Total Alkalinity came down from 300+ to 0!

[Zeerob]

Hi, sorry to have to bother you again. Early May, I have refilled my pool after a black algae affair.
And I have then switched to salt.
I have filled with borehole water that had a very high TA (300+).
I have followed the recommended procedure to lower TA: aerating to lower PH and adding MA.
This worked quit well. After adding a lot of MA (38L) the TA was down to 175.
After that I have not added any more MA manually, but the salt water system has an automatic PH- pump (and a PH sensor) that pumps MA.
I have set the automatic PH level to 7.3 and that level was kept well. However, the MA consumption by the automatic pump was still very high. A 20L can was gone in 4 weeks or so.
Then I went away for 3.5 weeks, and someone kept an eye on the pool. Then the MA consumption suddenly dropped very much. From about 5 L per week to almost nothing.
When I came back, I have measured the TA again, and it is zero (with my Lovibond meter and tablets). I have measured twice.
It looks as if the MA has killed all the TA. I feel a bit silly now. First the TA was way to high, and I did go through a lot of trouble to lower it, and now it seems to be all gone?
The PH is okay, 7.2 of 7.3 when I measure it.
So, should I now start adding baking soda? Will that then also be consumed by the MA that the automatic pump adds? Will I constantly have to add baking soda to compensate for what the MA eats? What is the problem with the low TA?
Other parameters: FC = 3.4, TC = 4.3, CA = 250, CYA = 25
Another thing is that I have not found backing soda for a reasonable price here in Portugal.
Thanks!

 

[ajw22]

If the pH is in the 7s the TA cannot be 0. You have a testing problem with the TA test.

Don’t add chemicals specifically for the TA if your pH is good. pH is what matters, not TA.

 

[jblizzle]

I would also note that trying to keep the pH down around 7.3 is a little unrealistic. The pH is generally much more stable up around 7.8. Doing that will greatly reduce the acid consumption, assuming that the CSI is still reasonable. Given your numbers I think it might have been quite negative which is bad for your grout between tiles.

 

[Zeerob]

If the pH is in the 7s the TA cannot be 0. You have a testing problem with the TA test.

Don’t add chemicals specifically for the TA if your pH is good. pH is what matters, not TA.

Thanks! You are probably right. But the TA test has worked okay, not so long ago.
The Lovibond is a digital meter that works with tablets.
I also have test-stripes for KH=TA, and they also say 0 now
So, I am not sure what is going on.
Do you have an explanation for the sharp drop in Acid consumption?
It went from about 5L/week to almost 0.

 

[Zeerob]

I would also note that trying to keep the pH down around 7.3 is a little unrealistic. The pH is generally much more stable up around 7.8. Doing that will greatly reduce the acid consumption, assuming that the CSI is still reasonable. Given your numbers I think it might have been quite negative which is bad for your grout between tiles.

Thanks, I will try a higher PH. The acid consumption is already very low at the moment.

 

[ajw22]

Don't know about your TA tests to comment on what is going on. Your TA may be below whatever limit the tests can read so it gives you 0.

Likely your pH and TA reached an equilibrium. If your pH staying steady at 7.3 then then you need little acid. Finding that equilibrium is exactly what you are looking for when lowering the TA.

 

[jblizzle]

There is a reason we do not trust digital tests

Chemistry will not allow a 0 TA and a pH of 7.3. If TA was really 0, then the pH is in the 4s.

 

[Zeerob]

Don't know about your TA tests to comment on what is going on. Your TA may be below whatever limit the tests can read so it gives you 0.

Likely your pH and TA reached an equilibrium. If your pH staying steady at 7.3 then then you need little acid. Finding that equilibrium is exactly what you are looking for when lowering the TA.

Thanks, sounds reassuring

 

[JoyfulNoise]

This is also why blindly relying on an acid dosing system is a bad idea. pH and TA go hand-in-hand, when you add acid to lower the pH or keep it at a specific setting, you consume carbonate alkalinity (the majority component of TA). Once the carbonate alkalinity bottoms out, your water is only left with cyanurate alkalinity (pKa ~ 6.5). Cyanurates do not have as much buffering capacity and can quickly be one exhausted with the further addition of acidic chemicals. Once the cyanurate shifts to cyanuric acid, the pH will crash to 4.5 or less.

