Total alkalinity question

[Wake9909]

Hi! Our pool was recently built (completed March, 2020). We have been fighting our pH rising ever since. We add at least a gallon of muriatic acid to our pool weekly. My question has 2 parts. 1. Isn’t the sole purpose of total alkalinity to keep the pH stable, so it doesn’t go too low? 2. Since I don’t believe our pH will be getting too low anytime in the relatively near future, how low can I allow the total alkalinity to get? We have quite a bit of aeration that adds to the pH rise as well (bubblers, spa spillover).

 

[Richard320]

You should keep TA above 50. But before you start to work driving it down, plug numbers into poolmath and see what the CSI is. If the TA gets to low you may drive CSI too low and etch the plaster.

A certain amount of aeration is inevitable when you have to keep fresh water in the spa and plumbing. You might be able to minimize it by reducing the spa flow some and especially by reducing the hours. Pool pros often recommend running the pump 24/7. It doesn't hurt anything but your electric bill and they're not paying. But obviously, 1/4 the hours means 1/4 the aeration. I manage with 2 hours a day. You'll need to experiment to see how short you can go.

How low are you trying to keep the pH? If you expect it to stay at 7.4, you'll be fighting it more than if you were content to let it rise to 7.8 or even 8 before adjusting back to 7.6.

Curing plaster will also create an insatiable thirst for acid for about a year.

 

[magiteck]

Looking at your logs, it looks like you’re adding MA when your pH gets to 7.8. My pH loves to settle around 7.7 - 7.8. If I lower it to 7.2, it will quickly come back there.

I would try waiting until your pH rises to 8, and then using MA just to lower it to 7.6. That should slow down the TA drop and hopefully allow you to use a lot less MA.

 

[mgtfp]

Yes, total alkalinity acts as a pH-puffer, reducing pH-fluctuations. This is particularly important when using pucks (trichlor) to chlorinate the pool as trichlor is very acidic. It is less important when chlorinating with bleach or a SWG, as these have no net effect on pH.

A side effect of total alkalinity (or more specifically the carbon alkalinity part of total alkalinity) is that high TA means high amount of dissolved CO2 in the water which wants to out-gas, driving the pH up.

You want a balance of these two effects, enough TA as a buffer, but not too much out-gassing. TA also has an influence on the CSI, water low in TA is more corrosive to plaster surfaces.

You should keep TA above 50ppm.

 

[Wake9909]

You should keep TA above 50. But before you start to work driving it down, plug numbers into poolmath and see what the CSI is. If the TA gets to low you may drive CSI too low and etch the plaster.

A certain amount of aeration is inevitable when you have to keep fresh water in the spa and plumbing. You might be able to minimize it by reducing the spa flow some and especially by reducing the hours. Pool pros often recommend running the pump 24/7. It doesn't hurt anything but your electric bill and they're not paying. But obviously, 1/4 the hours means 1/4 the aeration. I manage with 2 hours a day. You'll need to experiment to see how short you can go.

How low are you trying to keep the pH? If you expect it to stay at 7.4, you'll be fighting it more than if you were content to let it rise to 7.8 or even 8 before adjusting back to 7.6.

Curing plaster will also create an insatiable thirst for acid for about a year.

Thank you for your response! We wait till the pH gets to at least 7.8 before adding more acid. Sometimes it’s 8.0 or even a little higher. We have to add it about every other day or so. It averages out to about a gallon or a little over that over a week’s time.

So, hypothetically speaking, what would happen to our pool if we let the TA get down to 30, for example? Would it hurt anything? What’s the downside?

 

[mgtfp]

So, hypothetically speaking, what would happen to our pool if we let the TA get down to 30, for example? Would it hurt anything? What’s the downside?

Your water would start to get corrosive, especially once water temperatures start to get colder. Hack the numbers into PoolMath and play with the the influence of parameters on CSI. Activate the options "track CSI" and "track temperature" in the App.

 

[Wake9909]

Your water would start to get corrosive, especially once water temperatures start to get colder. Hack the numbers into PoolMath and play with the the influence of parameters on CSI. Activate the options "track CSI" and "track temperature" in the App.

Thank you! I’ll do that!

 

[mgtfp]

... and you would also get noticable fluctuation on your pH. We say bleach has no net impact on pH, meaning that the whole chlorination cycle is pH neutral. Adding bleach will slightly increase your pH, and the subsequent use of chlorine (by killing germs and algae, oxidizing stuff and by UV-decay) will bring it back down where it was (plus the drift due to out-gassing that had happened in the meantime). When your TA becomes too low, the buffering effect will be too low, and the pH fluctuations will actually become noticeable. The whole cycle will still be pH-neutral, but there will be noticeable fluctuations within the cycle.