CYA, pH, and Pool Water Treatment Realities

For at least 40 years, many pool service companies have been successfully maintaining their residential pools on a once-a-week visitation basis. Empirical evidence has shown that pool water can be kept safe and properly sanitized without harmful bacteria and algae developing with weekly treatment programs. (An important and often unrecognized side benefit of this professional pool service is that it keeps hazardous chemicals out of the hands of pool owners and their children.)

One reason weekly pool service works is due to the use of various feed or auto-systems which usually use either bleach, Cal hypo, salt, Trichlor, or bromine for sanitizing, sometimes with supplemental Ozone and UV systems. And the use of cyanuric acid (CYA), which slows down chlorine loss, also helps to provide the ability for less frequent chemical treatments.

An important aspect of this weekly sanitizing program for residential pools is proper pH maintenance. One thing that worries some service techs is when pH drifts higher in the days between service visits. It is an unfortunate but common myth that a pH of 8.0 to 8.4 makes chlorine in pool water completely ineffective and unable to properly sanitize and kill bacteria and algae. That simply is not true.

It is well known that CYA reduces the amount of “active” hypochlorous acid (HOCl) in the water.  But what is not well-known is that when the pH rises slightly in pools containing CYA, the amount of HOCl remains almost the same, and therefore, has nearly the same sanitizing efficacy and bacteria killing power.

Robert Lowry, a well-known industry chemical expert and author of 14 books including 3 training manuals for IPSSA and a frequent contributor to AQUA Magazine has led the effort to build a better understanding of the chlorine-CYA relationship in the pool service industry. Mr. Lowry’s chemical charts confirm that when the pH rises from 7.5 to 7.8, HOCl drops by only 10%, and when the pH rises from 7.8 to 8.2, HOCl drops by another 10%.

Does that lower amount of active chlorine make the water unsafe? Not really, and any difference in HOCl can be easily compensated for. While a 10% lower amount of HOCl wouldn’t make a lot of difference in sanitizing, the difference can be overcome by simply maintaining the chlorine 10% higher.

For example, pool water with 2 ppm of chlorine, with 50 ppm of CYA, and a pH of 7.8 is considered acceptable by industry standards. All one needs to do to obtain the same killing power and chlorine efficacy when the pH is maintained at 8.2, is to raise the chlorine level by 10% or to 2.2 ppm.  Obviously, that isn’t very much additional chlorine in a 15,000-gallon residential pool.  Only about 3.2 ounces of pool bleach.

Let’s also understand that if the chlorine is at 3 ppm in the above example with a pH of 8.2, it means that there is more sanitizing (killing) efficacy than having the chlorine at 2 ppm with a pH of 7.5, or even at a pH of 7.2.  This example shows that pool water can be well-sanitized at a pH of 8.2 (due to higher chlorine levels), and this higher pH does not automatically render chlorine incapable of killing bacteria and algae.

Plus maintaining the chlorine a little higher results in a higher reservoir of OCl- ready to convert into the more active HOCl.  So service techs can stop fretting about a pH rising above 7.8 between visits. In fact, pool service companies for many years have been keeping pool water clean and clear while maintaining the pH from 7.8 to 8.2.

A 40-year veteran of the pool industry, Kim Skinner is president of Pool Chlor, a chemical service firm with offices throughout the Southwest, and is also part of the consulting group onBalance, which researches swimming pool water chemistry and plaster issues.

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