CYA Loss -- Bacteria?

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C

COBrien

0
May 6, 2015
184
Chickasha, OK
It's been a bit since I've been around... A testament to the fact that my pool has been trouble free this year. That is, except for one thing...

When I opened my pool May 1, I did a full battery of tests. Everything was normal, except 0.0 FC and 0.0 CC. CYA was 40 ppm.

I SLAM'ed the pool, that took about 2 weeks. Ran another full battery of tests post-SLAM, and everything was still fine. CYA at 40 ppm.

I noticed a couple of weeks after my SLAM, though, that I was beginning to see more FC loss from week to week. I started out losing my usual 2.0 ppm FC daily (dosing for 6.0, never actually re-checked, probably ended up slightly higher, though). A couple weeks later, I was losing 2.5 ppm FC daily. A couple weeks after that, I was losing 3.0 ppm FC daily. When I got to the point where I was losing 4.0 ppm FC daily (2 weeks ago), I re-tested CYA.

CYA showed less than 20 ppm. I filled the tube to the top and could still see the dot. It was a bit cloudy, but I could see it plainly. Pool Math called for 67 oz. (weight) of CYA. I bought a 5.5-lb. container and added approximately half (trying to be conservative). FC loss dropped from 4.0 ppm daily to 2.5 ppm daily within 48 hours of adding CYA. I have not re-tested yet (out of reagent...), but now my FC loss has gone back up to 3.0 ppm daily.

I should note that I have not replaced any water and we have not had enough rain to overflow the pool. I have backwashed my sand filter once so far this summer, and that was a week after I added the 2.75 lbs. of CYA. I am adding about 1-1.5" of water per week due to evaporation.

Should I add the rest of the CYA I have on hand? Should I be concerned about this CYA loss? Last year I added CYA up to 40 ppm early in the summer and never saw a decrease. What's different this year?
 
There'll be a professional along in a minute but as I posted a CYA question today thought I'd try and chip in.

Your rising FC usage looks normal to me for this time of year, are you saying that this rise is different from previous years?

I had a CYA reading of <20 twice today....until I let the tube sit for 5 minutes and then I got a reading of 30. You say you've not been able to test again whilst you wait for reagents so is there a chance the reading could be wrong or did you allow it to fully dissolve and follow all the other 'back to the sun, waist height etc' guidelines?

You had a drop in FC usage after the CYA addition but maybe this was down to overcast/cloudy weather and the sun not being able to eat up your FC?

I've seen my CYA drop from 55 on 14th May to 40 19th June and now 30 today (also with only 1 backwash) so there does seem to be something eating up the CYA but I'm also at a loss as to why.

Just putting a few possibilities out there whilst you wait for someone more knowledgeable to come along.
 
We've had a few overcast/rainy days this summer (and those days I do see 0.5 ppm less FC loss), but the majority have been bright, sunny, and HOT. We had several days >100° in June (we usually don't hit the century mark until mid-July...).

I did the CYA test just as I have always done. Mix everything up, shake for a solid 10-15 seconds, let it rest while I conduct my FC/CC test (takes about 1 minute), then shake continuously while I walk outside. Back to sun, test tube at waist height. I test, pour back into squeeze bottle, and re-test at least 2x to get an "average." The last time I filled the tube up all 3 times and could still see the dot.
 
One thing for sure, if you're getting some of this brutal heat where you're at, a low CYA will open-up that pool for trouble. So definitely do what you can to increase the CYA right away. If you're still showing <20, consider adding another 4 lbs of stabilizer to get you back to the 40 range. From there, I would simply adjust FC and maybe consider doing an OCLT just to be safe since you had so much fluctuation. Always better to rule-out any excessive organics after something like this. It's always possible that your batch of stabilizer simply was bad. Last year I went through a weird time where my CYA didn't seem to want to hold until I switched brands for a bit. Your testing method seems to be accurate, so I would do those things for now and see how your pool reacts.
 
I agree. I'll be buying more CYA (of a different brand... hadn't thought about that). What I bought a couple weeks ago was from the Clorox Pool & Spa line. 100.0% CYA, should have been good stuff.

My bigger concern is what could have caused a 20 ppm CYA decrease over a period of 8 weeks.

That's 2.5 ppm per week, almost .5 ppm per day!
 
That is odd. I was confused last year as well. We know from experience that CYA will drop for any of the following:
- Water exchange
- Water leak that causes an exchange
- Significant splashout/backwashes, etc
- Minor monthly oxidation/use (2-4 ppm "perhaps" depending on weather)
- Reported posts of bad product (unexplainable drops in CYA)
- For newbies there is the +/- 15ppm testing variance

In your case I'm leaning towards the product. Hopefully a different brand will work for you. In my case (last year) it was the HTH brand. I switched for a while then came back to HTH this year and haven't had any problems yet.
 
Just to add my CYA loss to the pile, a drop of 25ppm in 54 days (14th May to 6th July) is 0.46ppm a day so very similar to COBrian and I haven't ever added CYA (and no leaks/significant splash out etc).
 
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