Spas, Splash pads, and Waterpark rides…
Why Aeration Causes pH Rise in Swimming Pools
Ever wonder why the pH in certain pools always seems to run high, no matter how much acid you add?
Spas.
Splash pads.
Swimming pools with fountains or water features.
Pretty much everything at a waterpark.
They all share one common characteristic:
Aeration.
When water is aerated, turbulence is created at the surface. That turbulence accelerates the release of dissolved carbon dioxide (CO₂) from the water into the atmosphere. As carbon dioxide escapes, the chemical equilibrium of the water shifts and the pH rises.
This process is well documented in aquatic chemistry and water treatment literature and is governed by what scientists refer to as the carbonate equilibrium system. In water, carbon dioxide exists in balance with carbonic acid (H₂CO₃), bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻). When aeration causes CO₂ to leave the water, the equilibrium shifts and hydrogen ion activity decreases, which results in a measurable increase in pH.
In practical pool operation, aeration is the only common operational method of raising pH that does not directly increase Total Alkalinity. That makes aeration both a useful tool and, in some situations, a persistent operational challenge.
At Aquatic Facility Training & Consultants, we believe it’s important to understand the “why” behind pool chemistry. When you understand the science driving water balance, swimming pools become significantly easier to maintain.
Understanding why pH rises in swimming pools, particularly in highly aerated environments, is a core concept taught in professional Certified Pool Operator (CPO) training. Pool professionals who understand the chemistry behind pH drift can diagnose problems faster and avoid unnecessary chemical adjustments.
A Simple Example of Aeration Changing Pool pH
At my own home pool, the opposite problem exists.
My pH naturally tends to run on the low side.
The reason is environmental. My yard has long-leaf pine trees. Pine needles are naturally acidic. Even when they do not fall directly into the pool, rainwater dripping from the needles carries organic acids into the water. Left alone, my pool’s pH would settle right around 7.0.
Several years ago, I decided to run a simple experiment on aeration in swimming pools.
I loosened the collar on one of my return jets and angled the return upward so the water disturbed the surface and created a ripple across the pool.
Within three days, the pH in my 15′ × 30′ pool rose to 7.6.
Between the aeration from the return jet and the mild acidity from the surrounding pine needles, my pool has remained remarkably stable at 7.5-7.6 for years without the need for chemical adjustment.
This small example illustrates how powerful aeration as a pH-adjustment tool can be when you understand the underlying chemistry.
When Aeration Causes Persistent High pH
Now think about the opposite scenario.
That pool on your service route where the pH is always high, and you’re constantly adding acid.
The pool might not even have a water feature.
Next time you’re there, take a look at the return jets.
Many homeowners point their return jets upward because they like to see the water move across the surface.
Unfortunately, what they have unintentionally done is create a continuous aeration system.
What appears to be a mysterious pH problem is often simply the technician fighting the chemistry of CO₂ outgassing.
Aim the return jets downward so they circulate water through the pool rather than disturbing the surface; often, the pH problem disappears almost immediately.
The pool wasn’t the problem.
You were simply fighting basic water chemistry.
Aeration and Competition Pools
During the summer months, competition pools sometimes utilize water cannons to cool the water.
USA Swimming guidelines recommend competition pool temperatures between 25–28°C (77–82.4°F). Water cannons help keep temperatures within that range by throwing water through the air and increasing evaporative cooling.
These systems work extremely well for temperature control.
However, they also generate substantial aeration and turbulence.
As we discussed earlier, turbulence accelerates the release of dissolved CO₂ from the water, raising pH.
Remember, though, that aeration increases pH without increasing Total Alkalinity.
To correct the rising pH, operators add muriatic acid.
That part makes sense.
The complication is that acid lowers both pH and Total Alkalinity.
Since the aeration raised the pH without raising the alkalinity, repeated acid additions can eventually drive Total Alkalinity too low.
It is not uncommon for large aquatic facilities to install sodium bicarbonate feed systems to restore alkalinity lost from continuous acid dosing.
CO₂ Injection as an Alternative
Another approach used at some large aquatic facilities is the injection of carbon dioxide (CO₂).
When CO₂ dissolves in water it forms carbonic acid (H₂CO₃).
Carbonic acid lowers pH effectively and is sometimes used as an alternative to muriatic acid.
However, injecting CO₂ has a different side effect.
Unlike muriatic acid, carbon dioxide increases Total Alkalinity while lowering pH.
Over time, this can cause alkalinity to rise too high, often leading operators to add muriatic acid to restore balance.
As you can see, both approaches ultimately interact with the same carbonate buffering system.
The Myth of Lowering Alkalinity Without Affecting pH
There is a persistent myth in the pool industry that adding acid can lower Total Alkalinity without affecting pH.
This is simply not supported by chemistry.
The term pH comes from the German phrase potenz hydrogen, meaning “power of hydrogen.”
pH is a logarithmic measurement of hydrogen ion activity in water.
Whenever acid is added to water, the concentration of hydrogen ions increases.
Increasing the hydrogen ion concentration must lower the pH.
Likewise, when alkalinity is reduced by adding acid, pH is affected because both values are linked through the carbonate buffering system.
The only operational method available to pool operators that raises pH without directly increasing Total Alkalinity is aeration.
Even then, aeration can have secondary impacts such as temperature change and evaporation, which can influence other aspects of pool water balance.
