High PH - Spa spillover raising PH in SW Pools?

[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.

[chem geek]

If your fill water is high in TA or if you have a lot of water evaporation, then the evaporation and refill can increase the TA level as well (everything in the fill water gets added to the pool). Since your water is very hard, the TA may be high in the fill water as well.

Anyway, it is a known fact that increased aeration will result in faster pH rise. The solution is to get the TA as low as possible since that reduces the rate of this carbon dioxide outgassing. However, an SWG has other ways of raising the pH such as having chlorine gas that doesn't fully dissolve in the water so outgasses from the (usually closest) return instead. And I already mentioned how you are adding way too much acid if it's full-strength since it would lower the pH too much. That just causes faster see-sawing. Also, half a gallon of full-strength Muriatic Acid in 13,000 gallons should lower the TA by 19 ppm while half-strength would lower the TA by about 9 ppm. I'm telling you, something just isn't right about your situation and I suspect some information (maybe test results) are off.

Look on the label of the Leslie's acid that you have and tell us what is lists for the ingredients. Is it this stuff? If so, then that should be 31.45% hydrochloric acid (i.e. full-strength Muriatic Acid).

What kind of test kit are you using? You aren't using test strips by any chance, are you? Or are the numbers from a pool store?

[jaimslaw]

Yes, it is Leslie 31.45% acid.

I eliminated the fill water component as no fill water was added for much of the spring season. Test kit is the Home Depot off the shelf chem test tube type - I stay away from test strips. I replace it each year with a new kit. I would like to get the TA lower - is this just a function of gradually adding acid each day? I've seen some confusing posts about this process.

[chem geek]

Lowering the TA is ultimately through adding acid, though the process is accelerated by aeration (which you already have from the spillover) and having the pH be lower during the process, but in your case what is strange is that you are adding LOTS of acid but not having the TA drop. I simply do not understand this. Since you've got a lot of pH rise and you add acid frequently, your TA should be dropping. If you had new plaster or a recent replaster, then this would make sense since plaster curing increases pH, TA and CH, but you never mentioned anything about any sort of new or replaster job.

There can be some pH rise from undissolved chlorine gas outgassing and that will also raise the pH, but to have the TA be stable when adding half a gallon of full-strength Muriatic Acid every week, that would require almost 14 ppm FC to completely outgas as chlorine gas from the pool which seems inconceivable.