High PH - Spa spillover raising PH in SW Pools?

[jaimslaw]

1st Question: I understand salt water pools run PH high- but some, like mine, run a lot higher than others. I know this can be due to various factors (BTW, my TA = 70). One factor that I read about is the part aeration plays in the SWG chlorine producing / higher PH process. Does the aeration caused by a dual spill over spa add to the elevated PH situation? I mean if aeration in the SWG operation contributes to enhanced PH levels, is not the aeration caused by a dual spill over spa enhancing the rising PH siutation? (and if so, I guess I l will live with adding about two gals a week if I want to maintain optimum PH levels).

2nd Question: Does anyone know the potential PH rising properties of flaking sandstone from above-pool surface walls surrounding the pool? Seems it was able to move PH higher by putting some in the PH test tube and letting it dissolve. Could this be combing with other factors to drive up my PH?


Thanks in advance for anyone who can contributing to this (and to those answering, you will be spared when when it comes time that we take over your planet).

[PoolDoc]

You're adding 2 gallons of acid -- muriatic acid? -- per WEEK to your pool?

You'd better post your actual test results and pool volume. Something's not right here.

PoolDoc

[CarlD]

In other words, despite the fact that the answer is "yes" to both assertions that SWCG pools can constantly raise pH and that the spillover will also raise pH, the answer is "no" to whether that should result in you adding 2 gallons of muriatic acid every week. You may well be having to add acid, but that amount is far beyond what should be required. Therefore something else is going on.

[chem geek]

Two questions: what is the size of the pool in gallons and is it a new pool or a newly re-plastered pool? The curing of plaster significantly raises the pH and requires a lot of acid in the first month though continues with noticeable curing for about a year.

[madwil]

also, how much sandstone is flaking off? is it dissolving in the pool, or mostly just chunks that get removed fairly quickly?
Sandstone can change ph, but only if dissolving- if whole chunks are in but removed before substantial dissolving, shouldn't make "much" difference...
I didn't use 2 gallons acid all last year, much less in a week!

[jaimslaw]

also, how much sandstone is flaking off? is it dissolving in the pool, or mostly just chunks that get removed fairly quickly?
Sandstone can change ph, but only if dissolving- if whole chunks are in but removed before substantial dissolving, shouldn't make "much" difference...
I didn't use 2 gallons acid all last year, much less in a week!

My pool info: 11 year old 13K plaster pool/spa in good shape; (as mentioned, dual spill over spa) So. Calif. very hard water; TA 60-70; Conditioner 80; Salt levels 3200; Starite DE filter and Heater; 2 HP Whisperflo single speed pump;

Sandstone flaking off weekly - I would say total would be maybe a cup - maybe two pool wide (hard to estimate this) - could be that small pieces do get to the DE grid and slowly dissolve there (am going to seal the Sandstone next month). Am not adding a lot of fresh water, which otherwise would cause a PH bump.

I will add 1/2 gal on Saturday, then PH is up past 7.8 by Tues. ; add another 1/2, PH back up past 7.7 a feww days later - so maybe its more like 1 1.2 gallons over, say, a 10 day period.

I asked the question as to the aeration effect of a dual spill over spa (increasing the PH in SWG) and want to follow up with this question: is there anyone out there with a dual spill over spa / SWG set up in So. Cal (ie using hard water) that has no Hi/ PH problems? On several other forums, I have read plenty of posts about the SWG/hi PH situation; I am coming to think that those with a SWG and no PH problem has a pool configuration different from others, such that there lack of a rising PH problem is more a function of their pool's configuration as opposed to TA levels and the like. (I mean, my TA levels are low, but that "fix" does not work for me).

Thanks for any input/insight you may have as to some of my "theories."

[chem geek]

Unless you've got some strong base source such as plaster curing, then your pH rise really is coming from two main sources. The increased aeration such as from a spillover most certainly causes faster carbon dioxide outgassing and that causes the pH to rise. People with spillovers, fountains, waterfalls, etc. often find the pH rises more. However, lowering the TA does help, but 70 ppm isn't actually that low in this case. See this thread where pH stability was attained at a TA near 50 ppm.

