TREATMENT TRAINS

ERDC TN-DOER-T7

October 2004

Mechanical dewatering is not presently used as a standard operating procedure in dredged

material management. However, as the available capacity in existing CDFs has diminished,

there has been greater interest in beneficial use of dredged material to reclaim or preserve

disposal area capacity. Given the time required for materials to consolidate sufficiently so that

they can be excavated and recovered from a CDF, mechanical dewatering may offer a viable

means of continuing to operate in a CDF that is nearing the end of its design life. It is envisioned

that with proper management these facilities would become "rehandling" stations, and dredged

material would be treated as a resource rather than a long-term storage problem. Where

sediments are too contaminated for either beneficial use or CDF disposal, savings in

transportation costs to a permitted disposal facility may justify the dewatering processing cost.

Whatever the motivation, mechanical dewatering is being considered more frequently in dredged

material management evaluations.

TREATMENT TRAINS: There is a variety of equipment from which to choose in developing a

dewatering treatment train. Equipment selection will be based on such variables as sediment

characteristics (e.g. grain size distribution, organic content, plasticity), available staging area,

capacity requirements, and ultimate disposition of the dewatered dredged material. Available

transportation, disposal, and beneficial use alternatives will determine the specifications for the

dewatered cake and the importance of volume and weight reduction. A typical treatment train

will incorporate the following three primary stages: 1) sand and oversize removal, 2) thickening,

and 3) dewatering. Each of these stages may require one or more different pieces of equipment,

depending upon the specific requirements of the application and the characteristics of the

sediment.

Sand and Oversize Removal. Removal of sand, trash, and oversize materials is typically the

first operation in a dewatering circuit, as for any physical separation treatment train. Physical

separation and dewatering treatment trains may, in fact, be almost identical, with the only

difference being the principal processing objective. Sand may be removed first, using a sand

screw with sump and conveyor, followed by light trash removal (bark, grass, and plastic) on a

scalping screen. Alternatively, oversize materials (e.g. coarse gravel, rocks, debris, and light

trash) may be removed first on a grizzly or a vibrating wet screen, followed by sand removal

downstream using a hydrocyclone. Advantages of the first approach are:

The sump on the sand screw provides some surge capacity, which is important in coupling

the dewatering treatment train to a dredging process.

The sand is separated at a point in the system where this will occur without additional energy

input, and where energy would otherwise be required to prevent its settling during process

stoppages.

The overflow passes from the sump of the sand screw with sufficient energy to be passed

onto the scalping screen for removal of light, floatable trash, and collection of the fine slurry

in a mixing tank. Conversely, when the trash and oversize material are separated first on a

wet screen, the underflow (sand and fines slurry) must be pumped to the next stage. The

presence of coarse organics in the sand, however, may necessitate the use of a hydrocyclone

to achieve effective removal of the organics. Also, the fine slurry leaving the mixing tank