Pool Water Chemistry

[JasonLion]

Nearly everything is still true at CYA levels over 100, but some of the secondary effects start getting to be major problems when CYA is that high, making it difficult to take care of the pool properly. For example, the FC levels required start to cause the PH test to not work. The amounts of chlorine required to fight algae also get so large that fighting algae starts to become impractical.

"Cyanurics, benefactor or bomb" is an entertaining mix of good information, misleading statements, and stuff that is just plain wrong. As CYA rises the ORP readings for various FC level get closer and closer together. Eventually you can't distinguish the different FC levels because the change in ORP from changing FC levels becomes smaller than the noise in the system.

ORP is related to sanitizing power, but not in a straightforward way. Some of the factors that change the ORP level directly affect sanitizing, while other factors that change ORP do not affect sanitizing at all. If you could magically hold everything else constant, and only FC varied, then you can use ORP as a proxy for sanitizing ability. But if some of the other factors start varying, and they will if you are using a SWG, ORP becomes far less useful.

"The convergence of the ORP suggests to me that the relatioship between FC and CYA may change at high levels of CYA." No, you are speculating about the wrong things. The relationship between FC and CYA is different when CYA is really really low, but is just as we describe when CYA is high. The ORP effects are an artifact of how ORP works, not anything to do with CYA directly. You really do need extreme FC levels when CYA is high, many people have tried it and confirmed that this is true.

 

[chem geek]

Regarding "Does this formula for working out HOCl above hold up at CYA levels over 100?", yes, the formulas are based on equilibrium chemistry and borne out by experiments done by real scientists/researchers. The paper you refer to from PPOA has things in it that are just plain wrong, partly because it is looking at commercial/public pools with high bather loads where such bather load has more chlorine demand than the breakdown from sunlight so little benefit is seen from CYA levels above 30 ppm while Mark did experiments showing greater CYA shielding benefit at higher CYA levels. More has been written about this article in this thread and this thread. The ORP data is older using sensors that had more problems than some newer ones and ORP is not an accurate absolute standard anyway as I indicated in an earlier post in this thread where even the mV increase per doubling of FC isn't consistent between sensors so using ORP as an absolute measure doesn't make sense.

I have a link in the CPO thread to real ORP data from real pools that does not show the convergence he described (when looking at calculated HOCl levels, not FC alone) even though many data points were in the 80-100 ppm CYA range (one was 150 ppm), but even if it did ORP is not a direct measure of HOCl anyway and ORP sensors tend to not measure lower ORP values well, such as those that occur from low FC/CYA ratios. In this real ORP study of 620 samples from 194 pools, comparing ORP readings from a portable Oakton ORP measuring device vs. the built-in ORP controllers in many of the pools, 30 out of 130 (23% of those that had built-in ORP controllers) were off by more than 100 mV measuring the same pool water at the same time! Also, the sensors need to be regularly cleaned and it may be that higher CYA levels have some direct effect on the sensors requiring more frequent cleaning (some other people have noted that).

I have another link in the CPO thread to a scientific peer-reviewed paper measuring kill times for protozoan oocysts where they used amperometric sensors for more direct measurement of HOCl and it also tracks what is predicted from the fundamental chemistry reasonably well. Also, the other scientific papers showing kill times for bacteria and inactivation rates for viruses and protozoan oocysts all track as predicted by the fundamental chemistry and some tested CYA levels above 100 ppm.

Do you have any scientific basis for why the laws of science and equilibrium chemistry breaks down at higher CYA levels such that the FC/CYA ratio is no longer a reasonable proxy? Also, the chlorine/CYA tables aren't based on FC/CYA, but rather on the detailed chemical equilibrium equations calculated in this spreadsheet. The FC/CYA ratio is an approximation and falls apart at higher FC/CYA ratios. The shock values in the table are based on the accurate calculations, not the FC/CYA ratio. This ratio is reasonable until the FC gets to around 50% of the CYA level in terms of FC and CYA combinations with the same FC/CYA value having the same hypochlorous acid concentration. In terms of FC/CYA at a pH of 7.5 as a proxy for the equivalent FC with no CYA (and where roughly half of this ratio is the hypochlorous acid itself), this starts to fall apart above an FC that is around 30% of the CYA level, but again, the tables were done using accurate calculations, not the FC/CYA approximation.

