Effect of cyanuric acid on the inactivation of Cryptosporidium parvum under hyperchlorination conditions

Abstract

Cyanuric acid (CYA) is a chlorine stabilizer used in swimming pools to limit UV degradation of chlorine, thus reducing chlorine use and cost. However, CYA has been shown to decrease the efficacy of chlorine disinfection. In the event of a diarrheal incident, CDC recommends implementing 3-log10 inactivation conditions for Cryptosporidium (CT value = 15 300 mg·min/L) to remediate pools. Currently, CYA's impact on Cryptosporidium inactivation is not fully determined. We investigated the impact of multiple concentrations of CYA on C. parvum inactivation (at 20 and 40 mg/L free chlorine; average pH 7.6; 25 °C). At 20 mg/L free chlorine, average estimated 3-log10 CT values were 17 800 and 31 500 mg·min/L with 8 and 16 mg/L CYA, respectively, and the average estimated 1-log10 CT value was 76 500 mg·min/L with 48 mg/L CYA. At 40 mg/L free chlorine, 3-log10 CT values were lower than those at 20 mg/L, but still higher than those of free chlorine-only controls. In the presence of ∼100 mg/L CYA, average 0.8- and 1.4-log10 reductions were achieved by 72 h at 20 and 40 mg/L free chlorine, respectively. This study demonstrates CYA significantly delays chlorine inactivation of Cryptosporidium oocysts, emphasizing the need for additional pool remediation options following fecal incidents.

Figure 1

(a) Microscopical field with C. parvum developing stages labeled with a parasite-specific fluorescent monoclonal antibody; (b) counterpart image with C. parvum life stages enumerated by ImageJ Software.

Figure 2

(a) Time (hr) versus log₁₀ inactivation of C. parvum (95% confidence interval) at 20 mg/L free chlorine (FC) for cyanuric acid (CYA) concentrations of 0–48 mg/L. Note: solid lines represent the linear trend estimated by a linear mixed model; graphical representations of log₁₀ inactivation utilize linear interpolation to communicate experimental results and should not be viewed as suggesting a kinetic model for the actual oocyst disinfection process. (b) Time (hr) versus log₁₀ inactivation of C. parvum (95% confidence intervals) at 40 mg/L FC for CYA concentrations 0–46 mg/L. Note: solid lines represent the linear trend estimated by a linear mixed model; graphical representations of log₁₀ inactivation utilize linear interpolation to communicate experimental results and should not be viewed as suggesting a kinetic model for the actual oocyst disinfection process. (c) Time (hr) versus log₁₀ inactivation of C. parvum (95% confidence intervals) at 20 and 40 mg/L FC for 100 mg/L CYA and oxidant demand-free (ODF) water control. Note: points represent the observed raw data and solid lines represent the approximated smooth curved based on raw data.