It appears that changes to the pool industry’s recommended parameters for chlorine and pH levels is forthcoming.
The current range of 1 to 4 ppm for chlorine will likely be raised when pool water contains cyanuric acid (CYA), and the pH range will be broadened beyond the current limited range of 7.2 to 7.8. The reason for this change is due to a new understanding of the science of stabilized pool water.
Some years ago, Richard Falk (known as Chemgeek on pool forums) wrote about the relationship between CYA, Chlorine, and pH, and communicated that information to the Council for the Model Aquatic Health Code (CMAHC) that adopts standards for public and commercial pools. As a result, two years ago the CMAHC formed an Ad-Hoc committee with Ph.D. chemists, micro biologists, and other knowledgeable industry members to study that topic.
At the recent World Aquatic Health Conference in October, Richard Falk, representing the CMAHC Ad-Hoc committee, presented scientific data regarding the benefits of CYA and on the chlorine/CYA/pH equilibrium relationships, including bacterial kill rates, and other factors affecting pool sanitizing by chlorine when CYA is present.
To summarize, the CMAHC is tentatively suggesting higher minimum amounts of Free Chlorine should be maintain and proportionately based on the amount of CYA present in pool water. For example, at 40 ppm of CYA, the minimum Free Chlorine level should be at least 2 ppm, and at a CYA level of 90 ppm, the minimum chlorine level should be 4.5 ppm.
It should be noted that the CMAHC is aware of the EPA’s chlorine regulation minimum of 1 ppm and maximum of 4 ppm. However, the CMAHC believes that higher Free Chlorine levels are necessary to achieve safer and healthier pool water. Also mentioned was that a higher pH (above 7.8) does not significantly affect chlorine efficacy (killing power) when CYA is present.
It was clear from statements made by various CMAHC and CDC representatives that standards should be adjusted or changed as science becomes better understood. They also emphasized that flexibility in standards are needed (when scientifically supported) to allow more effective water maintenance.
At some point, the CMAHC and CDC will probably present this science to the EPA in hopes that higher chlorine levels will be adopted when CYA is present. And a broader pH range (due to applicable science) may be forthcoming also.
A simplified explanation of the science regarding Chlorine/pH Efficacy when CYA is present in pool water.
Dr. Stan Pickens (Ph.D. and water chemistry consultant) has also written a very detailed and scientific paper on “Relative Effects of Chlorine and pH on Disinfection.”