The scientific peer-reviewed paper on the oxidation of organics indicates that the chlorine bound to CYA may be 1/100th as strong as HOCl, at least for the organic studied (so it is not known as to whether this would apply to algae kill rates or clearing by oxidation). So at high CYA levels with proportionately high FC there may be some effect from the chlorine bound to CYA. At 50 ppm CYA this would be roughly a doubling of the effective oxidation rate compared to looking at HOCl alone (since the HOCl concentration would be roughly 1/100th the FC which is roughly the chlorine bound to CYA) at normal (non-shock) levels. At shock levels where the FC is roughly 40% of the CYA level, the chlorine bound to CYA is 100 times the HOCl concentration when the CYA is around 80 ppm. So yes, there may be some effect of the chlorine bound to CYA at least for oxidation of some organics, though we haven't seen this in terms of algae inhibition levels (but this is a subtle effect since many pools have different algae growth rates and the table statistically tries to work for nearly all pools). However, the shock levels are somewhat arbitrary anyway since higher FC/CYA ratios just oxidize faster. One is already beyond what is needed to kill algae faster than it can grow so it's more a matter of having it be high enough to break through larger masses of it so that there aren't any regions that aren't exposed to the chlorine and to be able to clear the pool in a reasonable period of time.

Richard

 

[Freelancer]

"as ppm Cl2" is just the units that everyone uses for FC, ie just use the FC level straight from the test kit.

(FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

If FC is 5 and CYA is 50, HOCl is 5/(2.7*50-4.9*5+5) = 5/(135-24.5+5) = 5/115.5 = 0.043

This is a question to Jason and Richard regarding the above formulae.

Now I fully understand that the above formulae is calculating HOCl not bound to CYA.. IE that which is available for sanitisation. Given that PH is an important part of how much HOCl is created when chlorine is introduced into a pool why therefore is not the PH measurement taken into consideration or is it that this formulae is based upon a set PH of 7.5?

 

[chem geek]

If you look at where I wrote the original formula in the first post in this thread, I wrote:

The following is an approximate formula you can use so long as your CYA ppm is at least 5 times your FC (the formula really falls apart terribly below a ratio of CYA/FC of 3).

(HOCl as ppm Cl2) = (FC as ppm Cl2) / ( 2.7*(ppm CYA) - 4.9*(FC as ppm Cl2) + 5 )

and if you are interested in the FC for a given HOCl (to construct the equivalent of Ben's table, for example), you can use the following which just solves for ppm FC from the above.

(FC as ppm Cl2) = ( 2.7*(ppm CYA) + 5 ) / ( 4.9 + 1/(ppm HOCl) )

The constants in the above formulas are for a pH of 7.5 (which is the only parameter that significantly affects these constants). With the spreadsheet I can easily calculate the constants for other pH, but remember that the above formulas are approximate. For example, with FC of 3 and CYA of 15 the formula gives HOCl as 0.098 when the correct answer is 0.095. That's not terrible (about an 3% error). However, with FC of 5 and CYA of 15 the formula gives HOCl as 0.239 while the correct answer is 0.199 (about an 20% error) which isn't as good.

A rough rule of thumb that applies at a pH of 7.5 is that the effective chlorine level is reduced by a factor about equal to the ppm of the CYA. So, a CYA of 30 ppm reduces the disinfecting chlorine (HOCl) level to about 1/30th of what it would be with no CYA.

As noted, the formula is for a pH of 7.5 as the constants depend on pH. There isn't a simple accurate formula for this, but since CYA acts as a hypochlorous buffer, it doesn't change hugely with pH, at least not like it does without CYA (see the charts in this post). For an accurate calculation, you can use my Pool Equations spreadsheet. Note that I have the temperature dependence on the chlorinated isocyanurate equations turned off as this came from Wojtowicz and I am not sure of its accuracy (though I do turn that on when roughly calculating spa sanitation conditions -- this is at the line labeled "Use Temp. Dependent Cl-CYA" currently at line 226 in the spreadsheet).

The following table gives the factors on CYA and FC for the first formula as a function of pH (I got these from the end of my spreadsheet near lines 571 and 572 for factors "A" and "B"):

pH ..... CYA factor .. FC factor
7.0 ........ 2.0 .............. 3.6
7.1 ........ 2.1 .............. 3.9
7.2 ........ 2.3 .............. 4.2
7.3 ........ 2.4 .............. 4.4
7.4 ........ 2.6 .............. 4.7
7.5 ........ 2.7 .............. 4.9
7.6 ........ 2.8 .............. 5.1
7.7 ........ 2.9 .............. 5.2
7.8 ........ 2.9 .............. 5.4
7.9 ........ 3.0 .............. 5.5
8.0 ........ 3.1 .............. 5.6

Richard

 

[Freelancer]

Richard,

Do you know that before posting I'd just read your first post on the subject, but for some reason my brain just did not process the information regarding PH. I remember finding the fact that your CYA needed to be 5 times your FC and interesting point and was still pondering the ramifications of this information when I carried on reading.

