Environmental Engineering Reference
In-Depth Information
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Example 6.1
Problem
: Using the results recorded below, calculate the MPN/100 mL of the
example.
Sample in Each
Positive Tubes/
Serial Dilution (mL)
Tubes Inoculated
10. 0
5/5
1. 0
5/5
0.1
3/5
0.0
1/5
0.0 01
1/5
Solution:
1. Select the highest dilution (tube with the lowest amount of sample)
with all positive tubes (1.0-mL dilution). Select the next two higher
dilutions (0.1 mL and 0.01 mL). In this case, the selected series will be
5-3-1.
2. Include any positive results in dilutions higher than the selected
series (0.001-mL dilution, 1/5). This changes the selected series to
5-3-2.
3. Using the first three columns in
Table 6.4
, locate this series (5-3-2).
4. Read the MPN value from the fourth column (140).
5. Note that, in
Table 6.4
, the dilution series begins with 10 mL. For this
test, the series begins with 1.0 mL.
10 mL
1mL
MPN/100 mL
=
140 MPN/100 mL
×
=
1400 MP
N/100 mL
Membrane Filtration Technique
The membrane filtration (MF) technique is highly reproducible, can be
used to test relatively large volumes of sample, and yields numerical results
more rapidly than the multiple-tube procedure. MF is extremely useful in
monitoring drinking water and a variety of natural waters; however, the
technique does have limitations, particularly when testing waters with high
turbidity or non-coliform (background) bacteria. For such waters, or when
the MF technique has not been used previously, conducting parallel tests
with the MTF technique to demonstrate applicability and comparability is
desirable.
