Environmental Engineering Reference
In-Depth Information
Colony Counting
Upon completion of the incubation period, the surface of the filter will have
growths of both fecal coliform and non-fecal coliform bacteria colonies. The
fecal coliform will appear blue in color, while non-fecal coliform colonies
will appear gray or cream colored. To count the colonies, the entire surface
of the filter should be scanned using a 10× to 15× binocular, wide-field dis-
secting microscope. The desired range of colonies for the most valid fecal
coliform determination is 20 to 60 colonies per filter. If multiple sample dilu-
tions are used for the test, counts for each filter should be recorded on the
laboratory data sheet.
•
Too many colonies
—Filters that show a growth over the entire sur-
face of the filter with no individually identifiable colonies should be
recorded as “confluent growth.” Filters that show a very high num-
ber of colonies (greater than 200) should be recorded as “too numer-
ous to count” (TNTC).
•
Not enough colonies
—If no single filter meets the desired minimum
colony count (20 colonies), the sum of the individual filter counts
and the respective sample volumes can be used in the formula to
calculate the colonies per 100 mL.
Note:
In each of these cases, adjustments in sample dilution volumes
should be made to ensure that future tests meet the criteria for obtaining
a valid test result.
Calculation
The fecal coliform density can be calculated using the following formula:
Colonies counted
Sample volu
Colonies/100 mL
=
× 100
mL
(6.2)
me (mL)
■
Example 6.2
Problem:
Using the data shown below, calculate the colonies per 100 mL for
the influent and effluent samples noted.
Sample
Influent Sample Dilutions
Effluent Sample Dilutions
Sample (mL)
1.0
0.1
0.01
10
1
0.1
Colonies counted
97
48
16
10
5
3
