Containment area effluent

ERDC TN-DOER-E8

June 2000

Containment area effluent. In this case, the larger, heavier solids will settle near the inflow

point for the CDF and only finer particles will be present in the effluent. The permit limitation will

frequently be on the TSS at the limit of a mixing zone some distance from the effluent weir

discharge. A laboratory settling column and test procedure has been developed for prediction of

the effluent TSS (Montgomery 1978; Montgomery, Thackston, and Parker 1983; Thackston and

Palermo 1988; Palermo and Thackston 1988a,b,c; Headquarters U.S. Army Corps of Engineers

(HQUSACE) 1987; and U.S. Environmental Protection Agency (EPA)/U.S. Army Corps of

Engineers (USACE) 1998). In this procedure, the sediments are mixed with water from the site to

a slurry concentration approximating the dredge discharge concentration, pumped into the 20-cm

(8-in.) by 180-cm (6-ft) column, and allowed to settle. This settling column test is also routinely

used, along with the modified elutriate test described by Palermo and Thackston (1988b,c), to predict

the concentration of contaminants in the effluent from diked disposal areas.

The same laboratory settling column test can be used to develop the data for a correlation of TSS

versus turbidity. The column test procedure for CDF effluent quality evaluation requires that

samples for TSS be taken from the column from a series of sampling ports along the length of the

column (HQUSACE 1987; EPA/USACE 1998). Therefore, the data needed for the correlation

curve can be produced by simply performing a turbidity analysis on the samples taken for TSS

analysis during the column test. The samples of most importance will be those taken at times near

the expected average hydraulic retention time of the containment area, when the TSS and turbidity

are reduced to values approximating those in the effluent from the containment area.

If this settling column is not available, other types of settling containers may be substituted, such

as large graduated cylinders, but accuracy may be compromised. The most important factors are

the column diameter, which should be at least 15 cm (6 in.), and preferably 20 cm (8 in.), in order

to avoid wall effects, and sampling just below the surface in a manner that does not cause mixing.

The recommended procedure for producing a TSS-turbidity correlation curve would then consist

of the following steps:

As soon as the slurry has started to settle, begin taking samples from the uppermost column

sampling ports or from just below the water surface in the settling column. A suggested

sample volume is 50 mL.

Continue to take additional samples from the uppermost sampling ports remaining below

the water surface or from just below the water surface as the test progresses. It is not

necessary to maintain a precise or uniform interval for sampling, but a larger number of

samples should be taken at first with an increasing interval as the test progresses. The

optimum sampling intervals will depend on the settling behavior of the specific sediment.

At least 20, and preferably 50, samples should be taken within the first day or two of the

test, and samples should be taken until the supernatant becomes clear and the TSS and

turbidity have obviously dropped well below the limits set in the permit. An initial interval

of 30 minutes to 1 hour is recommended for the first 8 to 16 hours of the test, with increasing

intervals of 2 hours, 4 hours, etc., as the test progresses.

Analyze each sample for TSS and turbidity.