As others have mentioned, you need to keep your pH much higher at 7.6-7.8. Terrestrial water sources, whether from streams or bore-holes, have dissolved carbonates in them. Their pH naturally settles to a higher value, about 8.0-8.2. When you try to hold your pH at 7.3, you are only forcing the water to be in an unbalanced chemical equilibrium. A human being can comfortably swim in water that has a pH anywhere from 7.0 to 8.2. So there is no good reason to artificially keep the water’s pH where it does not naturally want to be. Aiming for a pH setpoint of 7.7 would use far less acid.

The only reason why you are using less acid is because you have exhausted the alkalinity of the water. Without that buffer, your water could quickly become very acidic and corrosive to your pool surface and equipment. You absolutely need a measurably TA level above 50ppm to be safe.

 

[Zeerob]

This is also why blindly relying on an acid dosing system is a bad idea. pH and TA go hand-in-hand, when you add acid to lower the pH or keep it at a specific setting, you consume carbonate alkalinity (the majority component of TA). Once the carbonate alkalinity bottoms out, your water is only left with cyanurate alkalinity (pKa ~ 6.5). Cyanurates do not have as much buffering capacity and can quickly be one exhausted with the further addition of acidic chemicals. Once the cyanurate shifts to cyanuric acid, the pH will crash to 4.5 or less.

As others have mentioned, you need to keep your pH much higher at 7.6-7.8. Terrestrial water sources, whether from streams or bore-holes, have dissolved carbonates in them. Their pH naturally settles to a higher value, about 8.0-8.2. When you try to hold your pH at 7.3, you are only forcing the water to be in an unbalanced chemical equilibrium. A human being can comfortably swim in water that has a pH anywhere from 7.0 to 8.2. So there is no good reason to artificially keep the water’s pH where it does not naturally want to be. Aiming for a pH setpoint of 7.7 would use far less acid.

The only reason why you are using less acid is because you have exhausted the alkalinity of the water. Without that buffer, your water could quickly become very acidic and corrosive to your pool surface and equipment. You absolutely need a measurably TA level above 50ppm to be safe.

I choose 7.3 because Zodiac (manufacturer of the SWG) advised 7.2-7.4, and with a TA of about 150 that gave a reasonable CSI.
Also, I have just had black algae and I figured a lower PH will make the chlorine more effective. I am still afraid the algae may come back.
So, do you think I should add baking soda now?
I can also start topping up with borehole water, instead of tap water that I am using now to top-up.

 

[jblizzle]

You need reliable test results to know what needs to be adjusted.

Also, lower pH does not make the chlorine more effective in a significant sense when CYA is present in the water. Just one more piece of half-truth that the pool industry feeds everyone.

 

[Zeerob]

This is also why blindly relying on an acid dosing system is a bad idea. pH and TA go hand-in-hand, when you add acid to lower the pH or keep it at a specific setting, you consume carbonate alkalinity (the majority component of TA). Once the carbonate alkalinity bottoms out, your water is only left with cyanurate alkalinity (pKa ~ 6.5). Cyanurates do not have as much buffering capacity and can quickly be one exhausted with the further addition of acidic chemicals. Once the cyanurate shifts to cyanuric acid, the pH will crash to 4.5 or less.

As others have mentioned, you need to keep your pH much higher at 7.6-7.8. Terrestrial water sources, whether from streams or bore-holes, have dissolved carbonates in them. Their pH naturally settles to a higher value, about 8.0-8.2. When you try to hold your pH at 7.3, you are only forcing the water to be in an unbalanced chemical equilibrium. A human being can comfortably swim in water that has a pH anywhere from 7.0 to 8.2. So there is no good reason to artificially keep the water’s pH where it does not naturally want to be. Aiming for a pH setpoint of 7.7 would use far less acid.

The only reason why you are using less acid is because you have exhausted the alkalinity of the water. Without that buffer, your water could quickly become very acidic and corrosive to your pool surface and equipment. You absolutely need a measurably TA level above 50ppm to be safe.

Hi, I have now topped up with borehole water (instead of tap water). This in order to regain some TA. I have added 4 m3 in a pool of 89 m3.
The borehole water has a TA around 300. After that the TA in the pool measured 25, instead of 0 before.
So, it looks like the carbonate alkalinity was indeed exhausted, and I now have some back. By continuing to top up with borehole water, I should be back to a reasonable TA soon. I have not used any acid since my last post, and I have actually turned off the acid pump. The PH has been 7.2-7.3. I hope/expect it to crawl up slowly. When it gets to 7.7 I will start the acid pump again, with a setpoint of 7.7 as you suggested.