Soil washing – a suitable pretreatment process for vacuum distillation during soil remediation

Soil washing – a suitable pretreatment process for vacuum distillation during soil remediation

Soil washing and vacuum distillation are two different methods used for soil remediation. It is because of their differences that they compliment each other where large volumes of contaminated soil need to be remediated in short time.

Soil washing is a chemical-physical process which is commonly used for soil contaminated with heavy metal, mineral oil, tar and other organic pollutants. Soil washing can achieve high throughputs at relatively low operating costs.

The objective of the soil washing is an increase in contaminant loading in a reduced portion of soil. For this purpose the soil is mixed in water to break up large lumps into individual soil particles. The mechanical forces also separate some of the pollutants from larger soil particles.  They are then partially reabsorbed on smaller particles and partially removed with the wash water. The main cleaning effect is achieved by finally separating the clean coarse fraction from the contaminated fine fraction (< 100 µm). Thus, the contaminated part of the soil can significantly be reduced to 10-30%.

The result is a large volume of clean soil and a smaller volume of heavily contaminated fines. The clean soil can be reused as building material or as backfill material. The contaminated fines have to be landfilled as hazardous waste or treated further.

The VacuDry® vacuum distillation is especially suitable for this purpose. Because the VacuDry® treatment results in clean fines that can also be reused, it is superior to other options such as oxidative-thermal or biological treatment in accordance with the legal requirements of the Recycling and Waste Management Act in Germany. The combination of soil washing and vacuum distillation thus achieves that 100% of the contaminated soil can be reused.

The vacuum distillation is a thermal process and is especially suitable for heavily contaminated materials. The material is heated under vacuum until the boiling points of the contaminants are reached.  The evaporated contaminants are then removed from the heating chamber and collected as condensates. No harmful exhaust fumes are produced in the process. In addition, the vacuum achieves a reduction in boiling points which results in lower heating temperatures and reduced energy demand. The only limitation are heavy metals with the exception of mercury. They cannot be evaporated because of their high boiling point and require additional stabilisation to reduce their leachability.

On the basis of a compact and modular design, the combination of soil washing and vacuum distillation enables a decentral on-site recovery of contaminated soils. Consequently, landfill and transport costs can be saved and high throughput rates up to 20t/ h can be reached at the same time. Among the most efficient remediation methods, this combination is the most environmentally friendly and economic solution for the rehabilitation of contaminated soils.

 

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