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Active Chlorine Level and Disinfection By-Products (DBPs)

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Key Takeaways:

  • Cyanuric Acid (CYA), aka stabilizer or conditioner, significantly reduces the active chlorine (hypochlorous acid) concentration by orders of magnitude.
  • Higher active chlorine levels are associated with faster reactions rates with chlorine including those producing disinfection by-products, oxidizing skin, hair, swimsuits and bather waste.
  • Most pools without CYA (such as most indoor pools) have at least 10 times the active chlorine level as pools with CYA.
  • There is controversy over whether this huge difference in active chlorine levels is appropriate and where the tradeoff should be with respect to oxidation rates vs. rates of creation of disinfection by-products.
  • If a lower active chlorine level is used in higher bather-load situations or for pools not exposed to sunlight, then supplemental oxidation is often required to prevent a buildup of organics (especially urea) to a higher steady-state level.

 

As described in the Chlorine/ Cyanuric Acid (CYA) Relationship discussion, the active chlorine (hypochlorous acid) level in a pool with CYA in the water is very low.  This has implications for the rate of creation as well as the specific composition of disinfection by-products (DBPs).

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The current focus on disinfection by-product removal (or creation) tends to be on the circulation system including UV (chloramine reduction, but possible THM creation), ozone (DBP reduction), filters (possible DBP creation from biofilms), etc., I believe one should not neglect what is going on in the bulk pool water itself in timeframes far shorter than that of even one turnover of water.

 

In this paper, it was shown that skin and bacteria both react with chlorine to produce chloroform which is usually the dominant trihalomethane (THM) in chlorinated pools.  An arm immersed in 30 liters of water produced around 3 µg/L (ppb) of chloroform per hour.  Bacteria and skin produced THMs at roughly the same rate (when scaled for absolute amounts).

 

In this paper, it was shown that total trihalomethane (TTHM) is correlated with bather load, total organic carbon (TOC) and temperature.  There was a great variation of TTHM within pools indicating that the creation of THMs is relatively fast compared to circulation of the water. 

 

Other studies have shown that chlorination of algae and of humic and fulvic acids (such as come from rich soil) produce THMs.

 

So while circulation systems that can reduce THMs already formed in the water are important (say, via activated carbon or extreme oxidation techniques), it is also important to reduce their formation in the first place.  Reducing the active chlorine level in the water should proportionately reduce this rate of THM production, including that of skin.  Today, CYA is not used in indoor pools so the active chlorine concentration is much higher than in most outdoor pools.  At 1-2 ppm FC with no CYA, the active chlorine concentration is 5-20 times higher than that of outdoor pools with an FC that is 10-20% of the CYA level.  Removal of organic precursors is also required or else they can build up to higher levels toward a steady-state with chloroform outgassing.  Intentional aeration with greater air exchange during off-hours (say, at night) might also help reduce bulk levels.

As described in technical detail in this post, a higher active chlorine level not only results in a faster rate of disinfection by-products including chloramines, but also results in a larger absolute total amount of nitrogen trichloride (trichloramine) being produced.  For oxidation of ammonia (and possibly also urea as well to some extent), the active chlorine (hypochlorous acid) level determines the relative amounts of the three chloramines.

 

Higher active chlorine levels result in lower monochoramine and dichloramine levels but at the expense of higher nitrogen trichloride levels.  Since the latter is the most irritating, volatile, and potentially harmful at the lowest concentrations, one can "tune" these relative amounts.  An active chlorine level in the neighborhood of 0.3 ppm FC (at pH 7.5) gives a balance of the three that are equally proportioned to their odor detection limits though one would want an even lower level such as 0.1 or 0.2 to minimize nitrogen trichloride.  It is very hard to achieve such a low active chlorine level without the use of Cyanuric Acid (CYA) as a hypochlorous acid buffer.  An FC that is around 10% of the CYA level gives the rough equivalent of 0.1 ppm FC with no CYA.  For higher oxidation rates as well as disinfection rates for commercial/public pools, an FC that is around 20% of the CYA level for the equivalent of 0.2 ppm FC with no CYA may be more appropriate.  Note that 0.2 ppm FC with no CYA is at the low end of the German DIN 19643 standard when ozone is also used -- the standard is 0.2 to 0.5 with ozone; 0.3 to 0.6 without ozone.

I’m trying, but I’m finding this discussion a bit confusing. For instance, you stated with links: 

Higher active chlorine levels result in lower monochoramine and dichloramine levels but at the expense of higher nitrogen trichloride levels.  Since the latter is the most irritating, volatile, and potentially harmful at the lowest concentrations, one can "tune" these relative amounts

Breakpoint chlorination tells us that for inorganic chloramines, you start with ammonia, add chlorine and you get monochloramine; add more chlorine, you get dichloramine; add more chlorine, you get trichloramine, add more chlorine, and you get the end products.

 

This used to be all we knew, and to our frustration, often didn’t work very well no matter what we did. Organic chloramines came into the picture; the German Din Standard 16943 put a pH dependence on the types of chloramines that can be produced, basically saying that trichloramines are produced predominantly below a pH of 5; and now we have this with the amount of HOCL being the predominant cause as to what form of chloramine we are going to produce.

Now research is showing us that even the standby of UV isn’t all that great either, as it has it own set of pathways, that both eliminates and creates it own set of DPB’s. The fact is there appears to be multiple pathways of trichloramines production, from breakpoint all the way to its destruction of and the reformation by UV.

 

You also stated:

For oxidation of ammonia (and possibly also urea as well to some extent), the active chlorine (hypochlorous acid) level determines the relative amounts of the three chloramines.

If we take the premise of breakpoint being true in how it is supposed to work, then higher levels of chlorine,(HOCL) is what is needed to take the end reaction to completion, and what you are showing is what I would expect to see; low levels of mono and dichloramnines with higher levels of  trichloramines. And vice versa, with low levels of chlorine, one would expect to show more incomplete reactions…higher amounts of mono and dichloramines, along with much smaller amounts of trichloramines…because it just takes longer to complete them. Is it a fair statement to say tthat they will still all occur over time, with the speed of the reaction playing a major role. Isn’t it just adding to the over-all chlorine demand that could eventually build up in a busy pool?

