B.4.2 Column settling test procedure

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