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Key Takeaways:

  • In properly chlorinated pools and spas, breakpoint chlorination is continuous and does not get "stuck".
  • The rule for raising the FC to 10x the CC level is wrong (it is correct only when referring to chlorine oxidation of ammonia, not CC).  Higher FC will make chlorine reactions go faster, but the actual required amount of FC to complete oxidation of monochloramine is 0.5 to 1x the CC level while for monochlorourea it is 2-3x the CC level.

 

The chlorine oxidation of ammonia goes through a phase known as "breakpoint chlorination" (technical details in this post).  This only occurs when one is adding chlorine to ammonia (or to already existing monochloramine) to have it oxidized.  Initially, chlorine (hypochlorous acid) combines with ammonia to form monochloramine very quickly, in seconds to a minute or two (depending on active chlorine level).  As additional chlorine is added, it more slowly reacts with monochloramine to form dichloramine.  When even more chlorine is added, nitrogen trichloride (aka trichloramine) is formed and reacts with the dichloramine to not only get oxidized to nitrogen gas, but also to release hypochlorous acid.  This occurs over 10 minutes to several hours, depending on active chlorine concentration.  This is the "breakpoint" where the built-up monochloramine drops and no more chlorine needs to be added.

 

However, the above is not what happens in properly managed pools and spas where there is always some Free Chlorine (FC) available.  In this situation, breakpoint chlorination is continuous.  The pH and active chlorine concentration determine the rate of most of these reactions as well as the relative quantities of the monochloramine, dichloramine and nitrogen trichloride.  Nothing gets "stuck" by not using a high enough chlorine level so long as there is always measurable Free Chlorine (FC) in the water. -- things just happen more slowly.

 

The industry rule regarding raising the FC level to 10x the CC level in order to reach breakpoint is wrong on several counts.  Not only is it wrong based on continual breakpoint, but it is wrong because 1) CC is not ammonia, but monochloramine (or chlorourea) so already contains one chlorine in it and 2) the units of CC are ppm Cl2, not ammonia units of ppm N.  There is a factor of 5 difference between the units of CC and that of ammonia.  The 10x rule came from the following net equation for chlorine oxidation of ammonia:

 

3HOCl + 2NH3 ---> N2(g) + 3H+ + 3Cl- + 3H2O

Hypochlorous Acid + Ammonia ---> Nitrogen Gas + Hydrochloric Acid + Water

 

The molar ratio of chlorine to ammonia is 1.5:1 or 3:2, but since ammonia is measured in ppm N units while chlorine is measured in ppm Cl2 units, with the factor of 5.06 difference this is a ppm ratio of 7.6 to 1.  Because forming dichloramine requires 2 moles of chlorine for 1 mole of ammonia and because of side reactions that can occur, the actual chlorine to ammonia ppm ratio is around 8-10x which is where the 10x rule came from.  However, this is wrong since CC is in ppm Cl2 units (so no factor of 5.06) and monochloramine already has 1 of the 1.5 chlorine attached to it already.  To oxidize monochloramine, it takes from 0.5 to 1.0 times the CC level.  Even if the CC were urea, it takes 2-3 times the CC level, not 10x.  Of course, the higher the FC level the faster reactions occur, but there is no magic 10x amount.

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Replies to This Discussion

Richard,

This is a start on a topic that was hinted at some of your other discussions, (Active Chlorine Level and Disinfection By-Products (DBPs), and in older articles by others, such as ( http://www.poolspanews.com/2009/051/051breaking.html), and am curious on your take on a discussion on the realities of Continuous Breakpoint? Does it really exist? Is it overstated, and all too often, mis-applied or insinuated by various claims made by companies trying to sell their products...such as chemical controller manufacturers? If it does truly exist, what are its potentials for positive attributes, and what are its potential negatives if approached the wrong way?

 

If so, what does that mean for many of our commercial pools "if" this "continuous breakpoint" could really be accomplished with relatively realistic low levels of chlorine, of let's say1.5 to  2 ppm, that is maintained 24/7 by HCF feeders, which are large enough to meet any demand as it is introduced...in minutes of actual feed time rather than hours.

Is the current chemistry, as perceived today through research, (and through your interpretations), supportive of this approach as being a sensible first step to use to help fight our many battles with chloramines and DPB's, or is it not really necessary or even desirable?

