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Mobile plant cleans Belgian clay pit toxins

MOBILE, MODULAR plant for rehabilitating contaminated landfill - claimed to be the world's first - has been put into operation at a former terracotta clay pit near St Lenaerts Brecht, Belgium.

The outfit has been designed and run by Belgian soils cleaning company Soils NV (part of dredging group DEME). Polluter pays legislation introduced in 1995 means that remediation of contaminated soils is now 50% cheaper than dumping. However, because the contamination at St Lenaerts Brecht, known as 'Terra Cotta', is historical, the BEF700M (£11.7M) clean-up is being funded by the state.

Static plant to remove toxins and heavy metals is increasingly common in Belgium. Soils runs a remediation centre at nearby Kallo which processes material from as far away as northern France and the Netherlands. However, a system that can be located on the contaminated site has not been available before.

Soils developed the BEF180M (£3M) mobile plant to tackle nearly 50 years of uncontrolled dumping. The 30ha Terra Cotta site is contaminated with a mix of domestic and industrial waste, heavy metals, oil and petro-chemicals, and hydrocarbons. 'There is a long term public health issue', says Soils managing director Luc Ponnet. Unless removed, toxins would in time have leached through the clay sub-stratum to contaminate an aquifer supplying Antwerp and region.

Of the 200,000m3 to be excavated, half will be refilled after cleaning. The plant has a handling capacity of 60t/h and after treatment, toxin concentrations are just above legal background levels set to German DIN standards. Because of long term subsidence, however, the site will be landscaped and reserved as a public green space.

Development of Soils' 'integrated recycling plant' took six months, but draws on nearly 10 years experience in the industry. It is run by five operatives with the aid of purpose-designed computer software that monitors the kind and degree of contamination entering the system. According to plant manager Rudy van Driessche the system combines processes and off-the-shelf components common in the dredging and mining industries. 'The skill here lies in their integration,' he says.

After input, material is sieved, and particles larger than 50mm are crushed. Plastics and larger organic parts are removed, and after re-sieving ferrous metals are electromagnetically extracted. Phase two adds water, where lighter elements are removed from the surface and stones washed. Another sieving separates remaining light particles. In the third phase silt and sand are washed, and sand removed. Additives can be added to neutralise contaminants. All cleansed materials are either used as re- fill or recycled as building or landscaping products.

In phase four a rotating screen removes the silt fraction and the remaining slurry is dewatered and belt-pressed. Finally the departiculated water is immobilised and recycled.

Because it is a 'kit of parts', the installation can be tailored to meet conditions in different places. Soils is hopeful it can redeploy the St Lenaerts Brecht plant on a project at Zagreb in four years time. The company is also looking at sites in Belgium in neighbouring European countries.

Andrew Mylius

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