What is Washing?
This is a mechanical process which uses liquids to scrub hazardous contaminants out of polluted soil.
How Does Soil Washing Work?
Hazardous contaminants tend to bind to fine soil particles of silt and clay which typically adhere to the larger and courser sand and gravel particles. Soil washing uses liquid (usually water or a mixture of water and possibly other chemical additives) to separate the contaminated silt and clay from the coarser sand and gravel particles. When completed, the majority of the hazardous contaminants are isolated into a small volume of fine silt and clay which can then be treated by other methods such as bioremediation or disposal according to state and federal regulations. The larger volume of soil made nontoxic can then be safely reused as backfill material.1
Is Soil Washing Difficult?
The equipment to do soil washing is transportable so the process can be conducted where it is needed.
The first step of the process is to dig up the contaminated soil and move it to a staging area where it is prepared for treatment. The soil is then sifted to remove debris and large objects such as rocks. The remaining material enters the soil scrubbing equipment where the soil is mixed with a washing solution and agitated.
Large scale soil washing equipment can process over 100 cubic yards of soil per day.1 In this process the heavier sand and gravel particles settle out and are tested for contaminants. If clean, this material can be reused on the site or taken elsewhere for backfill. If traces of contaminants are still present, the material may be processed through the soil washer again, collected for alternate treatment or taken to an off-site disposal facility. The washwater, (which now also contains contaminants) is treated by wastewater treatment processes so it can be recycled for further use.
When is Soil Washing Appropriate?
The process is useful in treating a wide range of contaminants, such as metals, gasoline, fuel oils and pesticides. It works best when the soil contains a large amount of clay and organic materials.1
What are the Advantages to Soil Washing?
There are several notable benefits:
- Provides a closed system that remains unaffected by external conditions.1 Thus the system permits control of the conditions (such as the pH level and temperature) under which the soil particles are treated.
- Allows hazardous wastes to be excavated and treated on-site.
- Has the potential to remove a wide variety of chemical contaminants from soils.1
- Is cost-effective because it can be employed as a pre-processing step, significantly reducing the quantity of material that would require further treatment by another technology.
- Creates a more uniform material for subsequent treatment technologies.
- A Citizen’s Guide to Soil Washing, EPA, (http://www.epa.gov/tio/download/remed/soilwash.pdf), Retrieved October 16, 2014