My pool loves low TA

[BillyBumbler]

It's an 18000 gal. inground pebbletech finish pool. There is a rock waterfall that I have set to run for 5 minutes every couple days to keep the water in the recesses from getting algae. Yes it is a SWG system. I keep the CYA at between 40-50ppm and the salt level at 2600-3200ppm. Calcium is at around 250-300ppm. FC always around 3 and CC always at 0. Water looks beautiful. In the past I've used baking soda to get the TA back up which resulted in the need for acid to reduce PH, which in effect lowered the TA. Kind of a Catch-22 situation. It could be that I use to much acid? I like to use enough to get the PH between 7.0-7.2 so that I go go a few extra days without checking.

[chem geek]

The main problem is that you are trying to maintain a low pH of 7.0-7.2. Why is that your target your pH? Normally, you would target 7.4-7.6 and that will be much easier to maintain as there will be less carbon dioxide outgassing (and pH rise) at that pH.

[BillyBumbler]

Well not really trying to maintain it that low. Since the PH is always on a steady incline I set it there to give me more time before another acid dose. I add acid at 7.8-8.0. Is this all wrong?

[chem geek]

Well, it's not "all wrong", but adding acid to get down to 7.0-7.2 will cause the pH to strongly rise from that point. The rise in pH will slow down as the pH gets higher so setting a target of 7.4 or 7.5 would be better. At the lower TA, you won't see the rise in pH being as rapid and won't have to add as much acid. If you decide to stick with the low TA, you might want to bump up your calcium hardness (CH) a bit closer to 400 ppm though that is not essential (your water is a little more corrosive with the lower TA and having a higher CH compensates for that).

I suggest you take a look at this thread and consider adding 50 ppm Borates (Borax) to your pool. This should allow you to lower your SWG output (since the Borates will kill algae so less chlorine gets consumed) which will lower the tendency of the pH to rise. The SWG generates not only chlorine, but also hydrogen gas and these bubbles pull out the carbon dioxide from your water and that causes the pH to rise. This is why you see less of a problem at lower TA, because there is less carbon dioxide in the water at lower TA (think of the pool as a carbonated beverage!).

What is the recommended level of CYA that your SWG manufacturer says you should have? Is it 70-80 ppm? We have a current controversy as to whether a high CYA of 80 ppm makes the salt cell more efficient. The problem is that it is easy to add CYA and not easy to get rid of it. So using the Borates is probably a better bet.

Richard

[CarlD]

Chem_Geek is of course correct about what's going on, but here's a simplification and SLIGHT correction about what to do (sorry Richard ):

By keeping your pH at 7.2 or lower, the waterfall pushes the pH up. You add acid, pH and T/A come down, you run the waterfall, it pushes the pH up without pushing up the T/A as well.

So. I would suggest (and I RARELY do this) that rather than use Borax to raise your pH, you use Washing Soda (Sodium Carbonate). It's the same as "pH UP!" and other pool products that use soda ash to raise pH, but it costs FAR less. Arm&Hammer Washing Soda is usually next to the Borax in the detergent department. We don't usually recommend Soda Ash to raise pH PRECISELY because it raises T/A at the same time as it raises pH.

Alternatively, if you DO use Borax (and there's nothing wrong with that--I use it all the time) then you can equally boost your Total Alkalinity by adding ordinary baking soda (Sodium BIcarbonate).


It's cheaper to use washing soda because, pound for pound it's even cheaper than 20 Mule Team Borax--and you don't have to buy baking soda.

But either way is fine. The Borates, may, as Richard says, help your SWG run effectively at lower chlorine levels, so that may be a good way to go. After all, even if you use Borax and Baking Soda from the supermarket, it's still pretty cheap.

So:
1) NEVER let your pH go below 7.3 unless you WANT to lower T/A
2) Raise pH by adding either Soda Ash (washing soda) or Borax.
3) Raise T/A to over 100 but not over 125 by adding Soda Ash (which you did in 2) ) or adding baking soda.

[chem geek]

I agree that if you want to raise pH while also raising TA, then using pH Up / Washing Soda / Sodium Carbonate will do that. It just seems that BillyBumbler's pool has a very strong tendency to rise in pH and that a large part of that is due to aeration from his SWG. I don't see a problem with his operating at a rather low TA level since that does help reduce the pH rise from carbon dioxide outgassing, but as you and I both said, he shouldn't lower the pH so much since that just exacerbates the pH rising problem.

I think you knew this, but just wanted to be clear, that the 50 ppm Borates option is not about using Borates to increase the pH, but rather to essentially add an algicide and another pH buffer to the pool. When initially adding Borates in such large quantities, a LOT of acid will be required to keep the pH constant until all of the Borates are added. I just wish I could be more confident in whether the Borates would help him. It helped waterbear and some others so I presume it could help in this situation.

Richard

[CarlD]

On the other hand, higher T/A levels DO tend to stabilize pH, so the out-gassing should be less effective at raising pH when pH is already in the optimal range.

Yes, I did understand about the borates as an algaecide. While it couldn't hoit, I'm not sure it's necessary. Running a T/A of 80 I believe WILL cause bouncing pH--It's just not high enough to stabilize pH adequately.