Recurring algae, high CYA and algae in the light enclosure

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KMPoolOwner

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Jul 27, 2016
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North Potomac, MD
I'm new to the site, had pool guys managing my pool until I retired a few years ago. They just added trichlor pucks and algaecide once in while and so that's what I did, just checking with test strips. After several summers of no algae ever, not even using any algaecide, I started to have problems with it three years ago. The guys at the pool store said everyone was having problems and so I used commercial shock products and algaecide. This year I got tired of all the chemicals and looked for an alternative. After studying TFPC, I think I have two problems. The first is a CYA of 110 (used a 1:2 dilution). I'm trying to delay changing out the water in the pool until September so we can use it without interruption and then I have time to get it rebalanced before closing.

Second, I noticed that when I sweep the pool (almost every day this year, trying to prevent algae), I find slimy flakes around my pool light every day. I realized that I had the bulb replaced the spring before all the problems started. It was replaced when the pool was opened that spring, and now that I think about it, I'm sure the pool wasn't balanced and completely clean yet.

So yesterday I opened up the light fixture and, wow, there was a lot of thick beige slime. Not sure what type of algae is that color, but there was lots of it. I added a copper algaecide to try to keep what was released into the pool from taking hold and then today started a SLAM. (BTW the pool has been perfectly clear after some initial work at opening, and but was murky after the light fixture was opened. It was clear again this morning.

I am sold now on nothing but liquid chlorine, and bought 18 jugs of 8.25 % hoping that would be enough. First, I read on a thread that chlorine works better at lower pH, so I added dry acid to bring the pH to 7.1. (My TA is about 125 so lowering it by adding acid is a secondary reason for this.) The reference table for FC levels said I'd need 39 ppm for the SLAM, but the calculator said 26 for shock. I used the higher number, adding 10 jugs of chlorine. Using 10 ml for the test sample and a dilution of 1:3, I got 38 for my initial level after 1 hour of circulation. Close enough. Then got about an inch of rain in a heavy storm. Checking an hour after the rain stopped, the level was 36. Added another jug of chlorine. After a couple of hours the FC was 34. Added 1.5 jugs and will check again before bed so I can see how much I lose overnight. I am doing all of this with the light fixture open. I don't want any algae hiding in there.

I have two questions:

1) Why is it OK to swim in the pool with such high levels of free chlorine? I guess the chemist in my wants to understand what the CYA is really doing that I can have 39 ppm in the pool and not have it be very irritating.

2) Am I on the right track? I'd really like the advice of all the experts out there. I have a chemistry degree, but that doesn't mean I understand all of this. Below is the most recent test results from before I started the SLAM.

FC 5.4
CC 0.2
pH 7.1
TA 125 (I corrected for high CYA the reading was 160 - .3(110) ~ 125
CH 260
CYA 110

FC during SLAM has not dropped below 34, and I am adding 8.25% bleach to keep it at 39. Will do OCLT tonight.

Thanks! I am so glad to have found this site!
 
Welcome to TFP! Good to have you here, and thank you so much for becoming a supporter right out of the gate. :)

Your shock level should be more like 44 ppm FC.

If you want to go deep on chemistry, scan this and then go back and read it closely. (for me around 7 or 8 times!) The amount of work behind this thread is amazing. Pool Water Chemistry

In a nutshell, high FC is not annoying or dangerous because the CYA is part of a system which holds the vast majority of the chlorine in reserve. The active chlorine, hypochlorous acid and hypochlorite ion, is released per an equilibrium reaction. They are both sanitizers but the hypochlorous acid does the lion's share of the work. At lower pH, a bit more of it is available active chlorine. We test free chlorine (FC) because it's a cheap, reliable test, but FC includes both active chlorine and chlorine held by the CYA. The active chlorine is what people often refer to as the "harsh" form of chlorine, and is the part which wears out swimsuits and fades things. The other thing people find annoying is combined chlorines, which are loosely referred to as cloramines, and these are what arise as organic contamination is removed.

You can run a high-CYA pool as you hope to do, but don't be shy with FC levels. If it was my pool, the bare minimum after the SLAM would be 8 ppm FC and a target of 12. If you study that thread and click through on a few links, you'll learn that your pool will have less of the "harsh" chlorine than a well-run commercial indoor pool that does not use CYA and which runs at 1 to 2 ppm FC. If you're diligent, you will find that your daily consumption of chlorine will not be any more (and may even be less) than if you had the pool at a lower level of CYA.

Thanks again for becoming a supporter. That's really helpful with keeping the site up and running!
 
Thanks so much for the explanation. I will definitely study that post you suggested. And I will up my target for SLAM to 44+.

This morning the FC had gone from 38 to 30, so I added another 2 gallons, this time of 10% liquid chlorine, and a half jug of the 8.25% I had left from last night. I should have been about 40. Then at 10 it was back down to 28. I can see I have some work to do.

But the SLAM can still work if I keep the 44+ FC even with the CYA high? If not, I would certainly bite the bullet and replace water now.

Thanks again.
 
I thought I should give an update. Passed the OCLT Sunday night (4 days of SLAM)! I am letting the CL drift down. So far, so algae, but I am still at FC 26. I will run about FC 12 until I get the partial refill done in September, then start managing the pool using TFC without fail. I have learned so much in just the last couple of weeks. Thank you, everyone who's experience contributed to my education! And for your quick, supportive responses, Needsajet!
 
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