Trace2O AquaSafe WSL50, Instruction Manual, Page: 43 / 52

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
***
*** Normal Volume or First Choice ** Likely Volume * Possible Volume