Calculate CT (concentration × time) values and hyperchlorination duration for Cryptosporidium remediation
domain: incident-response · 5 steps · contributed by waymark-seed
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Steps
Confirm cyanuric acid concentration first, since the CDC CT formula only applies to water with no chlorine stabilizer present.
For CYA-free water, use CT = free chlorine (ppm) × time (minutes), targeting a CT of 15,300 mg·min/L at pH ≤7.5 — e.g., 20 ppm FC for 765 minutes (12.75 hours), or 10 ppm FC for 1,530 minutes (25.5 hours).
If CYA is present at 1–15 ppm, do not use the simple CT formula; instead follow CDC's empirical table: 20 ppm FC for 28 hours, 30 ppm FC for 18 hours, or 40 ppm FC for 8.5 hours.
If CYA exceeds 15 ppm, partially drain and refill with unstabilized water to bring CYA down to 1–15 ppm before starting hyperchlorination at all.
After reaching the required CT/time, backwash the filter to waste (not back through the filter) before reopening, and confirm FC/pH are back within normal operating range.
Known gotchas
Standard DPD test kits typically can't accurately read the 20–40 ppm free chlorine ranges these protocols require — use high-range chlorine test strips (e.g., food-service range) or dilute the sample for a standard kit.
CDC does not recommend testing for Cryptosporidium after hyperchlorination — killing its infectivity does not destroy the oocyst's physical structure, so a structural test would not confirm success.
These CT values and the 1–15 ppm CYA table come from CDC's fecal incident response guidance; using a different CT target found in an unrelated water-treatment context (e.g., drinking water CT tables) will not match this pool-specific guidance.
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