CYA vs UV Chlorine Loss Test – Observations

[Tom ONeill]

If it were me I would limit my CyA to 50ppm and run my FC at 5ppm. I assume there is a requirement for super chlorinating. Good job on these pools. I love those FB HOA stories, do you have a golf cart with someone that goes around measuring the grass heights? I think there is a level of perceived mathematical accuracy here, I would record these figures as 99.3%, 0.003ppm HOCl, 0.01ppm OCl- etc., and then there would also be a level of accuracy to factor in.. Just to make sure we’re on the same page here, the rate of measured FC loss to UV is directly related to the concentration of OCl-. HOCl is mostly unaffected by UV degradation as is the HOCl bound to CyA. Note the term ‘mostly‘ as it is inferred in the literature that a small portion of HOCl may be affected and even a small portion of the bound HOCl. And although we believe the CyA concentration has a greater affect then pH, pH will still affect the HOCl/OCl- ratio and the amount of OCl- exposed to UV. And then there will always be a possibility that the system is influenced by something we haven’t thought of. Those golfers and nudists would be constantly swapping clothes while standing along a common fence that keeps moving. And then we’d need to account for skin type variations, sunblock usage, ground reflection etc. etc.

If it were me I would limit my CyA to 50ppm and run my FC at 5ppm. I assume there is a requirement for super chlorinating.

Good job on these pools. I love those FB HOA stories, do you have a golf cart with someone that goes around measuring the grass heights?



I think there is a level of perceived mathematical accuracy here, I would record these figures as 99.3%, 0.003ppm HOCl, 0.01ppm OCl- etc., and then there would also be a level of accuracy to factor in..




Just to make sure we’re on the same page here, the rate of measured FC loss to UV is directly related to the concentration of OCl-. HOCl is mostly unaffected by UV degradation as is the HOCl bound to CyA. Note the term ‘mostly‘ as it is inferred in the literature that a small portion of HOCl may be affected and even a small portion of the bound HOCl. And although we believe the CyA concentration has a greater affect then pH, pH will still affect the HOCl/OCl- ratio and the amount of OCl- exposed to UV. And then there will always be a possibility that the system is influenced by something we haven’t thought of.

Those golfers and nudists would be constantly swapping clothes while standing along a common fence that keeps moving. And then we’d need to account for skin type variations, sunblock usage, ground reflection etc. etc.

I leave the measuring of grass heights and weed counting to the owners. Most of them could do with more outdoor exercise anyway. I just wish they would pick up after their dogs.

We are currently running both pools at 50 ppm CyA +/- 2 ppm and 4 ppm FC, +/- 0.3 ppm through the day (it drifts, of course), with sparkling clear results. Although I would love to go to 60 ppm CyA, it wouldn't leave me much headroom (code compliance limit) if we needed to push it up a bit. In June - July, at 50 CyA, I could jump to 6 ppm FC overnight, reduce input in the morning and we'd be back to 5 or lower by maybe 10:00 AM. And as we have to test the pools twice a day 7 days a week, it's not a huge effort to do that, if needed. Last year in June - July we were running the pools at 40 ppm CyA and tracked chlorine use (mostly UV burn) and logged an average of 3.8 ppm FC/day use rate in each pool. It will be interesting to see how it goes this summer at 50 ppm CyA. The pools are open for use every day from 7:00 AM to 10:00 PM, subject to any significant maintenance or repair events.

As to rate of FC loss from UV - Yes, the OCl- is the dominant factor in the UV loss. And, the exposure of HOCl + OCl- to UV is governed by CyA. Lower CyA = higher proportion of HOCl + OCl- exposed to UV.
At FC/CyA of 3/30 and pH 7.5, about 97.25% of all the FC is bound to CyA (UV shielded) and only 2.75% of FC, as HOCl + OCl-, is exposed to UV. At Mark's starting test condition of 7.2/80 and 7.2 pH, only 0.93% of FC, as HOCl + OCl-, is exposed to UV. That's a decrease in the proportion of HOCl + OCl- exposed by a bit over 66%.

