From: Public Services and Procurement Canada
Ex situ soil washing, also known as solvent or chemical extraction, combines technologies that involve mixing contaminated soils with an extraction solution to dissolve or mobilize organic and inorganic contaminants, such as organic compounds and heavy metals from soils to the extraction solution.
Before the washing process, contaminated soils, sediments or sludge are excavated and generally separated according to their particle size. Subsequently, the contaminated soil, sediment or sludge is washed with an extraction solution that allows the extraction of contaminants until the desired concentrations are reached. Soil washing can be carried out using various agents selected according to the contaminants present. The extraction solution and the contaminants are then recovered and the active compounds (surfactant, alcohol, etc.) are recycled. Soils, sediments or treated sludge can then be returned to the site if the site-specific environmental criteria are met.
This technology Doesn’t destroy contaminants, but solubilizes, mobilizes, suspends and concentrates them in a reduced soil volume or extraction solution.
Soil washing can be done using a mobile washing unit that can be used directly on the site or at a local facility that can perform such treatment.
A soil washing unit, including the extraction solution treatment system, may be temporarily constructed directly on-site, or the contaminated materials may be transported to a centre with the soil washing facilities. In both cases, the contaminated soils, sediments or sludge are first segregated into grain size classes (the contaminants are usually concentrated in the finest grain size classes) and are inserted into a cell and combined with an extraction solution, which usually flows counter currently. The extraction solution can then be recovered by distillation and reused for further processing. When implementing the treatment system, it is important to consider the system's water and air management.
The implementation of a soil washing system may include:
The treated soil, sediment or sludge must be analyzed following the treatment, in order to evaluate the remaining contaminant concentrations and dispose of them appropriately. Soils that meet site-specific environmental criteria can be returned to the site.
A significant amount of water is used in the process and must be treated. Residues that are generated during this process may be considered hazardous materials.
Atmospheric emissions resulting from treatment may also be released from the treatment system.
Notes:
Laboratory tests are recommended to determine optimal extraction solution composition, contact time and treatment cost. It is also important to analyze the impact of temperature and pH on the extraction reaction.
Ex situ soil washing is not recommended in northern regions because the treatment system requires a certain level of equipment maintenance and residue management, which may be more difficult in an environment where access to public services or local labour is limited. In addition, the cold climate in northern regions could cause equipment breakage.
Remote sites also require greater mobilization, resulting in higher on-site monitoring costs. Equipment availability is limited, and work windows are relatively short. The volume of contaminants to be treated must be significant to justify the mobilization of a treatment unit.
None.
Soil washing Doesn’t transform contaminants, but rather concentrates the contaminants within the washing solution or a soil fraction. Treated soil must be analyzed and the extraction solution must be treated and disposed of appropriately.
The complex mixture and heterogeneous composition of contaminants in the material may complicate the formulation of an appropriate extraction solution to effectively remove the different types of contaminants. A sequential wash that uses different extraction solution formulations or different material ratios to be treated, may be necessary in this case.
Ex situ soil washing technology can be combined with many other remediation technologies such as free phase separation and removal or vacuum/distillation methods. Soil washing is generally preceded by soil particles size separation, which reduces the amount of soil to be treated.
Many private companies have developed their own soil washing procedures and extraction solutions. Many offer examples of their technologies.
The following sites provide application examples:
Ex situ soil washing is an effective remediation technology for a wide variety of contaminants. The performance of this technique depends upon the characteristics of the extraction solution and the soil to be treated, as well as the types of contaminants present in the soil.
Main exposure mechanisms
Applies or Doesn’t Apply
Monitoring and mitigation
Dust
Applies
Monitoring conditions favourable to dispersion during the excavation of the soil to be treated.
Atmospheric/Steam Emissions—Point Sources or Chimneys
Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements])
Atmospheric/Steam Emissions—Non-point Sources
Doesn’t apply
N/A
Air/steam—by-products
Runoff
Monitoring of stored materials, and of favourable conditions for runoff.
Groundwater—displacement
Groundwater—chemical/ geochemical mobilization
Groundwater—by-product
Accident/Failure—damage to public services
File checks and licensing prior to excavation, development of excavation and emergency procedures
Accident/Failure—leak or spill
Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions
Accident/Failure—fire or explosion
Other—Manipulation of contaminated soils, sludge, sediments or other solids
Other—Production of hazardous materials
Other—Handling of toxic materials
Composed by : Serge Delisle, Eng. M.Sc., National Research Council
Updated by : Karine Drouin, M.Sc., National Research Council
Updated Date : April 16, 2013
Latest update provided by : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée
Updated Date : March 22, 2019
