The effect of cyanuric acid on the disinfection rate of Cryptosporidium parvum in 20-ppm free chlorine

Cyanuric acid is used to stabilize free chlorine to reduce photodegradation in outdoor swimming pools. While there have been numerous studies examining its effect on the disinfection rates of bacteria and viruses, it is not known whether cyanuric acid can significantly impact the effectiveness of hyperchlorination for inactivating Cryptosporidium oocysts present in fecally-contaminated swimming pools. This study examined the effect of cyanuric acid on the disinfection rate of Cryptosporidium parvum under swimming pool hyperchlorination conditions (20 mg/ml free chlorine). When 50 mg/L cyanuric acid was present there was a 0.70-log₁₀ reduction in oocyst viability after 10 hours as compared to a 3.7-log₁₀ reduction without cyanuric acid. Aids to remediation, such as decreasing the pH to enhance the germicidal efficiency of the free chlorine and doubling the amount of free chlorine residual, were still unable to achieve a 3-log₁₀ reduction. Current public health recommendations for hyperchlorination and pool remediation are insufficient for pools using cyanurate-stabilized chlorine to achieve a three log inactivation of the parasite.