Whereas, a pH change alters the HOCl/OCl- ratio. At 3 ppm FC and 30 ppm CyA, and pH of about 7.5 the ratio is about 50%/50% and at pH 8 it shifts to about 25%/75%. Bang goes the OCl- portion of FC.

And that pH effect and its implications is a whole other topic. A topic I believe is highly applicable to pools using either Sodium Hypochlorite or a SWCG. An effect that is almost never even guessed at by those who have always used Trichlor for chlorination. I never did. With Trichlor we had to keep a close eye on the TA levels particularly, and LSI in general, to make sure the conditions weren't "eating" the plaster.

 

[AUSpool]

In the beginning it was assumed the conditions before and after the switch to bleach where similar but given the pools were poorly managed with high CyA resulting in algae blooms I would suggest that the FC didn’t increase when switching to bleach but rather the pools were not using enough prior to the switch. With elevated CyA, as much as 500ppm, just about all the FC would be protected from UV degradation leaving nothing available for sanitation and oxidation even though the DPD or FAS/DPD test was indicating there was still 5ppm FC available.

Given a similar set of FC and CYA concentrations with matching pH and temperature the UV degradation should be identical. Although using trichlor or dichlor will constantly shift (reduce) the FC/CyA ratio.

"the FC loss to UV is directly related to the unbound HOCl/OCl- . . ."
For clarity I should edit that to remove the “directly” which is an over simplification. The equations used by Pickens are tad more complicated. If it‘s still of interest Pickens talks about the Monsanto data (Nelson, G.D. Special Report No. 6862, Rev. May, 1975, Monsanto Indust. Chem. Co., p. VI-8, Fig. 41.).

And that pH effect and its implications is a whole other topic. A topic I believe is highly applicable to pools using either Sodium Hypochlorite or a SWCG. An effect that is almost never even guessed at by those who have always used Trichlor for chlorination. I never did. With Trichlor we had to keep a close eye on the TA levels particularly, and LSI in general, to make sure the conditions weren't "eating" the plaster.

See

CSI and LSI - Further Reading

www.troublefreepool.com

 

[Tom ONeill]

I appreciate your information on the CSI/LSI. Provides more detail on how it came about and how it applies. Thanks.

After the county shut our pools down, and I then took over, we drained the pools entirely, cleaned them, refilled with fresh water and started over. We added granular Cyanuric Acid to bring the pools up to 40 ppm CyA, and went on from there. Still using Trichlor. However, we also did partial water drains & refills periodically to maintain an average of 40 ppm CyA. A Lot of water down the drain.

Prior to the start of our summer season here last year, we installed the peristaltic pumps and began running with 10% - 12.5% pool bleach, attempting to save water, as we live in the arid Southwest here. That is when we noted we were using nearly twice as much bleach as I had calculated. A calculation based on previous (Trichlor) daily chlorine ppm usage converted to comparable fluid ounces of 10% bleach. At first, I thought the bleach we were using was not at it's advertised rating. I tested the bleach - it was up to strength. SO, why were we using nearly twice as much? That's when I started investigating, and eventually found the posting by Mark with his empirical test results. Once I understood the mechanics and relationships, did the analysis and math, etc., our situation ultimately became clear that it was due to the increase in pH, which exposed more of the HOCl/OCl to the sun's UV. Nothing else. Not more CO2 outgassing (actually it's H2CO3+NaOH→Na2CO3+H2O, decreasing Carbonic Acid [increasing pH] and creating more Sodium Bicarbonate [increasing TA]), nor is it "unicorn farts"(hydrogen gas generated in-stream in SWCG pools increasing CO2 outgassing - although a bit of that does occur, but does not with NaOCl). When using Bleach or a SWCG, NaOH is unavoidably generated - and for as long as you run the SWCG or the bleach peristaltic pump. The reaction with carbonic acid is rather immediate and might simply be viewed as "sequestering" the CO2, not taking it out of the pool through outgassing, but rather changing where it is - what state it's in. You can get it back to the prior state of carbonic acid and/or CO2aq by addition of HCl or some other acid to the water. Best if you can add acid at a comparable countering rate vs the addition of NaOH to maintain a more stable pH.

