b. Representative Sampling. Wastewater samples must be as nearly
representative of the entire body of wastewater being tested as possible. Errors in
sampling too often nullify the accuracy of laboratory tests. Intermittent pumping of
wastewater in the conducting channel causes the character of layers or stratas of
residue in the conducting channels to change, making it difficult to get representative
samples. Analyses should be conducted as soon after collection of the samples as
possible; otherwise, the biochemical processes that take place within the sample itself
may render the test results meaningless.
(1) Grab sampIes. A grab sample is defined as a sample taken one time at
any one point. The results of a grab sample indicate only the momentary condition
existing at the particular time of collection at the particular rate of flow past a particular
point and are useful for immediate control purposes. Some tests which require grab
sampling are dissolved oxygen, chlorine demand, residual chlorine, settleable residues,
pH, mixed liquor and return sludge concentration (for activated sludge), and digester
content tests. Grab samples are also used in making stream surveys to monitor the
effect of the plant effluent on the receiving stream.
(2) Composite samples. For determining total loading on a plant, the
overall treatment efficiency of the plant, and the pollution load on the receiving stream, it
is necessary to know the average daily composition of the wastewater as it passes
through the plant. For this purpose, a composite sample is obtained. To make a
composite sample, individual samples are gathered at regular intervals over a selected
time period (usually 16 to 24 hours), proportioned according to hourly flow past the
sample point, and composited for analysis. Composite samples are taken for the
following determinations: nonfiltrable residue; biochemical oxygen demand (BOD); and
total and volatile residue of sludge, grease, and nitrogen.
(3) Sampling points. Samples from channels are taken at two-thirds the
depth of the flow at a point free from back eddies. Samples of digester sludge are
collected at 3- to 5-foot intervals, starting at the top and working down to avoid agitating
the sludge from which the succeeding samples are taken. Figure 2-30 shows the
samplers used for wastewater effluent and sludge. Sampling for dissolved oxygen
requires special procedure and apparatus (see Figure 2-31) to prevent an increase in
oxygen content through contact with air.
c. Schedule of Analyses. Table 2-1 is a suggested schedule of sampling and
analyses. Unless otherwise indicated by footnotes in the table, grab samples and
analyses are made daily. Analyses requiring composite samples are made every 6
days at plants serving populations between 1,500 and 6,000; every 3 days for
populations between 6,000 and 20,000; and daily when the population served exceeds
20,000. Samples from digesters and lmhoff tank sludge compartments are taken
monthly and samples from streams receiving the plant effluent are taken weekly.
Where the population is less than 6,000, the minimum analyses to be made are daily
grab-sample tests for settleable residue, pH, and chlorine residual. It should be noted
that revisions may be necessary to comply with the appropriate Federal, state, or local
regulatory agency having jurisdiction.