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Treatment of Contaminated Land by Soil Washing

Soil washing for contaminated landSoil washing has become an important method for cleaning up contaminated land, and it was first propelled into the main stream as a process for this purpose in the summer of 2004. The reason for this was that the summer of 2004 saw the implementation of EU Landfill Directive requirements which slashed the available number of UK landfill sites available for all forms of waste disposal, and for the first time the disposal of soils from contaminated land to non-hazardous landfills was banned.

The cost for disposal of contaminated soils to landfill rose massively, and suddenly it made sense economically to carry out in-situ methods to clean up soils, with methods like soil washing, rather than transport them to the much more expensive landfills.

Then, in the summer of 2005 the rules tightened further, with the requirement that no wastes shall be sent to hazardous waste landfill in excess of 6% organic matter (carbon), and the new Waste Acceptance Criteria require that all wastes sent to hazardous waste landfill shall have been pre-treated.

Prices for waste disposal to hazardous landfill rose dramatically, to above £100/m3, and frequently in the region of £150/m3, and that, or higher, is where they will now stay.

The result is that since that time, the contaminated land remediation industry has been, wherever possible, using alternative methods to landfill disposal to achieve the clean-up of contaminated land, and one of those methods is “Soil washing” in order to treat contaminated waste in-situ.

“Biopiling” is another option, where the contaminant is biological.

Soil Washing Requirements

Soil washing requires a reasonably permeable soil with a fines percentage normally below about 30%. Large material is screened out using a suitable mesh size.contaminated-land-excavation02

As the name suggests the primary aim is to wash the granular fraction of the material through a water-wash solids scrubbing unit. Organic materials are removed, the clean larger particle sizes (gravels and sands) are washed, allowed to settle, separated and removed, Further sieving may be applied, and the result is that the washed clean sands and gravels are now non-hazardous and can be returned to the land.

The remaining silts and clay materials containing the contaminants are further treated, normally by coagulation and flocculation to achieve settlement in a lagoon, and the resulting sludge must then be dewatered to become a solid again.

Sludge presses, hydrocyclones and/or centrifuges are used for dewatering and a substantially reduced volume  of sludge “cake” remains, for which there is normally no option but to send it to landfill, or possibly in some circumstances to solidify in concrete, or incinerate.

Most of the water used is recycled, but some will require treatment and/or tankerage to suitable disposal at an industrial effluent treatment facility.

Regulatory Problems Persist

Complications which have to be overcome in order to treat contaminated wastes on-site include the complexities entailed in applying for Waste Management Licenses, and possibly PPC permits for these works, before work can proceed. It is not possible to obtain the necessary permissions just once for the process. A new application must be made for each site on a site specific basis.

Furthermore, even after the contaminants have been removed the material remains classified as a “waste”. Waste can only be disposed to licensed landfills, which is hardly the intention of on-site remediation, so bureaucracy demands that a Waste Exemption certificate also be applied for and issued by the Environment Agency (England).

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