If your system is making a significant change to its disinfection practice, it must calculate a disinfection benchmark using the procedure specified in the following table.
| To calculate a disinfection benchmark your system must perform the following steps |
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| Step 1: Using the data your system collected to develop the Disinfection Profile, determine the average Giardia lamblia inactivation for each calendar month by dividing the sum of all Giardia lamblia inactivations for that month by the number of values calculated for that month. |
| Step 2: Determine the lowest monthly average value out of the twelve values. This value becomes the disinfection benchmark. |