 

Now one could possibly throw in “The Law of Reactants”, which in effect insinuates, that if you double the amount of chlorine, you could potentially get 4 times the amount of chloramines. The reason being that you are maintaining more chlorine in the pool via a set point, like say from 2 to 4 PPM. Now I don’t know if this is the same thing as your chlorine/cyanuric ratio of 20%, which is maintaining reservoir of 4ppm’s of chlorine with the 0.2 PPM’;s of active HOCL? Will the end result of chloramine formation be the same? An example of this is the effect of acid on alkalinity…slugging vs spreading?

 

Sorry for the confusion.  Let me see if I can address each of your comments separately.

 

Breakpoint chlorination tells us that for inorganic chloramines, you start with ammonia, add chlorine and you get monochloramine; add more chlorine, you get dichloramine; add more chlorine, you get trichloramine, add more chlorine, and you get the end products.

Now research is showing us that even the standby of UV isn’t all that great either, as it has it own set of pathways, that both eliminates and creates it own set of DPB’s. The fact is there appears to be multiple pathways of trichloramines production, from breakpoint all the way to its destruction of and the reformation by UV.

 

I don't know why there has been so much confusion over the inorganic chloramines because the science behind them has been fairly clear though has changed in some ways over time.  I describe some of the dominant reactions in the latest as well as earliest models for breakpoint chlorination in this post.  All of the models share two primary characteristics.  One is that there is a three step process of chlorination of ammonia to monochloramine (very fast) to dichloramine (somewhat slow) to nitrogen trichloride (trichloramine; also slow).  The second is that there are somewhat slow destruction steps primarily for dichloramine to produce nitrogen gas.  The difference in the models is in the specifics of these destruction steps with older models essentially having hydrolysis of dichloramine while newer models having dichloramine react directly with nitrogen trichloride.  (By the way, these aren't just theoretical models, but were validated and adjusted to match experimental results.)

 

The bottom line with all models, however, is that the rate-limiting bottleneck is with the destruction of dichloramine.  So chlorination produces monochloramine and dichloramine which are backed up by a slower subsequent step ultimately oxidizing dichloramine to nitrogen gas.  So at higher active chlorine (hypochlorous acid) levels, the other "exit" of having chlorine react with dichloramine to produce nitrogen trichloride takes place more frequently with the net result that the monochloramine and dichloramine are lower but the nitrogen trichloride is higher.  The opposite is also true where low active chlorine levels essentially allow the dichloramine destruction "exit" a chance to occur rather than producing as much nitrogen trichloride.

 

It is absolutely, positively not true that nitrogen trichloride is only produced below a pH of 5.  Yes, there is a pH dependence, but the people quoting this are using the older model from Wei & Morris (1972) whereas the newer research from Hand & Margerum (1982), Jafvert & Valentine (1992) and Vikesland, Ozekin, Valentine (2000) show a different "exit" for dichloramine reacting with nitrogen trichloride that puts a cap on the nitrogen trichloride levels.  They increase as the pH is lowered, but only to the extent that there is more hypochlorous acid.  At around a pH of 6.5, there is very little increase in nitrogen trichloride from lowering the pH further.  Basically, at a lower pH the dichloramine level rises but that is a precursor for nitrogen trichloride creation as well as destruction so it cancels out.

 

Since the odor detection and irritation level of nitrogen trichloride is very low at 20 ppb, it doesn't take a low pH to create too much.  A sufficiently high chlorine level, say with no CYA, along with a sufficiently high ammonia level are all it takes.  For example, if I use 0.017 ppm N per hour of ammonia (equivalent to around 0.14 ppm FC consumption per hour) with 1 ppm FC with no CYA at a pH of 7.5, I get a steady-state result of 25 ppb nitrogen trichloride (and 1.5 ppb dichloramine and 9 ppb monochloramine).  Of course, in real pools, urea is dominant and can produce nitrogen trichloride more directly from its oxidation by chlorine.  Unfortunately, there are no good complete models for chlorine oxidation of urea (there are some old proposals, but they have clear flaws), though Blatchley is making first steps in this area.

 

Organic chloramines are of course important, but my point was that we may very well be chasing our tails focusing on reducing the Combined Chlorine (CC) measurement when we should really be concerned with what the CC is composed of.  If it's primarily chlorourea or even monochloramine, then this is far less of an issue than if it is nitrogen trichloride.  After all, much drinking water now uses around 1 ppm CC as monochloramine today for disinfection.  When the active chlorine level is low, then a higher CC may not be at all problematic though obviously with high enough bather load it still can be an issue even at low active chlorine levels -- supplemental oxidation can help in this case.

 

As for UV, I hadn't heard that it contributed to reformation of chloramines.  I had only read how it can lead to creation of trihalomethanes (THMs) such as chloroform.  As for chloramines, UV should be able to destroy them without direct consequence.  The THMs are coming from a separate mechanism, possibly from the creation of some free radicals.  If you have anything describing UV creation/reformation of chloramines, please send a link.

 

If we take the premise of breakpoint being true in how it is supposed to work, then higher levels of chlorine,(HOCL) is what is needed to take the end reaction to completion, and what you are showing is what I would expect to see; low levels of mono and dichloramnines with higher levels of  trichloramines. And vice versa, with low levels of chlorine, one would expect to show more incomplete reactions…higher amounts of mono and dichloramines, along with much smaller amounts of trichloramines…because it just takes longer to complete them. Is it a fair statement to say tthat they will still all occur over time, with the speed of the reaction playing a major role. Isn’t it just adding to the over-all chlorine demand that could eventually build up in a busy pool?

