Josh,
First of all, welcome to TFP!
As for persistent chloramines, I can't say that the theory I'm about to describe has been proven with indoor pools. The only thing that has been shown, in spas, is that using too much CYA leads to regular Combined Chlorine readings since the breakpoint reaction is slowed down too much. At the other extreme with too little CYA, the problem is that this produces more of the nastier and smellier dichloramine and nitrogen trichloride (trichloramine) products in addition to monochloramine, but this part has not been proven.
Basically, in a nutshell, CYA acts as a chlorine (hypochlorous acid, specifically) buffer and moderates its concentration significantly. You can see in
this post the traditional industry graph of HOCl/OCl
- which only really applies in a pool with no CYA and then I show the true graph in the presence of CYA. In a pool with 3.5 ppm FC and 30 ppm CYA and a pH of 7.5, for example, 97% of the chlorine is bound to CYA in a series of chemicals called chlorinated cyanurates that are not effective sanitizers nor oxidizers. 1.5% is hypochlorite ion, OCl
- while only 1.5% of the FC is hypochlorous acid which is disinfecting chlorine. Free Chlorine (FC) measures not only hypochlorous acid and hypochlorite ion, but also all of the chlorinated cyanurate species as well.
I write more about this including some analogies to hopefully make it easier to understand in
this post on another forum (just focus on the section "Chlorine and Cyanuric Acid (CYA)" unless you are interested in the rest).
The bottom line is that with 3-5 ppm FC in a pool with no CYA, you are over-chlorinating the water. This makes the breakpoint reaction happen too quickly so results in a higher production of disinfection by-products, according to breakpoint chlorination models I have (I use two different ones and am still looking for even better ones). As an example, let's say you have 4 ppm FC at 87F temperature and a pH of 7.5 and 0.1 ppm ammonia (as Nitrogen -- equivalent to 0.5 ppm combined chlorine if turned into monochloramine). The following table shows what happens with no CYA and with 10 ppm CYA. The numbers are in pairs since there are two breakpoint chlorination models I have and they produce different results -- I think the second in the pair might be more accurate. [EDIT] I've added a third number that is probably the most accurate since it is from the most recent and comprehensive model of breakpoint chlorination. [END-EDIT]
....................................
0 ppm CYA .................................................
10 ppm CYA
Chemical ........... Peak Time ....... Concentration ................... Peak Time ....... Concentration
Chlorine ................ 0 sec ............... 4 ppm FC ............................ 0 sec .................. 4 ppm FC
Ammonia .............. 0 sec ............... 0.1 ppm NH
3-N ................... 0 sec .................. 0.1 ppm NH
3-N
Monochloramine .... 3 sec .............. 360 ppb ........................ 10 / 10 / 27 sec ............ 360 ppb
Dichloramine .... 30 /140 / 48 sec ... 80 / 265 / 73 ppb ............ 1 / 6 / 7 min ............. 15 / 83 / 59 ppb
Nitrogen trichloride . 10 / 15 / 4 min .. 1.6 / 9.3 /
122 ppb ..... 20 / 40 / 8 min .......... 0.15 / 1.1 /
13 ppb
50% Breakpoint .... 140 seconds (2.3 minutes) ........................ 21 minutes
90% Breakpoint .... 400 seconds (6.7 minutes) ........................ 64 minutes
Note that after breakpoint is complete, the ammonia, monochloramine and dichloramine will go to zero, but the nitrogen trichloride will remain persistent (though it is quite volatile and very smelly and irritating). Though the above would explain why indoor pools with no CYA (as well as a lack of sunlight and poor air circulation) would have more problems with smelly chlorine and people having respiratory problems, eye irritation, and asthma, it doesn't really explain large quantities of combined chlorine.
I think that may be coming more from other compounds other than just simple ammonia. For example, urea, which is ammonia-like but may not behave as described in the above model, as well as other organics. Usually, chlorine doesn't combine that quickly with other organics unless the chlorine concentration is high, which is the case in pools without CYA.
So in order to reduce the rate of all chemical reactions with chlorine and to make the pool sanitation more consistent with that of outdoor pools, I propose using a small amount of CYA to moderate the disinfecting chlorine level. Even 10 ppm CYA will cut down the disinfecting chlorine level significantly.
4 ppm FC with 10 ppm CYA has the same disinfecting chlorine level as 0.5 ppm with no CYA. So all reactions, including the formation of perhaps more persistent combined chlorine, will be around 8 times slower and the final quantities will be about 1/8th in amount. It will still be relatively fast, but will reduce the creation of disinfection byproducts.
So while using the CYA makes a lot of sense to reduce corrosion, oxidation of swimsuits and hair, and production of disinfection of byproducts (all of which depend on the effective disinfecting chlorine or hypochlorous acid concentration), I don't know if it will significantly reduce the production of Combined Chlorine that you are seeing. My guess is that it will, but we don't have any test case that has proven that (as I said before, we've seen the opposite case of too much CYA causing Combined Chlorine, specifically monochloramine, and then lowering the CYA to a more reasonable level of 4 ppm FC with 20 ppm CYA made the CC go away, but the case of no CYA vs. a small amount of CYA we haven't proven in real pools yet).