The table that you posted is most useful indeed.

Anyway you have once again fully explained what I needed to know and again I thank you for your time and patience.

 

[chem geek]

In this post, I wrote about chlorine breakdown from the UV in sunlight, but what I wrote was an oversimplification. In reality, the UV in sunlight breaks apart chlorine into hydroxyl, oxygen anion and chlorine radicals:

HOCl + hv ---> OH• + Cl•
OCl^- + hν ---> O^-• + Cl•
O^-• + H₂O ---> OH• + OH^-

The hydroxyl radicals are very powerful oxidizers though very short-lived. They probably explain why outdoor pools exposed to sunlight tend to be in better shape and not build up organics as quickly, at least in low bather-load pools. Mostly, the radicals end up extinguishing primarily from chloride ions and bicarbonate/carbonate ions acting as scavangers, producing oxygen gas and chloride ions:

Cl^- + OH• ---> Cl• + OH^-

Cl• + H₂O ---> H₂OCl• ---> HOCl^-• + H⁺
HOCl^-• ---> OH• + Cl^- + H⁺
or
Cl• + Cl• ---> Cl₂
Cl₂ + H₂O ---> HOCl + H⁺ + Cl^-

OH• + OH• ---> H₂O₂
H₂O₂ ---> HO₂^- + H⁺
OH• + HCO₃^- ---> OH^- + HCO₃•
HCO₃• + HO₂^- ---> HO₂• + HCO₃^-
HCO₃• ---> CO₃^-• + H⁺
HO₂• ---> O₂^-• + H⁺
CO₃^-• + O₂^-• ---> CO₃^2- + O₂(g)
plus other similar termination reactions (this link gives some other hydrogen peroxide decomposition reactions)

It is possible that some of the chlorine bound to CYA might break down as well. If that happens, then the following may occur:

HClCY^- + hv ---> •HCY^- + Cl•
•HCY^- + HOCl ---> HClCY^- + OH•
--------------------------------------
HOCl + hv ---> OH• + Cl•

If instead the following occurs:

•HCY^- + HOCl ---> H(OH)CY^- + Cl•

then chlorine would still get consumed but CYA would be partially degraded (this seems less likely given CYA's apparent stability).

 

[gboulton]

Richard,

You are one of the truly great resources around here. Thanks!

(My old HS AP Chem teacher would adore you...and probably disown me. :lol: )

 

[imrodee]

Thank you. I'm saving this thread to come back to..

If I lived 100 more years, I couldn't cram all this in my hard drive... all at once. :study:

 

[Swampwoman]

^yes, I'm with you, but I truly appreciate the opportunity to attemp to wrap my head around it now and then so thanks also, Richard. Around our house, the phrase "Chemgeek says" has become popular when discussing the viscitudes of pool chemistry

 

[chem geek]

Thanks for the kind words. They are most appreciated.

 

[jpm]

I'm interested in your ORP conclusions. It seems like ORP is a more effective way to measure whether chlorine is really disinfecting and not bound up by CYA etc. Looking at the OBrien paper there's so many equilibrium reactions that anything could be happening in the water, while ORP seems to measure the available disinfecting power of whatever is in the water.

This paper concluded that meeting the minimum pool code did not assure meeting an ORP >= 650 mV.
It also notes that eight states include ORP, and that in Iowa, an ORP < 650 mV could close a pool/spa regardless of other parameters. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646482/
Interesting also that Germany enforces ORP in their swimming pool standards (probably because Europeans have so much more experience with ozone etc.]

In the CDA Model Aquatic Health Code, it seems to confirm that CYA reduces ORP readings, ie. reduces sanitizing power, by actively lowering the ORP of the water. This corresponds to what's known about high CYA lowering disinfection rates.

A portable ORP tester might be an "inexpensive" investment - $100 will get you a Hanna ORP/pH/temp meter on Amazon. The Oakton meter (used in the study) seems to go for $200, ORP only, but doesn't seem to need as much calibration and storage solutions. They don't say how long the electrodes last, so could be pricey unless you can get several hundred tests out of them. As a bonus, the meter would tell you if your ozonator is working or contributing.

 

[chem geek]

Welcome to TFP!

I'm interested in your ORP conclusions. It seems like ORP is a more effective way to measure whether chlorine is really disinfecting and not bound up by CYA etc. Looking at the OBrien paper there's so many equilibrium reactions that anything could be happening in the water, while ORP seems to measure the available disinfecting power of whatever is in the water.