 

Are there drawbacks in this approach of rapid chlorine feeds... maybe...perhaps pertaining towards the dichloramine destruction pathways relating to higher formations of NCL3, due to the higher chlorine feeds in the pipe...or it's effects on urea and monochlorourea compounds...or in some other way you can think of? My sense is that their are more positives than negatives here, but still open on this. I've seen too many pools turn around to think otherwise, but not sure on the chemistry involved as to exactly why.

 

Now, in fair disclosure, I am a relatively small pool company, who craves information and products that will truly make a difference in how we run our pools. Currently, I'm not an advocate of medium pressure UV, but am on the fence for low pressure UV, and understand the principles behind your stance on chlorine /cyanuric acid ratios.  I do believe in HCF types of chlorine feeders, as I've seen too many pools show vast improvements, without the use of UV, and therefore, am a dealer for SureWater. http://sureh2o.com/solution.html . With that said, they are not the end-all solution for every pool, even though I truly think they could benefit every pool more so than not.

I am also supportive of using Sphagnum Moss, and would love to be a dealer for them, but currently, they are not in that mode yet to deal wth pool companies. I am also supportive of improved ventilation systems, and improved filtration/coagulation systems, as being equally important additions to our arsenals of modern day weaponry...all of which wouldn't be as necessary if we could train our patrons to be more hygienically responsible.

So long as there is measurable FC there is continuous oxidation of ammonia and many nitrogenous organic compounds.  That's chemical fact.  The whole idea of "breakpoint chlorination" comes from the experiments and charts where one starts with an amount of ammonia and then adds chlorine to it.  Initially, the chlorine combines with the ammonia almost immediately (in seconds -- with CYA in the water in a minute or two or with a lot of CYA in the water perhaps 10 minutes) to form monochloramine so all added chlorine disappears almost immediately.  Then adding more chlorine will more slowly have it react with monochloramine to go through more stages and one of those stages has the chlorine from the bound chlorine (from dichloramine and monochloramine) get released which helps the reaction continue to completion to generate mostly nitrogen gas, some nitrate, and a small amount of nitrogen trichloride.

 

However, this is not what one wants to happen in a pool -- that is, having so much ammonia introduced that it wipes out the chlorine.  In a pool, one wants the FC level to ALWAYS be maintained so that if ammonia is introduced by bathers there is always an excess of FC available.  In that situation, one never sees "breakpoint" pre se, but rather there is a continual oxidation of the ammonia.  You might see some fluctuation in the FC level, but so long as you don't let it get to zero, the oxidiation is continuous.  Yes, at lower active chlorine levels the process is slower, but it doesn't stop or get stuck.

 

The issue in a high bather-load pool is that there can be a buildup of intermediate bather waste products that aren't getting oxidized quickly enough.  With ammonia getting introduced and assuming you are adding chlorine fast enough to maintain a consistent FC and active chlorine level, then the buildup is mostly monochloramine and some dichloramine.  In the case of urea getting introduced, it much more slowly reacts with chlorine to form monochlorourea so the buildup in this case is mostly urea and then some monochlorurea.  Further oxidation after quadchlorurea is briefly formed (if that is the mechanism -- no one knows for sure) then goes through a path similar to ammonia oxidation where you can see monochloramine, dichloromine and nitrogen trichloride.

 

So the irony is that when the bather load is higher, then the buildup of some of the intermediates registers as Combined Chlorine (CC) which has one often raise the active chlorine level in order to accelerate its oxidation.  However, doing so will increase nitrogen trichloride production.  If the CC is mostly monochloramine and monochlorurea, then they may not be a real problem in the short run because, as you know, utilities often use 1 ppm or more of monochloramine with chloramination and hardly anyone really notices it in their tap water.  Monochlorurea is also likely to be well tolerated in the few ppm range.  However, if one doesn't oxidize or remove these intermediates, they will continue to build up if there is still high bather-load and at some point can get to a level that is a problem and where dichloramine is too high as well.  The problem is that the current 0.2 ppm CC limit is a standard based on high active chlorine levels (indoor pools with >2 ppm FC and no CYA).  At low active chlorine levels (say, because of CYA in the water or because one is using a DIN 19643 approach at the low end with 0.2-0.4 ppm FC) a higher CC is not usually a problem because even with a few ppm CC there is very little nitrogen trichloride being produced, but as I noted if the bather load stays high then the CC will continue to increase and will become a problem even with low active chlorine level.  Essentially, the rate of production of by-products is the product of the active chlorine concentration with the intermediates concentration so even if the former is low, if the latter is high enough the product gets too high.

