Environmental Engineering Reference
In-Depth Information
and (2) the
membrane filter
(MF) technique for members of the coliform group.
The multiple-tube method is still used in many labs, because the membrane
filter method is not applicable to turbid samples; however, the membrane fil-
ter method takes less time and provides a more direct count of the coliforms
than the multiple-tube method does. It also requires less laboratory equip-
ment. The bottom line? Drinking water practitioners need to understand the
essential differences between these two tests, but before we explain the dif-
ferences, we need to lay the groundwork by explaining testing preparations.
Note:
Both methods are still recognized as sufficiently reliable with rela-
tively simple techniques and economic equipment to be run as often as
required by the monitoring activity of water quality control.
We discuss each of these procedures briefly in the following sections; how-
ever,
do not attempt to perform the fecal coliform test using the summary informa-
tion provided within this text.
Instead, refer to the appropriate references cited
in the
Federal Register
or the current
Standard Methods and Procedures
for a
complete discussion of these procedures.
Note:
Because the MF procedure can yield low or highly variable results
for chlorinated water, the U.S. Environmental Protection Agency (USEPA)
requires verification of results using the
most probable number
(MPN) pro-
cedure to resolve any controversies.
Testing Preparations
Whenever microbiological testing of water samples is performed, certain
general considerations and techniques are required. Because these consider-
ations are similar for each test procedure, we review them here prior to our
specific discussion of the two test methods:
•
Reagents and media
—All reagents and media used in performing
microbiological tests on water samples must meet the standards
specified in references cited in federal regulations.
•
Reagent-grade water
—Deionized water that is tested annually and
found to be free of dissolved metals and bactericidal or inhibitory
compounds is preferred for use in preparing culture media and test
reagents, although distilled water may be used.
•
Chemicals
—All chemicals used in fecal coliform monitoring must be
American Chemical Society (ACS) reagent grade or equivalent.
•
Media
—To ensure uniformity in the test procedures the use of dehy-
drated media is recommended. Sterilized prepared media in sealed
test tubes, ampules, or dehydrated media pads are also acceptable
for use in these tests.
