The effect of CYA on Free Chlorine

One piece of pool folklore that ‘floats’ around the industry and becomes a conversation point in the busy summer outdoor pool season relates to the degree to which CYA (stabilizer) reduces the effectiveness of free chlorine. When added at an appropriate dose, CYA prevents the rapid deterioration of free chlorine in the presence of UV light from the sun so it’s still available as an active sanitizer. There is a very sensitive correlation between the increasing level of CYA in water and longer time required for chlorine to kill pathogens in the water. The Centre for Disease Control (U.S. Department of Health and Human Services) recommendation is only use unstabilized chlorine for a fecal contamination response and reduce CYA to below 15 ppm to deal with diarrheal incidents. 50 ppm is the maximum limit set in the AB Pool Standards.

The term ‘free chlorine’ is a measured value that includes both the complete and ‘active’ hypochlorous acid compund (HOCl) and the lesser effective hypochorite (ClO). The pH has a significant role to play in how much ‘active’ HOCl is present vs the less effective ClO. In the world of pool water treatment, we need fast kill times so it’s critical to have most of our free chlorine as HOCl. It’s important to note the key concept here is both compounds register as ‘free chlorine’. If we take the analysis one step further, we can calculate active chlorine from free chlorine to take into account other chemistry variables. This is where we can elevate our understanding and make factual comparisons of active chlorine levels that factor in the effect of pH and CYA on the disinfection capacity of the water.

There’s some great data out there from as far back as 1965 showing the effect of different chlorine levels with variable pH & CYA values on kill times but if you want a tool to handle all the math for you, give the free LABCONNECT app or desktop assistant program a download. This app can connect to a PoolLab photometer via Bluetooth but on its own it has a couple useful stand alone features include the ‘Active Chlorine’ calculator.

See below for some sample calculations showing high CYA with a drop in pH (Figure 1 & 2) and the effect of moderate CYA to no CYA (Figure 3 & 4)

Stanley R. Pickens, “Relative Effects of pH and Cyanurate on Disinfection”, Journal of the Swimming Pool and Spa Industry, 6 (1), 6-19, 2019.

Danial L. Harp. Current Technology of Chlorine Analysis for Water and Wastewater. Hach Company, 2002.

Photo Credit: Centers for Disease Control and Prevention