 

Now one could possibly throw in “The Law of Reactants”, which in effect insinuates, that if you double the amount of chlorine, you could potentially get 4 times the amount of chloramines. The reason being that you are maintaining more chlorine in the pool via a set point, like say from 2 to 4 PPM. Now I don’t know if this is the same thing as your chlorine/cyanuric ratio of 20%, which is maintaining reservoir of 4ppm’s of chlorine with the 0.2 PPM’;s of active HOCL? Will the end result of chloramine formation be the same? An example of this is the effect of acid on alkalinity…slugging vs spreading?

 

I don't know where this misconception came from of the breakpoint reactions somehow not getting to completion and getting stuck somewhere.  That is simply not true and never has been in any of the models.  It is only if you do not use enough FC (not active chlorine, but all available chlorine) where you can end up with, for example, monochloramine, but simply adding more chlorine will pick up where it left off.  There is no getting stuck, period.  At least not with the inorganic chloramines.  You are right in saying that it's really about the rate of reactions.  The higher amounts of monochloramine and dichloramine I referred to at lower active chlorine levels is talking about steady-state amounts -- they will still get fully oxidized once the bather load has subsided.  Now there are such things as "persistent chloramines", but those aren't inorganic chloramines which can readily be oxidized in minutes to hours depending on active chlorine concentration.  These persistent chloramines are most likely to be various organic chloramines -- some like chlorourea are not a problem themselves while others might be more of a nuisance.

 

Yes, the chlorine demand may build up in the form of some chloramines, mostly monochloramine in the case of the oxidizing ammonia, but so what?  If 1 ppm monochloramine is not objectionable in drinking water, then why is this such a big deal in pools?  This is another example of where people looked at chlorine levels without CYA in the water and jumped to conclusions that CC was a proxy for nitrogen trichloride, which it sort of is (in a proportional way), but that this proxy relationship changes dramatically with active chlorine level such that 0.2 ppm CC may be very bad at high active chlorine levels while 2 ppm CC may not be bad at all at low active chlorine levels.

 

The Law of Reactants doesn't work the way you describe in this case because it's more of a linear relationship.  Doubling the active chlorine concentration cuts in half the monochloramine and dichloramine steady-state levels, but doubles the nitrogen trichloride steady-state level.  It is the active chlorine, HOCl, that matters.  The FC is completely irrelevant -- it just acts as a chlorine buffer to ensure that you don't run out of chlorine, but it has very little to do with reaction rates, including disinfection rates, oxidation rates, etc.

 

I just want to reiterate again that the organic chloramines, such as chlorourea, changes the analysis.  It doesn't invalidate what goes on with the inorganic chloramines, but it adds other reaction paths that based on Blatchley's experiments tend to give higher nitrogen trichloride levels.  I'm still working on developing a model that would fit that data while still working with the inorganic chloramine models -- there are inconsistencies between the two -- not disastrous, but not great either.

 

As for the effect of acid on alkalinity with slugging vs. spreading, that is a myth where you can read more about this in this discussion that talks about what really goes on and a much more efficient way of lowering TA a lot when that is required.

 

The following is a reply to this discussion.  I am putting the reply here since it is more technical.

 

Unfortunately, the chemistry isn't selective about which chemical reactions with chlorine are going to get faster when you choose a level of 2 ppm FC with no CYA vs. having 4 ppm FC with 20 ppm CYA for the equivalent of 0.2 ppm FC with no CYA (which, by the way, I am NOT recommending people do in commercial/public pools since current regulations in many states do not allow CYA in indoor pools -- I'm just discussing it as something to look at and compare against German DIN 19643).  Though with 2 ppm FC and no CYA you are speeding up the kill rates for pathogens and are speeding up the oxidation of bather waste, you are also speeding up the oxidation of your skin, hair, swimsuits, corrosion of metal, outgassing rate of chlorine (hypochlorous acid), and change the balance and increase the rate of creation of disinfection by-products.

 

My wife has personally experienced this difference between our own outdoor pool with roughly a 3-6 ppm FC at 40 ppm CYA vs. an indoor pool at a community center at 1-2 ppm FC with no CYA.  Her swimsuits degrade over just one winter 5-month season of use (mostly the elasticity gets shot) and her skin is flakier and hair frizzier while in our own pool the swimsuits over many 7-month summer seasons do not show significant signs of wear and her skin and hair don't have the same problems either.  The active chlorine (hypochlorous acid) level in the indoor pool is over 10 times higher than in our outdoor pool.  In fact, many of the complaints with indoor pools compared to similar outdoor pools may not just be due to the poor air circulation and lack of sunlight, but also at least partly due to much higher active chlorine levels in indoor pools not using CYA compared to most outdoor pools that often do use CYA.

 

When I was at last year's NEHA conference, there were several people talking about techniques they were using that were successful at controlling chloramines in high bather-load indoor pools.  All of the supplemental oxidation systems you described were used (separately with different people using different systems), including use of MPS and all worked to improve air quality and reduce CC measurements, but as you point out they each have their pros and cons.  The irritating component in Dupont's MPS is potassium persulfate (aka peroxydisulfate) at around 3% of Dupont Oxone product compared to the main ingredient which is 43% of potassium monopersulfate (aka peroxymonosulfate).  Interestingly, the use of silver ions, such as in the Nature2 system for residential spas, catalyzes the destruction of this irritant which is probably why the low/no chlorine MPS-based Nature2 system doesn't irritate residential spa users.

 

As for kill times for pathogens, they are very fast for most (see the table and references in this post I wrote at TFP) where even having an FC a little more than 10% of the CYA level which is equivalent to 0.1 ppm FC with no CYA does a 3-log (99.9%) kill of fecal bacteria including Escherichia coli, Enterococcus faecalis and Staphylococcus aureus in under one minute and Pseudomonas aeruginosa in around 90 seconds.  Viruses such as adenovirus and influenza are around 6 minutes while coliphage MS-2 is around 2 minutes.  The protozoan oocysts take much longer with Giardia lamblia at around 150 minutes and of course Cryptosporidium parvum is essentially untouched (i.e. months).  Yes, having a 2 ppm FC with no CYA increases these rates of kill by a factor of up to 20 for the kill rates just quoted (or a factor of 10 compared to using 4 ppm FC with 20 ppm CYA), but not without side effects of increasing ALL chlorine reaction rates.

