cyanuric acid level too high

[Rabbs]

I have a public indoor pool of 25,000 gal that we keep getting a cyanuric acid level of 100ppm or higher every 2 to 3 months and has to be drained to remove it, I was wondering if there is anything I can try to get the levels down or at least find out why this keeps happening.
Thanks
R Meyer

[chem geek]

The reason this keeps happening is that you are using stabilized chlorine, most likely Trichlor pucks/tabs possibly in an inline feeder. You may also be shocking the pool with Dichlor powder/granular.

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Cal-Hypo, it also increases Calcium Hardness (CH) by 7 ppm.

If you use chlorinating liquid or 6% unscented bleach as your source of chlorine, then you won't increase either CYA nor CH over time. You would need to add this regularly, depending on your bather load, possibly daily or with heavy loads more than once a day. Officially, indoor pools are recommended not to use stabilized chlorine at all, but if you don't have any CYA in the water at all then the active chlorine level is very high and you can't maintain a low 0.2 ppm FC level since that's not enough in reserve to handle bather load locally which is why the minimum FC guideline is 1 ppm. So using a SMALL amount of CYA, say around 20 ppm, can be useful, but that is not industry-standard practice (though would be a lot better than the 100 ppm you are seeing now).

If I assume it takes 2.5 months to get from 0 to 100 ppm CYA, then your chlorine usage is (100/0.6)/(2.5*30) = 2.2 ppm FC per day which sounds about right for a moderate bather load in an indoor pool (this may be due to roughly 50 bather-hours per day if swimming or more bather-hours than that if wading). It should be pretty easy to add chlorinating liquid daily to maintain a reasonable FC level. If your CYA were 20-30 ppm, then you could start out at 4 ppm FC and end up with just under 2 ppm FC at the end of the day. Again, this is not industry-standard practice as they recommend not having any CYA at all, but your regular patrons will complain of swimsuits, hair and skin getting affected by the chlorine. My wife goes through this every winter season when she uses an indoor pool with 2 ppm FC and no CYA while in our own pool during the summer with 3-4 ppm FC and 30-40 ppm CYA there is no problem.

If you want to avoid having to add chlorine manually every day, you can get a peristaltic pump to feed chlorine on a more continuous basis. Or you can get a saltwater chlorine generator (SWG) system.

You could use Trichlor for around 20 days to get to around 20-30 ppm CYA and then switch to using chlorinating liquid. If you do some ongoing dilution of the water over time, which is recommended for public pools, then you can go back to using Trichlor for a short time when the CYA drops. You can measure the CYA down to 20 ppm and also measure chlorine accurately with the TF100 test kit from tftestkits(dot)com here. It is mostly based on the Taylor K-2006 test kit, but has a better CYA test (measures down to 20 ppm instead of 30 ppm). If you already have the K-2006, you can just get the CYA test from that same website I linked to above.

If you have a plaster pool and your calcium levels are low in the fill water, then with regular dilution your Calcium Hardness (CH) level could drop so you can increase it easily by using Cal-Hypo as your source of chlorine for a while. Note that you do NOT every put Cal-Hypo in the same feeder that was used for Trichlor (it can start a fire). Generally you use granular/powdered Cal-Hypo that you pre-dissolve in a bucket of pool water and add over a return flow with the pump running (similar to how you add chlorinating liquid -- over a return flow with the pump running).

Note that when using Trichlor, you probably have your Total Alkalinity (TA) up high to help compensate for its acidity, but when using chlorinating liquid you won't need that and to have more stable pH you'll want the TA to be lower (ironically). This is because higher TA leads to faster carbon dioxide outgassing (pools are intentionally over-carbonated) and that makes the pH rise (or the pH drop more slowly when using net acidic chlorine sources).

You can learn much more about managing a pool at the Pool School at Trouble Free Pool. The reason that the industry practice says not to use CYA in indoor pools is that they only talk about CYA's ability to protect chlorine from breaking down from the UV rays in sunlight and they don't talk about how CYA is a buffer for hypochlorous acid, significantly lowering its concentration. They do talk about it making chlorine less effective and increasing kill rates, but most everything except protozoan oocysts are killed fairly quickly with very low levels of active chlorine so using CYA isn't a problem there if it doesn't get too high. Not using any CYA at all makes the FC level TOO strong. FC does NOT measure active chlorine concentration. The science behind this has been known definitively since 1974 and the chlorine/CYA relationship is described in this paper.

Richard