 

The only real way out of this damned-if-you-do (i.e. increase active chlorine level but produce more nitrogen trichloride and oxidize swimsuits, skin, hair, equipment, etc. faster) and damned-if-you-don't (i.e. keep active chlorine level low, but build up intermediate CC chemicals) is to remove either the precursors (such as urea) or intermediates (such as monochloramine or monochlorourea).  Coagulation/filtration tries to do this by physical removal (with chemical assistance from coagulants).  Supplemental oxidation systems such as ozone or MPS (or UV for breakdown of some chemicals, though not oxidation, per se) or advanced oxidation systems (e.g. boron-doped diamond electrodes that product hydroxyl radicals) try to do this by oxidation without producing chlorine disinfection by-products (because they are oxidizing without using chlorine).  Outdoor pools do better not only because of the better air circulation, but also because of sunlight's UV that breaks down chlorine into hydroxyl radicals (and chlorine radicals).  The hydroxyl radicals are very powerful oxidizers that help keep organic intermediates in check, at least at low and moderate bather loads, but at high bather loads even outdoor pools can get overwhelmed.

 

Basically, your HCF chlorine feeders fix the most basic problem of not keeping up with fluctuations in bather load that would lead to monochloramine buildup and zero FC.  However, beyond that they alone can only maintain whatever active chlorine level you choose, but that's about it.  It's just one parameter to play with but with higher bather-load you have to have the other approaches as you have mentioned.  By the way, even clean bathers would not be enough, though would certainly help.  That might just improve things by a factor of 2 or perhaps somewhat more, but won't be enough in a high bather-load pool.  Typical chlorine demand is around 2 grams per bather-hour if they are reasonably clean to begin with and you don't have other sources of chlorine demand (see below).

 

And then there are the other sources of chlorine demand and associated disinfection by-products such as bacterial biofilms in sand filters that Howard talks about.  That's an entire additional layer to worry about and address.

Until now, I've primarily been a "lurker" on the site. I've read almost every post on the site, and have one main frustration: "unshockable" chloramines. We have an 18,000 gallon therapy/instructional pool that sees an average of 150 bathers per day, and that commonly doubles in the summer. In the past, we'd had zero luck with textbook breakpoint chlorination. I'll post our base numbers at the bottom. Let's say if the combined chlorine (using Taylor FAS-DPD reagents) was reading 1.2ppm, everyone says get the free chlorine above 12 ppm and everything should be good, right? We had failed attempts (sometimes bringing the free chlorine up to 30ppm and letting it go overnight) where the combined would just stay there, or in many cases actually climb.

This was with maximizing the outside air & exhaust. We haven't actually gone for breakpoint chlorination according to the textbook in almost 8 months because of the multiple failed attempts. That was with maintaining the pH at 7.4-7.5.

The only way we've been able to control it (and I mean bare minimum, keeping the combined chlorine under WI state max of 0.8ppm) has been a combination of weekly enzyme use and dilution - instead of backwashing the filter for 4-5 minutes, I go for as long as 20. Whenever the pool is staffed with guards and the weather is conducive, we also prop the outside doors to help blow out the air.

I have also had some success with running the pool vac in the surge tank, on the idea that the cleanest floor in your house can only be a certain % cleaner than the dirtiest floor in your house.

Here's a breakdown of what has worked & what hasn't:

Breakpoint Chlorination: 20% success (1 in 5 attempts actually gets the combineds to drop)

Enzymes: 80% success (keeps things under control, but never gets combined below 0.4ppm)

Surge Tank Cleaning: 50% (when things get really bad, has gotten combined from 1.2 to 0.8)

Filter Sand De-grease: 50% (same results as surge tank cleaning)

We've entertained quotes for UV and Ozone systems, and once we are presented with the right numbers, we'll probably be forced to make a jump in one of those directions. Given our bather loads vs. pool volume, I don't know if we have much else to choose from. I look forward to hearing any other ideas. I've gone back & forth with Al on this in the past (thanks again Al, I do appreciate it!). Other than looking at just the bather loads, I'm also guessing that most of what ends up in our pool doesn't fit the classic ammonia breakpoint curve. If I had cats peeing in the pool, it'd probably be easier to manage than it is now! From what I've read so far on urea and many of the creatines & proteins and how they react with chlorine, simply going for breakpoint isn't going to cut it.

