Pool Disinfection Information

The length of time it takes to disinfect a pool depends, for example, on the type of fecal accident and the chlorine levels chosen to disinfect the pool. If a fecal accident is a formed stool, the following chlorine levels will determine the times needed to inactivate Giardia:

These times are based on a 99.9% inactivation of Giardia cysts by chlorine, pH 7.5, and 77°F (25ºC). The times were derived from the EPA Disinfection Profiling and Benchmarking Guidance Manual [2]. These times do not take into account “dead spots” and other areas of poor pool water mixing.

If the fecal accident is diarrhea, the following chlorine levels will determine the times needed to inactivate Cryptosporidia:

A CT value is the concentration (C) of free available chlorine in parts per million multiplied by the time (T) in minutes (CT value = C x T ). The CT value for Giardia is 45 and the value for Cryptosporidia is 9,600. If a different chlorine concentration or inactivation time is used, CT values must remain the same. For example, to determine the length of time needed to disinfect a pool at 15 ppm after a diarrheal accident, the following formula is used: C x T = 9,600. Solve for time: T= 9,600 ÷ 15 ppm = 10.7 hours. It would, thus, take 10.7 hours to inactivate Cryptosporidia at 15 ppm. You can do the same for Giardia by using the CT of 45.

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