The second source is possible chlorine gas outgassing. By any chance is the distance between your SWCG and the first pool return rather short? If so, then chlorine gas may not have a long enough time to dissolve in which case it can outgas from the pool and that would cause the pH to rise. If you have eyeball returns, you might try pointing them down, at least for the one closest to the SWCG.

Also note from this chart that a higher TA and a lower pH are way over-carbonated so if you are lowering your pH down to 7.2 instead of just to 7.5 or 7.6, then that could be part of the problem as well. If you are using full-strength Muriatic Acid (31.45% Hydrochloric Acid), then a half-gallon in 13,000 gallons with a TA of 65 and CYA of 80 would lower the pH from 7.8 to 6.8 which is way too low. Even with half-strength Muriatic Acid (15-16%), that would still get down to 7.2 which is too low. The pH will fairly rapidly come up from such low pH, but then slow down in its rise as the pH gets higher. Also, the full-strength acid addition would lower the TA by 17 ppm while the half-strength by about 8-9 ppm so you'd have to be adding baking soda regularly to prevent the TA from dropping (but you didn't mention that, so is that what you are doing?).

As for the sandstone flaking, only if your water was fairly aggressive would it dissolve and yes that would raise the pH, but also the CH level. Given your water parameters, your saturation index is probably OK at the higher pH, though too low soon after you add acid. You said you had very hard water, but what is your measured Calcium Hardness (CH) level? I figured it was probably around 500 ppm.

If you do end up letting your TA get lower to 50 ppm and if you don't try and lower the pH as much -- just lower it to 7.5 only and let it rise to 7.8 -- then you can consider using 50 ppm Borates in the pool which will provide additional pH buffering and also be a mild algaecide that may let you cut down your SWCG on-time if you happen to be fighting nascent algae growth. What is your current target Free Chlorine (FC) level with your 80 ppm CYA? If it's below 4 ppm, then that could be part of the problem since a higher SWCG on-time results in more hydrogen gas bubbles for more aeration and in potentially more chlorine gas outgassing as well.

[jaimslaw]

Very informative reply. Also points out just how little I know about all of this.

My swg is a fair distance to the first non eyeball return - probably a good 10 feet.

As far as there TA, I should work on that a little. Must be sure how to lower it properly, ie, not bringing the ph way down, as has occurred a few times. I have always been a little vexed by the process of adding acid - I will study up on your post and see if I can make some progress.

Your take onthe aeration seems about the most thorough explanation I have seen to date.

Will post more about the CH levels when I get those tested.

Thanks for an erudite treatment of my problem.

[chem geek]

My swg is a fair distance to the first non eyeball return - probably a good 10 feet.

That's actually quite close. A large distance would be more like 30-50 feet as occurs in my own pool (though I don't have an SWCG). Too bad it's not an eyeball return where you could force the emerging bubbles to spend more time mixing with the water.

The TA should be dropping over time on its own from your acid additions. You shouldn't have to explicitly work on it. Just don't add the baking soda (or Alkalinity Up) that you have probably been doing that has kept the TA from falling. Do I have that right that you have been adding something to keep the TA up?

It's also possible that the acid you are using is very weak. Check the label to see if it gives a percentage of "Hydrochloric Acid" and tell us what it says.

[jaimslaw]

The TA lelvel today was 80 - PH was high (past 8.0), so I have added my usual 1/2 gallon of acid. I don;t get the Home Depot watered down stuff - what a waste of money. Leslie's pool supply is my acid source. I think its around 34% but I could be wrong on that.

I have not ever added anything to raise the TA - I dod freak out once when I added a little too much acid and the PH was off the charts low, so I did add a little baking soda that one time just to avoid any corrosive effects of that really low PH. OTher than than, have never done anything to raise PH or TA.

As I posted before, I will read up again on the technique for lowering the TA correctly to see if I can tame the PH situation a little.

I suspect that the sandstone flaking issue that I alluded to earlier is not a very big contributing factor. But I will still seal it anyways.

My money is still on the aeration effect of the dual spill over spa and remain curious (an element of my nature) as to who else out there with a SWG and dual spill over might be having some high PH control problems. It would sure be reassuring and diagnostically significant if someone with my same configuration weighed in as to whether or not they have the same or similar ph problem.