The application of soil washing to the remediation of 

contaminated soils

Mike Pearl, Marc Pruijn and Jan Bovendeur

Abstract

Soil washing is an ex situ treatment technology for the remediation of contaminated soil. It has been applied to a variety of inorganically, organically, and even radioactively contaminated soils. Although it is a well established technology in continental Europe and North America, there are very few applications in the UK. However, the picture in the UK is changing as the Landfill Regulations 2002 start to bite – particularly with respect to the rising costs of disposal of contaminated soil.

Soil washing is a volume reduction/waste minimization process in which the majority of the contamination in soil is removed as a small fraction of the original soil volume. The remaining ‘clean’ fraction of the treated soil can then be: (i) recycled on the site being remediated, as relatively inert backfill; (ii)

used on another site as fill; or (iii) disposed of relatively cheaply as non-hazardous material. It should be remembered that the ‘clean’ fraction is not totally contaminant-free, so for soil washing to be successfully applied, the contaminant concentration in this fraction should be below a ‘legislative’ trigger

value, or a site-specific limit (e.g. based on risk assessment).

The process equipment used for soil washing comes mainly from the mineral processing industry. Full-scale soil washing plants exist as mobile/transportable plant and as centralized treatment facilities. UK applications are generally of the former type, where equipment is transported to the contaminated site, soils treated, and the clean product reused on the site, whilst only the contaminated fraction is transported to hazardous waste landfill. Although designed in modular units for ease of assembly and disassembly, mobile plant operations require additional mobilization and demobilization activities compared to centralized plants. These additional mobilization and demobilization activities can significantly add to the overall operational costs. Thus it is uneconomic to treat relatively small quantities of soil (e.g. <5000 m3) with mobile equipment. In comparison, relatively small volumes of contaminated soil can be economically treated in centralized plants which avoid many of the mobilization and demobilization costs. However, centralized plants require all the contaminated soil from a site to be transported to the plant, and the associated transport costs can be considerable.

Not all soils are amenable to treatment by soil washing. Potential application of the technology is usually ascertained by laboratory treatability tests. Most of these are based on particle separation techniques (based on differences in grain size, specific gravity, surface chemical properties and (rarely) magnetic properties) where the separated fractions are chemically analysed for the contaminants of concern to ascertain whether ‘clean’ fractions can be produced and to determine their proportion relative to the mass of the original soil.

As a rule of thumb, soil washing can potentially be applied to soils where the clay and silt contents are less than 30–35 weight % of the soil.

Key words: contaminated soil, remediation, soil washing

© 2007 EPP Publications Ltd

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