Risks with high alkalinity?

[CarlD]

You may get some scaling.

Please look at Ben's method for lowering Alk. I think keeping pH that high is a bad idea--it drastically lowers chlorine's effectiveness.

Basically, you lower pH to 7.2 or less, which pulls down the alk, then you AERATE to raise the pH. Repeat the process until Alk is where you want it. Then use Borax to raise the pH to the desired level.

[Rangeball]

You may get some scaling.

I've not noticed any at all, but will keep an eye on it.

Please look at Ben's method for lowering Alk. I think keeping pH that high is a bad idea--it drastically lowers chlorine's effectiveness.

Basically, you lower pH to 7.2 or less, which pulls down the alk, then you AERATE to raise the pH. Repeat the process until Alk is where you want it. Then use Borax to raise the pH to the desired level.

I've looked at that so many times I think I can probably recite it I'm familiar with the process, have done it several times, and frankly am tired of doing it. With such a high starting Alk, by the time I get it down it's time to add more water and the whole process starts all over again.

The elevated PH and reduced chlorine effectiveness is my main concern, but in Ben's "run a high PH pool" page, I get the feeling I may actually be ok, provided I stay ahead of my pool. I'd just feel more comfortable doing so if I knew what the risks of high Alk were.

[chem geek]

There are two problems with high alkalinity and both can probably be addressed. The problem with scaling can be addressed by having a lower calcium hardness and by keeping the pH lower (but that will be hard due to the other problem with high alkalinity). The other problem is that you will be outgassing carbon dioxide until your pH is at a higher point which is pretty much what you've been seeing.

First of all, just out of curiosity, since you haven't seen any scaling (and don't have cloudy water), what is your Calcium Hardness (CH) level? One thing you can do to avoid even the potential of scaling is to lower CH significantly (unless it is already). If you are going to have a pH of 8.0 with the 320 ppm alkalinity, you could have your CH as low as 50 or even lower (staying in the range of 10-50 would be nice, but even 100 or more may not cause you problems).

As for the pH affecting chlorine effectiveness, that is true, but it doesn't have as much as an effect as you might think because the CYA acts somewhat like a chlorine buffer resisting changes to the chlorine level from pH. Specifically, the disinfecting chlorine (HOCl) level gets reduced by about 15% when you go from a pH of 7.5 to 8.0 (assuming a CYA level of 30 ppm) so that's not too bad and can be easily overcome by running slightly higher levels of chlorine. I suggest you just use Ben's Best Guess CYA chart and add about 20% to his numbers to adjust for your 8.0 pH (assuming your CYA is at least 30).

If you look at the following chart, CO2.png, you can see how higher alkalinity leads to more outgassing of carbon dioxide and that causes the pH to rise. Your levels are off the charts, but I calculate that at a pH of 8.0 and alkalinity of 320 the equivalent outgas rate (the left-hand side Y-axis) is about 10 which is usually pretty stable. You would probably have a hard time keeping your pH much below 7.8

Richard

[aquarium]

One thing you can do to avoid even the potential of scaling is to lower CH significantly (unless it is already).

How is that done?

[chem geek]

How is that done?

Unfortunately if the CH isn't already low then the only way to lower it would be a drain and refill. If the fill water is high in CH (we don't know that; we only know it's high in alkalinity), then there isn't a good solution. So you are right that I really need to know not only what the current CH is in the pool but also what the CH is in the fill water. My hope is that the CH in both are already low (which would explain why there's no scaling nor cloudiness).

If the fill water is high in CH, then it's an extra bad situation because any filling after evaporation will introduce even more calcium in the pool. Let's see what the numbers are for CH before panicking. I'm also very interested because there appears to be no evidence of scaling nor cloudiness in this situation so this could give an even higher "limit" to the saturation index for a real-world pool.

Richard

[aquarium]

Okay,

That's what I thought, but was hoping maybe you knew something I didn't about getting CH lower.

High alk and CH usually come together in the fill water, which is my situation. About 220ppm alk, 430ppm CH.

I lower the alk each time I top up, but I know the CH is just getting worse and worse due to evaporation. So far not seeing a problem.

BTW, it's easy to use one of these to aerate each time during a top-off. I just screw it in for a day or so, then remove it:
EDIT - And add acid too. END EDIT

[chem geek]

High alk and CH usually come together in the fill water, which is my situation. About 220ppm alk, 430ppm CH.

Tom,

I am interested in what pH range you are operating with. It will be interesting to see what the relative CO2 outgassing rate is and the saturation index. More real-world data -- I love it!

Richard