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Help With Chlorine Problems


sharrowm

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I'm a new spa owner and we have been using it almost daily for about a month now. I have been doing a lot of research on spa chemistry. I purchased a Taylor test kit (K-2006) to do my testing. I have found it to be fairly easy to keep everything balanced except the combined chlorine. Here is my latest test results:

PH: 7.5

TA: 100

Free chlorine: 2ppm

Combined chlorine: 7ppm

Based on my research the CC should be less than .5. According to the guide that came with my test kit I need to super-chlorinate the spa with enough chlorine to create 10 times the CC level or 70ppm. And if I'm reading the tables & doing my conversions correctly then that would require 6.5 cups of ultra bleach (6%) for my 350gal spa. That just doesn't seem right to me. Am I doing something wrong or is it OK to add that much bleach to my spa. Also, how did the CC get so high in the first place? Any help is appreciated.

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I assume you have no CYA in your spa (i.e. don't use Trichlor tabs or Dichlor powder). If you do use Dichlor, then you need to test your CYA level, but generally you don't want to use CYA in a spa unless it's going to be uncovered in direct sunlight (during usage or at other times). [EDIT] We now know that you DO want to use Dichlor initially or add pure CYA to get to around 30 ppm CYA; otherwise the active chlorine level is too strong and too harsh on spa covers, swimsuits, skin, outgasses faster, etc.) Killing the bacteria Pseudomonas aeruginosa that causes hot tub itch requires rather high levels of active chlorine and using CYA will significantly cut down active chlorine levels.

As for your combined chlorine, this is what is produced when chlorine combines with ammonia/urea and organics in your water. My guess is that your chlorine level was too low at some point given how many people were in the tub (i.e. high bather load) and got overwhelmed so combined chlorine built up. If the chlorine level is high enough, then the combined chlorine will go to breakpoint which releases nitrogen gas and produces chloride ion (i.e. salt) at which point the CC drops back down close to 0. If you had too much CYA in your tub, then it can be very hard for the chlorine to reach breakpoint quickly enough -- the creation of combined chlorines is a rather fast process (especially with ammonia/urea), but achieving breakpoint is generally much slower (about a half-hour if no CYA). So this is yet another reason not to use CYA in your hot tub (it makes achieving breakpoint that much harder). If you weren't checking your CC regularly, then at some point it started to build up and if you don't catch it early, it will continue to grow and grow (because the FC is not 10 times the CC so you build up harder-to-breakdown intermediate products instead of simple chloramine).

In an outdoor pool, exposure to sunlight helps achieve breakpoint so that is how pools, even with CYA, avoid having CC rise. Also the bather load (people per volume of water) is much, much lower in a pool so the chlorine is much less likely to get overwhelmed (i.e. the FC is always > 10 times the CC). If you were able to expose your hot tub to UV from sunlight (or UV lamps), you could breakdown some of the CC, but that's probably not an option for you so...

As for how to rid yourself of the CC, yes you need to add 10 times it's level in chlorine, but as you point out that's awfully high. I can't say if 70 ppm would bleach out anything in your tub. It might be easier, in your case, to just dump and refill the water which is something you should be doing every so often anyway (after a month of daily use, your water could probably use a full refresh). In the future, monitor the chlorine levels, including CC, more closely and when the CC is anything more than 0.5 ppm, shock with more chlorine. At that point, adding extra chlorine when the CC is, say, 1 ppm is not a big deal as 10 ppm isn't too high. If you still find it difficult to manage avoiding CC buildup, you can use a non-chlorine shock such as potassium monopersulfate (KMPS) which will combine with ammonia/urea and organics before chlorine and avoids CC completely. This also has the advantage of reducing disinfection by-products (DPBs) which would be greater in a spa without CYA and at higher temperatures.

