What's eating my CYA now?

[Water_man]

Last year my CYA was 60, after years of trichlor use (or abuse). I opened this season with zero. So did two other gunite IGP owners who live within half a mile from me. None of us had ammonia. I figured we "joined the club" of those afflicted by winter CYA eating bacteria, or another common CYA consuming "stuff" that affects IGPs when they have their winter dormant season.
After opening this season I added CYA to 30, and FC consumption was reasonable for more than a week, but suddenly I saw a drop in FC. I measured CYA, and it dropped to 23. No ammonia is present.
My buddy, who also adjusted his CYA from 0 to 30 just recently, also had a drop to 20!
What happened to my CYA? I also started to add borates. So far added half the calculated amount.

CYA 23
FC 4
CC 0
TA 90
pH 7.3
CH 240
Ammonia 0
Borates assumed 25 ppm, not verified yet.

 

[chem geek]

I don't know (at least when FC is always present). There is a known slow oxidation of CYA by chlorine so maybe some sorts of conditions catalyze it. Normally though, it takes a very high pH (closer to 9) and very high chlorine levels to see the effect happen more quickly. Normally, the drop in CYA is accounted for by dilution of the water.

 

[JasonLion]

The CYA test is +-15, so a result of 20 is consistent with a result of 30. The test can not reliably make fine distinctions like that, so there is no way of saying if the CYA level went down or stayed the same.

Ammonia is volatile and can evaporate out of the water if given an opportunity. The CYA could easily have been turned into ammonia by bacteria, and then the ammonia could have evaporated, giving the results you observe.

 

[Water_man]

Based on what you're saying, here's my plan of action:
1. Raise CYA to 33.
2. After a few days get consitent readings of CYA 30-33 with the test kit that starts at 20.
3. Monitor CYA for a week or so after level has been established at around 30. If I get consistent readings of 20 (three at least) - shock. Does it make sense?
I just don't know what should be the FC level of that shock.

 

[chem geek]

I don't think it makes any sense to shock a pool when the CYA is dropping unless the FC is very low (near zero) to begin with. The bacteria that convert CYA to ammonia do not live if there is any practical amount of FC in the water. It takes an incredibly low amount of chlorine to kill bacteria faster than they reproduce.

 

[Water_man]

Then, since there's no dilution and there's always FC present and no CC, I assume it will be reasonable to conclude, as Jason said, that the fluctuations between 30 and 20 are meaningless, because the accuracy of the reading is +- 15.

 

[Lershac]

I tell you, you could sell the CYA eating bacteria if you could bottle em!

Imagine being able to lower your CYA without draining and refilling!

 

[Water_man]

I tell you, you could sell the CYA eating bacteria if you could bottle em!
Imagine being able to lower your CYA without draining and refilling!

Yup. Chem geek definitely had a chance. See his post here. He also had lots of ammonia, but not enough to make a fertilizer or something more exotic. :lol: