Pool Water Chemistry (Warning: Can Get Technical)

This thread presents my findings so far on pool water chemistry including the following:

1. More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI)
2. Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)
3. Determination of pH and Alkalinity changes when adding chemicals to the pool
4. Various reaction rates including chlorine breakdown by UV

Disinfecting Chlorine (HOCl) vs. Total Free Chlorine (FC) and Cyanuric Acid (CYA)

The most important finding was how little disinfecting chlorine (HOCl) is left after chlorine combines with Cyanuric Acid (CYA) to get "stored" as chlorinated cyanurates (aka cloramides). The chart at the following link shows this relationship. (I recently discovered that all forms of chlorine are measured as ppm equivalents of chlorine gas, so all charts, graphs and the spreadsheet have now been updated to reflect this.)

HOCl Chart

Note that the red in the linked chart above represents a cutoff of 0.011 ppm HOCl which roughly corresponds to the 650 mV ORP level that the U.S. and WHO set as the minimum required for disinfection. The green color is a guess at 0.05 ppm HOCl of the minimum level of chlorine needed to prevent algae. The actual number may be quite different, from 0.02 or less to 0.1 or more, but based on Ben's "Best Guess CYA Chart" which is based on real-world experience, I suspect the actual number will be somewhere in this range. So, red means bacterial growth while green means possible algae growth. Blue is the safe area.

The following shows this same data in graphical form with lines showing the same two (probably correct) "bacteria" and (totally a guess) "algae" levels.

http://richardfalk.home.comcast.net/pool/HOCl.gif

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) - 6.6*(FC as ppm Cl2) + 6 )

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) + 6 ) / ( 6.6 + 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 0.112 when the correct answer is 0.095. That's not terrible (about an 18% error). However, with FC of 5 and CYA of 15 the formula gives HOCl as 0.370 while the correct answer is 0.199 (about an 86% error) which isn't very 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.

The inverse of the above chart may be seen at this link:

FC Chart

The chart columns from 0.02 to 0.1 ppm HOCl roughly correspond to "Ben's Best Guess CYA Chart". Ben's chart converted to show HOCl may be found here where you can see that the rough Min FC corresponds to 0.03 ppm, the rough Max FC corresponds to 0.07 ppm (implying an ideal target of 0.05 ppm) and the shock table is not consistent, but probably implies a minimum of 0.3 ppm, at least for green algae. User experience indicates that hard-to-kill yellow or mustard algae (and maybe black algae) may need 1.0 ppm HOCl for shock. User experience with black algae indicates that keeping active black algae from growing requires around 0.07 ppm HOCl.

A comparison of the "traditional" HOCl/OCl- graph with the same graph in the presence of CYA may be found at this post. This also shows how CYA is a "chlorine (specifically HOCl) buffer" that makes HOCl concentration about half as sensitive to changes in pH.

The original source for the equilibrium constants was done in 1973 (and published in 1974) where the recommended maximum CYA level was 25 ppm:

J. O'Brien, J. Morris and J. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate”, Chapter 14 in A. Rubin, ed.,
Chemistry of Water Supply, Treatment and Distribution, 1973 Symposium, (published 1974), Ann Arbor Science, Ann Arbor, MI, pp.
333-358.

[EDIT] See document here. [END-EDIT]



A Little CYA Goes A Long Way

NOTE: The mechanism of protection of chlorine from sunlight by CYA is currently under review in this thread. Higher CYA levels may protect even proportionately higher levels of chlorine more, especially in deeper pools.

The following is a graph showing that a large amount of the benefit of CYA protection of chlorine from UV (sunlight) is already there at around 20 ppm. This data is approximate, not only because it is dependent on the amount of sun exposure, but because the rate constants themselves change with FC level (because there is a mix of two different rates of destruction -- one from HOCl and the other from the chlorinated cyanurates which are more stable, but still breakdown from sunlight). The limiting half-life for HOCl/OCl- is 35 minutes which is consistent with pool studies, but some experimental studies give 11.6 minutes. The limiting half-life of the chlorinated cyanurates is 8.4 hours though some other data shows it could be 6 hours.