There is an unfortunate catch-22 with hot tubs in that, to avoid hot tub itch, they require higher levels of active chlorine than pools and that means using little or no CYA. You also need enough FC to not "run out" from the heavy bather load so that probably means at least 3 ppm and probably closer to 5 ppm FC. However, such high levels of active chlorine will definitely wear on swimsuits. My wife uses an indoor pool in the winter where, apparently, no CYA is used (it normally isn't used indoors), and her swimsuits last only one winter when using that pool -- the rubber wears out first and then the swimsuit shows signs of fading. In our own outdoor pool that uses CYA, the swimsuits never show any signs of wear. I haven't really figured out a good answer for hot tubs. Perhaps using a very, very small amount of CYA might be OK, but not more than 10 ppm. Unfortunately, the test kits only measure down to 30 ppm on most kits including the K-2006 (though the K-1720 measures down to 20 ppm).

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Great info! Thanks Chem Geek.

Actually, I have been using dichlor for sanitizing and bleach for shocking. After reading your post I checked for CYA and didn't detect any. However, as you pointed out, the k-2006 will only detect above 30ppm. The test sample was very clear even with the vial completely filled so I would guesstimate that CYA is probably not a problem. I think that I will take your advice and start over with a water change.

Question: I tested my tap water for chlorine and the FC is .5 & the CC is 1.5. Is this normal? I'm thinking that I should shock it immediately after a water change to get rid of the CC. Does that make sense?

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Your drinking water may be sanitized with chlorine and as the water travels through the pipes it may encounter pathogens or organic material and convert the FC into CC. It is also possible that your drinking water is being sanitized with monochloramine which would immediately register as CC (though that wouldn't explain the FC you are seeing). Either way, yes, you can simply shock after you refill in order to get rid of the CC (you may still register a small amount, perhaps 0.2, but that's OK).

As for the CYA, it doesn't make any sense at all that you measure virtually no CYA if you have been using dichlor for sanitizing and have been doing so for a month. For every 1.0 ppm of FC increased by using Dichlor, you introduce 0.9 ppm of CYA. Perhaps you only used 5 or 10 ppm of FC over that month, does that sound right (it seems too small to me)? Such a small amount might not show up in the CYA test. At least the first 5 ppm of CYA might not show up cloudy at all as the first "precipitate" is actually a little soluble.

[EDIT] A few additional thoughts I had. First, make sure you really shake the mixture of the CYA reagent with your spa water for 30 seconds. It won't form a percipitate unless it is mixed quite well. Second, I assume that your spa is kept covered except when you use it and that it is not exposed to sunlight when it is used. If for some reason it is exposed to sunlight, then you would need to use some CYA but as I mentioned in the earlier post, that's a bad catch-22 since you want a lot of disinfection for a hot tub (which CYA reduces). Third, there is a bacteria that will breakdown CYA, but it is anaerobic (meaning it lives in oxygen-free environments) and that is unlikely in your hot tub (it's more common in soil and in still pools in very cold water, which is probably why CYA levels sometimes drop in pools over a winter). [END-EDIT]

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Great info! Thanks Chem Geek.

Actually, I have been using dichlor for sanitizing and bleach for shocking. After reading your post I checked for CYA and didn't detect any. However, as you pointed out, the k-2006 will only detect above 30ppm. The test sample was very clear even with the vial completely filled so I would guesstimate that CYA is probably not a problem.

The CYA test is temperature sensitive! I assume your tub is kept heated? Let the sample reach room temp (between 70-80 degrees F) before doing the CYA test. Lower temps can slow the preciptation reaction down considerably (often seen when testing pools in the winter) and, from my understanding, higher temps cans inhibit it. Both conditions can cause low readings on the test.

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The CYA test is temperature sensitive! I assume your tub is kept heated? Let the sample reach room temp (between 70-80 degrees F) before doing the CYA test. Lower temps can slow the preciptation reaction down considerably (often seen when testing pools in the winter) and, from my understanding, higher temps cans inhibit it. Both conditions can cause low readings on the test.

Hmm...I did not know that the test was temp sensitive. No, I did not let the sample cool down before testing, but I did shake it well for a good 30 seconds. Too late now...I just finished changing the water.

Chem geek: Yes, I keep my spa covered exept during use, and as of yet have not used it during daylight hours. I can't believe that the low CYA readings are due to using too little dichlor. In fact, I think I might have been using too much if anything. I'll just assume that I screwed up the CYA test :P

It sounds like it might be better to just skip the dichlor and use bleach to sanitize and shock. I know that the bleach is unstablized, but my tub is rarely exposed to sunlight. Are there any other issues with usiing bleach to sanitize that I should be concerned about?

