In order to determine if water is being treated properly, the number of coliform

Question

In order to determine if water is being treated properly, the number of coliform indicator organisms are measured. Dilutions of the untreated and treated water are plated in replicate using the membrane filter technique to measure coliforms. These were on the small plates so use dilutions with 5-50 colonies. A) (02 pts) What are coliforms and why do we measure them). B) (0.3 pt each) Using the following results determine the numbers of coliforms in the 1) untreated and 2) treated water your answer should be expressed as cfu/mL water

Explanation / Answer

a.They are bacteria. The concentration of coliform bacteria in water is measured to determine the contamination by microbiological organisms. The coliform are not pathogenic (they don’t cause disease), but they are commonly found alongside pathogenic organisms. The measurement is used as an indicator because it easier to test for coliform than for pathogenic organisms.

B.

In untreated water:

Use the values for the dilution 1:1000. The average value is

1000 x (152+189+165)/3= 170 000 cfu/mL   (use 2 significant figures)

In treated water:

Use the values for 100 mL. The average value is

(1/100) x (98+75+89)/3 = 0.87 cfu/mL

C.

Removal efficiency is

100 x (170000-1)/170 000 = 99.9994 %

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