15
4.1 Collecting the Sample for Analysis
Samples can be collected using the sterilised
sampling cup of the
Membrane Filtration
Unit (MFU)
.
It is supplied
with a cord
to allow the
sampling cup
to be lowered
into a water
course, well or
storage tank.
IMPORTANT
: Always rinse the sterile
sampling cup with some of your sample
water before taking the final sample. This
eliminates any residual methanol left over
from the sterilisation process.
Care must be taken not to introduce floating
matter or material from the edge of the water
course into the water sample. It may be
preferable to attach the sampling cable to the
sterilised sampling cup and take the sample
from a bridge or other overhanging location.
Alternatively, the cup may be cast into the
water from the edge and pulled slowly and
carefully back towards the operator.
Alternatively, any suitable sterile container
or sample bottle can be used in place of
the sampling cup.
When sampling from a river or stream take
the sample as near as possible to the main
flow and not too close to the edge where
the water may be still and unrepresentative
of the sample as a whole.
When sampling from a tap or outlet that
provides water for a consumer remove
any tap attachment. Clean the tap/outlet
with a dry cloth before allowing to run for
1 minute prior to sample collection.
Once collected, the sample must be processed
immediately or as quickly as possible. The use
of a portable field test kit makes this possible.
However, if the delay between sample collection
and analysis is between 2 and 6 hours, chill
the sample rapidly to about 4°C with ice
blocks in an insulated container/cool bag.
Resuscitate the sample prior to full incubation
using the automated routine contained
within the Wagtech incubator software.
Even if the sample is kept cold, the
maximum sample storage time is 6 hours.
Analysis of samples not stored under these
conditions or processed after a period of 6
hours are unlikely to reflect the bacteriological
conditions at the time of sampling.
If chlorinated water samples are being
collected, sodium thiosulphate (not supplied
with kit) should be added to the sample
bottles to neutralise chlorine.
4
Taking a Sample
4.0 Taking A Sample
The optimum volume of sample is that which will allow the most accurate enumeration of bacteria.
The technique of
membrane filtration
is unsuitable for natural waters containing very
high levels of suspended material, sludges and sediments, all of which could block the
filter before an adequate volume of water has been filtered.
For potable or treated water samples the number of faecal coliform bacteria should ideally
be zero in 100ml, indicating a microbiologically safe (or more accurately LOW RISK) water
supply. The preferred sample volume is 100ml.
For raw source waters and partially treated waters, including those derived from ground
water, it is sometimes useful to reduce the sample volume to obtain faecal coliform counts
in the optimum range. This may be a reduction of the sample size to 50ml, or even 10ml in
more contaminated water sources. To aid this, the filter funnel of the membrane filtration
unit has two internal graduations at 50ml and 100ml.
100ml
50ml
