We have had many people do partial or full drain/refill to reduce CYA and never seen a significant CYA "sticking" problem. We've also had others who do Reverse Osmosis (e.g.
Pool Services Technologies) and they never reported any need to brush or lower the pH to lower the CYA level in pools. That doesn't mean that some small amount of CYA does not adhere to some surfaces, but the amount must be too small to show up with any significance. If lowering the pH and brushing are part of the process of removal, then that implies that the "bond" is one of the negatively charged cyanurate ion since lowering the pH has it convert mostly to uncharged cyanuric acid. The following shows the concentration of the relative species at pH 7.5 vs. 6.0 (in the following, "CY" is the core de-protonated cyanuric acid ring so H
3CY is cyanuric acid while H
2CY
- is singly negatively charged cyanurate ion):
CYA Species ..
pH 7.5 .......
pH 6.0
H
3CY ............. 17.71% ....... 87.03%
H
2CY
- ............ 82.27% ...... 12.97%
HCY
2- ............ 0.014% ....... negligible
CY
3- .............. negligible ... negligible
For simplicity, let's assume a 16'x32'x4.5' rectangular pool. The surface area of pool plaster is 944 square feet (87.70 square meters). The volume is 65242 liters so if 100 ppm (mg/L) of CYA were left on pool plaster surfaces that would be 6.52 kilograms (14.4 pounds) of cyanuric acid. The density of solid cyanuric acid is 2.50 g/cm
3 so the volume of solid cyanuric acid would be 2616 cm
3 so the thickness on the pool plaster surfaces would need to be 0.03 cm or 0.3 mm. Of course, the cyanuric acid wouldn't be binding to itself as it is soluble in water (saturation is several thousand ppm) but presumably bonded in some way as cyanurate to the pool plaster, but this amount seems unrealistically high.
This paper describes various cyanurate derivatives and there are calcium and magnesium cyanurate salts, but they are moderately soluble in water as we know otherwise one could not have CYA in a pool with hard water without producing a precipitate. Pool plaster is largely a mixture of calcium silicate hydrate (C-S-H) and calcium carbonate. I can find nothing in the scientific literature with regard to cyanurate bonding to calcium silicate hydrate or pool plaster in general. That does not mean it does not occur, but that it has not been studied.
We are generally very skeptical of these sorts of assertions ("CYA builds up in the surface of the pool") because there are so many myths in the pool/spa industry that turn out to be wrong. Not many people get their pools up to 800 ppm CYA so if this effect is a small one it might not show up significantly until the CYA levels get quite high as in the example given by TexasJeff. Also, if a neglectful pool owner never brushed their pool on a regular basis, especially if they had calcium scaling, then it is possible for such calcium scale to incorporate other compounds incidentally though the volume of CYA we are talking about here does again seem to make that implausible. Finally, if the CYA were over 800 ppm CYA, then ending up with 100 ppm CYA could easily happen if the pool was not drained completely since 100/800 = 12.5% so assuming (for simplicity) a 50/50 split 6 foot vs. 3 foot deep pool, this implies having no water in the shallow end and leaving a little over 1 foot of water in the deep end of the pool or the equivalent of that including the water found in pipes and filter (which is probably negligible compared to pool water volume). Is it possible that the people who claimed to have drained their pools did not fully drain them?
Others have claimed that CYA settles to the bottom of pools yet we have not measured significant CYA concentration differences in pool water after any added CYA was thoroughly mixed. Note that without circulation, fresh rains can certainly cause a bifurcation of concentrations by diluting surface water leaving water at depth to be more concentrated and only mixing by diffusion which is very slow.
Note that
PPOA refers to "puddling and absorption in filter media, pipes, plaster and elsewhere retains small amounts", but the small amount was a result of 2 ppm CYA after draining from 50-60 ppm.
I am not saying that the experience that is seen is not real -- just that the explanation for it may not be CYA bonding to plaster. We can certainly keep a close eye on this from the hundreds of pools that have partial or full drain/refill to see if any of them result in higher-than-expected CYA levels on a consistent basis.
