DEWATERING EQUIPMENT

ERDC TN-DOER-T7

October 2004

must also be pumped to the next stage, so a pump will either precede or follow the sand

separation unit operation. Although the bulk of the water reports with the fines, the solids

flow rate will be lower for a pump to transfer the fine slurry from the mixing tank to the

thickener, than for a pump to transfer the wet screen underflow (sand and fines) to the

hydrocyclone. Pump abrasion will also be decreased with removal of the sand.

The sand coming off the screw is essentially self stacking, and can be periodically removed

with a front-end loader. Additional dewatering might be required for the underflow from a

hydrocyclone (sand fraction), and a sand screw might be used for this purpose, as well as to

convey material from beneath the hydrocyclone. The sand screw followed by the scalping

screen therefore requires less equipment overall when additional washing of the sand is not

required to remove coarse organics and agglomerated fines.

Thickening. Fine-grain slurries normally require the addition of polymer to coagulate the

solids and facilitate dewatering. Polymer may be added to the slurry in a tank and agitated to

encourage flocculation, but it is often added in-line, between the sand separation unit and the

dewatering stage. At this point, a thickener may be used to settle solids to optimum solids

content for the dewatering equipment. The thickener also acts as a clarifier, recovering the free

water from this initial dewatering step, and reducing the suspended solids in filtrate returned

from the dewatering equipment. The clarifier overflow may be recycled through the system for

necessary process dilutions, or returned to the receiving water body. Alternatively, thickening

and initial dewatering of fines can be done with an inclined wedge wire screen. However, the

inclined screen is sensitive to changes in grain size of the feed, resulting in process upsets and

variability in the solids content of materials leaving the screen. Sandy materials may fall through

the screen entirely, rather than flowing down the face of the screen to the next stage.

Dewatering. Once sand and oversize materials have been removed, and the material thickened

if necessary, the fine slurry can be dewatered using a centrifuge or filter press, each of which are

available in a variety of configurations. Selection of the best equipment and configuration for

the application will depend upon the degree of water removal and volume reduction required, the

ability of the equipment to handle the type and volume of material being processed, and the rate

of production. Bench and pilot scale testing are typically used to make this determination and to

size the equipment. Depending upon the dewatering equipment used downstream of the

thickener, additional polymer may be required to further coagulate the solids for the final

dewatering step. Insufficiently coagulated or low-solids-content slurry may result in poor solids

capture in the dewatering step. Overtreating with polymer can be equally undesirable with

respect to processing cost and wastewater quality.

DEWATERING EQUIPMENT:

Sand Screw. A sand screw typically consists of a sump equipped with a flat or angled auger to

transport settled sand from the sump, and a conveyer to feed the sand from the auger (Figure 1).

Alternatively, sand may simply be allowed to stack at the base of the sand screw and be handled

with conventional earth-moving equipment. Slurry is fed into the top of the sump, and the coarse

materials settle onto the auger. Fine and organic materials are carried out in the overflow. The

sump may be equipped with counter-current flow capability, to improve the removal of the fines

and organics from the sand fraction. The sand fraction can be coarsened by increasing the