I was wondering if there were any discussions on this site about the potential for high levels of cyanuric acid to deteriorate plaster or cementitious surfaces in a pool. I probably didn't search the best way and maybe Polyvue or someone else can post a link.

Although not documented, it seems that there is a relationship between high cyanuric acid levels and rapid plaster deterioration. This speculation comes from a frequency of symptoms observed from a variety of pools over a long time period. I have searched the NPC and ASPA and been unable to find any studies or discussions of this question.

If this hasn't been discussed, it would be great to hear some of the chemists veiwpoints on this speculation.

Thanks in advance. Rod

My understanding is that maintaining a high level of CYA affects pH; and in the calculation of CSI/LSI the CYA level is used in determining the Carbonate Alkaline portion of TA, so it's a factor in water balance. But since you're talking about "corrosion" of marcite/cementitious surfaces and not scaling I would think that it's peripheral and not causative. Perhaps the acidity of Trichlor is more culpable for any degradation of plaster... but this is just an educated guess. The ion/anion folks will likely have something to say about this. Let's see if there's any discussion of troweling technique.

The following threads include a Pool and Spa News article from 2008, a brief summary of the Arch study written in 2005 and a couple of TFP links where [the possilbity of] this and/or adsorption into plaster has been noted:

http://www.poolspanews.com/2008/052/052acid.html

http://findarticles.com/p/articles/mi_m ... n15932555/

viewtopic.php?p=19483#p19483

cya-adsorption-into-pool-plaster-t11116.html

My memory is that the plaster studies were questionably designed and not supported by follow up studies. However, it has been some time since I last looked into this, so I am not sure that I am remembering correctly.

In any case, we strongly discourage CYA levels above 80.

See chem geek's response in this topic for slightly more information.

I am very rarely a "deep end" visitor but I could easily offer a "shallow end" hypothesis.....

Very high CYA pools could easily mean poorly managed pools wherein most, if not all, parameters were seldom tested or adjusted. Neglect or ignorance of pool management might well be the culprit over a period of time.

Indeed, high CYA levels have a reasonably strong correlation with low PH, which is the most common cause of plaster deterioration.

In addition to the info in the post I wrote in the thread that Jason referred to, I agree with the previous posts that the main reasons for high CYA pools having plaster problems is 1) the typically too low pH from continued use of Trichlor tabs if the pH is not adjusted regularly and 2) too low a saturation index due to not accounting for CYA's contribution to TA (i.e. not using an adjusted TA in the LSI/CSI calculation).

how high can you get CYA while still being able to disinfect pool with chlorine?

The answer depends on what it is you are trying to kill and how quickly you are trying to kill it. Most heterotrophic bacteria are very easy to kill. With a 99% kill CT value of 0.08, this corresponds roughly to a 50% CT of 0.012 so with a 15 minute generation time (most bacteria double in population in 15-60 minutes), this implies a Free Chlorine (FC) level with no CYA of 0.0008. This is less than 1 ppm FC with 1000 ppm CYA. So preventing uncontrolled bacterial growth with simple planktonic (free-floating) bacteria is incredibly easy.

In practice, bacteria are often in clumps or attached to other material and can begin to form biofilms on surfaces if not killed right away. Nevertheless, even in real pools as shown in the Pinellas County, FL Pool Study, you can see a substantial drop-off in bacterial counts above an active chlorine (hypochlorous acid) level of around 0.002 ppm.

Some pathogens require higher chlorine levels (see this table) and algae clearly requires more as we know from the chlorine/CYA chart. Then there's person-to-person transmission where you want to kill pathogens faster than they can be transmitted. At the rough algae inhibition level where the FC is around 5% of the CYA level and corresponds roughly to 0.05 ppm FC with no CYA, this is a 99% kill time for most heterotrphic bacteria of around 1-1/2 minutes, though Giardia would take 5 hours (it's a protozoan oocyst so doesn't reproduce in pool water). Commercial/public pools could probably use a somewhat better safety factor and also higher oxidation rates so perhaps an FC that is 20% of the CYA level (say, 4 ppm FC with 20 ppm CYA), though there really isn't a single perfect answer. Higher chlorine levels can produce disinfection by-products more quickly (including more nitrogen trichloride).

So what we normally recommend for algae prevention is pretty reasonable for sanitation in residential pools. You can have high CYA levels, but you'd need correspondingly higher FC levels and that can become impractical (i.e. 25 ppm FC or more with 500 ppm CYA). At some point, the CYA can begin to precipitate out of the water, which I believe occurs more at lower pH (some sources say solubility is around 2000 mg/L so that's 2000 ppm).

Richard

The largest problem with very high CYA levels is that it becomes very difficult to measure what the CYA level actually is. None of the common CYA tests are especially accurate above 100. It also becomes difficult to measure the PH, since high CYA levels require high FC levels, and that starts to affect the PH test. Then there are the astronomical amounts of chlorine required to fight algae at very high CYA levels. All of this very quickly gets you far away from "trouble free".

There are a couple of pool services that use very high CYA levels to reduce the frequency of chlorine addition down to once a week. Doing this requires specialized test equipment and extensive knowledge of what you are doing. I don't recommend it at all, but it is possible.

Thanks Polyvue, you are amazing. In a matter of less than 40 minutes you managed to find the best available answers to the substance of all my questions!

Jason Richard and Dave, I can't tell you enough how great it is to have this website as a resource for quality information. Thanks for all that you contribute.

Rod