Soil washing or soil scrubbing is a water based process for remediation of excavated soils. Soil washing removes contaminants from soils two ways:
1. By dissolving and/or separating, suspending contaminants on soil particles into the wash water, wash water can be dosed with chemicals to improve the washing characteristic (such as pH adjustment, surfactants, etc),
2. By concentrating contaminants into a smaller volume of soil through particle size separation and attrition scrubbing.
The majority of inorganic and organic contaminants generally bind, either physically or chemically to fine particles (clays, silts, organic matter). These silts and clays in turn are attached to sand and gravel particles in the soils. Soil washing separates the contaminated silts and clays from the "clean" sands and gravels, thereby reducing the volume of contaminated material requiring further treatment or disposal.
The most important factor affecting the soil washing process is the percentage of fines (particles with a diameter less than 0.063mm) in the soil, if the percentage of fines is high then there will only be a small volume reduction in the amount of contaminated material and the efficiency of the soil washing process will be low.
Generally it is considered that if the fine content of the soil is above 25% then soil washing will not be effective. More granular soils are better suited to soil washing than cohesive or semi cohesive soils.
Soil washing has been shown to be able to remediated/reduce the volume of contaminated soil, contaminated with:
- Diesel range organics (DRO)
- Petroleum range organics (PRO)
- Volatile organic compounds (VOCs)
- Semi- volatile organic compounds (SVOCs)
- Heavy metals (lead, chromium etc)
After excavation contaminated material undergoes soil processing to remove over sized objects. Contaminated material then passes to the first stage of the process, a soil scrubbing unit. Wash water and soil are vigorously mixed, pH buffering and dosing with surfactants can be included to improve soil washing.
Free phase contaminants removed from the soil by the mixings and the 'dirty' water are separated from the soil and treated, the water can then be recycled. The coarser "clean" fraction gravels are separated at this stage and are usually suitable for reuse on site.
The remaining mixture of water, fines (sludge) can be further sieved to remove heavy metal containing compounds, the sludge then under goes dewatering in preparation for further remedial treatment.
Water treatment is also an essential part of soil washing and modular plant similar to those described in the pump and treat section of our website are commonly used.
Commonly used equipment in soil washing includes:
- Soil processing (screens, sieves, trommels, etc)
- Shear mixers
- Dewatering units
- Counter current washers
- Control systems
- Water tanks
- Free product recovery tanks
- Dosing pumps
- Water treatment plant
Advantages and Limitations
- Reduction in contaminated soil volume requiring further treatment (cost saving)
- Can treat a wide range of contaminants
- Proven track record in sandy/gravely soils
- Dependent on fine content less than 25% by weight
- Requires a water treatment plant
- Produces concentrated contaminated sludge (potentially difficult to remediate or dispose)
- Can require large volumes of wash water