 

The CC levels in high bather load outdoor pools are often easier to control even when CYA (in reasonably small amounts <= 30 ppm) are used and is probably due to the exposure to the UV in sunlight.  If a UV system were to simulate the UV in sunlight rather than the far shorter wavelengths, then it would break down some more chlorine to produce chlorine and hydroxyl free radicals which may be what enhances oxidation rates in such outdoor pools and keeps a check on chloramine levels.  That's just speculation on my part, but something is clearly different in addition to better air exchange.  Of course, there are expensive diamond-doped boron electrode systems that can produce more hydroxyl radicals directly and are also known as advanced oxidation systems.

 

As for keeping up with bather load, only a portion of the chlorine demand varies with bather load in the short-run (all does eventually in the long-run).  Ammonia is only about 8% of bather load while urea is about 80% (4% is creatinine, 3% is amino acids, and 5% is other organic compounds).  The ammonia in sweat and urine reacts with chlorine very quickly, in seconds to a minute (depending on chlorine level) to form monochloramine.  Chlorine reacts with skin to produce trihalomethanes (THMs) continually and especially reacts with humic acids in dirt/soil to produce even more THMs.  The complete oxidation of ammonia (monochloramine) is 90% complete after 9 minutes with 2 ppm FC and no CYA to 90 minutes with 4 ppm FC and 20 ppm CYA.  The oxidation of urea, however, is slow even at 2 ppm FC with no CYA taking days to react so it builds up over weeks to a long-term steady-state level.  Blatchley's experiments with urea measured the slow initial step of chlorine combining with urea to form monochlorourea.  Judd had earlier measured the fate of nitrogenous bather load compounds in pool water and saw that organic carbon (mostly as urea) reached a steady-state after 200-500 hours of operation.  Note that this simulation was without exposure to UV in sunlight -- outdoor pools may not build up as much urea as a result.

 

Using the Jafvert & Valentine model (with updates from Vikesland) for chlorine oxidation of ammonia and a high bather load of 2 people per 1000 gallons and using 2 ppm FC with no CYA, the steady-state concentration of chloramines just from the ammonia (i.e. ignoring urea for the moment) of monochloramine is 0.01 ppm (9 ppb), for dichloramine is 1.5 ppb, and for nitrogen trichloride is 50 ppb (note that 20 ppb is an odor and irritation threshold).  If one uses 4 ppm FC with 20 ppm CYA for the equivalent of 0.2 ppm FC with no CYA, then monochloramine is 0.1 ppm, dichloramine is 14 ppb, nitrogen trichloride is 5 ppb.  As you can see, the monochloramine and dichloramine are increased by a factor of 10 while the nitrogen trichloride is decreased by that same factor of 10.  Of course, the Combined Chlorine (CC) level that is actually measured is higher because it most likely consists of monochorourea and in the steady-state this level will be about 10 times higher in the low chlorine level case as will the built-up concentration of urea, but the net oxidation rate on a percentage basis will be the same -- 10 times lower chlorine with 10 times higher urea results in the same net chlorine usage rate.  So the CC level will likely be about 10 times higher in the low chlorine case but the question to ask is whether this really matters.  Monchloramine levels of around 1 ppm are often used in tap water without ill effect.  Chlorourea has rabbit conjunctiva irritation at 10 mg/L compared to monochloramine at 3 mg/L and FC at 20 mg/L.  So is even a 1 or 2 ppm CC level really a problem when the active chlorine level is low?  That is the million dollar question.

 

Now let's take a look at chlorine demand from bather load in the short vs. long run.  The chlorine demand from the nitrogenous compounds in sweat and urine in a pool at a very high bather load of 2 people per 1000 gallons is roughly 2 ppm FC per hour.  However, as was noted earlier, the fast reacting portion of this bather load is ammonia that only consists of 8% of that load, so around 0.25 ppm FC per hour.  The urea that makes up 80% of the bather load builds up to give a background FC demand that will persist even when bather load is not present.  There is additional short-term bather load from oxidation of skin (by which I include skin oils, etc.).  In this paper, an arm in 30 liters of tap water starting at around 4 ppm FC with no CYA and a pH of 7.2 had a chlorine demand of roughly 1 ppm FC per hour starting at a level of 2 ppm FC.  So a full bather in 500 gallons (1893 liters) might be another 0.3 ppm FC per hour using this link to figure that the arm is 4.5%/(100%-4.5%-4.5%) = 4.95% of total body area excluding the head.  So with this very high bather load situation, figure 0.5 ppm FC per hour delta from the bathers.  However, the background urea oxidation level is over 1.5 ppm FC per hour though this isn't realistic since bathers aren't actually in a pool for 24 hours of every day.  If we figure the full bather load is for 12 hours and the pool is closed for 12 hours, then that's a built-up background chlorine demand of 0.75 ppm FC per hour.  So overall, we've got a roughly 0.75 ppm FC per hour starting point, then bathers get into the pool and the chlorine demand rises to 1.3 ppm FC per hour until the pool is closed again.

 

So what's the bottom line with this?  The higher active chlorine level of 2 ppm FC with no CYA results in a lower Combined Chlorine (CC) measurement by up to a factor of 10 compared to 4 ppm FC with no CYA.  However, that higher active chlorine level also has the nitrogen trichloride level (at least from ammonia and likely similar from urea) be up to 10 times higher so even though it is so low as to not show up as CC, it is the most volatile and irritating of all the chloramines.  Also, a lower active chlorine level has the instantaneous rate of disinfection by-product creation be slower until the organic precursors build up, but this affords an opportunity to remove such precursors through water dilution and supplemental oxidation.  So having a low CC standard of 0.2 ppm may actually be a hindrance towards better water quality because it doesn't accurately reflect the worst disinfection by-products.  At lower active chlorine levels, such as with 4 ppm FC with 20 ppm CYA, the higher CC is not as objectionable.  This is a radical change of thinking and obviously isn't going to happen without a lot of research and experimentation, but unfortunately NO ONE looks at CYA in their disinfection by-product research today in spite of my asking people to do so.

