(3) Dechlorination. If the sample to be examined is from a chlorinated water
supply, a dechlorinating agent should be added to the sample bottle prior to collecting
the sample, in order to stop the bactericidal action of the chlorine and thus give a more
accurate indication of the true bacterial content of the sample. Sodium thiosulfate, in an
amount sufficient to provide a concentration of 100 mg/1 (0.1 ml of a 10 percent solution
of sodium thiosulfate to a 4-oz (118.28 ml) sample bottle), is a satisfactory
dechlorinating agent. The sodium thiosulfate must be added prior to sterilization. It is
imperative the samples intended for bacteriological examination be delivered or shipped
to reach the laboratory with 6 hours after collection. This is important because of the
extensive changes that may take place in the bacterial flora even though the samples
are stored at temperatures as low as 10C. (See paragraph 3-4c)
(4) Procedure. The sample bottle must be kept closed until the moment at
which the sample is taken. The stopper and hood are removed in such a manner as to
prevent contamination, and the bottle is held near the base. The bottle is filled (leaving
an ample air space to facilitate mixing of the sample by shaking) without rinsing and the
stopper and hood replaced immediately. The following special procedures apply to
specific types of samples.
(a) If the water sample is being taken from a tap, the tap should be
connected to a service line from a main, rather than from a storage tank. The tap water
should be allowed to run for 2-3 minutes before taking the sample.
(b) If the sample is taken from a well serviced by a pump, the pump
should be operated at least long enough for all water to be flushed from the connecting
pipes.
(c) Samples taken from rivers, streams, lakes, reservoirs, or other
surface sources should be collected at a point which is representative of the water
which will be the source of supply to the consumer. Particular caution should be taken
not to disturb the bank of a pond or stream; otherwise, fouling of the water may occur
and the sample will not be representative. The bottle should be held near the base and
plunged, neck downward, below the surface. The bottle is then turned so that the neck
points slightly upward and toward the current. If there is no current, one may be created
by pushing the bottle, underwater, in the direction of the mouth.
3-2.
SEWAGE MICROBIOLOGY
Microbes play an extremely important role in sewage treatment. It is largely
through biological digestion that sewage is converted from a highly contaminated,
infectious liquid into a relatively stable, inert sludge and a harmless effluent needing
only chlorination before it may be discharged into a receiving stream, leaching bed, or
other disposal area. There are two biological processes involved in sewage treatment.
Aerobic digestion is exemplified by the activated sludge process, in which the wastes
from primary settling tanks are thoroughly aerated until active masses of
microorganisms settle out as sludge, leaving a clear effluent of relatively low organic
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