Adequate medium is necessary for the organisms filtered out of the water to grow
properly and provide valid results. On the other hand, the use of excess medium must
be avoided since excessive medium causes the colonies to run together.
Figure 3-3. Saturate absorbent pad with culture medium.
Rinse the funnel-assembly with 50 ml of buffered water.
(7) Place a sterile membrane filter disk with the grid side up on the filter
base and center it over the porous part of the membrane support plate (see figure 3-4).
A membrane filter is easily damaged. Using sterile forceps, always grasp the outer part
of the membrane filter to prevent damage to the part through which the sample is to be
Figure 3-4. Place sterile membrane filter on filter holder.
(8) Attach the funnel element to the filter holder. To avoid damage to the
membrane filter, never turn or twist the funnel element as you seat and lock it to the
filter holder. In securing the funnel element to a filter holder which has a bayonet joint
and locking ring, take special care to turn the locking ring sufficiently to give a snug fit,
but do not tighten it excessively.
Filter the first water sample. Pour the measured water sample into the funnel
(see figure 3-5); then enter in the appropriate space on DD Form 686 the volume of
sample being filtered. Since the primary objectives in this step are accurate
measurement of sample and optimum distribution of colonies on the filter, the methods