A P.S. to the above.  As was noted in the paper talking about the oxidation of skin, another type of disinfection by-product known as trihalomethanes (THMs) was discussed and was seen to be produced from skin at a rate roughly proportional to the active chlorine level.  So while bathers are in the water, using 2 ppm FC with no CYA would have roughly 10 times the rate of production of THMs compared to 4 ppm FC with 20 ppm CYA assuming that the skin organics leave the pool when the bather leaves the pool (i.e. that they are not like sweat and urine that remain in the water).  In reality, some of the skin oil probably leaves the skin, but most of the organic precursors in skin will probably leave with the bather.

 

So it's not just nitrogen trichloride that is reduced by lower active chlorine levels, but the trihalomethanes from direct skin bather load as well.  Of course, that's all just theory and correlation with specific lab experiments, but needs to be proven in real pools.  Since there is a significant build-up of slower to oxidize organics when the active chlorine level is lower, this really emphasizes the need for supplemental oxidation, coagulation/filtration, and/or significant water dilution when bather loads are high.  Of course, we need to figure out what CC level is truly acceptable when low active chlorine levels are used, because supplemental oxidation may not be needed as much as one would think if chasing a 0.2 ppm CC limit that is artificially too low for these conditions.  On the other hand, throwing a high active chlorine level at high bather load will certainly keep the CC lower, but will do so with negative side effects.

Another item of note is that the bather load "per bather" for competitive swimmers is about double that used in the calculations in the previous posts, but you don't get 2 bathers per 1000 gallons in that case.  The ways that the chlorine demand can go way up include bringing in lots of organics into the pool whether that be suntan lotion or dirt or blown-in pollen.

 

Another way that can have a huge effect is urination.  The assumption for typical bather load is around 200 ml of sweat with 992 mg/L N (so 198 mg N) and 50 ml of urine with 12,220 mg/L N (so 611 mg N), presumably per hour.  50 ml is 1.69 fluid ounces or about 1/5th of a cup so even this small amount of extra urination would significantly increase the bather load per person from around 2 ppm FC per hour to 3.5 ppm FC per hour, but this is the long-term chlorine demand since it is mostly from urea.  The fast reacting ammonia component is 50 ml of 560 mg/L N so in 500 gallons results in around 0.1 ppm FC per hour as ammonia is around 5% of urine content.

Richard,

Thanks for responding to this, although you could have left it where it was, as I was hoping to keep the discussion on the importance of meeting demand as it is introduced, and the effect this has on the oxidation requirements. The premise I am trying to show is that keeping up with demand, allows for more rapid and complete oxidation than what is normally seen in standard practice. Now if you factor in low chlorine levels predicated by chlorine/cyanuric levels, then this puts a spin on it, and raises the question of lower DPB formation and slower and more oxidation requirements as an end all. I mean this as wondering for a given scenario, the bather load contamination from a variety of sources, both inorganic and organic, are still going to be the same, so ineffective or very slow oxidation may bring about different intermediates, but are still going to be there and have to go to completion sometime. The end all build up of oxidation needs will be more extensive and get higher and higher, unless the bather loads goes down for a long period of time for the pool to catch up.

 

You always raise some good points, and it is quite evident that throughout your many years of promoting this concept of chlorine/cyanuric ratios as being beneficial, that you have had this discussion many times over in other forums, and are evidently very passionate on your stance that this is a good thing. I think you have convinced a lot of people, including me, that chlorine/cyanuric ratios can be a good thing, with more positives going for it than negatives in terms of achieving low chlorine levels. The ideology of low chlorine levels having the potential to produce a less amount of DBP’s of whatever type is in itself is not in dispute, and certain chlorine /cyanuric ratios are indeed one-way of getting to that point.

 

With that said, what gets lost in the rhetoric of your science…chemistry, and calculations of short-term / long-term bather loads is that; as good as all this sounds, the trade-offs of this approach are still significant enough to warrant caution in its wide-spread acceptance for most commercial pool settings here in the US that have varying loads put on it from day to day. The realization that has to take precedence is that not all commercial pools are the same.  Not all pools have operators that the same knowledgebase to know when in advance to forecast that supplemental oxidation is going to be needed for that day. Not all pools can afford UV, Ozone, Continuous use of Peroxolytes, or would even want to in the first place. Not all pools have the same circulation patterns that don’t have dead spots, so low chlorine levels would likely make those areas worse. Not all pools have test kits that can accurately measure cyanuric acid PPM’s to less than 30 with any degree of accuracy, so HOCL levels may not actually be where anticipated…and the list can go on and on. The point is that the type and quantity of loads put on commercial pools are variable in a number of ways, and it’s the accumulation of these long-term organic loads that can and do build up over time that is the potential problem, not so much the short-term loads of ammonia or the immediate needs for disinfection. The potential for things to get out of hand quickly is more likely to occur with effective low chlorine levels of 0.2 PPM’s and resulting low levels of ORP, than with 2 PPM of FC and high ORP that may allow oxidation to go to completion faster. Even though you know of thousands of residential pools using this approach, the variability of loads put on commercial pools has to put this in the realm of being theoretical, and not yet practical as a routine approach. If you’re responsible for a  200,000 gal indoor pool, and are using cyanuric, and are having problems, then it becomes very expensive to get rid of the cyanuric if you decide that it was a mistake…and more likely that mistake will be highlighted by issues with oxidation.  

 

Now let’s get back to oxidation, specifically long-term oxidation. Yes, you have mentioned briefly, as a disclaimer, that supplemental oxidation may be needed for some high use pool situations…but the importance of that is what is exactly lost in all the mind-boggling rhetoric that often precedes the statement.  (Think of some pharmaceutical advertising in magazines with the fine print on the bottom of the ad, or the mile a minute verbiage at the end of a TV commercial).

 

 I may as well respond to some statements you made above in previous postings to this discussion.

