Ex-situ soils washing, as its name suggests, involves the extraction of contaminated soil from the ground for processing or washing. A leased mobile plant can process the soil on the contaminated site itself, or static plants may be permanently based at specialist landfill sites. Where typically applied, the process can generate 80-90% clean, re-usable soil and only 10-20% contaminated material for disposal in a licensed landfill site. This means that instead of extracting, dumping and re-filling a site with clean soil, and all the expense that entails, site owners can opt for a processing plant which leaves a relatively small quantity of contaminated waste for disposal.
Factors to be taken into account include size of site, level of soil contamination and time available to clean the site.
Ex-situ soils washing is a water-based, volume reduction process whereby hazardous contaminants are extracted from the soil and concentrated into a small residual portion of the original volume. It relies on the well-documented fact that contaminants have greatest affinity for the fine particles – silts and clays – in a soil. This principle applies to a wide range of contaminant types, from heavy metals such as lead, zinc and mercury, to hydrocarbons and organochlorines. The washing process involves transfer of the contaminants and their hosts from the soil to the wash water and their subsequent removal therefrom. The technique can be applied successfully where contamination is complex with a cocktail of different contaminants being encountered.
The soils washing process can be divided into six distinctive steps:
No long-term monitoring is required. Once processed, analysis of cleaned material and validation of the excavated site will demonstrate final compliance with the standard of remediation laid down by statutory bodies.
Faced with a problem of unsaleable land contaminated by heavy metals such as mercury, zinc and asbestos and posing a threat to local groundwater, a client had two main options: removal, dumping and refilling the site, or soils washing. Soils washing proved to be the better alternative both economically and environmentally. A Linatex soils washing plant installed in March 2000 processed in excess of 50,000 tonnes of contaminated soil, of which 93% was cleaned and reinstated. Only 3,700 tonnes required disposal in a specialist landfill site. Once successfully cleaned, the site was validated by the Environment Agency as suitable for development.
A more complete understanding of soil washing can be acquired from a review of the composition of the soil and the typical distribution of contaminants.
Picture 1, A) Minor surfical contamination – physical adhesion resulting from compaction. Can be removed and transferred to wash water by mechanical scrubbing. B) Mineral silts and clays are principal hosts for absorption of hazardous contaminants because of their very high surface area to volume ratio and their strong cohesion properties.
Picture 2, C) Solid organic matter such as roots, leaves etc, has a high affinity for the collection of contaminants because of absorbent properties. D) Water-borne contaminants are present in suspended silt and clay particles and in solution.