MIL-HDBK-1005/16
11.5
Type of Sample. The type of sample collected depends
on the variability of flow, the variability of water quality,
specific handling and storage requirements of the designated
laboratory analysis, and the accuracy required. For compliance
monitoring, a specific sample type is typically specified in the
operating permit. In general, two types of samples are taken for
laboratory analysis, grab samples and composite samples.
11.5.1
Grab Sample. A grab sample is defined as an
individual, discrete sample collected over a period of time not
exceeding 15 minutes. The single sample is taken at neither a
set time nor flow and represents conditions or characteristics
only at a particular point in time. Grab samples can be taken
manually or with the aid of a suitable mechanical device, such as
a pump or vacuum.
11.5.2
Composite Sample. A composite sample is defined as a
sample formed by mixing discrete samples taken at periodic points
in time or by collecting a continuous proportion of the flow. The
number of discrete samples which make up the composite depends
upon the variability of the wastewater quality as well as
variability of flow. A sequential composite is defined as a
series of periodic grab samples, each of which is held in an
individual container, then composited to cover a longer time
period. Six methods are used for compositing samples. Table 18
lists those methods with their advantages and disadvantages. The
choice of composite type is dependent on the relative advantages
and disadvantages as they apply to local conditions.
11.5.3
Selection of Sample Type.
11.5.3.1 Grab Samples.
Use grab samples in the following
instances:
a)
When using samples such as batch dumps (i.e.
return-activated sludge [RAS], waste-activated sludge [WAS], or
digestor supernatant, etc.), the process stream does not flow
continuously.
b)
The water characteristics are relatively constant.
c)
The parameters to be analyzed are likely to change
with storage, such as dissolved gases (i.e., DO, residual
chlorine, soluble sulfides, oil and grease, microbiological
parameters [coliforms], some organic constituents, and pH).
d)
It is desired to corroborate composite samples.
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