Soil Washing: Advanced Remediation for Contaminated Soils

Introduction to Soil Washing Principles

Soil washing is a remedial process whereby contaminated soils are “washed” with water to remove the contaminant loading. This effective technique removes contaminants from soil through two primary mechanisms:

Dissolution and Suspension: Contaminants attached to soil particles are dissolved or suspended in wash water. The wash water can be chemically enhanced with pH adjustments, surfactants, or other additives to improve cleaning efficiency.
Concentration and Separation: Soil washing isolates contaminants by concentrating them into a smaller volume of fines (clays, silts, and organic matter) while separating out “clean” sand and gravel particles.

Contaminants often bind to fine soil particles, which adhere to larger sand and gravel particles. By separating these fines, soil washing reduces the volume of contaminated material requiring further treatment or disposal.

The effectiveness of soil washing is highly dependent on the soil composition, particularly the percentage of fine particles. Soils with more than 25% fines are less efficient for soil washing, whereas granular soils, such as sandy or gravelly soils, are better suited to this process. Soil washing is best suited to the treatment of larger volumes of impacted soils as the specialist plant required generally mean it is not cost effective for small soil volumes.

Contaminants Addressed by Soil Washing

Soil washing is a versatile technology capable of treating a wide range of contaminants, including:

Diesel Range Organics (DRO)
Petroleum Range Organics (PRO)
Volatile Organic Compounds (VOCs)
Semi-Volatile Organic Compounds (SVOCs)
Heavy Metals (e.g., Lead, Chromium)
Pesticides

This process is particularly effective in reducing the volume of contaminated soil, making it an essential technique for soil remediation projects.

Soil Washing Plant Description

The soil washing process begins with excavation and soil processing to remove oversized objects such as rocks, debris, and metals. Once processed, the contaminated material undergoes a series of treatments to separate contaminants:

Soil Scrubbing Unit: Soil is vigorously mixed with wash water to dislodge contaminants. The addition of pH buffers or surfactants can enhance the washing process.
Separation of Coarse Materials: Gravel and sand particles are separated as the “clean” fraction, often suitable for reuse on-site.
Fine Particle Processing: The remaining sludge, containing fines and contaminants, is further sieved to isolate heavy metal compounds. The fines are then dewatered in preparation for additional treatment.

Water Treatment: An integral part of soil washing, water treatment systems like those used in pump and treat solutions are essential to clean and recycle the wash water.

Example of Soil Washing in Action

VertaseFLI’s Innovative Soil Washing Project

VertaseFLI successfully completed a soil washing project during the reclamation of a former landfill in South Wales. This large-scale project demonstrated the efficiency of soil washing in reducing contaminated material volumes while recovering reusable materials.

Key Project Outcomes:

360,000m³ of industrial and household waste, along with contaminated soil, was excavated and processed.
Over 325,000m³ of material was cleaned and deemed suitable for reuse.
Approximately 15,000m³ of soil was isolated for further treatment via bioremediation and solidification/stabilisation.
31,000m³ of non-treatable waste (e.g., plastics, fabric) was disposed of at licensed facilities.
Recovered 250m³ of asbestos-containing materials for safe disposal.

The remaining materials were recycled on-site, contributing to the sustainable restoration of the landfill area.

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Advantages and Limitations of Soil Washing

Advantages:

Volume Reduction: Significantly reduces the volume of contaminated soil requiring further treatment or disposal.
Cost-Effective: By isolating contaminants, the cost of treatment or disposal is minimised.
Wide Applicability: Suitable for treating a broad range of contaminants.
Reusability: Clean fractions such as sand and gravel can be reused on-site for engineering fill or restoration projects.

Limitations:

Soil Composition Dependency: Less effective for soils with more than 25% fines.
Water Usage: Requires large volumes of wash water, necessitating robust water treatment solutions.
Concentrated Sludge: Generates contaminated sludge that may be challenging to treat or dispose of.
Site Space Requirements: Soil washing plants may require significant space, particularly for stockpiling and processing.

All works exceeded all expectations of our QHSE Team. Would highly recommend VertaseFLI for all ground remediation works for the future.

Louie Sharp

Civil Engineer, Vistry Northern Home Counties

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