Is there a certain window of time

[Rangeball]

To aerate when lowering alkalinity?

My fill water is high alk. Tests at 320. 3 times this year I've added acid and tried to lower it. First two times when I lowered the PH the alk dropped with it, from the lower 200s to 150. I had a suction side small air leak and was getting bubbling in my return flow that I thought would provide adequate aeration. Each time the PH rose, the alk rose along with it.

The 3rd time my PH dropped but the alk didn't really budge. I had my PH at 7.2, was aerating with return bubbling plus a bunch of normal pool activity to really stir things up. I notice after a dive or cannon ball, after the normal bubbling has stopped the surface of the pool looks like a glass of coke when you pour it and the foam goes away, carbon dioxide popping out of the pool. This is the desired result, right?

Is there a definite time frame during which the aeration must take place to get the alk to stay down?

The alk only goes down from the acid, not the aeration, right?

Pool water is crystal clear, but PH jumps back to 8ish only a few days after lowering it with acid. For some reason, the alk always goes back up with it. Am I doing something wrong?

Should I just keep dropping the PH and aerating and eventually the alk with lower and stay low?

Any help is greatly appreciated Oh, and I've not added any fill water since after the first time I added acid above.

[JohnT]

When the pH starts back up, your aeration lowering effect is diminished. You have to stay right at 7.2 (lower with concrete) to see much effect.

[Rangeball]

But is it the aerating that's lowering the alk, or the acid?

In other words, is it-

1- PH is 8, Alk is 200. Add acid, PH drops to 7.2, alk at 200, aerate, alk drops while ph rises. Keep lower PH and aerating until alk lowers to where you want it then stop.

2- PH is 8, Alk is 200. Add acid, PH drops to 7.2, alk drops to 140. Aerate, PH rises but alk stays at 140 because carbon dioxide is gassing off from aeration. Continue to drop PH and aerate until you get alk where you want it.

I had understood it to be #2. However, My PH is dropping, but alk isn't. And for some reason Alk increases along with PH when aerating.

Somethings not right

[Rangeball]

Can anyone help?

[Tredge]

I'm also dealing with a high TA.

Are you also adding CYA? I think this will make it look like your TA and PH are going up when they are not.

Aerating actually causes your PH to go up. The only way Aerating will work is to keep adding Acid....if you can keep the PH low for a period of time then the TA WILL drop. It can take weeks.

Shoot for a constant PH of 7.0 (6.8 for a plaster pool) and watch your TA closely. When it starts to drop it will drop significantly.
Also remember that TA isnt a thing, its a measurement of the waters resistance to changing PH. It can go back up and in my experience will bounce around a little while you are trying to stabalize the PH.

This is all from info I've gleamed out of this forum, I'm still working on it myself.

[Rangeball]

I'm also dealing with a high TA.

Are you also adding CYA? I think this will make it look like your TA and PH are going up when they are not.

Nope, only using bleach. CYA is about 20ish and holding.

Aerating actually causes your PH to go up. The only way Aerating will work is to keep adding Acid....if you can keep the PH low for a period of time then the TA WILL drop. It can take weeks.

Shoot for a constant PH of 7.0 (6.8 for a plaster pool) and watch your TA closely. When it starts to drop it will drop significantly.
Also remember that TA isnt a thing, its a measurement of the waters resistance to changing PH. It can go back up and in my experience will bounce around a little while you are trying to stabalize the PH.

I had to add 3" of water to make up for evaporative loss last night. Since my fill water is highly alkaline and lower PH than my pool, I waited until it was circulated and retested. PH was down, Alk was 190.

The Alk number doesn't bother me (vinyl liner), but the PH tends to shoot back up within days, so I'd like to get it down to 100 or so and see if my PH stays more stable. Water clarity and feel is much better when I keep the PH between 7.2-7.6 ish.

