Soil Washing
ZAR
The King of Environmental
Remediation
What is soil washing?
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Soil washing uses water to remove contaminants
from soils.
The principle of soil washing lies in separating
the most polluted portion from the cleaner portion
of the soil by scrubbing it.
This scrubbing process reduces the amount that
needs to be cleaned, and we get two fractions of
soil; clean and polluted.
What are the major advantages of
Soil Washing?
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Reduction of amount needing cleanup
Reduction of cleanup leads to reduction in cost of
cleanup and disposal of polluted material
Soil washing works for very polluted soils
Soil washing is relatively a low-cost alternative
for separating waste and minimizing the volume
of waste requiring more treatment
It is a transportable technology that can brought
to the site.
How does the soil washing work?
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The process works by either dissolving or
suspending contaminants in the wash solution.
The principle of soil washing lies on the fact that
we are able to separate two fractions clean and
polluted. Why this is true?
Description of the process
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First Soil is excavated
The soil is then sieved to remove debris and large
objects such as rocks, which can be disposed of on site
if free from contamination
The smaller size of sand and gravel enters a soil
washing unit or scrubbing unit, in which soil is mixed
with washing water.
The washwater is then drained from the washed soil.
The soil is then rinsed with clean water.
After washing, the heavier sand and gravel are allowed
to settle.
If clean, the sand and gravel are put back into the
ground.
Simple Soil-Washing Studies
Flowsheet
Cored soil from a railroad tie treating plant after
water flooding and after surfactant soil washing
http://www.surtek.com/wshcont.htm
Why Soil Washing can separate
clean and dirty fractions of soil?
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Soil can be categorized in two categories:
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Fine, such as clay and silt
Coarse, such as sand and gravel
The separation between the two lies in the fact
that the size is different. The sand and gravel are
soils for which their particles are recognizable by
the naked eye while fine soils cannot be seen.
Why Soil Washing can separate a clean
and dirty fraction of soil? (cont)
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The separation between two different sizes, fine
and coarse will lead to different groups with
different level of contamination.
Contaminants tend to adhere (stick) or sorb to
fractions of clay and silt.
Why Soil Washing can separate a clean
and dirty fractions of soil? (cont)
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However, clay and silt in their turn stick to gravel
and sand that contain no pollution.
If we separate the soil into sizes we will minimize
the pollution.
We use separation with water addition to clean up
(scrub) the gravel and sand from the clay and silt
that contains the pollution.
Why Soil Washing can separate a clean
and dirty fractions of soil? (cont)
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Other separation techniques can be used:
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We can use gravitational methods of separation when
the soil is contaminated with metals.
When the soil is contaminated with metals, it
becomes heavier, hence we can separated the heavier
part from the lighter part (the clean part).
Why then do we need water?
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Is separation by sieving the soil enough?
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No, sieve separation will not separate the sticky parts
of clay and silt and allow them to be cleaned up.
Water will help by separating the clay and silt even
more and dislodging the contaminated section from
the clean section of the clay or silt.
Why then do we need water? (cont)
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Sometimes you have many types of pollutants in
one soil. Here you would need to clean it with
different detergents. This process is called
sequential washing.
Sequential washing is the washing of the soil
several times, each time adding a different
detergent.
Why then do we need water? (cont)
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The addition of water, and other chemicals can
also dissolve some chemicals into the water
stream, hence minimizing their concentrations not
only in the coarse fraction but also in the fine
fraction of a soil.
What happens with the wash
water after use?
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Wash water is usually sent to insitu treatment
plant.
It is cleaned, and a portion of that washwater, if
there is no interfering chemicals, can be used for
more washing of soil.
Is there a concern about air
quality in soil washing locations?
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Unless there are volatile organic compounds
(VOCs) present there is not a concern.
Scrub units have a hood and the Environmental
Protection Agency (EPA) monitors the air quality,
to make sure that no harmful contaminants are
released to air.
What is the fate of the two
fractions of soil?
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The clean soil (the sand and gravel) is tested. If it
passes the requirements it is put back in place.
If it does not pass, it can be washed again and
treated further.
The polluted fraction is concentrated into a
smaller volume and is shipped to either a landfill
or special treatment plant.
Limitations
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Soil washing does not clean the soil, it separates
the contaminated fraction from the clean fraction,
hence minimizing the amount of soil need to be
cleaned.
It is a technique of concentrating contaminants
through separation
Soil washing does not destroy or immobilize the
contaminants
Limitations (cont)
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Sometimes soil washing alone does not clean the
polluted soil enough (therefore other methods
must be used after)
If there is a high organic content in the soil, it
may require pretreatment.
Since it does not destroy or immobilize the
contaminants the resulting contaminated soil must
be disposed of carefully.
Technology Development Status
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The technology is well established in European
Countries. It is extensively used in Europe. In the
United States commercialization is not yet
extensive.
EPA defines soil washing as a new or innovative
technique used, among other methods, to clean up
pollution at Superfund and other contaminated
sites.
It is being used at six Superfund sites and other
sites across the country
Applicability
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The soil washing process is used for soils that are
contaminated with:
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Semi volatile Organic compounds
Fuels
Heavy metals
Some selected Volatile organic compounds
Pesticides
Conclusion
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Soil washing is a treatment technology that uses
liquids (usually water, sometimes combined with
chemical additives) and a mechanical process to
remove hazardous contaminants from soil.