Options for reducing CYA

[Craig]

From what I have been reading, there are three ways to reduce CYA:

1- Drain pool and dilute mixture by adding water
2- Buy an expensive CYA reducer with questionable results
3- Close pool for winter and when reopening in Spring CYA should be reduced

Is this correct? Is the third option definitely true? I ask this question because I am located in New York and our pool season is approx 4 months. Being this said, if I get my CYA too high, and maintain the chlorine at the right levels based on the best guess chart, should I just wait until I close the pool to reduce my CYA level?

 

[JohnT]

The third option only works sometimes. Mine is always down in the spring, but it's because of rain dilution through my mesh cover. Some pools seem to eat it over the winter.

 

[Craig]

John,

Thanks for the reply. Unfortunately this is my first year with the pool therefore I am not sure what will happen to by CYA in the winter.

I have one of those safety covers, therefore, I do not believe I will get any rain through it. I believe with your cover, you are really falling into my first category of draining and diluting.

Thanks,
Craig

 

[frustratedpoolmom]

My CYA did not reduce over winter - infact 2 years of not draining to pool to close it for winter caused the CYA to build up too high over 110.

 

[Craig]

Thank you Frustratedpoolmom! Are you actually saying it increased over the winter or that it just built up over the 2 years?

I guess to answer my own question a little bit, I believe the CYA will be reduced when winterizing, since the water level will need to be lowered for freezing purposes, therefore, I will be doing option 1 like it or not. But I'm still looking for an answer of the winter phenomenon of automatic CYA level reductions.

Thanks,
Craig

 

[chem geek]

I think that the winter phenomenon of CYA reduction, at least when a pool is "let go" to zero chlorine over the winter, is due to anaerobic soil bacteria that convert CYA ultimately to ammonia. So unless those bacteria make it into your pool and are able to grow and consume the CYA, then nothing will happen. The downside is that the conversion of CYA to ammonia means that it takes a LOT of chlorine to break down all the ammonia upon opening. Drain/refill is a much more reliable method for CYA reduction and there are methods using a large plastic sheet or using a silage bag where one can do the drain/refill using less water than a continuous drain/refill.

Richard

 

[Craig]

Thanks Chem Geek! Just out of curiosity, how does anaerobic soil bacteria attack pool water? Additionally, how does the silage bag technique work?

 

[JohnT]

Thanks Chem Geek! Just out of curiosity, how does anaerobic soil bacteria attack pool water? Additionally, how does the silage bag technique work?

You put the silage bag in the pool with the opening on the deck. Add new water inside the bag and pump old water out from outside the bag. Don't let them mix until you have the full amount of water replacement done.

 

[chem geek]

Craig,

This link shows the degradation pathway. Some bacteria (mostly found in soil) have enzymes that speed up the degradation -- that's what those chemicals whose name end in "...ase" are -- those are enzymes. Without such enzymes, CYA is essentially stable under pool conditions. At high pH (9-10) and very high chlorine levels (well beyond any shock levels we ever do), CYA can break down in hours without the enzymes or bacteria.

Richard

 

[Craig]

Chemgeek,

I guess I shouldn't have asked. How does the bacteria actually get to the pool water? I assume it goes airborne?

Thanks,
Craig

 

[chem geek]

Yes, I suspect the bacteria get into the pool via blown dirt/dust in the air, though just stepping on the soil and going into the pool will do it as well. However, when there is chlorine in the pool, such bacteria should die very quickly (half the population dies in around 7 seconds if its typical heterotrophic bacteria). There were some reports of CYA loss over the winter even when chlorine levels were maintained so I can't explain that, but most pools with the large loss were "let go" to zero chlorine.

Richard

 

[rimshaker]

How about dilution via evaporation and then rain water? Albeit a slow dilution method.

 

[JasonLion]

Evaporation followed by rainfall or refilling to the original water level leaves your levels esentially unchanged (the PH tends to go down a little because rain tends to be acidic).

Splashout from swimmers followed by rainfall or refilling will eventually lower your levels, but this is normally pretty slow (unless you reguarly have lots of kids over).

 

[Thehobe]

I have also had CYA levels over 100, probably about 120. Over the last 6 months of avoiding the use of tablets containing CYA levels have dropped to 60. I started using Calcium hypochlorite and now using just liquid chlorine. My chlorine levels seem to be holding better as well.
I assume CYA naturally disappears over time so using the right amount of tablets in a well controlled pool may not build up and maybe the product has been designed to add the appropriate amount of CYA to make up for the loss over time when using tablets. In any event, CYA does appear to reduce over time if none is added.

 

[chem geek]

CYA doesn't go away very quickly. Chlorine slowly oxidizes it, but in a pool that's only around 2-3 ppm CYA per month (in a hot spa it's closer to 5 ppm CYA per month). You probably had some winter rain dilution if such rain is allowed to overflow the pool, though Los Altos doesn't get that much rain even in the winter. I use winter rains to dilute my pool water, mostly to keep the salt level in check. If you have a sand filter and backwash, then that dilutes the water some. If you really had CYA drop more quickly and didn't have water dilution, then there's some mechanism we don't understand (perhaps you've got some metals or other chemicals acting as a catalyst that has it degrade faster).

One generally cannot use only Trichlor tablets for chlorine in a pool without building up CYA too quickly. The reason is that for every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. So with a 2 ppm FC per day chlorine usage, that's an increase of 36 ppm per month. You'd have to have a pretty short swim season and some water dilution (say from regularly backwashing a sand filter, especially for a smaller pool) to not build it up quickly or you'd have to have a low FC/CYA ratio to have less than the 2 ppm FC per day loss from sunlight. If you have a pool naturally poor in algae nutrients, then you can get lucky and not have a problem as the CYA level rises and it will rise more slowly losing less chlorine per day.