Just as using MPS will artificially inflate the ORP which is not good for your automation of chlorine dosing, the use of CYA will also affect the ORP by lowering it, BUT it will be a legitimate measurement of what is going on because right now you are overdosing in chlorine. ORP sensors are unfortunately inconsistent, but 4 ppm FC with no CYA is 1.9 ppm hypochlorous acid and will report 812 mV ORP on Chemtrol, 803 mV ORP on Oaktron, 869 mV ORP on Sensorex. With 10 ppm CYA the same 4 ppm FC is 0.25 ppm hypochlorous acid and will report 746 mV ORP on Chemtrol, 719 mV ORP on Oakton, 622 mV ORP on Sensorex. It is still plenty for sanitation as most outdoor pools operate closer to 3.5 ppm FC at 30 ppm CYA which is 0.05 ppm hypochlorous acid and will report 695 mV ORP on Chemtrol, 654 mV ORP on Oakton, 428 mV ORP on Sensorex. The Sensorex data was taken from a table they posted on the web so I suspect it's just plain wrong. The Oakton data is from actual field data from a portable Oakton sensor. The Chemtrol is from data on the web, but where they appeared to do a decent job measuring their sensor in a variety of conditions. So you'll have to "recalibrate" your ORP controllers to achieve your target 3-5 ppm FC level after you've added the 10 ppm CYA since the target ORP will be lower.
By the way, the 10x rule for breakpoint of chlorine is wrong -- another one of those industry myths where it starts out with some correct information and then gets twisted somewhere along the way. The correct rule comes from the fact that it takes 1.5 times the amount of chlorine, on a molecule per molecule basis, to achieve breakpoint with ammonia as indicated below:
2HOCl + 2NH
3 --> 2NH
2Cl + 2H
2O
HOCl + NH
2Cl --> NHCl
2 + H
2O
NHCl
2 + NH
2Cl --> N
2(g) + 3H
+ + 3Cl
-
-----------------------------------------------------------------------------------------------------------------------
3HOCl + 2NH
3 --> N
2(g) + 3H
2O + 3H
+ + 3Cl
-
Hypochlorous Acid + Ammonia --> Nitrogen Gas + Water + Hydrochloric Acid (as ions)
Ammonia is typically measured in units of ppm Nitrogen which is 14.0067 g/mole while chlorine is measured in units of ppm Chlorine Gas, Cl
2(g), which is 70.906 g/mole. So it takes 1.5*70.906/14.0067 = 7.6 times more ppm of chlorine to oxidize (achieve breakpoint with) a ppm amount of ammonia. Original experiments found that using 8-10 times worked best so that's where the original 10x rule came from.
BUT, note that the above rule uses a measurement of ammonia based on ppm Nitrogen. The rule for pools uses Combined Chlorine which is a measurement in units of ppm chlorine (gas), NOT ppm ammonia-nitrogen. One monochloramine or other combined chlorine molecule will measure the same as one molecule of chlorine in water. So because they are the same units, the real factor would appear to be just 1.5, but even this isn't quite correct because Combined Chlorine isn't measuring ammonia but monochloramine (or it's equivalent). As you can see from the reactions above, one molecule of chlorine combines with ammonia to produce monochloramine so when measuring Combined Chlorine you've already used up 1.0 of the 1.5 you need. So technically, you only need 0.5 times the Combined Chlorine amount to achieve the rest of breakpoint and even rounding up and being conservative this would be just 1 for 1 -- a Free Chlorine (FC) that is equal to Combined Chlorine (CC).
In fact, in most pools and spas, there is continual breakpoint and it doesn't require anything near 10x the Combined Chlorine even when it measures a relatively small amount. I think that the 10x rule has persisted at least partly because in pools with CYA the breakpoint reactions are slower, as I had indicated earlier, so by using a higher level of chlorine one speeds up the reactions. At least with CYA in the water, one doesn't go overboard and produce a lot of disinfection byproducts. The bottom line is that hopefully with a little CYA in the water, you won't be seeing Combined Chlorine build up, but you could see it temporarily after bather loads and then drop down in the hours after such load has dropped off. At least that's the hope. And yes, air circulation is very important to helping the breakpoint reaction proceed well. When airing out the pool to reduce CCs, especially if you smell anything chlorine-like, then you should run the pump and point the returns up and otherwise aerate the water to remove the volatile compounds. Note that the pH may rise if you do this (depending on your TA level).
If you have a smaller pool you can use as a "test", then that obviously would be the best way to go. Be sure to let us know what happens and report your progress. It may take some time to get the Combined Chlorine down initially, but the key will be to see if it keeps getting created at the same rate when CYA is present -- that is, if it doesn't rise as much as you've been seeing.
There is no question that UV from sunlight keeps Combined Chlorine in check so that using a UV system should help in that regard considerably, but I hope you try the CYA to see what happens. Even if it doesn't help with the CC, then as long as it doesn't hurt it's actually much better and healthier for your swimmers and not as harsh on your equipment (for corrosion).
By the way, you can use Dichlor as a fast way to add both chlorine and CYA since it dissolves readily, but it is a more expensive source of chlorine and CYA (see
this link for a CYA cost comparison and
this link for a chlorine cost comparison). For every 10 ppm FC added by Dichlor it also adds 9 ppm to CYA. Pure CYA is the least expensive, but dissolves very slowly so the fastest way is to hang it in a panty hose or sock over a return flow so it will take a day or two instead of a week to dissolve. I only recommended 10 ppm CYA in what I wrote in this post so that your techs wouldn't freak out and would be able to more readily dilute a pool back towards zero CYA if you changed your mind, but really 20 ppm CYA is a more reasonable number if things work out for you. This is not only measurable with some CYA (manual) turbidity tests, but it further reduces disinfecting chlorine by a factor of 2 while still being plenty for disinfection (and algae prevention).
Richard