Yes, I've researched Boron - Borax, Boric Acid. I definitely does help hold down the rate of pH increase caused by the addition of hydroxide. It acts as a Lewis Acid by "capturing" OH- (hydroxide). The problem is that it can't do that forever and then you have to add the same amount of acid as you would have if you had been adding acid right along. It essentially 'buys you time", nothing more. Why bother?

BTW, I believe that Richard's PoolEquations.xls spreadsheet calculations of moles/liter for each of the chlorine/Cya states are a bit more spot-on than the EPA calculations.

 

[Tom ONeill]

A p.s.:

I also appreciated the graphic you provided on the half-life curves at various levels of CyA. I had, however, already seen that when I first started "caring" about our pools.

If you are running any chlorine into the pool at a relatively constant rate by any means (Trichlor, SWCG, Bleach-pump), trying to maintain a fairly stable FC level, half-life loss calculations are meaningless. Just as asking a dog what time it is and getting the answer "Now", a continuous chlorine feed results in the calculation of chlorine loss as being "Now". In other words, the rate of loss is the first instant of loss times however many "instances" you have - until you turn it off. So, it's not a declining-balance calculation. Interestingly, you could use an adapted amortization calculation to determine the half-life of chlorine. Long ago I was an accounting and finance guy. That was before I turned to mechanical engineering, manufacturing, computer systems engineering, and so - on from there.

 

[mgtfp]

Not more CO2 outgassing (actually it's H2CO3+NaOH→Na2CO3+H2O, decreasing Carbonic Acid [increasing pH] and creating more Sodium Bicarbonate [increasing TA]), nor is it "unicorn farts"(hydrogen gas generated in-stream in SWCG pools increasing CO2 outgassing - although a bit of that does occur, but does not with NaOCl). When using Bleach or a SWCG, NaOH is unavoidably generated - and for as long as you run the SWCG or the bleach peristaltic pump. The reaction with carbonic acid is rather immediate and might simply be viewed as "sequestering" the CO2, not taking it out of the pool through outgassing, but rather changing where it is - what state it's in. You can get it back to the prior state of carbonic acid and/or CO2aq by addition of HCl or some other acid to the water. Best if you can add acid at a comparable countering rate vs the addition of NaOH to maintain a more stable pH.

This is not correct.

You have the equilibrium

CO2 (aq) + H2O <-> H2CO3

This equilibrium is at pool pH very far on the left side, you hardly have any H2CO3, but mainly dissolved CO2 (aq). The CO2 (aq) is far above the amount that would be in equilibrium with atmospheric CO2, and it will outgas to reach equilibrium with the atmosphere, raising the pH in the process.

You can also do the opposite, and inject CO2 gas into the water as a way to lower pH.

There also is no NaOH formed when adding bleach. You are adding some Na+ ions and some OH-, but these ions don't combine in water. The Na+ just stays in the water, the OH- does its thing in water, going back and forth with H3O+ and H2O depending on pH.

The pH raising effect from adding bleach gets compensated when chlorine turns back into chloride. There is a small effective pH rise due to some excess lye in bleach. The bulk of the pH rise comes from CO2 outgassing.

 

[mgtfp]

The reason why outgassing CO2 raises pH is understandable when you look at all the Carbonate equilibrium equations:

CO2 (aq) + H2O <-> H2CO3
H2CO3 <-> HCO3- + H+

HCO3- <-> CO3-- + H+

As mentioned before, the first equilibrium is far on the left, and it is more convenient to combine the first two equilibriums to

CO2(aq) + H2O <-> HCO3- + H+

pH is just a logarithmic measure for the number of H+. The more H+, the lower the pH.

If CO2 gasses out, all the equilibriums have to reshuffle, and in the end some H+ got "swallowed", which means that pH rises.