Deep cleaning with down-to-earth drill: Britain tries a safer way to treat polluted industrial sites

A TECHNIQUE for cleaning up contaminated land that removes the risk and is cheaper than other methods has been used for the first time in Britain to reclaim an oil-shale refining site near Edinburgh.

The technology, called Mectool, involves drilling into contaminated soils and sludges while injecting chemical stabilisers and cement. At ground level the process is covered by a metal dome that traps the drilling dust and prevents air pollution.

Traditional methods of cleaning up polluted industrial sites, like this one at Pumpherston, West Lothian, owned by BP, involve either transporting the polluted soil to a landfill or treating the pollution on site by excavating the contaminated soil, treating it, and then backfilling the stabilised material.

'Using Mectool you don't have to dig up any of the contaminated land, which could generate dust and other hazards; the remediation process takes place underground,' says John Conway, remediation manager at UK Waste, a subsidiary of Waste Management International, which carried out the restoration.

Mectool was developed in the United States by Waste Management's parent.

Mr Conway added: 'Although safety is the primary advantage of using Mectool it does have economic advantages because it eliminates the costs involved in handling and transporting contaminated material and dumping it in a landfill.'

Because all the work is done underground, clean-up operations are not affected by the weather. When open excavation methods are used, dry, dusty conditions and high winds may increase exposure of staff and the environment to pollutants.

Oil-bearing shales were processed at Pumpherston from the 1880s to 1963. About a third of the total volume of rock remains after the oil is removed. This residue includes hard tars containing carcinogenic hydrocarbons that could be released in dust if the material were excavated.

Mectool can bore and mix a column of soil up to 30m (100ft) deep. The drill has a hollow stem for injecting the stabilising reagents into the ground, rather like adding ingredients through the top of a food processor. Sensors in the device control the rate of treatment and application of reagents and are linked to personal computers that continuously monitor treatment and provide documentation. The boring and mixing tool can be used for adding all types of reagents.

Mr Conway says a sample of the contaminated material is analysed in the laboratory in order to work out which reagents are needed to make the pollutants safe.

Because the top of the boring tool is shrouded, some pollutants can safely be drawn off for treatment. For example, volatile pollutants can be heated to vapourise them and the vapour collected inside the canopy.

A range of different treatments can be applied to the affected area using the same equipment. Apart from solidification and stabilisation, the equipment can also be used for bioremediation, in which bacteria are added to metabolise and break down contaminants.

Bioremediation usually involves removing the soil, adding the bacteria, and then turning the soil regularly to ensure the bacteria have an adequate air supply. This process takes six to 18 months to complete. Using Mectool, bacteria are added and the soil aerated while remaining on site.

In cases of extreme or untreatable contamination, Mectool contains the pollution by ring-fencing it with concrete columns, sunk down to a non- porous clay or rock base, preventing pollution seeping out.

'No one treatment method will solve all the problems at any one site,' says Mr Conway. 'This is another advantage of Mectool, which uses the same equipment to administer different treatments.'

Sites such as those at Pumpherston could never be restored to their natural state, allowing them, say, to be used for agriculture. But Mr Conway says the treatment makes them good for building on, because the added concrete makes the ground very stable.

Alternatively the site could be landscaped and used for recreation. Although no decision has been made on what to do with the Pumpherston site, there is a nine-hole golf course looking for another nine holes on the adjoining land.

(Photographs omitted)