This is absolutely not true that "there's so many equilibrium equations that anything could be happening in the water". Since when did having simultaneous chemical equations of equilibrium mean that the result was not specific and determinate? There is no question that the calculations are tedious, but they are nothing more than a straightforward application of 1st year college chemistry. See the Pool Equations spreadsheet that precisely calculates the active chlorine (hypochlorous acid) concentration based on pool water chemistry parameters (it also calculates ORP for a few of the different manufacturer's sensors). There are also standard chemical equilibrium programs one can purchase (CHEMEQL, EQS4WIN, MINEQL+, etc.) where one simply enters in the equilibrium constants and initial conditions and out comes an answer. Also see this post in this thread that derives the rough rule of thumb that a pool with Cyanuric Acid (CYA) has roughly the same active chlorine level as a pool with an FC that is FC/CYA when no CYA is present. So a pool with an FC that is around 10% of the CYA level has the same active chlorine as a pool with 0.1 ppm FC and no CYA.

This paper concluded that meeting the minimum pool code did not assure meeting an ORP >= 650 mV.
It also notes that eight states include ORP, and that in Iowa, an ORP < 650 mV could close a pool/spa regardless of other parameters. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646482/
Interesting also that Germany enforces ORP in their swimming pool standards (probably because Europeans have so much more experience with ozone etc.]

The paper was not surprising at all since it is well known that ORP is mostly affected by the hypochlorous acid concentration and not the chlorine bound to CYA. It is true that there is a lot of over-stabilization in some pools. However, if you look at bacteriological field studies, such as the Pinellas County 1992 (1994) pool study, you find that it takes a very low level of active chlorine to prevent uncontrolled bacterial growth. The issue has more to do with kill times to prevent person-to-person transmission of disease. For residential pools, the issue is more about algae growth since it takes much higher levels of chlorine to kill algae than to kill bacteria (at similar kill rates -- for bacteria you want to kill them a lot faster than they can reproduce while with algae you only need to be somewhat ahead of their reproduction rate).

It is not true that Germany enforces ORP in spite of what the article said. They don't, at least not as a primary determinant. DIN 19643 has specific tests for bacterialogical quality and for FC levels with no CYA in the water. ORP is a secondary measurement. Specifically, 19643 requires a 4-log (99.99%) reduction of Pseudomonas aeruginosa in 30 seconds. They require 0 CFU in 100 ml for E. coli, P. aeruginosa, an aerobic count of 100 cfu/ml and 0 CFU in 100 ml for Legionella. It has a pH range of 6.5 to 7.6 and requires Free Chlorine to be between 0.3 and 0.6 ppm (with no ozone; with ozone the range is 0.2 to 0.5 ppm). The Combined Chlorine (CC) is to be <= 0.2 ppm and TTHM is to be <= 20 ppm. The ORP is 750-770 mV, but this is not the absolute standard since it can vary for other reasons. Nevertheless, given the pH and FC ranges and not having CYA in the water, achieving that ORP is not difficult, but it depends on whose sensors you are using.

In the CDA Model Aquatic Health Code, it seems to confirm that CYA reduces ORP readings, ie. reduces sanitizing power, by actively lowering the ORP of the water. This corresponds to what's known about high CYA lowering disinfection rates.

You are correct that CYA decreases the active chlorine (hypochlorous acid) level and lowers not only ORP but also oxidation and disinfection rates. However, you need to look at this the other way around which is that most pools with no CYA, especially in the U.S. where the FC is 1-2 ppm or more, are OVER-CHLORINATED. They have too high an active chlorine level so oxidize swimsuits, skin and hair too quickly and react chemically with organics to produce disinfection by-products more quickly and to produce more nitrogen trichloride which is very irritating and volatile. The Germans intentionally lowered the FC target to try and minimize these problems, but it is very hard to maintain 0.2 or 0.3 ppm FC everywhere in the pool unless one has high capacity feed systems and outstanding circulation. Another alternative is to use CYA in the water, but in moderation, since CYA is a hypochlorous acid buffer. So one can have 4 ppm FC with 20 ppm CYA for the equivalent of 0.2 ppm FC with no CYA but with an ample reserve of chlorine nearly instantaneously available wherever it is needed to meet local chlorine demand.

My wife has personal experience with this over-chlorination problem where her swimsuits would degrade every year during the 5-month winter swim season when she swam in an indoor community center pool with 1-2 ppm FC and no CYA 3-4 times a week. In our own pool with 3-6 ppm FC with 30-40 ppm CYA during the 7-month swim season swimming nearly every day, her swimsuits last for around 7 years. The difference is that our pool has the equivalent FC of 0.1 ppm with no CYA so that the indoor pool is 10-20 times higher in active chlorine level and degrades her swimsuits much, much faster (and she reports her skin is flakier and hair frizzier when using that pool).

