Surface Water Treatment Rule
How much CT you need to inactivate giardia and viruses — read from the EPA's published tables, interpolated properly, and worked out on your device so it answers with no signal at all.
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Contact time needed to inactivate giardia and viruses, from the tables in EPA 815-R-99-013.
Disinfectant
Pathogen
Logs of inactivation
99.0 mg·min/L
Interpolated from the EPA tables. The manual's regression gives 107 mg·min/L for the same conditions.Every answer comes from EPA 815-R-99-013. Those tables are sampled on a coarse grid, so readings between grid points are interpolated — across temperature alone for most disinfectants, and across temperature, residual and pH together for free chlorine against giardia.
Conditions outside the published range are refused rather than extrapolated. Past the edge of the tables there is no measurement to extend, and a confident number there would be an invented one.
The tables are bundled into the app and the arithmetic runs on your device. There is no account, no server and nothing to wait for — which matters in a plant room, where the answer is needed and the bars are not.
Giardia from 0.5-log to 3-log inactivation in half-log steps, and viruses at 2-log, 3-log and 4-log — to 25 °C, for every disinfectant the manual tabulates. Cryptosporidium is not covered.
| Disinfectant | From | Varies over |
|---|---|---|
| Free chlorine | 0.5 °C | Temperature, residual and pH |
| Chloramine | 1 °C | Temperature |
| Chlorine dioxide | 1 °C | Temperature |
| Ozone | 1 °C | Temperature |
Know the number.
Not the page it is on.