Cya Levels When Using Dichlor/bleach Method

[chem geek]

For those of you using the method of 1-2 weeks of Dichlor usage after a fresh refill and then switching to unscented bleach until the next drain/refill, please do me a favor and measure your Cyanuric Acid (CYA) level (if you have a test kit that measures CYA) after 1 month, 2 months and 3 months (just before the next drain/refill). I want to make sure that the CYA level is being maintained and not slowly breaking down or anything like that. The idea is to maintain a constant CYA level during the whole time (except the buildup in the first week or two).

If for some reason the CYA were to drop, then short-term use of additional Dichlor would be needed. I'd like to know about this so that this method can be fine-tuned to be more consistent (if necessary).

Thanks,

Richard

[TinyBubbles]

I was already planning on doing this Richard. I, too, am curious if there would be a need to add dichlor at some point. I will keep you posted.

[Nat]

I actually found a VERY informative state government (PA) reference about cyanuric acid. It states that "stabilized chlorine" a.k.a. "dichlor" products should not be used in hot tubs, to prevent the build up of cyanuric acid. At least this coincides with what I have been reading on the Forum here. I do understand that up to 30 or so ppm cyanuric acid is beneficial. Its going over this concentration by continued use of dichlor that is not good. The entire document is worth reading. See:

http://www.dsf.health.state.pa.us/health/c...80&q=234664

Item #9.

"Should cyanuric acid be used in hot tubs or spas? – At even moderate levels of cyanuric acid, the amount of time it takes chlorine to kill pseudomonas aeruginosa (the bacteria that causes “hot tub itch”) can be as much as a hundred times as long as in a hot tub or spa without cyanuric acid. For this reason, the Pennsylvania Department of Health does not recommend the use of cyanuric acid or stabilized chlorine in any hot tubs or spas."

Another source said up to 50 ppm was allowable (this may have been for pools, though).

What baffles me is that all the hot tub manufacturers say to use dichlor, and none of them even mention cyanuric acid levels, or testing for it, as far as I know.

--Nate

[TinyBubbles]

I agree Nate, that is confusing. Maybe they are assuming that we drain and refill our water so frequently that the cya levels can't get very high. In the real world, we know this doesn't happen. On my first fill, my cya levels were through the roof when drained the spa. Maybe it's more of an issue in public used spas than in residential spas. Regardless, I don't want to take any chances.

[chem geek]

The pool and spa industry tends to not promote disseminating information based on scientific studies and field data, and even when they do they tend to go towards the extremes with the manufacturers saying there is no problem and CYA doesn't matter while the health agencies say to ban it in public situations. The latter is the case in the example you gave in Pennsylvania (which as you surmise is directed more towards commercial spas) and in New York where an outbreak of Cryptosporidium had the state ban CYA in public pools (search for "stabilizer" in this link). Even when there are studies, conclusions are drawn to benefit those doing the study such as the Pinellas County, Florida pool study led by Occidental Chemical (OxyChem) that concluded that FC alone was the only important factor yet my reading of that study would not conclude that at all. It takes a very low level of chlorine to kill most bacteria so that really wasn't the issue anyway, but to ignore the numerous studies showing CYA's reducing chlorine's effectiveness is at best irresponsible.

When Pennsylvania talks about chlorine being 100 times less effective against hot tub itch bacteria, they are talking about very high levels of CYA as would be found after 1-2 months of Dichlor use. Nevertheless, even this old paper from 1969 shows the effect of CYA on kill times for the hot tub itch bacteria (but for swimming pools). The chemical relationship between chlorine (hypochlorous acid) and Cyanuric Acid was very precisely determined and presented at a conference in 1973 (published in 1974). It's not like these discoveries are new.

I honestly do not know if the 20 ppm CYA level is the "right" amount for proper disinfection. The paper I linked to above implies a relatively fast chlorine kill time for hot tub itch bacteria -- a 3-log (99.9% kill) CT value of 0.15 compared with the 4-log (99.99% kill) CT value of 30-50 I was using from other sources (here and here). That is a HUGE discrepency (the 3-log equivalent of the second source would be around 23-37). All I can say for sure is that using no CYA would be overdosing in too high a disinfecting chlorine concentration and would be terrible for covers, disinfection by-products, etc. I can also say that using too much CYA will reduce chlorine's effectiveness, but do not know the best cutoff. We know for pools that algae will not be inhibited in growth unless the FC level is at least 7.5% of the CYA level (this rule works for phosphate levels up to around 3000 ppb) and the 4 ppm FC with 20 ppm CYA that I recommend is conservatively higher. Perhaps it would be fine to use 50 ppm, but clearly using Dichlor alone would raise the CYA too high, well over 100 ppm.

Richard

[TinyBubbles]

Ok, I'm at the one month mark for using bleach. The cya has not changed. The ph level has dramatically increased with the use of bleach, so I am now adding ph down. It was always on the low side with dichlor. The bleach definately does not hang around as long as the dichlor. We had friends over for New Year's eve and we used the tub, so I used dichlor after the soak and still had a residual the following day, which I don't get with bleach, no matter how much I use. After adding the dichlor I got foam, which I haven't had since using the bleach. I had high readings for cc when using dichlor and they were hard to get rid of with shocking. I have not had this problem with bleach, I don't know if it's just a coincidence. So far, I don't have any complaints with the bleach. The only difference is using ph decreaser instead of increaser, and the bleach is less expensive.

[chem geek]

Some of things you mentioned are explainable; others are a mystery. There is no question that using bleach is pH neutral while using Dichlor is acidic so the outgassing of carbon dioxide which makes the pH rise will show up much more when using bleach than using Dichlor. The solution to this is simply to lower the TA level to reduce the amount of carbonates in the water. Your addition of acid (combined with carbon dioxide outgassing) will slowly accomplish this, but you can speed up the process by lowering the pH to 7.0, running the jets, and adding acid when the pH goes up. If you lower the TA to 60-70 ppm, you should find a lower tendency for the pH to rise and this is still plenty of pH buffering for the water when you are using bleach as your source of chlorine. After you have lowered your TA, you can target 7.7 as your pH since there is less carbon dioxide outgassing at higher pH. If you tell me your current pH and TA levels and the spa water volume, I can tell you how much total acid you will need to get to a specific TA.

As for the chlorine not hanging around as long, that is most likely due to the lower CYA level. At high CYA levels, the disinfecting chlorine concentration is low (after the weeks of Dichlor-only use) and this slows down chlorine's reactions with chemicals in the water as well as slows down its outgassing (as hypochlorous acid) and breakdown from high temperature so it lasts longer. It also slows down the breakpoint reaction with ammonia so would lead to a buildup of Combined Chlorine (CC) -- specifically monochloramine -- which also can smell and outgas (it's more volatile than chlorine).

I don't have any explanation for the difference in foaming. Maybe there are some impurities or binders in the Dichlor product that lead to foaming. The usual solution to prevent foaming is to raise the Calcium Hardness (CH) to around 100-150 ppm.

Richard