Remediation and Management of Contaminated Soil - Sustainable Practices in Geoenvironmental Engineering - page 406

be an increased resistance to erosion. Depths can be a few to more than 10 m and widths
are in the range of 500 to 1500 m. As they are illed, capping is used.
Another approach is to place the material in woven or nonwoven permeable synthetic
fabric bags, geo-tubes, or containers (NRC, 1997). Costs at the demonstration trial in
California were approximately $65/m 3 (Clausner, 1996). The contaminants must not seep
through the fabric into the water, and these uncertainties must be further investigated.
11.3 Extraction Processes
11.3.1 Physical Separation
Physical separation processes are generally technically simple methods for separation of
solids on the basis of size and density and are often used as pretreatments. These processes
have been applied in the separation of contaminated fractions from the clean coarser par-
ticles. As coarser particles, such as sand and gravel fractions, have less contamination on
their surfaces, washing is often enough to clean for beneicial use. This is important to
reduce the amount of material to be disposed of. The most contaminated fractions may
require further treatment or restricted disposal. The volume of the ine residuals may be
minimized using mechanical dewatering techniques (Olin-Estes and Palermo, 2001).
Physical separation processes include centrifugation, locculation, hydrocyclones, screen-
ing, and sedimentation. Hydrocyclones separate the larger particles greater than 10 to 20 μm
by centrifugal force from the smaller particles. Fluidized bed separation removes smaller
particles at the top (less than 50 μm) in countercurrent overlow in a vertical column by
gravimetric settling and lotation, which is based on the different surface characteristics of
contaminated particles. Addition of special chemicals (lotation agents) and aeration causes
these contaminated particles to loat. Screening is most applicable for particles larger than
1 mm. Magnetic extraction also may be used. If the solids content is high, mechanical screen-
ing can be used. Gravity separation or sedimentation is applicable if the contaminated frac-
tion has a higher speciic gravity that the rest of the soil fraction. According to the U.S. Army
Engineer Detroit District (U.S. Army Engineer Detroit District, 1994), costs are in the range of
$ 3 0 -70/m 3 for quantities in the range of 7600 to 76,400 m 3 and for soils/sediments with 75%
sand and 25% contaminated silt or clay. The expense is only justiied if the soil contains more
than 25% sand, which is rare (NRC, 1997). Physical techniques only concentrate the contami-
nants in smaller volumes and are thus useful before thermal, chemical, or other processes.
In Japan, similar techniques and processes are used to obtain aggregates from soils for
concrete. The soils taken from mountainous areas are washed. The ine and light frac-
tions are separated from coarse particles (concrete aggregates) in a centrifugal tank, and
dewatered using the ilter presser or belt presser. The water content is usually controlled as
about 40% from the energy cost and treatability of the materials. The technology can also
be used for dredged materials.
11.3.2 Soil Vapor Extraction
Soil vapor extraction (SVE) (Figure 11.2) involves the removal of VOCs and some fuels
with a Henry's law constant greater than 0.01 or a vapor pressure greater than 0.5 mm
Hg through either air injection or vacuum vapor extraction. SVE is an in situ unsaturated