Here's a snapshot of the pool's stats:

18,000 gal, 88-89 degrees, depth 2.5 - 4.5 ft, 1900 cfm outside air

Filter flow rate ranges from 85 gpm super dirty to 110 gpm clean, most of the time 100 gpm

Free chlorine ranges 3.2 - 6.2 ppm depending on bather load, ORP 760 mV

Combined chlorine ranges from 0.4 to 1.0ppm, likes to stay right at 0.8 most of the time

pH ranges 7.3-7.5, TA 60-80ppm, Calcium Hardness 480 ppm, TDS 1450

Average daily bather load 100-150 winter, 160-250 summer, max 54 at one time (by code), usually 20-30

Sanitizer is 12.5% sodium hypochlorite with muriatic acid for pH control

Weekly to twice-weekly applications of SeaKlear PRS for filter aid

Weekly to twice-weekly applications of pool enzymes for supplemental oxidation

Thanks for the real-world example of persistent CC's.  You are absolutely correct that most of what goes into a pool is not ammonia which is what goes through the classic breakpoint curve and is also oxidized more readily, even in hours at low active chlorine levels and faster at higher levels.

 

I just happen to post something related to this in a LinkedIn discussion so I'll copy some of that here.  The following Table 4.1 in the WHO "Guidelines for Safe Recreational Water Environments" Volume 2 "Swimming Pools and Similar Environments" shows the primary nitrogenous compounds in sweat and urine:

Nitrogen- ........... Sweat ............................ Urine
containing ..... Mean content ............... Mean content
compounds .... mg/l .. % of N ............... mg/l .. % of N

Urea ............... 680 ..... 68 ................. 10,240 ..... 84
Ammonia ....... 180 ...... 18 ...................... 560 ..... 5
Amino acids ..... 45 ........ 5 ...................... 280 ..... 2
Creatinine .......... 7 ........ 1 ...................... 640 ..... 5
Other ............... 80 ........ 8 ...................... 500 ...... 4
compounds
Total nitrogen . 992 .... 100 ................ 12,220 ... 100


Urea is by far the largest component and is also very slow to oxidize when there is no exposure to sunlight as described in "Reaction Mechanism for Chlroination of Urea" by Blatchley and Cheng.  I speculate that the UV in sunlight in outdoor pools breaks down chlorine into hydroxyl radicals that are very powerful oxidizers and probably help to oxidize urea which is one reason why this problem is more often seen in indoor pools.


In addition to UV, Ozone might be an alternative that would work.  According to "Chemistries of Ozone for Municipal Pool and Spa Water Treatment" by Rip G. Rice, he states that "Although there are a few organic compounds which are rapidly oxidized by ozone to destruction (e.g. formic acid, phenol), the great majority of organic compounds are only partially oxidized, even by as strong an oxidizing agent as ozone, in aqueous solutions, particularly under the conditions which exist in swimming pool and spa water. Most organic compounds, particularly those which are refractory in nature (i.e. organo-nitrogen compounds -- urea, creatinine; organo-chlorine compounds -- chloroisocyanurates, trihalomethanes), are only slightly reactive with ozone, and are not destroyed by ozonation, except upon greatly extended reaction times (up to hours), which are not practical in pool and spa treatment."

He further notes that partial oxidation can still be helpful by forming more polar molecules that in the presence of polyvalent cations such as calcium and magnesium can combine and become insoluble through microflocculation. He also notes that "when the amino acids and urea are treated with chlorine, the corresponding N-chloro-derivatives are produced" and "are much more susceptible to ozone oxidation than are the non-chlorinated nitrogen-containing compounds."


The fact that enzymes are helping would point to organic compounds as being the culprit.  Your ORP seems low if you have the FC level you indicated and there is no CYA in the water.  I would have expected something closer to the 790-815 mV range, though this depends on the specific ORP sensor you are using.  However, your having such a high FC with no CYA means that there is likely to be more nitrogen trichloride being produced.  Do you notice any bad chloramine odor?  Ironically, using a lower active chlorine level would likely reduce that, but by itself it won't help your CC reading.


Indoor pools often need supplemental oxidation and most especially high bather-load indoor pools.  Your typical bather load of 25 people in 18,000 gallons is 720 gallons/bather while the max with 54 is 333 gallons/bather.  A rough rule-of-thumb is that < 1000 gallons/bather is high, 1000-5000 is medium, and >5000 is low.


You should first make sure that your chlorine is getting delivered quickly and maintained correctly, as I'm sure Al has already told you.  If that is already the case, then UV or ozone supplemental oxidation is probably your best option unless you wanted to go with a saltwater chlorine generator (which may only partially help the problem) or use coagulation/filtration/backwash techniques (as is done in Europe).

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