Risks with high alkalinity?

[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