SOIL WASHING UNITS

Description

Soil washing is a water-based, volume reduction process whereby contaminated soil or dredgings are separated into a small contaminated fraction and a larger cleaned fraction. The larger fraction represents 70-90% of the original and can be redeposited on site. The concentration of the contaminants into only 10-30% of the original volume means that the scale of any subsequent treatment or disposal operation is much reduced.

Soil washing takes advantage of the greater affinity of contaminants for fine particles. Contaminants are attracted to the surface of soil particles, and so the high surface-area-to-volume ratio of fine particles allows them to hold a relatively high concentration of contaminants.

Typically, soil washing segregates a soil into a small volume of highly contaminated silt and clay and a larger volume of cleaned coarser soil. The segregation is achieved by screening, crushing, attrition scrubbing (grinding the soil particles together to remove the surface contamination), and using hydrocyclones, gravity settlement, dissolved air flotation and mechanical dewatering. Chemical additions enhance the dissolution, transfer and flocculation of contaminants.

Ultimately, soil washing unit will produce the following four fractions: (i) a decontaminated and dewatered granular soil for redeposition, (ii) a dewatered clay and silt contaminant to be treated or disposed of, (iii) a dewatered organic contaminant fraction to be treated or disposed of, and (iv) wash water that can be stripped of contaminants and recycled.

Application

Soil washing is a first-step treatment method for land based soils, such as those associated with gas works, fuel storage, chemical plants and metal finishing, as well as for river and harbour sediments.

Typical hazardous contaminants which can be effectively removed include petroleum and fuel residues, radioactive contaminants, heavy metals, PCBs, PCP, pesticides, cyanides, creosote and asbestos.

The process can be an effective and economical remedial technology when the contaminated soil or sediment contains no more than about 40% silt, and no more than 20% organic content by volume. Soil washing can be justified in the majority of cases with more than 10,000 m3 of contaminated soil. The concentration of contaminants into a small soil fraction reduces any further transport and landfill/incineration costs.

Sizing

A typical soil washing unit sized to wash 25 tonnes of dry solids per hour would have an electrical power requirement of about 100 kW. The electrical requirement is primarily accounted for by pumps, with a smaller proportion needed for drive motors.