http://richardfalk.home.comcast.net/pool/HalfLife.gif

The following graph combines the two concepts of needing more chlorine at higher CYA vs. the greater protection of chlorine by CYA. The graph shows the total chlorine (FC) loss rate in ppm/hour vs. CYA at different HOCl levels. Remember that this rate of loss will slow down as chlorine gets used up. Nevertheless, the absolute loss of chlorine is greater at higher CYA levels (keeping HOCl constant) and is the downside to a "high CYA & high Chlorine" approach. However, the primary reason to have higher CYA and Chlorine is to have a sufficient buffer of chlorine to prevent it from dropping to dangerous levels. There is obviously a tradeoff here. Though using no CYA results in the least amount of chlorine loss, the fact is that you simply can't maintain a pool with only 0.05 ppm chlorine everywhere in it -- hence a minimum level is needed as a buffer.

http://richardfalk.home.comcast.net/...lorineLoss.gif

Salt Water chlorine Generation (SWG) pools seem to require a higher level of CYA, about 70-80 ppm, to operate efficiently. The theory is that the CYA is slow to "store" the chlorine as it is being generated so without enough CYA there is a build-up of chlorine that degrades the performance of the salt cell. I would prefer that the SWG manufacturers offer a larger lower-power (per length) cell that would work efficiently at lower CYA concentrations.

pH Rising

If you find that your pH wants to keep rising, this may be due to your pool outgassing CO2 to the air. The rate of outgassing increases with lower pH, higher alkalinity, and aeration of water (splashing, water fountains or slides, high wind, jets pointed up, etc.). The aeration of pool water is a physical process that will vary greatly from pool to pool, but the following chart shows the relative outgassing rate as a function of pH and Total Alkalinity. It is possible that the hydrogen gas bubble production from SWG systems contributes to significant aeration and is a source of rising pH in such SWG pools. The rate is actually a function of Carbonate Alkalinity so this chart is for a CYA of 30, but the variation with different amounts of CYA is not large. Note that there is a large variation with pH (the Y-axis is logarithmic). I have drawn a somewhat arbitrary "Limit" line at a relative rate of 15 that I have found is roughly the tolerance limit where many people start complaining about rising pH, but again aeration is a factor I cannot predict.

CO2 Chart

http://richardfalk.home.comcast.net/pool/CO2.png

Spreadsheet For Detailed Calculations

The link to the spreadsheet (in a ZIP file) that calculates all of the above data is PoolEquations.zip and was last updated 29-Jul-2010. It also does some of the things that BleachCalc does, but is not for novice users.

Also see Equations for Chlorine Chemistry.

Also see Oxidation-Reduction Potential (ORP) vs. HOCl

(I will continue to edit this post to add more detail and discussion.)

These are great - personally I find I need to stare at them quite a while to grasp them (I guess it's because you're dealing with 3 variables on two-dimensional graphs) but they convey more than words can.

Just a couple comments / question for clarification:

--What are the numerical values corresponding to "algae" and "bacteria" on your first graph? Do they correspond to the colors of the numbers in the table (i.e., red is below bact, green is above algae, black is in between)? Do these levels of HOCl correspond to particular levels of "oxidizing power" or "disinfecting power" that we commonly see quoted?

--Taking that algae line as a given, I read (roughly) the following minimum chlorine PPM for various CY PPM:

CYA..........Chlorine
5...........1
10..........2
20..........3
30..........4
50..........6

(Could you do another graph or chart which transforms the data into this format?) Unfortunately, these look even a bit higher than Ben's chart - discouraging!

--On the second graph, would you say the "take-away" is really that most of the chlorine-retention benefit of CYA is *already there* by 20 ppm, rather than "starts at" 20 ppm?

--The "no CYA" and "infinite CYA" entries are really points, right, not horizontal lines? (Hard to show clearly on this graph, though, I'll agree.)

--You talk about the "half-life" of chlorine, which gets at a concept you mentioned in that other post and I asked about there. I'm assuming that half-life here means the time in which the free chlorine ppm drops to 50% of what it was before. Perhaps you could also recast this graph to illustrate your point that "with twice as much chlorine, you lose it twice as fast." To me, this emphasizes the "double gotcha" with CYA - it forces you to bring your chlorine levels higher, which in turn means that your chlorine loss each day will be higher.

Thanks.

Quote:

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Originally Posted by stualden

These are great - personally I find I need to stare at them quite a while to grasp them (I guess it's because you're dealing with 3 variables on two-dimensional graphs) but they convey more than words can.

Just a couple comments / question for clarification:

--What are the numerical values corresponding to "algae" and "bacteria" on your first graph? Do they correspond to the colors of the numbers in the table (i.e., red is below bact, green is above algae, black is in between)? Do these levels of HOCl correspond to particular levels of "oxidizing power" or "disinfecting power" that we commonly see quoted?
Yes, below the "bacteria" line are red numbers in the chart and above the "algae" line are green numbers in the chart. The "bacteria" line corresponds to 0.011 ppm HOCl which appears to be about 650 mV ORP which is the minimum standard for disinfection in the U.S. and by WHO (though Germany has higher standards). I will add more data to my post to support this assertion, based on the Oregon Commercial Spas Study. However, the "algae" level is a guess since I cannot find good data to determine the level of chlorine needed to inhibit all algae, so I set an agressive level of 0.05 ppm HOCl. The real number could be 0.03 or less, or 0.1 or more, but given that Ben's table is based on reality, the real number is probably somewhere between 0.02 and 0.1 which correspond to the ranges in Ben's table (for the most part -- the shock section is the most inconsistent). I have been in communication with Ben on this and eventually he'll update his tables, but we want to get more real-world experiences from users first -- I plan to start another thread (non-technical) to get such data.

--Taking that algae line as a given, I read (roughly) the following minimum chlorine PPM for various CY PPM:

CYA..........Chlorine
5...........1
10..........2
20..........3
30..........4
50..........6

(Could you do another graph or chart which transforms the data into this format?) Unfortunately, these look even a bit higher than Ben's chart - discouraging!
See my response above. The 0.05 ppm HOCl level I set for "algae" is a guess so don't be distressed by it. Ben's table has low to high ranges that are roughly 0.02 ppm HOCl to 0.10 if you take combinations of "high chlorine & low CYA" to "low chlorine & high CYA". Please do not change any dosing behavior away from Ben's chart unless you are willing to take risks. Though I believe you will be fine from a disinfection point of view, we simply do not know enough about the algae prevention level to make a determination. You also don't want to "run out" of chlorine at any time, so Ben's approach of having a sufficient "buffer" of chlorine (stored in chlorinated cyanurates) is both conservative AND prudent.

--On the second graph, would you say the "take-away" is really that most of the chlorine-retention benefit of CYA is *already there* by 20 ppm, rather than "starts at" 20 ppm?
Yes, that is better phrasing and I will edit my post accordingly. Thank you.

--The "no CYA" and "infinite CYA" entries are really points, right, not horizontal lines? (Hard to show clearly on this graph, though, I'll agree.)
Yes, this is also true. Perhaps I can "truncate" the lines and/or dash them so that they look more like asymptotes which is what they represent.

--You talk about the "half-life" of chlorine, which gets at a concept you mentioned in that other post and I asked about there. I'm assuming that half-life here means the time in which the free chlorine ppm drops to 50% of what it was before. Perhaps you could also recast this graph to illustrate your point that "with twice as much chlorine, you lose it twice as fast." To me, this emphasizes the "double gotcha" with CYA - it forces you to bring your chlorine levels higher, which in turn means that your chlorine loss each day will be higher.
This is true, though I'm not sure how to graph this combination. Perhaps I can target a fixed ppm HOCl level and then the graph will show the relationship between FC and CYA that produce that HOCl level. On the same graph I can show the half-life of FC as a function of CYA (so CYA will be the X-axis and FC and half-life will be Y-axes). I'll try something and you (and others) can give me feedback/suggestions.

Thanks.

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Thanks a million for you comments, questions, and suggestions. I have only just started this post and will add more and hopefully make things clearer. Right now I'm going to add a link to the spreadsheet that is sure to scare the bejesus out of most people! I'll also explain how the HOCl table and graph are only for a pH of 7.5 and will also post approximate formulas for calculating HOCl or FC given CYA levels.

If I understand the first chart correctly.

Red numbers denote - Level to low to kill bacteria or algae

Black numbers denotes - no bacteria but algae possible

Green numbers denotes - algae also killed.


If this is the case, perhaps you should switch the Black and Green colors.
A small point I know, but most people will associate the color green with algae growth. And it should help a layman to comprehend the chart.

Quote:

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Originally Posted by haze_1956

If I understand the first chart correctly.

Red numbers denote - Level to low to kill bacteria or algae

Black numbers denotes - no bacteria but algae possible

Green numbers denotes - algae also killed.


If this is the case, perhaps you should switch the Black and Green colors.
A small point I know, but most people will associate the color green with algae growth. And it should help a layman to comprehend the chart.

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Yes, you understand the chart correctly. I was thinking of red being "bad" and green being "good", but I see your point that green, in the context of pools, probably means algae to most people. I'll change the colors in the table when I make some other changes I'm working on.

Quote:

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in the context of pools, probably means algae to most people

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I personally made the "green means algae" association so I mentioned it.

But now you have me thinking completely in context of pools. How about these for full visual association?

Red = Germs
Green = Algae
Blue = Clean water

.

If the pH is lowered to 7.2 do the charts change significantly?

THanks for taking the time to post this information. Having an understanding of the relationships is helpful

Following the Best Guess Chart has kept me out of trouble to this point. Through real world experience , I know it provides a margin of safety . I also know that things are better when I have FC towards the upper end of the scale. Now there's a little more insight as to how and why.

This data is very interesting. Thank you for doing this research.

I downloaded the Excel spreadsheet and began playing with it. Only problem is when I click on the "Calculate pH/TA" button, I get an error message "Compile Error - Can't find project or library". When I click OK, it takes me to the debugger. It highlights "SolverReset" in the "Sub Calculate_pH_TA_FC_CYA()" section of Module1. Any idea what the problem is on my end?

Thanks,
Jim

Quote:

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Originally Posted by aquarium

If the pH is lowered to 7.2 do the charts change significantly?

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I can best describe the change by an example. At a pH of 7.5, FC of 3.0 and CYA of 30.0 (and TDS of 550, Temp of 80ºF), the HOCl is 0.045.

At a pH of 7.2, with everything else the same, the HOCl is 0.052 which is not a huge change, but is still about a 15% change. The best thing to do is to download the spreadsheet and put in your actual numbers, but remember that except for disinfection, this is VERY preliminary so use with caution. Stick with Ben's best guess chart for now.

Richard

Quote:

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Originally Posted by jereece

This data is very interesting. Thank you for doing this research.

I downloaded the Excel spreadsheet and began playing with it. Only problem is when I click on the "Calculate pH/TA" button, I get an error message "Compile Error - Can't find project or library". When I click OK, it takes me to the debugger. It highlights "SolverReset" in the "Sub Calculate_pH_TA_FC_CYA()" section of Module1. Any idea what the problem is on my end?

Thanks,
Jim

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Jim,

You might have downloaded the spreadsheet before I edited my post to give the following instruction:

The “Pool Equations” spreadsheet uses macros to compute the reverse computations (calculating pH and alkalinity when you add chemicals) so to get this to work on a PC using an older version of Excel you will need to go into your “Tools – Add-Ins...” menu in Excel and check the box for “Solver Add-In”.

The error you are getting sounds like the Solver toolpak is not installed so try adding it in as I describe above and let me know what happens.

Richard

I am using Office 2003, the latest version. I have all the addins turned on, including "Solver Add-In" and still get this error.

Quote:

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Originally Posted by jereece

I am using Office 2003, the latest version. I have all the addins turned on, including "Solver Add-In" and still get this error.

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I'm sorry you are having trouble getting it to work. I created it on a Mac in Microsoft Excel 2004 for Mac Version 11.2.5 (060620) and I also tried it on a PC that has Microsoft Excel 2000 (9.0.6926 SP-3) running Windows 2000 Professional. It worked for me on the PC though I had to add the Solver Add-In and I also had to change the security level to Medium and accept the warning about macros running.

[EDIT]I have been very frustrated with trying to get Solver to work consistently when running from a macro on the PC. So, I'm going to delete my later posts that tried to explain workarounds since they are a pain and may not work. Instead, I'm going to try to rewrite the macro to do the Solver function myself. I've only got two "goals" with two "variables" so that shouldn't be too hard to do with a 2-variable partial differentiation (Newton-Raphson) approach. I hope that doesn't end up being as hard as it sounds![END-EDIT]

[SECOND-EDIT] Since around 8/23/06 (see later posts) Solver is no longer used. [END-EDIT]

Richard

I updated the PoolEquations.zip spreadsheet as well as the graphs and charts in the first post in this thread (and the links) to reflect the fact that FC (and CC and TC) are measure in ppm of Cl2 equivalent and not as HOCl. I also changed the HOCl concentration to also be in ppm of Cl2 equivalent so all chlorine is measured in the same way and this made the changes to everything rather minor.

I also added the Borax and Borates (ppm Boron) to the spreadsheet, though I need to do some cleanup to make it easier to use. I still need to replace the Solver calculation with my own iteration so that the spreadsheet will work reliably on a PC (it works fine on my Mac).

[EDIT] I made an initial error and have now corrected it so be sure that the first line of the spreadsheet not only gives the date 8/15/06, but also has the word "CORRECTED2" after this date. [END-EDIT]

Richard

Quote:

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Originally Posted by chem geek

I updated the PoolEquations.zip spreadsheet as well as the graphs and charts in the first post in this thread (and the links) to reflect the fact that FC (and CC and TC) are measure in ppm of Cl2 equivalent and not as HOCl. I also changed the HOCl concentration to also be in ppm of Cl2 equivalent so all chlorine is measured in the same way and this made the changes to everything rather minor.

I also added the Borax and Borates (ppm Boron) to the spreadsheet, though I need to do some cleanup to make it easier to use. I still need to replace the Solver calculation with my own iteration so that the spreadsheet will work reliably on a PC (it works fine on my Mac).

[EDIT] I made an initial error and have now corrected it so be sure that the first line of the spreadsheet not only gives the date 8/15/06, but also has the word "CORRECTED2" after this date. [END-EDIT]

Richard

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Darn! The previous version was working fine for me (PC). Now this version gives me the "solver error". I must have installed it wrong, or did not remove the first one correctly. Any other issues reported with this new version.

Double darn!! The original version is no longer available. I can't even go back to that one. I'm drifting aimlessly now, I was getting very attached to that 'working' spreadsheet.

ubalr1,

I'm sorry you now get the Solver error. I assume this only occurs when clicking on the "Calculate pH/TA" button, right? This has been reported as very flaky on a PC and I was able to reproduce the problem on a PC and it sometimes would work and then it wouldn't, so I don't think it's only the version change but something very strange with PC's solver getting a spreadsheet created on the Mac.

I plan to replace the use of Solver, but am going on vacation (to Canada) for 2-1/2 weeks. I'm taking my computer and one of the things I'll do during down time (on the train, for example) is work on the spreadsheet. Yes, I consider that fun (shows how geeky I am).

I don't have the earlier version myself anymore. As soon as I replace the Solver with my own iterative code, I'll post it and let everyone know by a post to this thread. Again, sorry you got "hooked" with a working version. It's pretty neat when it works, eh?

I just put up the latest version just in case that one works for you. You can download it and try it out.

Richard

ubalr1 and others,

I just removed the Solver dependence and uploaded a new (modified) spreadsheet. Give it a try and let me know if it works. Unfortunately, it will still "think" that Solver is being used when you load the spreadsheet so it will give you two macro warnings and may require you to add the Solver Add-In, but it won't actually use the Solver itself (which was the main problem).

[EDIT] As of around 8/30/06 I removed the apparent reference link to Solver that made Excel think that Solver was used even if it wasn't. [END-EDIT]

Richard

Quote:

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Originally Posted by chem geek

ubalr1 and others,

I just removed the Solver dependence and uploaded a new (modified) spreadsheet. Give it a try and let me know if it works. Unfortunately, it will still "think" that Solver is being used when you load the spreadsheet so it will give you two macro warnings and may require you to add the Solver Add-In, but it won't actually use the Solver itself (which was the main problem).

Richard

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Hope you had a fine vacation Richard.

The speadsheet is working fine for me now. I had to uncheck the Solver in "Add-Ins". Thanks.

Couple of questions:
1) Can I change the concentration of Sodium Hypo? You have 11.9% in there. I use the 6% bleach.
2) I don't know what my Total Sulfates or Total Borates are. Can I still get accurate results using the spreadsheet?

Quote:

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Originally Posted by ubalr1

Hope you had a fine vacation Richard.

The speadsheet is working fine for me now. I had to uncheck the Solver in "Add-Ins". Thanks.

Couple of questions:
1) Can I change the concentration of Sodium Hypo? You have 11.9% in there. I use the 6% bleach.
2) I don't know what my Total Sulfates or Total Borates are. Can I still get accurate results using the spreadsheet?

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I finally figured out how to remove Excel's thinking that there were references to Solver (when there weren't any more) so will update the spreadsheet with that fixed soon [EDIT] just uploaded an update today 8/27/06 with the Solver references removed [END-EDIT] [EDIT] and again today 8/28/06 with further improvements listing different concentrations for bleach in a pull-down menu. [END-EDIT]

Yes, you can get accurate results without the Total Sulfates or Borates entered, unless you have intentionally added lots of Borates (say, more than 30 ppm). The sulfates are there in case one has very high sulfate water (> 100 ppm) in which case this starts to affect the calcium carbonate water balance.

Richard

Richard,
I've been following this thread for quite a while and am wondering what is the latest date (version) on your spreadsheet. I've downloaded it several times but I'm not sure I have the latest.

I have printed out all the charts and graphs so I can study them --- my old brain take awhile sometimes.:o

I really appreciate the time you have put into this.

Quote:

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Originally Posted by fcfrey

I've been following this thread for quite a while and am wondering what is the latest date (version) on your spreadsheet. I've downloaded it several times but I'm not sure I have the latest.

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Whenever I upload a newer version of the spreadsheet, I update the date near the end of the first post in this thread (near where I have the link to the PoolEquations.zip file) as well as the date in the first line of the spreadsheet and currently it's at 9/12/06 though I've made some minor changes since but haven't uploaded them yet -- mostly relating to this confusion about Trade % vs. % Sodium Hypochlorite for liquid chlorine and bleach. [EDIT] I uploaded a new update 10/30/06, but check the end of the first post to see the date of the latest update. [END-EDIT]

Richard

Responding to this post.

Brad,

You can first check the first post in this thread on this forum that I created to explore the relationship between CYA and FC (check out the link near the end of the post where I have analysis about ORP as well). There's a lot of info there, but I also list the original source document that determined the equilibrium constants between the chlorinated cyanurates, cyanuric acid (CYA) and hypochlorous acid (HOCl). These constants are also found in this EPA document. There is also this article written by Kent Williams for the Professional Pool Operators Assocation (PPOA), but several people have said that Kent had an agenda as he was promoting ORP sensors, but my independent spreadsheet calculations (the spreadsheet is also found at the end of my post linked above) show that his data was essentially correct. Finally, this research paper indicates that the chlorinated cyanurates do not have disinfecting capability and in fact actually lower the effectiveness of hypochlorous acid -- in other words, not only does CYA significantly lower hypochlorous acid concentration (i.e. chemical equilibrium), but that small remaining hypochlorous acid is less effective in the presence of chlorinated cyanurates (but this is probably something unique to cysts that get some sort of sensitization from the presence of the chlorinated cyanurates). If you look at the table on page 1160 of the paper (click on the button for that page on the link), you can see reductions of "titrable Cl2" which is FC and that for CYA levels that are about the same (in ppm units -- the table shows Cy as moles/liter x10^5) the HOCl level is cut down by over a factor of 5. The situation is far worse when CYA concentrations are much higher than chlorine concentrations. By the way, I ran some of the numbers in the table in my spreadsheet and there is some error -- for example, the table at pH 7, Cy 7.0, Cl 14 shows HOCl as 1.88 while I calculate 2.18, but that's not a terrible error (though I wonder if the older equilibrium constants are off a small amount or if this paper's methodology is off or different...at any rate, it's not as if there is anything terribly wrong).

I am most proud of the fact that my results from theory match what Ben had discovered through observation in creating his Best Guess CYA Chart which I compare with calculated hypochlorous acid concentrations in a link in my post. If the calculations weren't borne out in the real world, they would be pretty worthless. The correlation with Ben's chart is pretty amazing for the Min/Max levels, above about 30 ppm, and the difference below 30 ppm is mostly due to Ben requiring minimum levels of chlorine so you won't run out, not just for disinfection. The shock levels don't track as well, but recent experience with nasty mustard/yellow algae on this forum has borne out the theory as well (some day, Ben's chart may be adjusted, but we'll need more validation before that -- and besides, it's his chart and his forum).

The bottom line is that the chemistry involved has been known for a long time and though a bit complex, it isn't quantum physics. I was able to do an accurate calculation in a spreadsheet. It is also possible to approximate the effects just looking at dominant species and only 2 of the 9 chemical equations (6 of which are independent). From my contact with various manufacturers, I would say that at least some are aware of these effects (one had done a spreadsheet themselves in order to create some of the data for the EPA link above), but that this information does not get out to the industry as a whole, probably for economic reasons. If people truly understood the relationship of CYA and FC to disinfection/oxidation, they would not use Trichlor as their sole source of chlorine (unless they backwash or drain/refill frequently) nor Dichlor as a frequent source in a spa. If people used chlorinating liquid or bleach (or an SWG) and did so properly so as not to get algae, then who would buy the expensive premium products (Trichlor, Dichlor, algaecide, clarifiers, flocculants, degreasers, stain removers)?