BTW: Thanks for the replys...great info!

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There aren't any signficant issues with using bleach for shocking, per se. But if you don't use ANY dichlor and have no CYA in your tub at all, then your effective active chlorine levels will be much higher. That's good for killing bacteria that causes hot tub itch, but it will also wear on your swimsuits more and you may notice more chlorine smell. If these become issues for you, then using a small amount of dichlor -- about 5 ppm FC worth -- would cut down the active chlorine some, but not too much.

You might think that without CYA you could just keep your FC down really low, but you need enough FC to be in reserve as it gets used up especially while you are in the tub (sweating which contains urea, for example). So having some CYA will let you keep your FC up for the reserve but not having too much CYA will keep the active chlorine effectiveness up. A dicey balance, but I'm much more "geekily" concerned about this than you need to be.

So if I were you I would try out the bleach-only for a while and see if there are side effects for you. If there are, then use a little dichlor. That sounds like a decent compromise.

waterbear can probably tell you a lot more since he has a spa as part of his pool with a spillover (or do I have your pool/spa mixed up with someone else?). He uses an SWG now, but I believe used bleach before the SWG was installed. By the way, waterbear, it looks like we've got another chemical mystery on our hands that needs to be sorted out. Namely, the use of dichlor but with no CYA getting measured. I suspect you are quite right that the higher temperature dissolves more precipitate so that would make the CYA test quite temperature dependent indeed! Gee, it would have been nice for Taylor to mention that. I think I'll send them an E-mail -- they always like hearing from me! (actually, they do). [EDIT] It looks like mealmine cyanurate (which is the precipitate) is soluble in water at 0.002g/100ml at 20C (68F) which works out to 20 mg/l or 20 ppm. So if this is correct, you don't even START to see cloudiness until 20 ppm (so how does my K-1720 test kit measure down to 20 ppm?) so that's another question to ask Taylor.

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suspect you are quite right that the higher temperature dissolves more precipitate so that would make the CYA test quite temperature dependent indeed! Gee, it would have been nice for Taylor to mention that. I think I'll send them an E-mail -- they always like hearing from me! (actually, they do). [EDIT] It looks like mealmine cyanurate (which is the precipitate) is soluble in water at 0.002g/100ml at 20C (68F) which works out to 20 mg/l or 20 ppm. So if this is correct, you don't even START to see cloudiness until 20 ppm (so how does my K-1720 test kit measure down to 20 ppm?) so that's another question to ask Taylor.

Actually, LaMotte (the OTHER water test kit company) does specify that the CYA test should be carried out between 70-80 deg F in the instructions for some of their colorimeters/turbidity meters for accurate results.

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Another thought occurred to me. Have you used any MPS ( Potassium monopersulfate, non chlorine shock) in addition to the dichlor and bleach? MPS will test as Total chlorine and can give readings such as you posted. Taylor markets a kit to remove the interferance to the total chlorine test to be used in conjunction with their other test kits for those that use MPS so that accurate readings can be obtained. Most people ignore Total chlorine readings (or should at any rate) when they use non chlorine shock instead of chlorine or in addition to chlorine for shocking. It works by a different chemical means than chlorine does and needs to be used differently to be effective. It will NEVER cause breakpoint chlorination to occur (means it won't burn off chloramines) but needs to have a residual concentration in the water at all times to burn off ammonia and organics BEFORE they combine with the free chlorine to produce chloramines.

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Another thought occurred to me. Have you used any MPS ( Potassium monopersulfate, non chlorine shock) in addition to the dichlor and bleach?

No MPS, but I did shock once a couple of weeks ago with lithium hypochlorite.

I used my spa last night for the first time since changing the water. I used bleach to bring the FC up to 6ppm before getting in. I noticed a bit of a chlorine smell while soaking. I never recalled a chlorine odor when I was using the dichlor. Is this normal? The smell was not bad enough to be irritating, but it was definitely noticable.

