Table 10-7.Procedures for Manual Wastewater Sample Collection
1. Samples should be taken where wastewater
1. Weirs are not good sampling points since
is well mixed.
settling of solids is enhanced upstream and
greases and oils build up downstream from the
2. Sampling should be done in the center of the
2. Solids often build up near the sides and
flow channel. To avoid floating scum, the
bottom of the flow channel.
mouth of the container should be held below the
3. A representative sample should be taken.
3. Raw wastewater should be sampled after
screening and grit removal. Deposits or
nonrepresentative materials such as grease or
scum should be excluded from the sample.
Particles larger than 0.25 inch (6 mm) in diameter
should be excluded.
4. When compositing samples into other con-
4. If dissolved gases or volatile substances are
tainers, the contents of each should be well
to be tested, turbulence may be produced by
mixed before pouring.
5. The sampling containers and sampling devices
5. Before the sample is taken, the container
should be clean, uncontaminated, and suitable
should be rinsed several times with the waste-
for the planned analysis.
6. Sampling places should be easy to reach
6. Proper sampling equipment should be
and safety precautions should be observed.
samples are also used when a batch dump or
changes in proportion to the flow. The flow-
sludge discharge is seen.
A composite sample yields data about the
wastewater over a longer span of time. A series
of grab samples may be taken over a certain
amount of time and combined to form a com-
posite sample. These samples should show the
time and frequency of the sample; for example,
an 8-hour composite of 30-minute grab samples.
The composite sample is used to find BOD, COD,
suspended solids, and nutrients.
The composite may be flow proportional. For
this type of sample, the volume of the sample
proportional composite sample is most often run
for 24 hours with a 2-hour interval between each
collection. To collect this kind of sample, the
volume needed for the tests and the average
daily flow for the plant must be known. Table
10-8 shows the volumes required for some tests.
The following formula may be used to find the
volume of sample to be taken at each interval.
For example, to collect an 8-hour composite
sample with a 2-hour interval, five samples would
be needed. If a total sample of 2 liters was needed,
the average daily flow was 60,000 gallons (227
cubic meters), and the flow at the first sample time
was 45,000 gallons per day (170 cubic meters),