I went to the World Aquatic Health Conference (WAHC) October 11-12th and spoke to those associated with the CDC Model Aquatic Health Code (MAHC) regarding their desire to eliminate CYA entirely from indoor pools and spas and high-risk venues. They are reconsidering this since it is an extreme reaction to fears of not being able to deal with the protozoan oocyst Cryptosporidium parvum that has consequences, especially when retaining the higher FC levels. By limiting the CYA level, not eliminating it, one can still shock to kill Crypto and there are other methods such as using chlorine dioxide that might be easier.

A portable ORP tester might be an "inexpensive" investment - $100 will get you a Hanna ORP/pH/temp meter on Amazon. The Oakton meter (used in the study) seems to go for $200, ORP only, but doesn't seem to need as much calibration and storage solutions. They don't say how long the electrodes last, so could be pricey unless you can get several hundred tests out of them. As a bonus, the meter would tell you if your ozonator is working or contributing.

If you looked at the ORP post and the links to manufacturer's own tables of ORP vs. FC in various conditions, it's all over the map. That means using ORP as a reliable measure of disinfection isn't very useful except on a broad scale. The ORP data is all over the place. Also, as for the Oakton meter, it was used in a study of 194 pools taking 620 samples as shown in this post. Note how the ORP is much more highly correlated to the calculated hypochorous acid concentration (which is roughly half the FC/CYA ratio) than it is to FC alone. This is, of course, expected, yet the 2009 paper you referenced didn't even bother doing the calculations for HOCl. Also note that in the pool study using the Oakton meter, many pools also had built-in ORP systems yet in 23% of those measuring the same water they showed 100 mV or more differences, some higher some lower. This wasn't just an issue of calibration either -- the Oakton was regularly calibrated for each day's measurements.

ORP measures a thermodynamic quantity, not reaction rates which is really what is relevant, and even then it doesn't actually follow the Nernst equation for chlorine and no one has explained why. The closest fit implies a chemical reaction with 0.6 to 0.8 electrons instead of the expected 2-electron reaction. Also, many chemicals including dissolved hydrogen gas from saltwater chlorine generators or the use of non-chlorine shock (potassium monopersulfate) can all affect ORP readings but do not result in the corresponding implied changes in disinfection rates. At best, ORP can be used for process control, as a setpoint, when accurately measuring FC, CYA and other water chemistry parameters.

Why spend hundreds of dollars on an ORP sensor when given the FC and CYA at normal pH near 7.5 you will know the active chlorine level? All you have to do is choose your FC/CYA ratio for your purposes. For those on this and other pool forums, the goal is to have enough chlorine to kill green and black algae faster than they can grow. In manually dosed pools, this is roughly an FC that is 7.5% of the CYA level (in saltwater chlorine generator pools it's roughly an FC that is 5% of the CYA level). If you were wondering what the kill times are like, this post lists them for an FC that is around 10% of the CYA level -- the equivalent of 0.1 ppm FC with no CYA. For commercial/public pools, a decent minimum would be no lower than this and might be an FC that is 20% of the CYA level so equivalent to 0.2 ppm FC with no CYA. This balances disinfection and oxidation times against minimizing disinfection by-products and oxidation of swimsuits, skin and hair and of corrosion rates.

 

[jpm]

Welcome to TFP!

Thanks!

This is absolutely not true that "there's so many equilibrium equations that anything could be happening in the water". Since when did having simultaneous chemical equations of equilibrium mean that the result was not specific and determinate? There is no question that the calculations are tedious, but they are nothing more than a straightforward application of 1st year college chemistry.

I think I oversimplified. My thoughts were that if an Iowa pool can be closed (according to the reference) with ORP < 650 mV, but other all other parameters being normal i.e. FC, then perhaps you don't know what's going on because of measurement problems, other wastes in the water tying up the sanitizer, etc. These are only models after all - do you have K constants for urea, sunscreen, defoamer etc?

It is not true that Germany enforces ORP in spite of what the article said. They don't, at least not as a primary determinant. DIN 19643 has specific tests for bacterialogical quality and for FC levels with no CYA in the water. ORP is a secondary measurement.

Thanks for the DIN reference, this is just what I'm interested in.

I have to check at the library for DIN 19643 as it's not online directly. and there appears to be some sub standards or revisions of it. What I could find through google listed physical and chemical "requirements" for poolwater. ORP was listed in those requirements. But it also lists other requirements such as turbidity etc. This is second hand through translation, and I also don't know if DIN standards are in turn enforced by German law. BTW DIN 19643 leads to some interesting reading on German wikipedia through google translate.

