Figure 9. Flocculated slurry, supernatant and cake

ERDC TN-DOER-T7

October 2004

Figure 9.

Flocculated slurry, supernatant and

Figure 10. Decanting centrifuge (Photo provided

cake (Photo provided courtesy

courtesy Centrisys)

Centrisys)

Physical and chemical characterization of the in situ material should include grain size

distribution, water content, percent solids, bulk specific gravity and specific gravity of the solids,

organic content, Atterburg limits, and chemical analysis of expected processing streams. If

contaminant concentrations are of concern, and size separations are to be done prior to the

principal dewatering step, chemical concentrations should be measured in the resulting solid and

aqueous process streams. Estimates of filtrate suspended solids and total and dissolved

contaminants will typically be of interest. Control of volatile emissions may also be a permit

requirement. Process flow diagrams with material mass balances will normally be needed.

Polymer Testing. Conditioners (flocculants) are used to produce optimum slurry suspended

solids in the feed to the dewatering equipment. Equipment performance is dependent upon

selection of an appropriate conditioner. The floc must have enough shear strength to minimize

solids losses in the dewatering equipment, but must be permeable enough to permit free

drainage. Chemical conditioners often constitute a major operating cost of a dewatering circuit,

so this step is very important with respect to process economics. Typically multiple conditioners

will be evaluated at bench scale to identify those that produce the best sludge for the least cost.

The best conditioners identified in bench tests will then be evaluated with the bench- and pilot-

scale dewatering equipment. Bench-scale testing may consist of a fairly simple funnel and filter

apparatus to evaluate gravity drainage characteristics of the flocculated material. It is common

to test a large number of polymers in order to identify the optimum conditioner and dosage.

Toxicity of conditioners may become an issue if filtrate and process water are to be discharged to

a water body without further treatment.

Crown Press. The Crown PressTM is a bench-scale piece of equipment intended to model the

belt filter press (Figure 11). The press facilitates evaluation of conditioners and belt materials,

and the action of multiple rollers can be simulated to achieve a specific cake dryness and density.

Migration of sludge on the belt can be measured. The belt tension applied on the bench press

translates to the units of force per inch. This information is used in adjusting the belts on a full-

scale press. Process variables of interest are: solids throughput rate, filtrate quality and