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B.4.2 Column settling test procedure
The following test procedure should be used:
Step 1. Mix the sediment slurry to a suspended solids concentration C equal to
the expected concentration of the dredged material influent Ci. The slurry should
be mixed in a container with sufficient volume to fill the test column. Field studies
indicate that for maintenance dredging of fine-grained material, the disposal
concentration will average about 150 g/L. This concentration should be used in the
test if better data are not available.
Step 2. Pump or pour the slurry into the test column using compressed air or
mechanical agitation to maintain a uniform concentration during the filling period.
Step 3. When the slurry is completely mixed in the column, stop the
compressed air or mechanical agitation and immediately draw off samples at each
sample port and determine their suspended solids concentration. Use the average
of these values as the initial slurry concentration at the start of the test. The test is
initiated with the drawing of the first samples.
Step 4a. If an interface has not formed during the first day, flocculent settling
is occurring in the entire slurry mass. Allow the slurry to settle and withdraw
samples from each sampling port at regular time intervals to determine the
suspended solids concentrations. Record the water surface height and time at the
start of the sampling period. Analyze each sample for total suspended solids.
Substantial reductions of suspended solids will occur during the early part of the
test, but reductions will decrease with longer retention times. Therefore, the
intervals can be extended as the test progresses. Recommended sampling intervals
are 1, 2, 4, 6, 12, 24, 48 hr, etc., until the end of the test. As a rule, a 50-m/L
sample should be taken from each port. Continue the test until either an interface
can be seen near the bottom of the column and the suspended solids concentration
in the fluid above the interface is less than 1 g/L, or until the suspended solids
concentrations in extracted samples shows no decrease.
Step 4b. If an interface forms the first day, zone settling is occurring in the
slurry below the interface, and flocculent settling is occurring in the supernatant
water. In this case, samples should be extracted from all side ports above the
falling interface. The first of these samples should be extracted immediately after
(a) the interface has fallen sufficiently below the uppermost port to allow
extraction, or (b) a sufficient sample can be withdrawn from the surface without
disturbing the interface. This sample can usually be extracted within a few hours
after the beginning of the test. Record the time of extraction, water surface height,
and port height for each port sample taken and analyze each sample for suspended
solids. As the interface continues to fall, extract samples from all ports above the
interface at regular time intervals. As before, a suggested sequence of sampling
intervals would be 1, 2, 4, 6, 12, 24, 48, 96 hr, etc. The samples should continue to
be taken until either the suspended solids concentration of the extracted samples
shows no decrease or for a maximum time of 15 days. For this case, the suspended
solids in the samples should be less than 1 g/L, and filtration will be required to
determine the concentrations. The data should be expressed in milligrams per liter
B13
Appendix B Column Settling Test and Effluent Elutriate Procedures

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