CYA disappears over the winter

[chem geek]

This thread is a (vain) attempt to understand why Cyanuric Acid (CYA) levels drop precipitously over the winter for many pool owners. Before starting to figure this out, take a look at this thread that talks about the anaerobic bacteria (i.e. that prefer oxygen-poor environments) that can consume CYA and break it down.

I recently received information (from a document found in this post) about Patent 4,075,094 which is in this link that indicates that Sodium Hypochlorite can break down Cyanuric Acid (CYA) in a molar ratio of 4.5:1 (which is 2.6:1 by weight and where optimal reaction conditions have a molar ratio of 6:1 to 8:1) and at a pH of 9-12 (with optimal reaction conditions at a pH of 9-10).

Now in a normal pool environment, the amount of CYA far exceeds the FC so the chlorine is mostly combined with CYA to form chlorinated isocyanurates and is unlikely to breakdown the CYA, at least not rapidly. So while this may not explain the wintertime reduction of CYA, it did lead me to another thought. Namely, that the regular addition of chlorinating liquid or bleach may, during part of its dilution, create conditions conducive to the breakdown of CYA. The following table shows the theoretical amounts of FC, CYA and pH at various stages of dilution (mixing) of chlorine with pool water. The thought was that at some point the pH will be in the optimal range and the stoichiometry will be such that the overwhelming amount of chlorine could break down some amount of CYA. I assume standard initial pool conditions of 100 ppm TA, 30 ppm CYA, 7.5 pH, 3 ppm FC (initial). The "Dilution" refers to the added chlorine, which is 12.5% chlorinating liquid, so 100:1 means 100 parts of pool water with 1 part of added chlorine. The "max. CYA" refers to the largest amount of CYA that could be completely oxidized by the amount of chlorine (FC).

Dilution  FC      pH   max. CYA (FC/2.6)
 5000:1    28    8.50       11
 2000:1    65    8.78       25
 1000:1   128    8.95       49
  500:1   253    9.12       97
  250:1   503    9.31      193
  100:1  1253    9.65      482
   50:1  2503   10.03      963

So during dilution of added chlorine into the pool water, the pH and FC to CYA amounts are favorable for destruction of CYA at dilutions of 1000:1 or higher. If any of this destruction were to occur, then one would see that the amount of FC rise in their pool after adding an alkaline/basic source of chlorine (such as hypochlorite) would be less than theoretically should be achieved and this difference would be the amount of CYA that is destroyed (with the amount of CYA destroyed being 1/2.6 the amount of FC that is consumed).

The major flaw in this speculative hypothesis is that the patent indicates that contact time for this degradation to occur in substantial quantities is on the order of hours, while the amount of time that the dilution is in some optimal range is probably on the order of a few minutes at the most. So the amount of degradation of CYA from added hypochlorite sources of chlorine is probably very small, but is possibly more than from acidic sources of chlorine such as from an SWG (which produces chlorine gas that then hydrolyses to hypochlorous acid plus hydrochloric acid).

So the point of that exercise was really just to eliminate a possibility. The anaerobic bacteria explanation still seems plausible, though I can't explain how such conditions are favorable unless such bacteria are resistant to chlorine levels normally found in pools and that dissolved oxygen levels somehow get lowered to favor such bacteria.

The other possibility is that CYA degrades on its own (or at normal pool conditions in the presence of chlorine), albeit slowly, and is only noticed over the winter since it is unexpected, while during the summer one expects some loss due to splash-out and backwash (and we may add more CYA to make up for that). I am currently monitoring the loss of CYA in my own pool over this winter and will continue to do so during the summer. I use a cartridge filter so never backwash and I will monitor CYA and CH levels since the CH level is a decent indicator of any dilution that might occur from the small amount of splash-out.

For anyone doing the CYA test, keep in mind that it is temperature sensitive, so at colder temperatures it takes longer for the precipitate to form while at higher temperatures the precipitate is more soluable. Let your pool water come closer to room temperature before performing the CYA test.

Richard

[fcfrey]

Hi Richard.