Why Understanding This Matters
Understanding the relationship between aeration, carbon dioxide loss, and rising pH in swimming pools is fundamental for anyone responsible for maintaining safe water.
Rather than constantly reacting to pH readings with chemical additions, experienced pool professionals learn to identify the source of the problem.
Often that source is simply turbulence at the water surface.
This principle is one of the key concepts taught in Certified Pool Operator (CPO) certification courses, where technicians learn the science behind water balance, aeration effects, and carbonate equilibrium.
When pool operators understand the chemistry rather than fight it, they can stabilize pH, reduce chemical use, and maintain safer swimming environments.
And that is ultimately the goal of professional pool water chemistry training.
References
Stumm, W., & Morgan, J. J. (1996). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters (3rd ed.). New York: Wiley-Interscience.
This foundational text in aquatic chemistry explains the carbonate equilibrium system and how degassing of dissolved carbon dioxide shifts equilibrium, raising pH in aerated waters.
Emerson, S., Russo, R. C., Lund, R. E., & Thurston, R. V. (1975). Aqueous carbonate equilibrium calculations: Effect of pH and temperature. Journal of the Fisheries Research Board of Canada, 32(12), 2379–2383.
This paper examines carbonate equilibrium relationships and the influence of dissolved CO₂ on pH in aquatic systems.
APHA, AWWA, & WEF. (2017). Standard Methods for the Examination of Water and Wastewater (23rd ed.). American Public Health Association.
The standard reference for water analysis, describing carbonate buffering systems, alkalinity, and the chemical relationships between CO₂, carbonic acid, bicarbonate, and pH.
Snoeyink, V. L., & Jenkins, D. (1980). Water Chemistry. New York: Wiley.
A widely cited water chemistry text that explains the carbonate buffering system and the role of carbon dioxide stripping in raising pH during aeration.
Hem, J. D. (1985). Study and Interpretation of the Chemical Characteristics of Natural Water (3rd ed.). U.S. Geological Survey Water-Supply Paper 2254.
This USGS publication discusses how aeration and degassing of CO₂ influence pH changes in natural and treated waters.
White, G. C. (2010). Handbook of Chlorination and Alternative Disinfectants (5th ed.). Wiley.
Discusses water chemistry principles relevant to aquatic facilities, including pH control and carbonate buffering in treatment systems.
World Health Organization. (2006). Guidelines for Safe Recreational Water Environments: Swimming Pools and Similar Environments. Geneva: WHO.
Provides technical guidance on water chemistry, pH control, and operational considerations for swimming pools and aquatic facilities.
Well if i had to take a guess I would say it would make the water colder. If the CO2 is leaving the water because the jets and I know more Co2 makes the fire hotter, then I would think the heat is leaving with it when the CO2 leaves the water.
Great line of thought! Thank you for commenting and thank you for reading!
My alkalinity is too high. if I add acid to reduce it, and end up with a ph that is too low I can use aeration to increase the ph without raising ta?
Correct!
will turning on the air bubbles in my spa pirate the water and raise the PH
Yes, it absolutely will.
I have a 20-25,000 gal pool in Phoenix, AZ. My ph is always too high, so I am constantly adding muriatic acid. This in turn lowers my alkalinity, so then I add sodium bicarbonate….which raises my ph. It seems like a never-ending and fruitless yo-yo! I use an aerator almost continuously in the summer, otherwise the water would be warmer than bathwater.
I thought getting the alkalinity up in the right range would keep the ph from fluctuating, but the ph just keeps on wandering higher and higher unless I constantly add muriatic acid.
Is this really how it’s supposed to go? Any insight or suggestions would be appreciated!
Thanks!
Hey Missy! The short answer is NO – it isn’t something that’s supposed to go that way, but it is a common chemistry conundrum. The bicarb you are adding probably isn’t as much to blame for a higher pH as you give it credit for (the chem doesn’t really raise pH by much). Realistically the sole culprit is the turbulence caused by the aeration.
A few things you can try:
Maintain a lower target Total Alkalinity (60 to 80 ppm)
Ask your local pool professional about establishing a Borate buffer Pool Acid Scarcity? Try a Borate-Bicarbonate Buffer
If you have a solar heater for your pool – try running it at night to cool the water temp
Ask your local pool professional about the use of CO₂ for pH control
I’m afraid there is not much we can do to STOP the pH from climbing due to aeration, however, any of the steps listed above could help to slow it down.
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This is regarding water chemistry, but not for pool water. My home is on a well that has a fair amount of iron in both ferric and ferrous iron form. After wasting money on a water softener, I invested in an iron backwash filtration system, as iron was staining my appliances and orange baths were not very appealing. The system requires the incoming, iron laden, well water to be injected with a solution of soda ash and/or muriatic acid to increase the PH to 7.7 to 8.0 PH. The issue here is that my well water has a HIGH total alkalinity. So, to raise the PH I must also raise the TA even higher. ( dry skin, ouch!) I just installed an aerator into the 15 gallon solution tank that injects the (high PH) water into the system and removes iron. My concern now is the aerator will raise the PH and leave the TA stand. (Good on that) I am thinking this will require me to add acid as the aeration is outgassing the CO2 generated from the acid. (Hydrogen ions) Any suggestions? This is tricky stuff and a little touch of soda ash and acid goes a long way in a 15 gallon tank. Any suggestions to get the balance correct? Thanks.
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