I don't know where this misconception came from of the breakpoint reactions somehow not getting to completion and getting stuck somewhere.  That is simply not true and never has been in any of the models.  It is only if you do not use enough FC (not active chlorine, but all available chlorine) where you can end up with, for example, monochloramine, but simply adding more chlorine will pick up where it left off.  There is no getting stuck, period.  At least not with the inorganic chloramines.  You are right in saying that it's really about the rate of reactions.  The higher amounts of monochloramine and dichloramine I referred to at lower active chlorine levels is talking about steady-state amounts -- they will still get fully oxidized once the bather load has subsided.  Now there are such things as "persistent chloramines", but those aren't inorganic chloramines which can readily be oxidized in minutes to hours depending on active chlorine concentration.  These persistent chloramines are most likely to be various organic chloramines -- some like chlorourea are not a problem themselves while others might be more of a nuisance.

 You mentioned different pathways of dichloramine destruction to trichloramine dependent on amounts of HOCL available; you mentioned that breakpoint doesn’t get stuck, that it just continues where it is left when more HOCL becomes available…and I said to myself, isn’t that what I was just saying also? I can’t yet figure out how you thought I was insinuating something else when I was talking about the importance of the speed of the reactions being able to go to completion faster, and what that means to the pool in term s of cleansing faster. All that was involving was that if the HOCL was available, then those reactions would go to completion faster than they would if the chlorine wasn’t available. Pretty much what you said in your response. Very confusing how you got so confused on what I was getting at, other than preconceptions that you may have had about what you thought I was going to say??  At some point you went on to say this:

Yes, the chlorine demand may build up in the form of some chloramines, mostly monochloramine in the case of the oxidizing ammonia, but so what?

Isn’t the “so what” pertaining to that this is an incomplete state of oxidation… that it will continue on when more chlorine is available, as stated above in breakpoint?

Also you mentioned:

As for UV, I hadn't heard that it contributed to reformation of chloramines.  I had only read how it can lead to creation of trihalomethanes (THMs) such as chloroform.  As for chloramines, UV should be able to destroy them without direct consequence.  The THMs are coming from a separate mechanism, possibly from the creation of some free radicals.  If you have anything describing UV creation/reformation of chloramines, please send a link.

What I was getting at here is from Blatchley's 2008 WAHC Presentation notes I sent you earlier on Pg 47:“Summary of Batch UV Experiments”, where it stated by the NCL3 bullet that NCL3 degrades photo chemically, that the Photoproducts yield NCL3 upon chlorination, ands therefore are competing reactions. Also on Pg 61, it shows an increase of percentage of NCL3 before and after UV.

 

You also have seemed to have mis-interpreted what I was asking when I referenced alkalinity and acid. I was only using this as an example of what might happen when comparing cyanuric /chlorine ratio’s with that of using no cyanuric in the amounts of HOCL that is availble for immediate use, and was wondering if the end amount of chloramine producton would be the same when all is said and done. Was just wondering if there was a co-relation, just like it is with allkalinity and acid slugging vs dribbling, knowing that it doesn’t matter if you slug in the acid or dribble it in. The end result is the same.

 

How about discussing the different types of supplemental oxidation products, and what effect they have on skin, swim suits, and hair. Are the results of oxidation from 2 PPM’s of FC the same as that from monopersulfates in the main body of water? In other words, do they create different DBP’s and THM’s.... or do they go directly to endpoint?

Just an added thought on low chlorine levels caused by incorporating chlorine /cyanuric ratios. You often compare this with the German Din Standard Pools requirements for low chlorine levels, as an attempt to give credence to those levels here in the US. The part you fail to convey consistantly is that those pools following the Din Standard 16943, are using extensive means of flocculating and coagulating the dissolved organics from the system prior to filtraton, which can drop the organic loading significanly. It’s also the premise of Howard Dryden’s  AFM Integrated System. Both are accompanied by very low flow rates and very large filters, both of which are not practiced here in the US. So the comparisons aren’t  exactly relevant or applicalble, other than saying low chlorine levels may produce less DBP’s and THM’s than high chlorine levels…which nobody is disputing. It all comes down to oxidation, which is addressed by Din Filtratrion pracitices, but not so much by the chlorine /cyanuric ratios.
We all know that the education of our patrons are going to be the best and most cost effective course of action in our battles with chlorine and DBP's in our pools. With all the rhetoric and on-going research, along with Richard's calculations, brings the issue of variable batherloads to a different level.  One would think our swim coaches and swim teams would know better, but the truth comes out here from Gold Medal Mel. This isn’t new…but pass it on anyway to those that need to know. Also, for those that missed it this old video of the comical relief of one life guard here
A higher active chlorine level will keep up with demand better, but really isn't keeping urea levels to zero (not even close).  There is still a latent bather demand there even with 2 ppm FC with no CYA and is why even when bathers leave the water that there is still a chlorine demand that persists.  So yes, the higher active chlorine level has faster oxidation, but I would not say it is more complete in that it will get more complete if bather load drops.  So long as there is ANY measurable FC, there is continuous oxidation occurring, it's just slower at lower active chlorine levels.  The buildup does not go up higher and higher forever -- it simply builds up until the oxidation rate equals the bather waste introduction rate.  This is that Law of Reactants you were writing about earlier:

 

Absolute Oxidation Rate = (Active Chlorine Level) x (Bather Waste Level)

 

So for something like urea, at a lower active chlorine level the urea level will build up, but it does not do so forever.  If the active chlorine level is lower by a factor of 10 then the urea level will build up by a factor of 10 at which point you have the same oxidation rate as before since:

 

Oxidation Rate = (1/10) x (10) = 1

 

As I pointed out, this may make the Combined Chlorine (CC) reading up to 10 times higher, but the CC will be composed of far less irritating monochloramine and dichloramine levels compared to far more irritating nitrogen trichloride.  It's a balance between these that is affected by the active chlorine level and in practice most of the CC will be chlorourea which is also less irritating.

