Aquasafe WSL50 Pro, Instruction Manual

Page: 49 / 193

SECTION 9: SELECTING THE OPTIMUM VOLUMES FOR
MEMBRANE FILTRATION
The optimum volume of sample is that which will allow the most accurate
quantification of bacterial colonies. This is achieved when the number of
faecal (thermotolerant) coliform colonies on the membrane following
incubation is between 20 and 200 colonies. If there are fewer than 10
colonies, then there exists the possibility of statistical error. Numbers greater
than 200 colonies are difficult to count with the naked eye.
Potable Waters
The number of faecal coliform bacterial colonies in treated water samples
should ideally be zero. Thus, the preferred sample volume is 100 mL, and a
count of zero faecal coliform bacteria per 100 mL is indicative of a
microbiologically safe water supply. If the count exceeds 1 faecal coliform per
100 mL, contamination is indicated. If the count exceeds 10 faecal coliforms
per 100 mL, action is urgently required.
Raw Waters
For source waters and partially treated waters, including those which are
ground water derived, it can be useful to adjust the sample volume in order to
obtain faecal coliform counts in the optimum range 10-200. It may also be
useful to process more than one quantity on the first occasion a particular
water source is sampled. In such cases it is not necessary to re-sterilise the
filtration equipment between different quantities of the same sample, provided
that the smaller volume is processed first. Typical volumes which may be
appropriate for various water types are shown in the following table. They are
only guidelines; there is no substitute for experience of a given source.
APPROPRIATE VOLUME
(mL)
SOURCE OF SAMPLE 100 50 10
Lakes, Reservoirs, & Rivers & other
surface sources
* ** ***
Wells, boreholes, other protected water
sources
* ** *
Water treatment plant partially treated
** ** *
Water treatment plant fully treated
***
Distribution system
***