A Theory About Rising pH in SWG Pools

[chem geek]

Kurt makes a good point. We have not gotten to the bottom of this significant rise in pH some users experience, though we are making progress. As far as I can figure out so far, the following are possible sources of pH rise:

1) High TA and/or Low pH and/or high aeration. This refers to my CO2 outgassing chart though this chart is very qualitative due to variance in aeration. This process has been proven through several users successfully eliminating their pH rise by lowering their alkalinity to 80 or even lower (or their pH to 7.2 in one case). This process can be distinguished from the others by the continual drop in TA over time (after adding acid to restore pH). NOTE: It's techically carbonate alkalinity that drives the CO2 outgassing, not TA, so adjustments should be made for CYA levels or Borates (Boric Acid; sodium tetraborate) that are used. My chart assumes no borates and 30 ppm CYA.

2) Curing of plaster (technically, the concrete in plaster/gunite). When enough Calcium Hydroxide has been produced from the curing process to raise the Calcium Hardness level by 10 ppm (which is probably the minimum detectable amount in test kits), this causes a rise (with a TA of 100) of 0.91 in pH so that 43 ounces of Muriatic Acid are needed (for a 16,000 gallon pool) to restore back to a 7.5 pH. So, the rise in CH will be relatively slow, though if you see it rise about 10 ppm after cumulatively adding 5-6 cups of acid over time (per 16,000 gallons), then this may be the cause of the pH rise. I also would expect that this curing process wouldn't last (in significant amounts) much more than 1 year at the most and that most of the curing would occur in the first 3-6 months. Don't forget that backwashing DE and sand filters as well as splash-out will dilute the CH (and everything else) over time so may hide the rise in CH (at a CH of 300, a loss of 10 is a 3.3% dilution or 333 gallons per 10,000 gallons in a pool). On the other hand, evaporation and refill will tend to increase CH unless the fill water has no CH.

3) SWCG systems that do not fully dissolve their generated chlorine gas. For every 1 ppm of chlorine that is generated and does not dissolve, this causes a 0.15 rise in pH which requires about 8 ounces (1 cup) of acid to restore pH (in a 16,000 gallon pool). My hunch is that after #2 above for newer plaster/gunite pools, that this is the main reason for rising pH in SWCG pools, but I have no proof of this (yet). One experiment to try is to use a lower power setting and run for a longer period of time (to make up for the lower power setting in terms of total chlorine produced in a day). In theory, the lower power setting should produce smaller bubbles and amounts of chlorine per volume of water flow and should have more chlorine dissolve in the water vs. outgassing to cause a rise in pH.

4) Other factors. These include fill water that is high in pH. Bather sweat is probably not very alkaline and rain is normally acidic so these are not likely to increase the pH.

Kurt's data showed that at least part of the pH rise was due to the outgassing of CO2 since his TA kept dropping. Kurt, if you are willing, continue to let it drop to about 50 (see this post for an example of where that worked for someone else. That still provides some buffering, but would virtually slow to a crawl the CO2 outgassing (at a pH of 7.5 or above). Someone who has an SWCG system and experiences rising pH can also try the experiment I mention in #3 above to lower your power setting and increase your time on your SWCG. Let us know what happens if you do this! Oh yes, and Kurt, how old is your pool if it's plaster/gunite?

Kurt, if you do conclude that your specific source of rising pH is the outgassing of CO2, then you might consider adding 50 ppm Borates (sodium tetraborate) to act as a substitute buffer replacing part of your carbonate buffer system. That would let you run with much, much lower TA readings. Before we get to that point, however, we need to see if you can get to a low TA level where the pH rise slows down significantly.

Richard