I don't know if turbidity lowers ORP, but if it does, that would be a good indicator of poor water quality where free chlorine seems to be a poor indicator. They used to say our chlorinated drinking water was good enough to drink when it was cloudy after torrential rains. Then they said chlorine doesn't work in turbid water, so they issued boil water advisories and they've spent a few hundred million dollars on municipal water upgrades.

Maybe it's better to say water quality is a combination of FC, combined chlorine, ORP, turbidity/colloids, TDS etc.

 

[chem geek]

I think I oversimplified. My thoughts were that if an Iowa pool can be closed (according to the reference) with ORP < 650 mV, but other all other parameters being normal i.e. FC, then perhaps you don't know what's going on because of measurement problems, other wastes in the water tying up the sanitizer, etc. These are only models after all - do you have K constants for urea, sunscreen, defoamer etc?

All the reference was really concluding was that the "standards" the states use are not the right ones for setting disinfection/oxidation rates. Having separate Free Chlorine (FC) and Cyanuric Acid (CYA) ranges is completely idiotic, yet that is what every state does because the industry has touted for so long that "only FC matters; CYA doesn't matter" so that the basic science known since at least 1974 was not looked at (or was intentionally ignored) and apparently is STILL not looked at in papers written in 2009! The states should be using an FC/CYA ratio range since that most closely correlates with the active chlorine (hypochorous acid) level for disinfection and with the unbound chlorine (hypochlorous acid and hypochlorite ion) level for oxidation (since hypochlorite ion is relevant for some oxidation reactions while hypochlorous acid applies to some others).

If the sanitizer is tied up oxidizing other materials, then it would not normally show up as Free Chlorine (FC) and would usually show up as Combined Chlorine (CC) instead. So using Free Chlorine (FC) along with the CYA number should be perfectly valid in determining the active chlorine level and in fact that was what I showed in the link to the graphs of real pools where ORP correlated to HOCl. The equilibrium constants for chlorine bound to ammonia, urea and other organics are irrelevant if they are not readily released and therefore measured during the FC test (instead they generally only show up as CC, though even that isn't relevant). Now it is technically possible for there to be other chemicals in the water that behave like CYA in that they bind to chlorine somewhat but not so much as to not get reported as FC, but in practice such chemicals aren't found in pools (e.g. glycoluril). Using the FC/CYA ratio is a very simple and inexpensive starting point for managing pool disinfection/oxidation rates. If one was going to spend money on a device to measure hypochlorous acid more accurately, then one should go all the way to getting an amperometric sensor for that purpose. If one wants just process control, then an ORP sensor is OK for that purpose.

By the way, the equilibrium and more importantly the rate constants for chlorine with ammonia and the resulting breakpoint chlorination model is known though there is more than one such model. See the thread Chloramines and FC/CYA (with a link to a spreadsheet you might be interested in) as well as Oxidation of Urea for more info.

Thanks for the DIN reference, this is just what I'm interested in.

Note the irony in the DIN 19643 approach in that they are trying to reduce the negative side effects of a high active chlorine level by setting a low FC target of 0.3 to 0.6 with no ozone or 0.2 to 0.5 with ozone, both with no CYA, yet it is very hard to maintain 0.2 or 0.3 ppm FC in a pool and such amounts can be locally exhausted from bather events (especially urination). CYA is an active chlorine (hypochlorous acid) buffer so lets one get the best of both worlds -- a low active chlorine level, but still high enough for sufficient disinfection and oxidation, but also a high level of chlorine buffer that can be released as needed anywhere in the pool very quickly. Half of the chlorine bound to CYA is released in 0.25 seconds if the unbound chlorine were instantly depleted. The key is not to overuse CYA and to manage the FC/CYA ratio.

I don't know if turbidity lowers ORP, but if it does, that would be a good indicator of poor water quality where free chlorine seems to be a poor indicator. They used to say our chlorinated drinking water was good enough to drink when it was cloudy after torrential rains. Then they said chlorine doesn't work in turbid water, so they issued boil water advisories and they've spent a few hundred million dollars on municipal water upgrades.

