Let each stand for 20 minutes and then flush the contents.
d) Dispose of each vessel as per local procedure or standard
operating procedure (SOP).
There is an alternate method of disinfecting the vessels and contents before
disposing of the vessels. Alternately, autoclave the vessels, with contents, at
121 C for 20 minutes in a "biobag" or similar container as noted in the SOP.
Then dispose of the vessels.
BACTERIOLOGICAL ANALYSIS BY THE MEMBRANE-FILTER TECHNIQUE
a. General. The membrane--filter technique is now considered the preferred
method for the bacteriological analysis of water, unless the turbidity of the sample is
sufficient to clog the pores of the filter. It permits: the examination of a larger volume of
sample than does the multiple--tube technique; a direct count of the bacteria in the
water, rather than a statistical approximation; and completion of the test in 24 hours,
whereas the multiple--tube method usually requires 48--72 hours for a confirmed test
(longer for a completed test).
b. Principles. The basic principles involved in the membrane--filter method of
determining the presence and number of coliform organisms in a given quantity of water
are as follows:
(1) A given quantity of water is filtered through a porous disk of polyester
cellulose acetate. The porosity of the filter is 79 percent, and the size of the pores is
0.45 microns (u). Since coliform bacteria are from 1 to 4 microns in length, they are
retained on the filter when the water passes through.
(2) Because the pores of the filter are so small, the water will not pass
through the filter by gravity. Therefore, a vacuum source must be available to enable
the water to penetrate the filter. In the laboratory, a hydrosol with a power vacuum
source is used. In the field, a hand-operated vacuum pump is used.
(3) After the water sample has been filtered, the filter is incubated at 35C
for 22-24 hours in contact with M--Endo medium in a petri dish. This culture medium is
favorable to the growth of coliform bacteria, but it inhibits the growth of other types of
bacteria. This gives it a selective characteristic. Although a few other types of bacterial
colonies may develop on the culture medium, the coliform bacteria will reproduce better,
forming readily identifiable colonies. Colonies may be counted using a low-power
microscope or a simple hand magnifier.
c. Procedures. The specific procedures for conducting the analysis are
outlined in succeeding paragraphs.