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Carbon Dioxide in Pool Water #4 of 4

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CO2 in Pool Water #4
So how long does it take for CO2 to off-gas after adding acid? Or, why does the “rebound” happen faster in some pools than others, or faster one time than another in the same pool? CO2 off gassing, and its relative speed, are dependent on several factors, which include water temperature, circulation, total alkalinity, ratio of water volume to air-exposed surface, and atmospheric pressure. These factors affect both the rate and the percentage of either off-gassing or absorption.

Factors which accelerate off-gassing (and speed pH rebound) include higher water temperatures, increased circulation, and especially increased aeration. Factors which inhibit off-gassing (thus slowing down pH rebound) include decreased aeration, decreased circulation, and low temperature. Factors which promote absorption of CO2 (which drops pH) include high total alkalinity, a pH above 8.4, and low water temperature.

Service techs can easily observe for themselves the speed at which CO2 off-gasses by watching the speed at which the pH increases, or returns to normal in their pools. They will detect how some pools have a major change in the pH within a day or so, maybe even in just a few hours, while some pools take over a week to have a significant change in the pH. Obviously, however, adding any more chemicals affects the pH and the process starts all over.

Another practical application relating to CO2 in water is the process of lowering high alkalinity levels in a short amount of time without allowing the pH to drop below the ANSI/APSP-recommended minimum of 7.2. The traditional methods involve either adding enough acid at a single time to remove the right amount of alkalinity (in which case the pH likely goes lower than desired), or adding smaller amounts of acid at intervals, allowing the pH to slowly rebound, and then repeating the small acid dose, over and over until the desired alkalinity level is reached, which takes hours or days.

The “CO2-savvy” method is accomplished by adding acid to water while off-gassing (by increased aeration and/or increased surface exposure) as much as possible. Aeration can be accomplished by turning on venturi jets, spa air bars, or other features or devices that create air bubbles in the water. Aeration is quite effective at rapidly reducing the CO2 in the water by increasing the surface area of water to air, which is where the off gassing occurs. Increased surface exposure includes longer circulation times, and using water features such as fountains – which might not technically aerate, but still increase the total amount of water exposed to air.

The routine involves adding just enough acid to lower the pH to 7.2, and then circulating and aerating the water as much as possible. The acid lowers the alkalinity, and the aeration accelerates the process of CO2 off gassing, thereby increasing the pH. Then more acid may be added, to pH 7.2, and so on – until the alkalinity reaches the desired target. In a relatively short period of time (at least, compared to traditional methods) the target alkalinity is reached without a) taking a lot of time, or b) allowing the pH to drop below safe and desirable ranges.

This is the final segment on Carbon Dioxide.
Provided by onBalance - Kim Skinner, Que Hales, Doug Latta

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Comment by Richard A. Falk on July 14, 2009 at 7:32pm
I agree that some clarity is needed. The following are chemical facts that are very counterintuitive, but when understood they help one know what is going on and how to adjust for it:

Hypochlorite sources of chlorine have a high initial pH so they raise the pH when added, BUT the consumption/usage of chlorine is an acidic process that exactly compensates for the initial rise in pH. The net result is pH neutral (or nearly so, except for a small amount of "excess lye" in some chlorinating liquid or bleach). This same result is also mostly true for saltwater chlorine generators (SWG), but there are other factors there including increased aeration and some outgassing of undissolved chlorine gas itself that leads to a faster pH rise (the latter not being able to be compensated by a lower TA).

TA is not only a pH buffer, but a SOURCE of rising pH itself, due to carbon dioxide outgassing. So higher TA increases this effect.

Dichlor is actually not net pH neutral when accounting for chlorine consumption/usage. Trichlor is even more acidic than it's initial pH would indicate due to chlorine consumption/usage. The same is true for chlorine gas.

So the bottom line net result is that for hypochlorite sources of chlorine, a lower TA leads to greater pH stability (i.e. a slower rise in pH over time). For the other sources of chlorine, a higher TA level is needed for greater pH buffering and to at least partially compensate for the acidity of the chlorine sources (and their consumption/usage). This is fundamentally where the general guidelines of 80-100 ppm TA for hypochlorite sources of chlorine and 100-120 ppm for other sources of chlorine comes from, but such guidelines don't "solve" the rising pH problem for the former nor the tendency of the pH to drop for the latter. A more aggressive TA adjustment (in the appropriate direction) can help provide greater pH stability, but will require appropriate adjustments to Calcium Hardness (CH) for the saturation index in plaster pools.
Comment by Alan Lewis on July 14, 2009 at 3:07pm
A discussion about the use of CO2 for pH reduction implies that in the main, we are talking about public pools rather than domestic pools. This is because in public pools it is possible to use bulk type facilities which are almost infeasible (though not impossible) in private pools. Like wise the inference (in parenthesis) that Richard Falk's comment below makes - that dosing of Chlorine gas (also infeasible in domestic pools) does not equire pH reduction at all - but rather the raising of pH.

I recommend that discussions on this site clearly define the type of pool(s) under scrutiny, at the outset.
Comment by Kim Skinner on June 27, 2009 at 8:23am
You are correct that higher TA results in faster CO2 outgassing. This concept was mentioned in #3, but should have also been mentioned in this blog #4.
Comment by Richard A. Falk on June 25, 2009 at 1:07pm
It should also be noted that a higher TA level results in faster carbon dioxide outgassing and therefore faster pH rise in pools that don't otherwise have acid added to them (including acidic sources of chlorine).

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