Advanced Waterworks Mathematics, 2019a

The information provided in the question statement will help determine which
CT table to use. In this case, it is Table C-3 which is specifically for inactivation of
Giardia by Free Chlorine at 10°C
The problem states that the water has a pH of 7.5 so you look in the column that
says pH = 7.5.
The supporting information (outlined in the boxes above) will help determine
which exact column and row you need to use to find the answer. For this
example, the chlorine concentration is 1 mg/L and the Log Inactivation is 1.0.
Therefore, the answer is 45 mg/L · min which is circled in red. It is the
intersection of the 1 mg/L row and the 1.0 Log inactivation column.
The CT Tables provide the required CT needed to inactivate either Giardia or viruses . The ratio
of the actual CT (calculated portion of the problem) and the required CT (found in the CT tables)
is then calculated to determine if the water has been properly disinfected. If the actual CT is
equal to or greater than the required CT then the ratio is equal to or greater than 1.0 and CT is
met. If the actual CT is less than the required CT then the ratio would be less than 1.0 and CT
would not be met.
Example:
If the Actual CT is 28 mg/L · min and the Required CT is 22 mg/L · min, has the
water been properly disinfected?
Actual CT 28 mg/L min
= 1.27
Required CT 22 mg/L min
Since the ratio is greater than 1.0, the water has been properly treated and the
CT is met.
Example:
If the Actual CT is 16 mg/L · min and the Required CT is 32 mg/L · min, has the
water been properly disinfected?
Actual CT 16 mg/L min
= 0.5
Required CT 32 mg/L min
Since the ratio is less than 1.0, the water has NOT been properly treated and the
CT is NOT met.
156 | A dvanced Waterworks Mathematics