Soil Washing

ZARZAR

The King of Environmental Remediation

What is soil washing? 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? 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? 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 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? Soil can be categorized in two categories:

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) 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) 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) Other separation techniques can be used:

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? Is separation by sieving the soil enough?

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) 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) 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? 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? 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? 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 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) 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 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 The soil washing process is used for soils that are

contaminated with: Semi volatile Organic compounds Fuels Heavy metals Some selected Volatile organic compounds Pesticides

Conclusion 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.