I love your deep studies of the chemistry in our pools. Having an understanding of the “why” seats the information much more permanently than memorizing the charts. Keep them coming.

Last fall I drained 6480 gallons out and refilled with well water to bring the CYA level from 45 to 30. I stabilized the chemistry in the pool (based on Ben’s BG Chart) for the winter (without shocking) to the following:

FCL 4.5 -- range 2.2 – 5.1
TA 70
PH 7.5
CYA 30 -- Range 25-35 --- NOT a SWG pool

The pool is covered with a mesh Loop Loc cover which allows the rain and fine dirt to pass through (keeps the big lumps out).

Since the winter (so far) has been very mild, I have been throwing the Aquabot in once a week and checking the chemistry. So far, the FCL has dropped very slowly, and I have made 1 adjustment (Less than a gallon of 6% bleach) since November to keep in my range.

I keep accurate rainfall records so I will know how much dilution from rain/overflow and will be able to predict fairly accurately what the CYA should be in the spring when I reopen the pool. If it is significantly different from the predicted level I will know that your theories have come into play.

Once again --- Thanks for sharing your knowledge, I’ll keep you posted.

[fofa]

Confessions of a cheapskate:
Living in Texas, I have found that below about 65F water temp, I don't need chlorine. Water temp seems to stablize between 50-55F usually in the winter if that makes any difference.
OMG NO!
So basically once I hit that number, I let the chlorinator run dry (so to speak) of the tri-chlor tabs, and pretty much the rest of the time just get leaves out of the pool until the water temp starts to go back up. I also cut the filter run time back to two 3 hours sessions a day from two 4 hour sessions. So pretty much from Nov. through March/April (depending on the weather and water temp) NOTHING gets added (unless I need water, but don't recall having to add water). Maybe a backflush (DE filter) maybe even two if pressure increases, but usually no more than one during the winter.
In the spring when the temp starts going up, I usually shock it, and then after a few hours take a water sample to my LPS (always the same one) for a reading. So far every spring the CYA reading is 20-25 lower than in the fall (usually a reading 1 month before the water temp reacheas my cutoff).
This spring it might be interesting to see if there is any differance since I did sprinkle a bunch of Cal-Hypo (shock) in letting it settle on some organic stains (didn't seem to make a differance to the stains) and see if that effected the CYA lowering at all.
Anyway just thought it worth a mention.
No covers, no nuttin except my automatic pool cleaner, my leaf rake and me all winter. 30K plaster IG

[Watermom]

You are fortunate that when your water gets to that temp that you don't have algae problems. We used to tell people that if your water temp was below 50, you didn't have to worry about algae. That was, until Poconos and I both had very slight algae when the water temps of our pools was in the 40s. Al actually has a pretty cool picture of algae growing in his pool under ice. I'm gonna see if I can find that pic and if I do, I'll edit this and put it in here. (or, I'll get Al to do it.)

Al's Edit: I never throw anything away. Don't know what the water temp was down at the bottom but it had a good layer of ice.
0004843AlgaeUnderIce.jpg

[nater]

Hey Richard,
How would the contact time vs. concnetration equations work within the cell of a SWC system? Would you expect more rapid CYA degredation?

[fofa]

That was, until Poconos and I both had very slight algae when the water temps of our pools was in the 40s.

maybe it is the winter wienie algae we have down here, can only handle certain temps? I just know I do not get an algae issue with the cooler water temps.
Not complaining mind you. Since it is too cold to swim, can't see where it would be problem (other than the dogs drinking it, but they drink out of ponds and other such things also).

[chem geek]

Hey Richard,
How would the contact time vs. concnetration equations work within the cell of a SWC system? Would you expect more rapid CYA degredation?

nater,

Sorry I didn't respond right away. This thread kind of slipped by so I didn't notice your post. Anyway, in an salt-water chlorine generation cell, the pH at the chlorine generating plate is very low (acidic) and very far away from the pH of 9-12 (optimal 9-10) that the patent described was when CYA broke down. So where the chlorine level is high in the salt cell, the pH is low. At some point in the middle as the water flows out of the cell, the pH is higher and the chlorine level is higher, but neither is in the range where the patent says that CYA will break down. Also, the contact time of water through the cell is measured in seconds so again wouldn't be likely to breakdown CYA (and even flowing through the pipe to your pool doesn't take very long).

