The traditional low-technology method for dealing with
contaminated sites has been to excavate the soil and remove it to
places licensed for depositing. However, with the increase in
building work on brown-field sites, suitable dumps are becoming
scarce. Added to this is the reluctance of ground operators to
handle large volumes of this type of waste. Also, where
excavations exceed depths of about 5 m, it becomes less practical
and too expensive. Alternative physical, biological or chemical
methods of soil treatment may be considered.
Encapsulation † in-situ enclosure of the contaminated soil. A
perimeter trench is taken down to rock or other sound strata and
filled with an impervious agent such as Bentonite clay. An
impermeable horizontal capping is also required to link with the
trenches. A high-specification barrier is necessary where liquid or
gas contaminants are present as these can migrate quite easily. A
system of monitoring soil condition is essential as the barrier may
decay in time. Suitable for all types of contaminant.
Soil washing † involves extraction of the soil, sifting to remove
large objects and placing it in a scrubbing unit resembling a huge
concrete mixer. Within this unit water and detergents are added for
a basic wash process, before pressure spraying to dissolve
pollutants and to separate clay from silt. Eliminates fuels, metals
and chemicals.
Vapour extraction † used to remove fuels or industrial solvents
and other organic deposits. At variable depths, small diameter
boreholes are located at frequent intervals. Attached to these are
vacuum pipes to draw air through the contaminated soil. The
contaminants are collected at a vapour treatment processing
plant on the surface, treated and evaporated into the atmosphere.
This is a slow process and it may take several months to cleanse a
site.
Electrolysis † use of low voltage d.c. in the presence of metals.
Electricity flows between an anode and cathode, where metal ions
in water accumulate in a sump before pumping to the surface for
treatment.
Physical Treatment of Contaminated Sub-soil