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SOIL TREATMENT TECHNIQUES

TECHNICAL & PRACTICE

There are a number of soil treatment techniques suitable for a range of conditions, most of which tend to be cheaper than landfill - depending on quantity and location. These include selective excavation and oxidation, soil washing, biological or thermal treatment, etc. Bizarrely, in many instances, excavating contaminated land and taking it to landfill, requires a higher degree of soil treatment than if it had been left in situ.

Soil washing Soil is removed to a treatment bath where it is mixed with water (sometimes with leaching agents, surfactants, etc) to form a liquid mulch. The fine soil particles are separated by settlement.

Thermal treatment Soil is taken to a treatment area that can be set up on site or in a dedicated facility, depending on soil quantities or contamination levels. The soil is heated so that organic contaminants and water within the mix evaporate and chemically join with a gas or vacuum system that takes the contaminant materials away.

High temperature techniques operate at around 320 to 560ºC Low temperature operate at 90 to 320ºC Biological remediation In a similar way that compost heaps transform everyday waste, so biological remediation occurs by microbial action breaking down the otherwise harmful organic material. Adding liquid and/or air will speed up cleansing action whereby the microbes remove or detoxify pollutants. These can be neutralised (usually transformed into carbon dioxide and water) or safely bonded to other soil materials.

Again, this process is generally carried out in dedicated facilities in which excavated soil is placed, alongside suitable humidity and temperature controls sufficient to encourage metabolic action. Toxic metals are not susceptible to bacterial degradation, but can be isolated in the soil. However, more than 30 different types of oil-degrading bacteria and fungi can be assigned to degrade hydrocarbon. Given that petroleum or petroleum-based materials are known to be the most pervasive site pollutants in this country, biological remediation is one of the more common processes.

Selective excavation This does exactly what it says on the tin. An assessment is made of the particularities of the site and clearly defined areas of contaminated land are dug up and taken away to landfill. As mentioned above, the landfill directive requires dumped material to be treated unless it is taken to expensive specialist contaminated landfill sites. Often, once the Soil Guideline Values have been assessed and the site appraised, the level of contamination permissible for it to remain in situ, is often higher than the level at which it is accepted in landfill sites.

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