Turbidity as well as Total Dissolved Solids (TDS) are generally irrelevant with regards to disinfection and oxidation by chlorine. Chemical reactions are based on concentration of chemical species and only a little bit on the ionic strength (especially for the reactions we are talking about) which is related to TDS. So the FC/CYA ratio rough rule-of-thumb applies equally well to a non-salt pool as to a pool with 3000 ppm salt for a saltwater chlorine generator. Also, while turbidity can be an issue for not being able to see someone drowning in the deep end of the pool, it really has nothing to do with water quality from a disinfection perspective. And besides, one doesn't generally clear turbidity with chlorine unless the cause of that turbidity is algae growth due to the FC/CYA ratio being too low. Turbidity caused by undissolved chemicals is generally handled via filtration, sometimes with the aid of coagulation.

Maybe it's better to say water quality is a combination of FC, combined chlorine, ORP, turbidity/colloids, TDS etc.

From a disinfection and oxidation point of view, such rates are based primarily on the FC/CYA ratio, somewhat on the pH, and then on the temperature and finally the ionic strength (related to TDS). The amount of Combined Chlorine (CC) is irrelevant to disinfection and oxidation, but is a marker that can relate to chloramines some of which can be irritating, but ironically at lower FC/CYA ratios one can have a higher CC and have the water and air quality be in better shape because there is more monochloramine, dichloramine and chlorourea with less nitrogen trichloride where the latter is what is the most volatile and irritating. In practice, in properly managed residential pools as on this forum, the CC is almost always very low, almost always <= 0.4 ppm and usually <= 0.2 ppm which is the lowest reading in the FAS-DPD test using a 25 ml water sample.

Are you asking these questions because you are managing a commercial/public pool or is this regarding a residential pool?

 

[RGB]

Here is the output from chem geek’s equations in the form I use directly to maintain outdoor pool effective chlorine (HOCl) levels.

For a prevailing pH (measured using phenol red or at test strip) and CYA (which is more stable and also read from a test strip), the chart shows the required FC level (from DPD reaction or a test strip) to maintain effective chlorine (HOCl) at 0.1 ppm (Cl2 equivalents), which in my pool is about the level needed to stop algal biofilm development during Summer.

The constants in the equations depend a bit on salt level (via TDS) but the reduction in the FC target level for a non-salt pool is very slight. I would love to see data from carefully controlled experiments on half-life for HOCl at various CYA levels, but I think the answers are going to be very conditional on season, location, pool depth etc. So for my money, with an SWG pool in the sub-tropics, 50 ppm CYA is plenty to provide buffering of FC around 10 ppm CL2 equivalents when the water is checked every few days, depending on amount of (domestic) use for swimming. I do have to add hypochlorite at times in Summer, when the SWG chlorinator can not keep up with chlorine demand.

If you are after lower HOCl levels (which work for me in Winter), here are examples of the output (requiring FC levels in the range my SWG chlorinator can supply). The calculations are almost unaffected by temperature across the range 10-30C.

 

[chem geek]

Thanks for the charts. I'm a bit surprised that you need such a high HOCl level to prevent algae growth. 0.1 ppm HOCl near a pH of 7.5 is close to an FC that is 20% of the CYA level (your graph shows closer to 18%, but I used my spreadsheet for the calculation rather than the approximate formula, though if one uses the Cl-CYA temperature dependence, then at 28C it's more like 15%). That's even beyond the 15% that is needed if one has yellow/mustard algae and doesn't do anything to completely get rid of it behind light niches, under removable ladders, etc.

The Pool School recommendations translate roughly to an HOCl of 0.03 ppm for non-SWG pools and 0.02 ppm for SWG pools.

 

[RGB]

Pool School and the FC:HOCl chart on page 1 of this topic are great at pH 7.5; but for a concrete pool my SWG manual recommends to be content with pH 7.8 (which saves a lot of HCl use fighting what seems to be nearer the pH equilibrium from effects in the SWG). Hence the charts with a pH axis. In case it is not obvious, I used chem geek's excellent spreadsheet to generate factors 'A' and 'B' at the pH levels of interest, then plugged these into his formula in the form: (FC as ppm Cl2) = ( A*(ppm CYA) + 5 ) / ( B + 1/(ppm HOCl) ). Thanks chem geek!

The Pool School recommended levels are probably spot on, under 'typical' conditions. So in case it helps, here is a graph for 0.025 ppm HOCl.

As an example of atypical conditions that may explain the occasional value of a chart for a higher HOCL level: I am often away from the pool for a week, and to avoid any noise nuisance for neighbours I do not run the pool pump - SWG chlorinator 24h/day. Also, I remove the 'thumper' wandering pool cleaner while I am away, so water circulation is primarily via the skimmer, under a thermal pool cover. The consequence of all this is that chlorine levels can drift down in my absence, letting the algae get a foothold on pebblecrete surfaces in Summer, and needing a higher chlorine level to knock them off than would have been required for prevention if I was present to adjust levels (and brush pool walls and floor) every 1-2 days.

