In a nutshell: Why the alk lowering process works

[Watermom]

Carl posted a very good explanation of why the process recommended here on the forum works to lower alkalinity. It is a very good post -- informative and explained clearly. I thought it deserved to be "stickied" so lots of people (especially our newbies) will be able to find it and read it. Below, is Carl's post. Great job, Carl, as usual!



Lowering total alkalinity is VERY confusing. Let me try to clear it up.

1) T/A and pH are linked. As pH goes up or down, so does T/A.

2) There are very few ways to move either T/A or pH MORE than the other.

We can raise T/A more than pH by adding baking soda. We can raise pH more than T/A by aerating.

That's it.

3) But we can ONLY raise pH via aeration when pH is 7.2 or less.

4) So we LOWER pH to 7.0-7.2, which pulls T/A down (see 1) again). We don't go lower than 7.0 because below 6.9 vinyl can be damaged. Concrete/Plaster/Gunite is not as sensitive to pH.

5) We then aerate which raises pH WITHOUT raising T/A--it's the ONLY way to do it. (Why does this work? Ask Chem_Geek--he's the BEST at explaining it)

6) We then repeat 4) and 5) again and again until T/A is where we want it.

Once you understand which action does what, it makes a lot more sense. Aeration does NOT lower T/A--that's frequently mis-understood--it's dropping pH to 7.0-7.2 that does it. Aeration raises pH without raising T/A.

[Tom Travis]

Thank you so much for explaining this. I have been aerating my pool for days trying to lower my alkalinity (is about 150 ppm) and it did nothing. My PH is at 7.5 and maintainig that. I will lower the PH and try it again.

[CarlD]

Thank you so much for explaining this. I have been aerating my pool for days trying to lower my alkalinity (is about 150 ppm) and it did nothing. My PH is at 7.5 and maintainig that. I will lower the PH and try it again.

Yup. That's why it didn't work. BTW, if you have a vinyl pool and don't have a heater or SWG system there's no reason to lower T/A. With vinyl pools the absolute max for T/A is 200ppm, not the 120-125 normally recommended. I like to think of the max as 180ppm, because it gives room if pH goes up or down.

So if your pool is vinyl, don't bother lower T/A. If you have a heater (gas or heat pump) and/or an SWG, or if you have concrete/plaster then you do need to lower T/A.

[aquarium]

Why would the pool construction make any difference? I lower TA (tapwater 250) so I don't have to fight it all the time to hold the pH down. Even then, it's mostly a matter of how much acid is used at the frontend to lower the TA, versus how much is used over the course of the summer to maintain pH, which tends to creep upward. Since I'd rather handle less acid less often, lowering the TA gets that handling done mostly in one effort.

BTW, I used the aeration method with great success last summer to drop it down from 250 (after a complete refill) to about 100. Testing this spring, it was down around 50. In an aquarium the water is considered adequately buffered at about 60, and there we are dealing with live animals in a small container.

[waterbear]

The only real advantage to having a higher TA is when you are using stabilized chlorine which is acidic. The higher TA (which will eventually get depleted) will keep causing the pH to drift upward and help stabilize the pH. With bleach or any other unstabilized chlorine running a lower TA will minimize your acid usage needed to maintain pH.

Edit: Carl is correct that there is no reason to worry about TA in a vinyl pool as long as your calcium levels are not too high. You might need to add acid more freqently if you are using bleach but there really aren't any other negatives. If the calcium levels are high then the combination of high TA and high calcium can lead to scaling or clouding of the water, especially if you let your pH get out of line and rise.

[CarlD]

Edit: Carl is correct that there is no reason to worry about TA in a vinyl pool as long as your calcium levels are not too high. You might need to add acid more freqently if you are using bleach but there really aren't any other negatives. If the calcium levels are high then the combination of high TA and high calcium can lead to scaling or clouding of the water, especially if you let your pH get out of line and rise.

Evan's right. Of course, too-high calcium is a problem for ALL pools, not just vinyl. If your calcium hits, say 500ppm and your T/A is 180 or 200ppm, you will almost definitely get a milk-white cloud.

But too-low calcium is NOT an issue for vinyl. And when it's low, higher T/A levels are fine.

[KurtV]

The only real advantage to having a higher TA is when you are using stabilized chlorine which is acidic. The higher TA (which will eventually get depleted) will keep causing the pH to drift upward and help stabilize the pH. With bleach or any other unstabilized chlorine running a lower TA will minimize your acid usage needed to maintain pH.

Good point.

Edit: Carl is correct that there is no reason to worry about TA in a vinyl pool as long as your calcium levels are not too high.

Except that, as Aquarium notes, that high TA will put upward pressure on your pH (as you note above); that's good in your example but bad when you're fighting constantly rising pH.

You might need to add acid more freqently if you are using bleach but there really aren't any other negatives...
I guess you're saying the same thing in a subtler way here.

I have my TA down to 60 in my ongoing battle against constantly rising pH and I'm still adding acid every few days. I'd hate to think how bad it would be at 180 ppm.

[waterbear]

But too-low calcium is NOT an issue for vinyl. And when it's low, higher T/A levels are fine.

One often overlooked reason for having calcium in a vinyl pool is to help prevent foaming. Soft water will have more of a tendency to foam than hard water. Personally, I think a calcium hardness of around 100-150 ppm would be more than enough. Might be in intersting experiment if anyone cares to do it! Other than that low calcium should not be an issue at all!

[CarlD]

One often overlooked reason for having calcium in a vinyl pool is to help prevent foaming. Soft water will have more of a tendency to foam than hard water. Personally, I think a calcium hardness of around 100-150 ppm would be more than enough. Might be in intersting experiment if anyone cares to do it! Other than that low calcium should not be an issue at all!

Well, my CH level is 50ppm this season, pretty low, but I haven't seen ANY evidence of foaming, ever. How low does it have to be before that can happen? Also, I'm a bear about keeping my chlorine levels stable, so that's probably preventing it as well.

[waterbear]

depends on a number of factors. Pools that develop a scum line tend to have more organics (from body oils, suntan lotion, etc.) and might have more of a tendency to foam in very soft water. Not really a major issue. Was just pointing out another reason to keep the water in the pool a bit 'hard'.