PHYSICAL SEPARATION PROCESSES

ERDC TN-DOER-C13

July 2000

The percentage of material in a given sample meeting the specifications Psi can be estimated based

on the overlap of the GSD curve for that sample and of the BU specification range. Figure 4a

illustrates a relatively large proportion of the material volume potentially meeting the specifications,

while Figure 4b illustrates a relatively small proportion meeting specifications. The proportion

meeting specifications (in percent) will determine the MRP. In cases where the BU specification

is given as a single D50 grain size, the value of Psi can be determined as shown in Figures 5 and 6.

Analytically, this can be expressed as:

Psi = (100 - % passing D50spec) 2

For D50 < D50spec :

(2)

Psi = % passing D50spec 2

For D50 > D50spec :

(3)

The tons dry weight represented by a sample Wsi can be estimated as follows:

Wsi = (0.0135)Vi (γi)

(4)

where

Wsi = dry weight of solids represented by sample i, (tons)

Vi = volume represented by sample i, cubic yards (determined by survey data)

γi = unit weight of the dry material for sample i, lb/cu ft (determined based on water

content or dry density measurement)

0.0135 = conversion factor

The calculation method shown here can be applied in the same way for an initial evaluation using

existing GSD data or following collection of additional samples. This relationship for calculating

MRP can be applied to individual samples representing an incremental volume of the total or an

average of several samples representing either a composited volume or the total volume of interest.

This will be determined, as previously mentioned, based on the variability of the site and the

expected compositing of the material for processing. Some data estimating approaches described

in Olin-Estes (2000) may also be helpful in segmenting the volume between data points.

CONSIDERATION OF RESIDUAL VOLUME CHANGES: Although a proportion of the total

mass dry weight of the material may be recoverable, the resulting increase in volume (bulking) of

the residual materials should be evaluated in determining the advisability of separation. For

example, if 50 percent by weight of the material in an existing CDF is removed, the resulting capacity

recovered within the CDF will be less if hydraulic methods are used in the separation process. The

residual fine fraction will increase in volume (bulking), at least in the short term. The same would

hold true if hydraulic separation methods were applied as a follow-on to mechanical dredging/

rehandling operations. Residual materials can be mechanically dewatered successfully, eliminating

the problems resulting from bulking, but mechanical dewatering is typically one of the more

expensive unit processes in a physical separation treatment train.