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