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Because you changed the water and used bleach instead of dichlor, you have no CYA in your spa (perhaps some small amount is there as a residual since it can stick to surfaces somewhat). Without CYA, your 6 ppm FC is about 2.5 ppm of the "active" form of chlorine (at spa temps of 104F). If you used Dichlor instead, then for every 1 ppm increase in chlorine, you also get a 0.9 ppm increase in CYA. So adding enough Dichlor to get 6 ppm would give you 5.5 ppm CYA and would result in about 1 ppm of the "active" form of chlorine, at least the first time you add Dichlor. Over time with each addition of Dichlor, your CYA levels increased because it doesn't go away while your chlorine did get used up. So after 6 uses of Dichlor (assuming you use enough to produce 6 ppm FC) you would have had 32.8 ppm CYA and the 6 ppm FC would result in only 0.088 ppm of the "active" form of chlorine.

So, the bottom line is that when you use Dichlor, your effective "active" chlorine levels are much lower, especially after a few additions of Dichlor. The smell of chlorine comes from its outgassing out of the water and the rate at which this occurs is proportional to the concentration of the "active" form of chlorine. So with Dichlor, you have less smell because you have signficantly less active chlorine.

So what does this mean and is it important? If you use Dichlor exclusively, then you will progressively reduce the amount of "active" chlorine in your spa. If you have too little active chlorine, then you won't get sufficient disinfection, especially to prevent "hot tub itch". On the other hand, if you use only bleach then you will have a lot of active chlorine in your spa which will certainly kill the bacteria that causes "hot tub itch", but will also smell much more like chlorine and will also wear out and fade your bathing suits much more quickly and you will be exposed to more disinfection byproducts.

From what I can tell, it takes an absolute minimum of 1 ppm FC at pH 7.5 to kill the bacteria Pseudomonas aeruginosa that causes hot tub itch (I couldn't find definitive CT times, but they appear to be on the order of 50 or so meaning that at 1 ppm it would kill the bacteria in about an hour which is OK for a residential situation). That means an active chlorine concentration of around 0.4 ppm is required. You can achieve such a concentration by have 6 ppm FC with 11 ppm CYA. That basically means you can use Dichlor twice to introduce 6 ppm FC into your spa (each time) and after that you should use a non-CYA source of chlorine, such as bleach.

So that is what I would recommend to reduce your smell of chlorine and have your bathing suits last longer and reduce your exposure to disinfection byproducts, while still killing the bacteria that causes hot tub itch. Use Dichlor twice to introduce 6 ppm FC each time and after that switch to bleach. Obviously start over with Dichlor again after you completely drain and refill your hot tub. In your current situation, you can just use Dichlor the next two times you need to add 6 ppm FC of chlorine. If you only need to add less FC, just keep track of the difference (i.e. increase in FC) and use a total amount of Dichlor corresponding to 12 ppm FC before switching over to bleach.

(I assume that there isn't anything strange going on such as CYA somehow breaking down at higher temperatures. For reasons I can't explain, waterbear didn't register any CYA even after using Dichlor for shocking and I would have expected some sort of measurement after several uses of Dichlor shock.)

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Another thought occurred to me. Have you used any MPS ( Potassium monopersulfate, non chlorine shock) in addition to the dichlor and bleach? MPS will test as Total chlorine and can give readings such as you posted. Taylor markets a kit to remove the interferance to the total chlorine test to be used in conjunction with their other test kits for those that use MPS so that accurate readings can be obtained. Most people ignore Total chlorine readings (or should at any rate) when they use non chlorine shock instead of chlorine or in addition to chlorine for shocking. It works by a different chemical means than chlorine does and needs to be used differently to be effective. It will NEVER cause breakpoint chlorination to occur (means it won't burn off chloramines) but needs to have a residual concentration in the water at all times to burn off ammonia and organics BEFORE they combine with the free chlorine to produce chloramines.

If you are smelling a chlorine smell I would test again for combined chloramines. You had a problem with them before and the drain and refill might not have eliminated the source if there is something that is introducing organics into the water! If you have no combined chloramines and are still noticing the chlorine smell then I would guess it is as chem_geek explained it.

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