Question

ENVIORMENTAL ENGINEERING

Using the attached table, determine the CT value to decrease the concentration of Giardia from approximetly 3,160 to 10 cysts/L, using a chlorine concentration =1.2mg/L at pH=7.5 , 15 degC

Answer 1

Log reduction is the same as counting the zeroes in a division of 10, 100, 1000, 10000 times:

If we can reduce to the desired value dividing by 10, then we're talking about a 1-log reduction.

If we can reduce to the desired value dividing by 100, then we're talking about a 2-log reduction.

In this case, dividing by 1000 gives us 3.16 which is still bigger than 1. We would need to divide by 10000 to get a value smaller than required (0.316 is smaller than 1). So the required log reduction is between 3 and 4. The exact value can be found simply using a calculator:

So, as I notice your chart is in 0.5 increments, the required log reduction value would be 3.5