CYA and Lifetime of Chlorine

[chem geek]

The formula for determining the light intensity passing through a solution is as follows:

I/Io = exp(-µ*l)

where µ (mu) is the absorption coefficient and is a function of wavelength (so the above formula is for a specific wavelength). "l" is the path length which for the units in the links I referred to is in centimeters.

There is also a formula for absorbance defined as follows:

A = -log10(I/Io)

and there is a molar extinction coefficient defined by the following equation:

A = ε*c*l

where ε (epsilon) is the molar extinction coefficient, c is the molar concentration in moles/liter, and l is the path length in centimeters.

so ε = µ / (log10(e) * c) = µ / (2.303 * c)

3 feet is about 91 centimeters and the molar concentration of 50 ppm CYA is 0.00039 moles/liter. So,

µ = 2.303 * c * ε = 0.00090 * ε

So to get any reasonable absorption from CYA (so that µ is near 1/91 so at 91 cm we have µ*l = 1) we need ε to be over 10. This source gives an extinction coefficient (for gaseous HOCl) of 123 M^(-1)cm^(-1) with an absorption peak at 220 nm. It looks like this may happen from the absorption spectra graph in the range of breakdown of hypochlorous acid. This link provides interesting detailed information about chlorine (and bromine and chlorine dioxide) in terms of half-life at various depths (no CYA present). Interestingly, there is quite a difference in half-life by depth at higher concentrations of chlorine indicating that the chlorine at shallower depths acts as a sacrificial shield to chlorine at lower depths and that this effect is more pronounced at higher concentrations. This makes sense, though it's something I hadn't thought of until I saw this data. It implies that the overall half-life averaged over all depths is longer at higher chlorine concentrations (but remember this is with no CYA at all). The difference in half-life by depth in summer at 0.53 ppm FC is 11 minutes at 0 meters, 26 minutes at 1 meter, 48 minutes at 2 meters, and 71 minutes at 3 meters (implying an extinction coefficient of 500) while at 17.6 ppm the half-life by depth was 9 minutes at 0 meters, 10.3 hours (not minutes) at 1 meter, 20.7 hours at 2 meters, and 31 hours at 3 meters. (implying an extinction coefficient of 74 in the first hour; later hours don't make sense). Thus, the chlorine is most depleted from water near the surface so having good circulation is essential in order to keep chlorine levels more uniform throughout the pool. It also appears, from the pH dependence, that perhaps hypochlorous acid (HOCl) is less susceptible to breakdown from sunlight than hypochlorite ion (OCl-). This implies that having a pool at lower pH results not only in more disinfecting chlorine, but has the chlorine last longer (though the effect may not be very strong from, say, 7.8 to 7.2).

Also, note that there is a non-linear effect from the concentration of whatever protective agent is present at the shallower depths (be it hypochlorous acid itself or CYA). So if I use a molar extinction coefficient of 10 and 50, then I would get the following for I/Io at 3 foot depth:

CYA (ppm) ... I/Io (10) .. I/Io (20) .. I/Io (50)
0 ................. 1.00 ........ 1.00 ........ 1.00
10 ................ 0.85 ........ 0.72 ........ 0.44
20 ................ 0.72 ........ 0.52 ........ 0.20
30 ................ 0.61 ........ 0.38 ........ 0.09
40 ................ 0.52 ........ 0.27 ........ 0.039
50 ................ 0.44 ........ 0.20 ........ 0.017
60 ................ 0.38 ........ 0.14 ........ 0.0077
70 ................ 0.32 ........ 0.10 ........ 0.0034
80 ................ 0.27 ........ 0.074 ........ 0.0015
90 ................ 0.23 ........ 0.054 ........ 0.00067
100 .............. 0.20 ........ 0.039 ........ 0.00030

So to see the dramatic change seen from higher CYA levels, the CYA shielding effect has to be strong enough to be the predominant effect. The shielding effect would "shield" not only unbound chlorine, but also chlorine bound to CYA. Note that using an extinction coefficient of 20 in the above table one finds the difference between 50 and 90 ppm CYA being a factor of 3.7 which is not far off from the factor of 4.2 that Janet was seeing. So perhaps adding an additional protection factor similar to the "20" column in the above table might be the thing to do. This link indicates that the chlorinated isocyanurates are unstable in sunlight, but it is unclear how much of that is due to breakdown from the equilibrium hypochlorous acid vs. direct breakdown itself. The study just shows that CYA is itself stable in sunlight. If the CYA absorption effect is really this strong, then deeper pools should be more protected at the same CYA level since more of their water volume will be at deeper depths "shielded" from the UV.