 

Since pools aren't typically used 24 hours a day, one has an opportunity to oxidize more bather waste at night with supplemental oxidation.  If one uses MPS for this purpose, for example, it can oxidize a lot of the bather waste and get completely used up before the next day so there should be no irritation.  There isn't necessarily a need to have MPS used during the higher bather load, as long as one isn't trying to chase a <= 0.2 ppm CC.  That was the point.  The idea of what CC really is and what level is truly an issue FOR A GIVEN ACTIVE CHLORINE LEVEL is what I believe needs to be looked at.

 

As for varying bather loads and dead spots of circulation, the chlorine bound to CYA releases from it quickly, in less than a second, so having 4 ppm FC with 20 ppm CYA actually has twice the chlorine capacity of 2 ppm FC with no CYA so would be even less likely to get all used up in a dead spot or where bather load spiked (i.e. urination).  I agree that a low FC level as with the German DIN 19643 system getting as low as 0.2 ppm gets risky because there that FC can potentially get wiped out, but that's not the situation with a higher FC used with CYA.

 

I interpreted the following as getting stuck:

 

... higher levels of chlorine,(HOCL) is what is needed to take the end reaction to completion, .... And vice versa, with low levels of chlorine, one would expect to show more incomplete reactions ...

 

I assumed by "levels of chlorine" you were referring to HOCL as you wrote implying that at low active chlorine levels the reaction won't reach completion ever.  I now understand that you just meant it would take longer and that the reactions would be more incomplete at a given bather load (i.e. show higher levels of some intermediates).  Technically, the reactions even at high chlorine levels never really complete since the rate slows down as the reactant levels get lower, but one can say how long it takes for 90% or 99% completion and that is certainly proportionately slower with a lower active chlorine level.

 

I now see the NCl3 summary from Blatchley's 2008 presentation.  So, the photoproducts of NCl3 may result in more ammonia or chloramine compounds that could then go on to produce NCl3 again, but that would likely be in lower quantities since some will get oxidized to nitrogen gas.  However, as noted on page 61, wavelengths of UV end up producing 21% more NCl3 than one started with which I presume to come from photolysis of other chemicals.  Though not great, NCl3 is the most easily controlled by lowering the active chlorine concentration and removing urea and chlorourea precursors.  The former can be done by using CYA while the latter requires special coagulation/filtration or oxidation.

 

I'm going to contact Dupont (I've talked to them before) about MPS and its rate of oxidation for urea and chlorourea as that is obviously very important.  As for disinfection by-products, they cannot produce chlorinated by-products (i.e. no chloramines or THMs) because they do not oxidize using chlorine.  They are an oxygen oxidizer, similar to other per-oxygen compounds including hydrogen peroxide.  Their main negative side effect is an accumulation of sulfates in the water and the irritation from the impurity persulfate (peroxydisulfate).  This impurity is still an oxidizer and its breakdown can be greatly accelerated by the presence of silver ions.  MPS is also more expensive than chlorine by a factor of 2-3 (at least for residential pool prices -- not sure about bulk quantities).

 

By the way, there is nothing wrong with using HCF in a pool with CYA.  One can still get a spike in chlorine demand and that spike can still be met with HCF.  Of course, the ORP target will be lower with CYA than without, but if one calibrates this to the FC level at a fixed CYA then one can still use ORP for process control which is all it's really good for anyway.  Different sensors measuring the same pool water vary a lot -- 23% of the sensors in the study noted in this post differed by over 100 mV.

Richard,

We could be having this discussion all day long, and never get anywhere. The reason is we agree on most points, and get thrown off the key aspect of the discussion by tangents related to specifics rather than realistic intended purposes. For example: you said

A higher active chlorine level will keep up with demand better, but really isn't keeping urea levels to zero (not even close).  There is still a latent bather demand there even with 2 ppm FC with no CYA and is why even when bathers leave the water that there is still a chlorine demand that persists.  So yes, the higher active chlorine level has faster oxidation, but I would not say it is more complete in that it will get more complete if bather load drops.  So long as there is ANY measurable FC, there is continuous oxidation occurring, it's just slower at lower active chlorine levels.

All  I can say to that is yaa. What I meant by going to “completion” before was in context to a general statement pertaining to the speed of expected reactions. That those reactions that could go to completion, (should have said, could/would/might…also meaning, being able, further in the reaction process of getting to, or even more advanced stages of completion …) would more likely occur with more chlorine, rather than with less chlorine that is associated with the chlorine/cyanuric ratios.  So emphasizing being absolutely complete was by no means the intention

You further went on to say:

The buildup does not go up higher and higher forever -- it simply builds up until the oxidation rate equals the bather waste introduction rate.  This is that Law of Reactants you were writing about earlier:

 Absolute Oxidation Rate = (Active Chlorine Level) x (Bather Waste Level)

 So for something like urea, at a lower active chlorine level the urea level will build up, but it does not do so forever.  If the active chlorine level is lower by a factor of 10 then the urea level will build up by a factor of 10 at which point you have the same oxidation rate as before since:  Oxidation Rate = (1/10) x (10) = 1

I suppose there are a number of Rate Laws out there that could be adapted to this. The “Law of Reactants” I was using was from other sources, of which I can’t find right now. However, it was pertaining to that if you double the amount of chlorine for instance, you would get the potential to create 4 times the amount of chloramines…I’m assuming chloramines being a generality for being a mixture of various types of DBP’s including chloramines. All it’s intent was to show that using more chlorine could create more byproducts, which is something we both agree on. I don’t know for sure if it is just linear, or somewhat exponential, but don’t really care. The basic premise of its use in my discussions is: Generally speaking…If you use more chlorine in a pool for any reason, you have the potential for creating more DBP’s, chloramines, or whatever species you want to choose. With that said, the premise I normally use this Law for is this: Is it better to maintain your pool at 2 PPM’s with the capability of meeting the chosen set-point (demand) more quickly, than keeping a buffer in the pool of 6 PPM’s or higher, which is exactly what some pools do by recommendations from health officials, to have some chlorine in the pool when it gets real busy. It’s also used for  quantities of shock. Putting in more to make sure you have enough isn’t always better as you have discussed many times before. Please don’t take the obvious intent where it is not meant to prove your points on cyanuric.

