END-EDIT] VOC (general volatile organic compounds) so there doesn't seem to be any way around getting something unless you use KMPS non-chlorine shock to get the organics oxidized before chlorine can combine with them. Fortunately, the rate of production for these nasties is low, especially with the use of CYA that makes effective chlorine levels very low, so whatever gets produced gets swept away by a light breeze quite readily. The same cannot be said for indoor pools that have no CYA and therefore high chlorine levels and generally poor air circulation. This is why I propose either using KMPS in indoor pools (as a preventative weekly oxidizer) and/or use a small amount of CYA (< 10 ppm) to keep the chlorine levels lower (while still being able to maintain a large FC residual). This use of CYA would also make swimsuits last longer which I know my wife would appreciate (the rubber in her suits deteriorates in the winter when she uses an indoor pool -- never had a problem when using our outdoor pool in the summer).

Richard