An experiment using shallow depth water with different levels of CYA will help isolate the two effects. If the CYA "shielding" or absorption is the main effect, then there should be little protection of chlorine in shallow water. If instead the chlorine combined with CYA has a longer half-life and that is the main effect, then higher CYA levels even in shallow depths should show significant protection and should roughly follow the curve in this graph. I suspect that there will be a some of both processes going on.

The original CYA patent by Fuchs may be seen at this link. There were interesting laboratory tests that appear to have been made at shallow depths and only show a small amount of the "depth" variation one sees with higher chlorine levels. The UV lamp they used appeared to have 1 ppm FC drop to 0.5 ppm FC in 1.7 hours so was not as strong as sunlight. The rate of chlorine loss seemed to track the amount of unbound chlorine, but with diminishing returns starting at a rate of 0.29 per hour at no CYA, 0.16 per hour with 1 ppm CYA, 0.13 per hour with 2 ppm CYA, 0.092 per hour with 5 ppm CYA, 0.071 per hour with 50 ppm CYA and an actual increased loss of 0.088 per hour at 100 ppm CYA. This is somewhat consistent with the original theory of a 35 minute half-life in direct sunlight with no CYA and an 8.4 hour half-hour limit when bound with CYA. This is probably where the industry got its original data for its tables. Note that CYA also has a protective effect on chlorine loss from oxidation of iron and copper. Though the patent speculates CYA may coat metals, it appears that the effect is explained by the reduction in disinfecting chlorine and therefore the rate of corrosion based on its concentration. It should be noted that in the patent "real pools" showed the greater protection effect of higher CYA levels by about a factor of 2 at 10 ppm CYA and over a factor of 3 at 50 ppm CYA. Thus there does appear to be a "shielding" depth factor for CYA protection separate from that explained solely by Cl and Cl-CYA breakdown. The fact that the chlorine levels were the same and only the CYA level increased, yet had a greater effect in a real pool with "depth" is very strong evidence.

The good news with this new information is that at sufficiently high CYA levels using a higher FC (to compensate for disinfection and prevention of algae) should not result in larger losses. Going from 30 ppm to 90 ppm requires about triple the FC level, but the loss rate may be cut down by a factor of 7 for a net overall savings of over a factor of 2. If we can validate this, then it should be possible to run a high CYA pool with high FC levels economically, especially in deeper pools.

Richard

[chem geek]

I got a response from Janet saying that the depth of her pool is 3.5 feet in the shallow end to 8.5 feet in the deep end (hopper shape with a long gradual slope). This is deeper than many pools with a 3 foot shallow end and a 6 foot deep end so could help explain why the higher CYA shows such a dramatic shielding effect in her pool. It should show this effect in the 6 foot pools as well, but would be more pronounced in a deeper pool. Her pH is usually 7.4 but tends to drift up towards 7.8 (I asked that since chlorine is more protected at lower pH). I think we're getting somewhere!

So now we just need to see if the SWG effect has anything to do with efficiency of the SWG cell or if it's just an issue of increased protection of chlorine from sunlight. That's not an easy test to do unless someone hasn't yet added high CYA levels to their SWG pool and is willing to increase those levels (and measure the SWG output rate overnight before and after the increase in CYA).

Richard

[mas985]

I was planning on slowly raising my CYA from 30 ppm to 80 ppm but to take advantage of solar, I am running my pump during the day.

One thought though is that pools with solar covers should have a considerable sheilding effect of UV. Experiencing an increase in production with a solar cover may indicate the same thing as doing the test overnight.

Another way to perform a shorter test would be to put the SWG into spa mode and run for a measured period of time. This would make it easier to test the chlorine levels since they should be higher and the test could be done in a much shorter period of time.

[chem geek]

Mark,

Agreed. The spa mode sounds like a much faster way to test for the SWG efficiency effect (either at night or with a UV-shielding cover) since the volume of water is much smaller. I'd hate for you to increase your CYA level in your pool only to find out that you may want to dilute it, but we already know that most high-CYA (70-80 ppm) SWG pools can run fine at lower FC (most are at 3 ppm) though some need to run closer to 5 ppm.

Running the SWG at night or with a cover should have the FC rise (rather than be maintained) so it will be any difference in the rate of rise (if any) that is what we are looking for.

Thanks,
Richard