Now for the Law of Oxidation Rate that you used. I haven’t really come across this one before. I understand on what you are saying, but have some questions.  Are you saying that, there is some sort of equilibrium here, in that bather waste can only get so high? What happens when you are hammered for days, and oxidation demands far exceeds the chlorine input? Doesn’t any excess demand that is not oxidized keep on building up, and isn’t this scenario more likely to occur with lower chlorine levels? All the example was showing that you used for this law, is pretty much obvious isn’t it? That oxidation is going on all the time there is chlorine in the water, with its speed being related to how much is available.

You mentioned:

As I pointed out, this may make the Combined Chlorine (CC) reading up to 10 times higher, but the CC will be composed of far less irritating monochloramine and dichloramine levels compared to far more irritating nitrogen trichloride.  It's a balance between these that is affected by the active chlorine level and in practice most of the CC will be chlorourea which is also less irritating.

Okay, but aren’t those moncochlormines, dichoramines, chlorourea intermediates, and with the addtion of more chlorine, will eventually go to advanced stages? As you said, it continues on when more chlorine is added, so aren’t overall demand levels just building up? Also, what happens to nitorgen trichloride after it is formed? What the next stage? Some is volatile and will actually gas off, but what happens with the rest?

You said:

Since pools aren't typically used 24 hours a day, one has an opportunity to oxidize more bather waste at night with supplemental oxidation.  If one uses MPS for this purpose, for example, it can oxidize a lot of the bather waste and get completely used up before the next day so there should be no irritation.  There isn't necessarily a need to have MPS used during the higher bather load, as long as one isn't trying to chase a <= 0.2 ppm CC.  That was the point. 

So how many pools do you know that have maintenance staff available after pool closings to do the MPS shocks almost everyday? Using 2ppm’s of FC under HCF as an example, what happens with the chlorine in our pools at night? You mention that it would create more volatile chloramines for a variety of reasons, which I don’t really dispute. This is due to slower oxidation rates. On the other hand, faster oxidation rates will create more volatile chloramines. If we use nighttime as an example, that’s not necessarily a bad thing is it? Let see, little volatilization due less water movement, so is it more likely that more reactions (like NCL3and others), will be able to go to the next stage and not be as volatile anyway?

I don’t understand your point on chasing the <= 0.2 ppm CC. Are you forgetting that commercial pools are regulated by codes, and those codes are what we have to abide by…period? So the arguments you make are, for the moment, theoretical and not proven yet to be viable by actual practice. Until that is changed, you almost have to have enough dialogue on the importance for sufficient supplemental oxidation requirements that are necessary for the chlorine/cyanuric ratios to be viable in our commercial pools, and you just don’t seem to have much enthusiasm to discuss that part of the equation. Or, how about a discussion on the realities of Continuous Breakpoint? Does it really exist? If so, what does that mean for our pools if that can be done with relatively realistic low levels of chlorine 2 ppm, that is maintained by HCF feeders in a manner to meet demand as it is introduced.   

 

You know, my whole argument of this discussion has been on the importance of the speed of reactions in terms of oxidation and its relative importance for our pools. For the most part, you are quite familiar with my viewpoint, as I am yours. In reality, they are not that much different. I’m including some of those points that we are in agreement with in discussions you made on another forum a few years back. They are taken out of context, and each paragraph is from different parts of the discussion here. I underlined what I thought was key points.

 Note that the above 1x or 3x rules simply refer to the minimum amount of Free Chlorine (FC) target needed to get the breakpoint reaction going in the worst case. It says nothing about the speed of that reaction. With CYA in the water, the active chlorine level is quite low so the speed of the breakpoint reaction can be slow as well.

 

So this entire idea of needing some factor of the CC as chlorine to be added is ridiculous from the point of view of "needing" that much chlorine. Since there is usually measurable FC in the water at the same time there is CC, the only purpose for increasing the FC is to increase the reaction rates to oxidize things faster. It isn't because it is "needed".

 

The concept of having a fixed amount of FC added all at once doesn't even make sense since real pools are maintained to keep a certain FC level so more chlorine is added, either manually or automatically, to maintain the FC. So in practice, breakpoint oxidation is occurring continuously and the only question or issue is how quickly it occurs.

 

In commercial/public pools with high bather load, it is bather load that is the fundamental problem since it will build up chemicals that need to get oxidized by chlorine. If one doesn't have CYA in the water, then the oxidation will go faster which tends to keep the CC lower, but will produce more nitrogen trichloride. If there is CYA in the water, then the oxidation will go slower, but will measure more Combined Chlorine (CC) since intermediate products will build up, but the nitrogen trichloride should be lower. If one freaks out with the higher CC and increases the FC level substantially, then one would speed up the oxidation, but would also produce more nitrogen trichloride.

 

How To Handle High Bather Load
When one measures CC, it is most likely that one is mostly measuring monochlorourea or possibly some dichlorourea. There may also be a smaller amount of monochloramine, but as indicated earlier, this does get oxidized by chlorine much faster than chlorourea. When the bather load is high, one will build up a lot of chemicals to get oxidized and over time with regular bather load a steady-state will be reached with a high amount of intermediate products and a balance between introduction of new chemicals to oxidize vs. the outgassing or removal of final products that are of concern. So the best approach to handling high bather load is to have supplemental oxidation for the higher bather load (e.g. UV, ozone, enzymes, non-chlorine shock, etc.) to get rid of urea, chlorourea, monochloramine, and other organics including organic chloramines. Water dilution is another way to deal with this and in practice a combination of supplemental oxidation with water replacement makes the most sense. To minimize the amount of nitrogen trichloride that is produced, one can use at least a small amount of CYA in the water -- for indoor commercial/public pools I think that 4 ppm FC with 20 ppm CYA is a nice sweet spot roughly corresponding to an equivalent of 0.2 ppm FC with no CYA. Of course, this is all theoretical speculation until SOMEONE does the experiments I've been asking for using CYA in the water to lower the active chlorine concentration.

 So instead of us beating a dead horse,  can we proceed?

 

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