As for your 42 ppm FC example that was without CYA, that's a whole heck of a lot of chlorine so there is no wonder that corrosion occurred. By the way, for your indoor pool installations with no CYA, what is the typical FC level? Also, how do you shock the pool to rid combined chlorine (CC) and do you find that CC buildup to be a problem? Or do you use a weekly maintenance dose of non-chlorine shock (potassium monopersulfate, KMPS)? Since the only reports of people getting sick from pools is from indoor pools (presumably without CYA) with asthma and other repiratory ills from competitive swimmers and small children, I propose actually using some CYA (10-20 ppm) in indoor pools to significantly cut down HOCl levels since that would cut down the rate of production of disinfection by-products (DPBs). Also, using KMPS would eliminate such DPBs by avoiding chlorinated organics altogether. Radical ideas, but I don't think anyone has tried them to prevent the reported problems. (I know I'm getting off the topic of corrosion and if we get into this further, we should start a separate thread or go offline).

Richard

Once I'm not so busy I'm going to read through those articles, thank you. I had only read "Benefactor or Bomb" before.

Residential indoor pools around here use a FC reading of 1-3 ppm normally. When using a SWC they normally do not ever 'shock' the pool, either by means of the SWC, liquid chlorine of monopersulfates. We test the water for them monthly and always have 0.0 ppm CC using FAS-DPD test kits. Customers that have used their indoor pools for years without a SWC almost always comment that the "chlorine smell" is gone for good once the SWC is running. New pools built with SWC's never mention anything about the smell, leading me to believe that it isn't there at all, and they just don't realize the difference the SWC is making.

Commercial indoor pools use a FC of 4-8-ish when using a SWC. We don't have inspectors around here to check, and our city (120,000 people) does not require anyone to have CPO training. We try to offer training as much as possible for our commercial customers, but we can't teach everyone everything. When we set up a new commercial pool with a salt system, we include pH and ORP controls so that they are never out of the proper range. As far as I know, these customers also do not 'shock' their pools.

For non-SWC customers, I would guess they have some CYA in their pools; from pucks. There are at least a few places that have used pucks, liquid shock, cal hypo, algaecide, flocs, coagulants, and monopersulfate - ALL in the past year. They will often let the chlorine drop to 0, the pool goes cloudy, they add a bunch of algaecide, clarifier and monopersulfate and when that doesn't work, they call us. We pour in some liquid chlorine and it clears up. They hardly listen to what I have to say, and do not feel that a SWC system would be worth the money. They are just content to have their pools shut down for a day or so every few weeks. These pools always have a strong chlorine odour; they just do not understand what they are missing.

I believe that most of the indoor pool related health problems, such as asthma, are related to combined chlorines. I also think that most corrosion of indoor pool areas is from CC's. I remember Sean saying once that they tested the chlorine inside their cell at above 80 PPM. This would suggest a constant shocking happening inside the cell, reducing combined chlorines. I think that when combined chlorines are gone, most of the health problems and corrosion of pool rooms dissappears.

Brad

Brad,

Thanks for the info. On this forum, feedback from users suggests that for outdoor pools, which have CYA and are exposed to sunlight, that even without an SWG the CC level is almost always at 0. After heavy bather load it may go up briefly, but will then go down on its own in a day (or two with really heavy bather load). For users that don't want to wait, they shock their pools with chlorine, but this only occurs 1-3 times in a swim season. Last year, I didn't have to shock my pool at all and never had CC above the one drop 0.2 ppm minimum on the test. Our bather load is not heavy with my wife swimming 2-3 times during the week and both of us using the pool 1-2 days over the weekend. So the combination of maintining chlorine levels and (possibly) having sunlight seems to keep CC in check. The chlorine levels suggested on this forum are higher than the 1-3 ppm range you describe. It's usually a minimum of 3 ppm at 30 ppm CYA and higher for higher CYA levels. That not only disinfects, but is enough to keep algae from growing (though some users reported needing 5-6 ppm at 30 ppm CYA to keep away mustard/yellow algae).

