Effluent During Disposal Operations

Effluent During Disposal

Operations

4.1 General Considerations

Effluent is defined for purposes of this manual as water discharged from a

confined disposal facility (CDF) during and as a result of the filling or disposal of

dredged material in the CDF (USACE/EPA 1992). Regardless of the manner in

which a CDF is filled, and especially if the CDF contains water or is hydraulically

filled, there will be an effluent.

Effluent evaluation procedures and tests are also presented in the ITM

(EPA/USACE 1998). For consistency and completeness, all effluent procedures

in the ITM are included in this manual in their entirety and with no technical

modification. However, this manual includes additional procedures for evaluation

of the effluent pathway that address a wider range of possible conditions and

additional computer-assisted tools for effluent evaluation.

4.1.1 Effluent Processes

A schematic of an active hydraulically filled CDF is shown in Figure 4-1.

Dredged material hydraulically placed in a CDF settles, resulting in a thickened

deposit of material overlaid by a clarified supernatant. The supernatant waters are

discharged from the site as effluent during active dredging operations. The

effluent may contain dissolved contaminants and suspended and colloidal particles

with associated (adsorbed or held by ion exchange) contaminants. A large portion

of the total contaminant load is particle-associated.

Supernatant waters from CDFs are discharged after a retention time that may

vary from a few hours to several days. Actual withdrawal of the supernatant is

governed by the hydraulic characteristics of the ponded area and the discharge

weir. Several factors influence the concentration of suspended particles present in

supernatant waters. Fine particles become suspended in the ponded water at the

point of entry because of turbulence and mixing. The suspended particles are

partially removed from the water column by sedimentation. However, particle

concentrations may be maintained by upward flow of water through the slurry

mass during settling. Wind and/or surface wave action may also resuspend settled

particles.

4-1

Chapter 4

Effluent During Disposal Operations