The International Information Center for Geotechnical Engineers

Soil Washing

 Main Concept and Description of Soil Washing

Soil washing is an ex-situ remediation technique that removes hazardous contaminants from soil by washing the soil with a liquid (often with a chemical additive), scrubbing the soil, and then separating the clean soils from contaminated soil and washwater (US EPA 1993, 1996). The concept of soil washing is based on the theory that contaminants are prone to bind to fine grained soils (silts and clays), which, in turn, are prone to bind to coarse grained soils. Therefore the main goal of soil washing is to separate these contaminated fines and washwater from the cleaned coarse grained soils (sands and gravels). The contaminated fines and washwater can then be treated or disposed of as necessary. The washed soils may be reused as backfill at the site if all contaminants are removed from the soil. The process of soil washing significantly reduces the volume of contaminated soil at a site, often making soil washing a pretreatment step for a different remediation technique (US EPA 1996, Sharma and Reddy 2004). Soil washing can be broken into six different steps (US EPA 1993):

  • Pretreatment
  • Separation
  • Coarse-grained treatment
  • Fine-grained treatment
  • Process water treatment
  • Residuals management

 

The following summarizes the process of soil washing, which is depicted in Figures 1-4 below.

Fig 3

 Figure 1: Schematic of Soil Washing Process (from US EPA 1996)

 

Plant Arial

Figure 2: Aerial View of Soil Washing Plant (from ART Engineering)

 

 

 Plant

 Figure 3: Typical Soil Washing Equipment (from ART Engineering)

 

 

shell separation

Figure 4: Separation Step (from ART Engineering)

 

Pretreatment

Pretreatment is completed after the excavated soil has been placed in a staging area and before it can be washed through a mechanical process. In this step, large objects are removed from the soil so that a homogenous (diameters less than 2 inches) soil is prepared for the washing step. Removal is done through scalping, mechanical screening, and jigging and tabling. The oversized materials can consist of anything from construction debris to large pieces of rock or gravels. These materials are usually not contaminated; however, if treatment is necessary crushing and grinding may be needed to reduce the size of the materials (US EPA 1993, Griffiths 1995).

 

Separation

Once the soil has been pretreated, it is ready to be washed in the soil scrubbing unit. Separation of the coarse and fine grained materials occurs in this unit. Since the coarse grained soils are likely not contaminated, they are removed from the washing unit. The particle size cut point is usually between 63 (#230 sieve) and 74 microns (#200 sieve). Separation is done since the coarse and fine grained soils will require different final cleaning procedures. Separation of the coarse grained soils is commonly done by using mechanical screening such as trommels, while the fine grained soils are sorted out by hydrocyclones or other methods (US EPA 1993, Griffiths 1995). Figure 5 below shows an example of the EPA mobile soil-washing system. The separation of the coarse fraction as well as the use of hydrocyclones is depicted.

Fig 2

Figure 5: EPA mobile soil-washing system (from Griffiths 1995)

 

Coarse-Grained Treatment

Once the coarse grained soils have been separated and removed from the scrubbing unit they may require additional treatment if contaminants have absorbed onto the soils. Common treatment methods include:

  • Surface attrition
  • Acid or base treatment for solubilization
  • Specific solvents for dissolving particular contaminants

The water (and any fine materials that were found in the coarse fraction) that is still in the coarse grained soils is removed and then added back into the system so that it can be treated along with the fine grained soils (US EPA 1993).

 

Fine-Grained Treatment

Contaminants are predominantly found in the fine grained soils. Chemical additives are often added to solution during the scrubbing process in order to clean the soils (as shown above in Figure 5). The soils are mixed vigorously with the solution and then settled out (US EPA 1993).

 

Process Water Treatment

The washwater used in the soil scrubbing unit will be contaminated and must be treated. Some of the contaminants that could be present in this washwater include (US EPA 1993):

  • Some coarse and fine grained soil
  • Dissolved salts
  • Leaves, twigs, and roots
  • Dissolved or solubilized heavy metals
  • Hydrocarbons or other contaminants on site

 

Treatment must be done for the washwater in order to be reused in the soil washing process or to be disposed of in sewers (although disposing requirements are more stringent, making the recycling of the water into the system the preferred choice as long as it does not interfere with the washing process). The most common types of treatment of the washwater are (US EPA 1993):

  • Neutralization
  • Carbon treatment
  • Ion exchange
  • Flocculation
  • Sedimentation and thickening
  • Dewatering
  • Volatile organics stripping

 

Residuals Management

The amount of residual material that is output during a soil washing process depends upon the grain size distribution of the original material.   Contaminated fine grained soils and sludges may be disposed of in a landfill or, if they are still considered contaminated by regulations, may require further treatment before disposal. This further treatment could include (US EPA 1993):

  • Incineration
  • Low temperature thermal desorption
  • Chemical extraction/dechlorination
  • Bioremediation
  • Solidification/stabilization
  • Vitrification

 

Contaminated feed material could contain leaves, twigs, roots, or grass that must be removed since it is likely contaminated due to the vegetative materials porous and adsorptive behavior. The outcome of soil washing will also produce clean soils that can be used as backfill at the site. These soils may require additional washing or cleaning prior to placement (US EPA 1993).

 

Overall, there are many different soil washing systems that have been developed and the systems can vary from site to site due to location specific constraints for soils or contaminants. Soil washing is often also used in conjunction with another remediation technique since it can be used effectively to concentrate the contaminants down into a smaller volume of soil which then could be easily treated by another technique (US EPA 1996, US EPA 1993, Sharma and Reddy 2004).

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