Since 1854, the 19-acre former gasworks site at Basford has been associated with the gas supply industry, providing employment, lighting and energy to the Nottingham area. It was the principal gas production facility for the City of Nottingham and also provided valuable by-products including coke, motor benzol, sulphuric acid and ammonium sulphate fertiliser before the works ceased producing gas in 1972. For many years the site continued to be used as a depot and training facility for British Gas.
Basford is the first large-scale use of soil washing technology for remediation in the UK.
Preliminary investigations of the site revealed that the soil was predominately granular. Working closely with the Environment Agency and the Environmental Services Department of Nottingham City Council, and taking into account the results from early site investigations, National Grid decided that soil washing could be the most appropriate way to clean up the site.
Prior to excavation, an intensive site investigation was undertaken to characterise the soils in situ on a 10m by 10m grid across the site. From the data recorded, a ground model was constructed and the excavated materials classified into eleven categories on the basis of physical and chemical characteristics, and the likely processes required to treat them.
The soil washing process works on the principle that most contamination, particularly of an organic nature, coats the soil particles and that contamination tends to be concentrated in particles with a high surface area to volume ratio, such as clay and silt. Soil washing separates finer material from coarser and scrubs the surface of larger particles, leaving a contaminated fine fraction plus a clean sand and gravel fraction. The process also floats off organic material within the soil, such as wood fragments and peat.
Although soil washing is at the centre of the remediation project, the overall philosophy is one of waste minimisation and recycling wherever possible. For example, significant quantities of ash and clinker have been recovered for use in steel and breeze block manufacturing processes.
Being a waste minimisation technique, soil washing substantially reduces the number of lorry movements to and from the site. At Basford this reduction was from 23,000 to 8,000 movements. After verification testing the cleaned material is used to backfill the excavations, thus reducing the need to import clean fill. Only contaminated soil not suitable for processing and residues from the process are removed for disposal.
