and 0.5 ml of ethanol. This mixture must then be heated with frequent agitation and
boiled until the solution is complete. Two ml of this solution are used on each filter pad.
DETERMINING FILTRATION VOLUME
The quantity of water sample to be filtered is governed by the expected bacterial
density. Sample volumes should be chosen to obtain at least one membrane filter with
20 to 80 coliform colonies. The total number of colonies of all types should not exceed
a. Data Available. If bacteriological data are available from previous tests,
water sample filtration volumes can be computed as follows:
(1) First, determine the arithmetic mean of the coliform counts; then
determine the volume of sample, which, on the average, may be expected to produce
20 coliform colonies. This is known as the Basic Filtering Volume (BFV).
Example: Previous data show that a given source has an average coliform density of
160 coliform colonies per 100 milliliters.
x 100 = BFV
Average coliforms per 100 ml
20 x 100 = 12.5 ml BFV
(2) If three sample volumes are to be filtered, as is preferred, then filter one-
third of the BFV, the BFV, and three times the BFV. (The computed amounts for
filtration in the example above would be 4.2, 12.5, and 37.5 ml. For convenience in
measurement, the samples should be filtered in the amounts of 4, 10, and 40 ml).
(3) If two sample volumes are to be filtered, filter one-half of the BFV and
one and one-half times the BFV. (The computed amounts for filtration in the example
above would be 6.25 and 18.75 ml. For convenience in measurement, the sample
should be filtered in amounts of 6 and 20 ml).
(4) If only one filtration is to be made (generally not recommended), filter the
BFV. (In the example above, the amount for filtration would be 12.5 ml or 10 ml for
convenience in measurement).
b. Data Not Available. In the absence of previous bacteriological data, the
following water sample volumes should be filtered: