Caramel Colored Stains

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AUSpool said:
Based on theory, we used to think that both pH and FC where higher in a SWG’s cell but in practice, due to the high flow through the cell, the pH is identical to the pool pH and the FC, at best, is 1-2ppm higher than the pool FC, which is nowhere near SLAM levels. I found this to be true by testing my SWG cell’s effluent. I would assume there still is a higher pH at the cell plate surface but I found no detectable change of pH in the return water.
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I know, we are not talking about superchlorination here, but FC and pH are slightly higher in the return water. I did some tests last year, FC was about 3ppm elevated, pH not quite 0.1:

mgtfp said:
FC in the return water was about 3ppm higher than the pool water.

pH was 0.07 higher in the return water. Was pretty reproducible. I was putting my pH-meter in and out of the eye-ball, and the pH kept jumping back and forth, always the same value. Before I started the the pH-test, I kept the meter for maybe 10 minutes in the water to make sure that the probe had adjusted to the pool water temperature
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Together with pH-rise due to increased aeration in the spa, that should show up in stain intensity when metals are in the water. It won't result in sudden massive staining, but it will favour stain build-up over the years in the spa over the pool. I think that's what happened in my pool/spa - but I wasn't around for the first 35 years or so of my pool's life. I only know that my spa was significantly more stained than the pool when we moved in.

But I agree, the main effect in the spa from the SWG would be the FC, pH only to a smaller degree. Aeration due to the bubblers is probably the larger culprit. Especially with TA being higher before I took over the pool.

The localised pH-rise within the cell that we refer to as being responsible for scaling in the cell is really only very localised around the cathode. The bulk water's pH in the cell doesn't change much apart from the small rise that is in line with the increased FC-level.
 
MGTFP: thank you for the reply. Yes, I have 4 returns in the spa and four in the pool (three on the walls and one on the floor of the deep end, I usually leave all of them open under normal pump operation). This is exactly the stain removal plan I intend to take on. I am really amazed by the tiny amount of AA required. 4-5 oz in 2300 gal! Can I ask: without sequestration, did your stains in the spa return? And if so, how long thereafter? I will have my fill water checked....as it seems to me this is the only source of iron in the water...sprinklers are the same source. My 1 lb container of AA will arrive tomorrow, hopefully in time for me to do this Monday evening. I have already isolated the spa yesterday ...mainly to get the CL down from 4 ppm...and it was around 3 ppm this morning. pH is at 7.5 and it takes only a tiny amount of MA to get the pH down to 6.8 to 7.0 (as I have learned accidentally the hard way LOL). My winter sump pump will likely take 1-2 hours I am guessing to drain the spa (2300 gallons). No worries I assume. Looking forward to diving into this and will report back the results. I'll post pre/post pix.
 
I would also say and agree this was a slow build-up of staining over the years as is stated herein, but it was far more noticeable this year when we pulled the cover at opening........certainly the spa is worse than the pool...and one more interesting comment....yesterday I put 6-8 1000 mG Vit C tabs into two socks each and let them sit for a minute each in 6 different places in the spa.......interestingly: Today, the now circular 1" diameter white areas of stain removal are far more pronounced THIS MORNING the yesterday directly following the application of the Vit C tabes. I guess it took a while for the iron to go into solution.
 
Just pondering: Is there any reason I cannot / should not use the water in the spa (after the AA Iron Treatment) to backwash my filter? Or use the pump to send it to waste? This would be far more convenient.... I assume the AA in the water will not damage the equipment.
 
My stains didn't return do far. But I don't have iron in my water. Never showed up in pool store tests, and also not in an iron test that I bought. I also never added a sequestrant, and what the previous owner might have added should have worn off by now. Which is why it took me a while to consider iron and do the vit C test and why I think it's a historical problem.

But my plaster is also very old and doesn't look perfect anymore, so I'm not chasing perfect
 
My pool was ill-cared for by the prior owner, in fact, had to replace the pump/filter/heater and all the tile and coping within one year after we moved in...but the plaster is not in bad shape. Rumor was the prior owner wanted to fill the pool in with dirt using an excavator, which is highly illegal. The 10-year-old boy from next door came over two weeks after we moved in and said: "wow, that's the first time I have ever seen the pool where it was not green"... LOL
 
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As the level in the spa gets lower during the backwash, run the hose in there to dilute the remainder as you will have to leave several inches of the old water in the spa to not lose suction.
 

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Well, pretty good success was achieved. I let the Spa get down to almost 0 FC, and dropped the pH to 7.0. Added about 5 oz of powdered AA, running the pump closed loop to the spa on filter recirculate. After about 30 minutes, there was some improvement and more after 1 hour. All in all I ran it for about 2 hours....then drained the spa on waste mode. Once it went low, I added some fill water....brought it up about 8 inches above the drains, and drained it a second time. The pictures tell the story. Thanks to all. IMG_0410.jpgIMG_0412.jpg
 
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I also wanted to point out that I double-checked my fill water and pool water for Iron. According to the tests yesterday at the local pool store, both Iron (and Copper) are at 0.0 ppm. So, I wonder where the Iron stains came from. I only did the Spa for treatment....see post above....As to the Pool...the most noticeable iron staining is in the shallow steps area in the pool. I dropped a little bit of powdered AA (like a tablespoon) down onto an area where there was the most apparent staining, 12 inches below the surface of the water...and saw improvement after 2-3 minutes. I'll work myself up to doing the entire pool later (maybe at closing this fall).
 
You might see if your water bureau offers free water quality reports. Mine does and it indicated a very slight amount of iron. It took about 3 years from when my liner was installed before the light amount of staining began to appear. Even a trace will eventually cause issues.
 
Do remember, iron is not an item required to be reported by the EPA. So few water treatment facilities measure or report it.
 
Yep, this must be a trace amount...as it's been a long slight slow steady staining process. And the "young pool dude" doing the test (I think he was like 16 and on his summer job) said the Iron reading "popped up" at 0.1 for a second on his screen, then settled out at 0.0 ppm. By the way...the "pool dude" also told me that my "high" phosphate number in the fill water (179 ppm) would tend to "dissipate your chlorine more quickly", to which I gave him a "knowing chuckle" and said "you shouldn't kid a kidder..... :p:p:p:p:p
 
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