I set up my sump pump to aerate, but we have a good chance at thunderstorms today so I'm waiting to add acid and start aerating until I see what the rain does to the water. If it doesn't have much affect or misses us again, I plan to attack the alk daily until it drops to where I want it.

This is all from info I've gleamed out of this forum, I'm still working on it myself.

Thanks. I try to read as much as possible before asking questions, but this one has been stumping me. I'm starting to think it's primarily the high starting alk that is making it so hard to keep alk down permanently (ie, without adding fill water). Once alk is lowered and properly aerated, it shouldn't rise sharply, right?

[waterbear]

But is it the aerating that's lowering the alk, or the acid?
TA is a measure of the carbonic acid/bicarbonate/carbonate buffer system in the water. Only the carbonate/bicarbonate part of it is measurable by testing. When you lower pH you shift the amounts of the three components to more carbonic acid and less of the other two. The amount of the buffer is unchanged. When you aerate you drive off some of the carbon dioxide (carbonic acid is just carbon dioxide gas dissolved in water....think club soda, exactly the same thing. Aerating...think shaking the bottle of club soda to make it go flat...exactly the same thing).
By driveing off the carbon dioxide you:
1 lower the amount of the buffer system in the water
2 this causes the pH to rise since there is a higher ratio of bicarbonate/carbonate to carbonic acid and less acid means higher pH (simplified a bit but essentially true)
3. once the pH rises the buffer system reaches a new equilibrium point and there will be less meaurable carbonates/bicarbonates in the water....you have now succesfully lowered the TA!

In other words, is it-

1- PH is 8, Alk is 200. Add acid, PH drops to 7.2, alk at 200, aerate, alk drops while ph rises. Keep lower PH and aerating until alk lowers to where you want it then stop.

2- PH is 8, Alk is 200. Add acid, PH drops to 7.2, alk drops to 140. Aerate, PH rises but alk stays at 140 because carbon dioxide is gassing off from aeration. Continue to drop PH and aerate until you get alk where you want it.

I had understood it to be #2. However, My PH is dropping, but alk isn't. And for some reason Alk increases along with PH when aerating.
Since the measured TA is dependant on the pH at which it is measured you might see this effect. If you measure the ALK at a pH of, say 7.8, and follow the procedure and then retest once the pH is back at 7.8 you will find that the measured TA is now lower. IF the TA is very high you need to keep the pH low as you aerate and keep adding acid to keep it low and also monitor the TA. You will see it start to decrease. When it has lowered then stop adding acid and aerate until the pH is where you want it, say 7.4. Your TA might read slightly lower at this point or not. The lower you can safely get the pH to and keep it there while you aerate the faster you will lower the TA in the water since you will maximize the amount of the buffer that is in the form of carbonic acid.

Somethings not right

Hope this doesn't confuse you too much!

[Rangeball]

Since the measured TA is dependant on the pH at which it is measured you might see this effect. If you measure the ALK at a pH of, say 7.8, and follow the procedure and then retest once the pH is back at 7.8 you will find that the measured TA is now lower. IF the TA is very high you need to keep the pH low as you aerate and keep adding acid to keep it low and also monitor the TA. You will see it start to decrease. When it has lowered then stop adding acid and aerate until the pH is where you want it, say 7.4. Your TA might read slightly lower at this point or not. The lower you can safely get the pH to and keep it there while you aerate the faster you will lower the TA in the water since you will maximize the amount of the buffer that is in the form of carbonic acid.

Hope this doesn't confuse you too much!

I believe I understood everything, and I believe I am following the procedure as outlined, what I don't understand is why I'm NOT seeing a signficant PH/ALK drop when I add the required amount of acid and aerate. Any ideas?

The only variable seems to be aerating WHILE trying to get the PH to drop, but I've eliminated this, so we'll see.

[Rangeball]

Tested PH at noon-

7.3, down from the 7.6 I got this morning. I plan to get more acid, and lower it until I have a 7.0 reading. At that time I'll retest ALK and start aerating again.