Is it possible to have too much of a good thing?
By Greg Schmidt
Photo Credit: © Can Stock Photo / Gudella
Cyanuric acid (CYA) is both a good and bad element in an outdoor pool. As an Aquatic Facility Operator course instructor and a pool operator for many years, I write this article to dispel some of the mystery around CYA and its reputation. Having just read a document by a respected source giving advice about CYA that is not consistent with the science presented here, I want to clear up any confusion about CYA that is still prevalent.
First, letâs look at why itâs a good thing. Hypochlorous acid (HOCl), or free chlorine, is subject to dissipation from UV rays in sunlight, which reduces residual potentially to levels below oxidation rates and even health-code minimums. HOCl is the form of chlorine that operators want in their pools. It sanitizes and oxidizes unwanted combined chlorine. Established standards for Oxidation Reduction Potential (ORP) in pools are 650 mV to kill bacteria and viruses, and 830 to 850 mV to provide excellent oxidation of combined chlorine. The goal of CYA use is to retain HOCl in adequate concentration to accomplish these two functions.
How does it work? CYA makes a loose chemical bond with HOCl to retain it and prevent sunlight dissipation. Itâs called a âstabilizerâ because this bond makes HOCl more stable, thus less likely to be lost to sunlight. Itâs the instability of the HOCl molecule that makes it such a terrific sanitizer and oxidizer. As such, I strongly encourage operators of outdoor pools to use CYA at the level of 10 to 15 PPM to retain 87 to 90 percent HOCl. The absolute maximum level of CYA in a pool should be 20 PPM. For best results, start diluting water at 15 PPM to avoid ever getting to 20 PPM. Do not pay attention to the higher retention rates at higher CYA concentrations. At 20 PPM, there will be about 95-percent retention, but thatâs where it has already become a bad thing.