So in theory, the salt cell is less likely to breakdown CYA than the addition of chlorinating liquid or bleach and the latter isn't very likely to breakdown much CYA either. Basically, the CYA is pretty darn stable under normal operating conditions of any pool. It's just this wintertime loss that we speculate is due to anaerobic soil bacteria (that got in the pool) that seems to be the culprit. I just don't like the fact that such bacteria seems to survive in chlorinated water -- this is what we're getting more info on (there were several users who reported CYA drops with pools over the winter that had constantly maintained FC levels, so I'm not disputing that -- I'm just trying to understand how that could be).

Richard

[chem geek]

FYI to those interested in obscure questions and answers. I had seen on several websites that the manufacturers of melamine cyanurate claim it has a solubility of 0.002g/100ml which would work out to 20 ppm. The Taylor K-1720 kit measures CYA down to 20 ppm so I thought "how is this possible?" and wrote to Taylor to find out. After all, if CYA were soluble at 20 ppm, how could you the black dot get obscured so you could get a 20 ppm measurement?

Taylor is a great company who answer any ridiculous question I ask (apparently) and they came back with the response that the standard solubility measurements are done in distilled water so they start out at a pH of 7. The CYA reagent used in the CYA test (which contains melamine) is buffered at a low (acidic) pH and apparently the solubility of the melamine cyanurate complex is less soluble at low pH and is essentially insoluble at the pH of the test so they can measure rather low levels of CYA in the test.

Richard

[fcfrey]

Hi Richard.

I love your deep studies of the chemistry in our pools. Having an understanding of the “why” seats the information much more permanently than memorizing the charts. Keep them coming.

Last fall I drained 6480 gallons out and refilled with well water to bring the CYA level from 45 to 30. I stabilized the chemistry in the pool (based on Ben’s BG Chart) for the winter (without shocking) to the following:

FCL 4.5 -- range 2.2 – 5.1
TA 70
PH 7.5
CYA 30 -- Range 25-35 --- NOT a SWG pool

The pool is covered with a mesh Loop Loc cover which allows the rain and fine dirt to pass through (keeps the big lumps out).

Since the winter (so far) has been very mild, I have been throwing the Aquabot in once a week and checking the chemistry. So far, the FCL has dropped very slowly, and I have made 1 adjustment (Less than a gallon of 6% bleach) since November to keep in my range.

I keep accurate rainfall records so I will know how much dilution from rain/overflow and will be able to predict fairly accurately what the CYA should be in the spring when I reopen the pool. If it is significantly different from the predicted level I will know that your theories have come into play.

Once again --- Thanks for sharing your knowledge, I’ll keep you posted.

As a follow-up to Richard's study:

I opened my pool to find crystal clear water and a CYA level of 25 PPM.
I measured 11.28 inches of melted precipitation (5414 gallons in my pool) over the winter and considering the dilution method was the excess water going out the overflow with very little mixing.

This was about what I expected considering I was able to keep the pool clean and treated until mid December when it froze over and then start cleaning and treating again in mid March.

I fully opened the pool last weekend (removed winter cover) and am currently using about a half gallon of 6% bleach per day in full sunshine and no swimmer load. We will see how it goes but I want to keep the CYA as low as I can afford.

[chem geek]

Thanks for the update. In my own pool, I kept the winter rains out (the pool has an automated opaque safety cover and I had the cover pump take the water into a drain, not into the pool. This winter, the CYA dropped from around 20 ppm to perhaps 10 ppm (it's hard to measure that low) and I maintained chlorine levels the entire time. That sort of change isn't very conclusive but it does appear that at least for some, keeping the chlorine level up prevents the CYA from at least completely going away. For those that let the chlorine go to zero, having the CYA go away is more likely. There are exceptions in both cases, but that seems to be the trend.

Richard