Question

The membrane filter technique was used to test a polluted water for the coliform group. Three different water sample volumes (5 ml, 50 ml, and 500 ml) were filtered through five filter membranes. The colony counts were as follows:

        

                  5 ml membranes:   4, 8, 5, 2, 3

                  50 ml membranes: 24, 38, 25, 32, 30

                  500 ml membranes: 250, 320, 360, 210, 280

         a. What are the most valid data obtained?

         b. Calculate the coliform count per 100 ml using the most valid data.

Answer 1

For 5 ml colonies count (mean data) is 4.4

For 50 ml colonies count is 29.8

For 500 ml colonies count is 284.

These data is used to extrapolate a graph (see below)

from graph it is identified, for 100 ml. Sample the colonies would be between 50 to 55 or ~50.