== == * You have two things that contribute to your high use of acid. The walls of your plaster pool actually absorb some of the acid over time as the walls continue to cure. This is very common in new pools. You may see a reduction in the amount of acid after a few years. Also, the salt system you use has a high pH level which contributes to your heavy use of acid. == == * Though the previous post about new plaster pools absorbing acid for curing may be correct (I don't know about that), I do know that the pH also increases due to other factors as follows:
1) Since all pools use a carbonate buffering system that is out of equilibrium with CO2(g) in the atmosphere, they will all tend to increase in pH (if nothing else was done) as H2CO3 goes to CO2(g) and H2O (water). Total alkalinity does not change (decrease in CO3 equals decrease in 2H's back to water), though adding acid to restore pH will result in lower alkalinity so you'll also add sodium bicarbonate. If you do not have a pool cover and/or you have a waterfall that aerates the water, then you will increase the pH more quickly.
2) Use of a chlorine generator makes the water more basic (alkaline) as the formula for generation is Cl- + 2H20 --> H2(g) + HOCl + OH-. Every 1 ppm of chlorine added to your system results in an increase in pH of around 0.024 (assuming your pH is around 7.5, total alkalinity of 100, etc.) which isn't much, but if your chlorine demand (and production) is 1 ppm per day, then you'll have an increase in pH of 0.1 in 4 days. This effect was stated in the previous post: "the salt system you use has a high pH level."
3) Some (though not much) of the chlorine in your pool will get out-gassed via the following formula HOCl + Cl- --> Cl2(g) + OH- which has a similar effect as item #2 above in that a loss of 1 ppm results in a 0.024 pH increase.
The only effects that counteract the above to make the water more acid are the oxidation of ammonia by chlorine (which returns HOCl to CL-, by the way) and the introduction of acids to the water from rain (remember "acid rain") or possibly refilling "tap" water (though usually such water is at least as alkaline as pool water) and of course other pool chemicals (I don't know what BioGuard Optimizer Plus is).
By the way, your 1 gallon of muriatic acid every 3-4 days in 60,000 gallons of pool water decreases the pH by about 0.28 which still seems excessively high even accounting for the above. Perhaps your Total Alkalinity is too high since that would account for a faster pH increase (due to more carbonate in the water so faster out-gassing to CO2) and a greater resistance to changes in pH from adding acid (or base). Are you anywhere near the recommended 100 ppm Total Alkalinity?
If you were at a Total Alkalinity of 100 ppm, then you would need to add about 5 cups of bicarbonate of soda after your gallon of acid, assuming you were going from a pH of 7.6 to 7.3. Does this sound like what you are doing? If so, then perhaps your chlorine demand is high (3 ppm per day).
I'd like to add the following update:
1) The 1 ppm loss of chlorine to the air (HOCl + Cl- --> Cl2(g) + OH-) actually increases pH in a 16,000 gallon pool by 0.1, not 0.024 as I said earlier. So, if you are generating too much chlorine and have an uncovered pool, you might be losing lots of it to the air.
2) A friend of mine had a very high acid demand, just like your situation, but his ended up being a combination of excessive chlorine generation (see above) from running his pump 24 hours/day and from way too low alkalinity (65 ppm). The alkalinity will make the pH swing more quickly, but won't change overall acid demand (higher alkalinity slows pH swing, but you need to add more acid to lower pH the same amount). Once he increased total alkalinity (to around 140) and cut down his pump hours (to around 8), his pH stabilized.
3) If your water is out of balance with too low total alkalinity or calcium hardness, you could be corroding (etching) your pool plaster which will also tend to increase pH (and alkalinity) as Calcium Carbonate from your plaster dissolves into the pool water. My friend also had that situation as noted above. For a salt pool, you need to have more total alkalinity and/or calcium hardness to maintain water balance due to the higher Total Dissolved Solids (from the salt).
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