 

[JasonLion]

A quick reminder for regular people: remember this is The Deep End. Most people don't have any reason to think about things at this level of detail.

 

[RGB]

One implication from chem geek’s analysis that seems to deserve faster diffusion to the shallow end is the changed effect of pH on HOCl:FC ratio, in the presence of CYA. Most sites continue to emphasise a pH target around 7.5. For example, in the Pool School Water Balance for SWGs page: “Monitor your pH and when it climbs to 7.8 add acid to lower it back to 7.5-7.6 (This is also IMPORTANT!)”. We can see from chem geek’s analysis that the difference in sanitising chlorine (HOCl) concentration will be minimal between pH 7.5-7.8 (even at 30 ppm CYA).

The Pool School Water Balance for SWGs page also recommends 70-80 ppm CYA (which seems to be above the level recommended by most SWG manufacturers and health authorities). Chem geek notes (http://www.troublefreepool.com/certified-pool-operator-cpo-training-what-is-not-taught-t18432.html) that this is to reduce the common problem of pH drifting up through SWG operation (probably because the production of H₂ and CL₂ in the SWC cell drives outgassing of CO₂, which consumes protons through the equilibrium CO₂ + H₂O <--> H₂CO₃ <---> H⁺ + HCO₃^-). CYAs pKa of 6.9 is a bonus for pool use, but the trade-off of increased FC requirement becomes problematic above 30-50 ppm CYA. It is hard to see how the optional addition of borate (pKa ~9.2) would help much to buffer at a target of pH 7.5; but it would have about 3-fold higher buffer capacity at pH 8. Chem geek has noted that outgassing of CO₂ would also be much less at pH 8.

Several considerations seem to point to an advantage of running (SWG-CYA) pools closer to pH 8 than pH 7.5. Why is it so “IMPORTANT!” to lower pH? Maybe for reasons other than sanitation or cost of chemicals, such as risk of scale formation or irritation to swimmers, or electrochemistry in the SWG, or because the common FC color test kits are calibrated for neutral pH (cart pulling horse), or just risk of the unexpected outside the range of past practice? It would be good to learn the reason in school.

 

[JasonLion]

RGB, practical considerations prevent us from making the recommended targets for PH too narrow or too high. Most relative novices are only able to read the PH test to +-0.2 or worse. Asking them to hit a narrow target leads to confusion and frustration. There are also dangers to using a PH target that is too high, since the range of the test isn't all that wide. The test only reads up to 8.2 and will show any level above 8.2 as 8.2. Worse, some common versions of it are only labeled up to 8.0. If we set the target PH at 8.0, the people with a kit that is only labeled up to 8.0 are very likely to mistake anything above 8.0 as 8.0 and accidentally let their PH get too high.

The wording in the Pool School article may not be ideal, but you are misunderstanding it's intent. The intention is to aim for a PH around 7.8, and only low it down to 7.5 when it does need lowering, rather than lowering it below 7.5, which was the old standard practice. To put that another way, the emphasis is designed to stress that people not follow the 7.2 to 7.8 with 7.5 being ideal advice given most other places, and is not intended to suggest that 7.5 is in any way ideal or the target level. We are trying to say that the range should be 7.5 to 7.8 with 7.7-7.8 being ideal.

When PH is constantly rising, as some SWG owners experience, there is a need to lower the PH to some fairly low value at regular intervals, least it get too high between adjustments. 7.5 to 7.8 is the narrowest range available that still allows a reasonable amount of time between PH adjustments.

There is wide disagreement among SWG manufacturers about the ideal CYA level. The recommended ranges from manufacturers go from 30 all the way up to 140. If you do a complete survey you will find that we are about in the middle of the range of what manufacturers recommend, with some much lower and some much higher and some in the same general area we are in. We recommend CYA be 70-80 because of extensive experience collected from a large number of members and because of practical concerns with how the test kit works, not because of any particular manufacturers recommendations.

The test kit becomes very problematic at CYA readings of 90+, which extremely high CYA levels sometimes being read as 90 or 100. That limits the upper target for CYA we can recommend, since anything over 80 is going to be prone to mistakenly raising CYA too much without noticing. Likewise, extensive experience has shown that CYA levels much below 70 result in significantly less stable PH and significantly more problems. Higher CYA levels help maintain more uniform FC levels through the day and provide more chlorine held in "reserve" to deal with issues that do come up. Higher CYA levels also lead to greater problems with shocking, but the dramatic reduction in algae incidents from the use of a SWG more then compensates for that.