Environmental Engineering Reference
In-Depth Information
Solution:
1.
Influent sample
—Select the influent sample filter that has a colony
count in the desired range (20 to 60). Because one filter meets this cri-
terion, the remaining influent filters that did not meet the criterion
are discarded.
48 colonies
0.1 mL
Colonies/100 mL
=
×
100
mL
=
48,, 000 colonies/100 mL
2.
Effluent sample
—Because none of the filters for the effluent sample
meets the minimum test requirement, the colonies per 100 mL must
be determined by totaling the colonies on each filter and the sample
volumes used for each filter.
Totalcolonies
=++=
1053 18
colonies
Totalsamp
le
=
10 0
.
mL
+
1 0
.
mL
+
0 1
.
mL
=
111
.
mL
18
colonies
11.1 mL
Colonies/1
00 mL
=
×
100
=
162
colonies/
100 mL
Note:
The USEPA criterion for fecal coliform bacteria in bathing waters is
a logarithmic mean of 200 per 100 mL, based on a minimum of 5 samples
taken over a 30-day period, with not more than 10% of the total samples
exceeding 400 per 100 mL. Because shellfish may be eaten without being
cooked, the strictest coliform criterion applies to shellfish cultivation and
harvesting. The USEPA criterion states that the mean fecal coliform con-
centration should not exceed 14 per 100 mL, with not more than 10% of
the samples exceeding 43 per 100 mL.
Interferences
Large amounts of t u r b i d it y, algae, or suspended solids may interfere with
this technique by blocking the filtration of the sample through the membrane
filter. Dilution of these samples to prevent this problem may make the test
inappropriate for samples with low fecal coliform densities, as the sample
volumes after dilution may be too small to give representative results. The
presence of large amounts of non-coliform-group bacteria in the samples
may also prohibit the use of this method.
Geometric Mean Calculation
Many National Pollutant Discharge Elimination System (NPDES) discharge
permits require fecal coliform testing. Results for fecal coliform testing must
be reported as a
geometric mean (average)
of all the test results obtained during
