It is the first time the technique has been used on a large scale in Britain, and tomorrow a gigantic Anglo-Dutch earth- washing machine is formally opened at the Basford works in the north of the city.
The tar-stained soil, contaminated with a wide range of toxic by-products from town-gas production in the past, is being extracted across an area as large as several football pitches, down as deep as 16 feet in places. It is then screened, crushed and washed.
Most of the contamination coats only the surface of the soil particles, and tends to be concentrated on the clay and silt fragments which are the smallest. The washing separates this finer material out, and scrubs the hazardous chemicals off the larger pieces of sand and gravel.
The water is continuously recycled, with the contaminants accumulating being extracted as a tarry paste. This has to be taken away for dumping in landfill sites along with the dirty silt and clay which remains. But about 80 per cent of the soil is left clean and can be spread back over the gasworks site, allowing it to be redeveloped by its owners British Gas Property. The company hopes that the pounds 4m washing project will work out cheaper than the main alternative - removing all the contaminated soil to a landfill site and importing fresh, clean material to replace it. But at another of its old gasworks in Sheffield, the company is testing the use of bacteria to break down polluting chemicals in the soil.
- Nicholas SchoonReuse content