I would agree that the smell is from combined chlorines, with monochloramine smelling bad, but trichloramine really smelling awful. Health problems generally don't come from monochloramine, but they can from trichloramine and especially from certain chlorinated organics including the trihalomethanes that include chloroform.

The SWG does superchlorinate a fraction of the water that goes through the cell. All of the water is not exposed to the high chlorine level -- if it were, then you would have 80 ppm FC emerging from the cell and that's not what happens. Nevertheless, this seems to be enough to keep CC at bay after multiple turnovers. There is a long and interesting discussion about this at this thread where I finally figured out (in the last post in that long thread) that regularly adding chlorine (which is very concentrated -- 60,000 to 125,000 ppm when added to the pool) doesn't superchlorinate the pool the way I would have thought because the pH is so high, so it doesn't get above about 14 ppm of disinfecting and oxidizing chlorine (hypochlorous acid) when no CYA is present and with CYA this amount is even lower. Though hypochlorite ion also has some oxidizing capability and is extremely high in concentration (when adding chlorine to the pool), it apparently is not as effective at eliminating combined chlorine.

As for health problems in indoor pools, the SWG keeping CC near zero may very well prevent these problems. Disinfection byproducts (DPBs) are probably being created faster than in outdoor pools because the CYA levels are zero in indoor pools (making the chlorine 10-30 times more reactive) but because they don't build up with the SWG, perhaps they are kept below dangerous levels that affect swimmers. It would be interesting to see if competitive swimmers report no problems in indoor SWG pools. As for corrosion, that should be related to the hypochlorous acid concentration, so not having CYA in a pool should make it more corrosive.

I will say that from my wife's personal experience, that her swimsuits degrade (the rubber falls apart and they start to fade) over a single winter of use in an indoor pool (presumably with no CYA) at our a local community center. She never has any problem at all with her swimsuits over a long summer in our own outdoor pool that has CYA. So this is part of the reason why I think that using a small amount (10-20 ppm) of CYA in an indoor pool would be beneficial since it minimizes exposure to chlorine in the pool and this would apply to SWG pools as well. This winter, I'm going over to the community center pool and will measure the chemistry to see if there's anything going on besides no CYA (i.e. is the FC super high).

Richard

I wasn't sure where to put this interesting piece of info, so I'm putting it here in the Pool Water Chemistry thread since it is related to the chlorine/CYA relationship. This link describes a patent where a fluid is initially viscous in order to carry particulate matter and then becomes less viscous (more fluid) over time in order to deposit or settle its particulate matter. The patent uses a thickener, a hypochlorite, and cyanuric acid. Differing ratios of the CYA to chlorine determine how quickly the thickener breaks down (which changes the viscosity of the water). It's interesting that this is a patent that DEPENDS on the chlorine/CYA relationship for its proper functioning in having only disinfecting chlorine (that is, chlorine not bound to CYA and probably only hypochlorous acid) break down the thickener and having that concentration (and therefore reaction rates) be dependent on the chlorine/CYA ratio.

Richard

I didnt think this post deserved its own thread, but it is some useful info regarding ozone and chlorine. waterbear (Evan) and I worked out how ozone could oxidize chlorine since there were some reports of people using ozonators finding greater chlorine usage (loss). The following is apparently what happens:

O3 + H2O + 2e --> O2 + 2OH- ............ Eo = +1.24V
OCl- + 2OH- ... --> ClO2- + H2O + 2e .. Eo = -0.66V
-------------------------------------------------------------
O3 + OCl- ...... --> O2(g) + ClO2- ........ Eo = +0.58V
Ozone + Hypochlorite ion --> Oxygen + Chlorite ion

O3 + H2O + 2e --> O2 + 2OH- ............ Eo = +1.24V
ClO2- + 2OH- .. --> ClO3- + H2O + 2e .. Eo = -0.33V
-------------------------------------------------------------
O3 + ClO2- .... --> O2(g) + ClO3- ......... Eo = +0.91V
Ozone + Chlorite ion --> Oxygen + Chlorate ion

With the net result being the following:

2O3 + OCl- ... --> 2O2(g) + ClO3- ......... Eo = +1.49V
Ozone + Hypochlorite ion --> Oxygen + Chlorate ion

So ozone will oxidize chlorine to form chlorate. This is a very strong (likely) reaction, but the above info does not determine the rate of the reaction.

Richard