CL:AIRE comes of age

CL:AIRE is 18 months old. The Contaminated Land: Applications in Real Environments Technology and Research Group has approved two technology demonstrations and one research project. The first demonstration, involving a field trial of low temperature thermal desorption (LTTD) technology, was reported previously in IEM - November 1999 and April 2000. Here, the following two.

200,000 tonnes of washing

This project involved the clean-up of ammonia, phenolic and poly aromatic hydrocarbon (PAH) contaminated soil at the Basford Gasworks. The work was completed in 1998. BG Property Holdings Ltd, working closely with the Environment Agency and the Environmental Services Department of Nottingham City Council, selected soil washing for the clean-up, based on a comprehensive site characterisation programme. The objectives of the cleanup were to:

  • remove contaminants and optimise re-use of treated soil;
  • improve safety conditions in the area by reducing vehicular traffic to and from the site; and
  • reduce the need for landfill and primary aggregates.

A pilot study was conducted on the site to select and scale the most appropriate unit processes. Output from the study was then used as a specification for the unit processes required to treat the materials encountered on site to a standard at least equal to the site contamination action values agreed with the Agency.

Substantial saving

Full scale clean-up took place during 1997 and 1998, resulting in the treatment of 200,000 tonnes of contaminated material. The soil washing programme at Basford was the first application of soil washing on such a large scale in the UK, with approximately 76% of the total excavation volume recovered for re-use – at 200,000 tonnes, a substantial saving in landfill void space and primary aggregate production.

A reduction in lorry movements brought wider environmental benefits in terms of dust, noise and fumes. Over 14,500 lorry movements were saved, compared to traditional dig and dump techniques. This saving, of nearly 700,000 miles, corresponds to approximately 86,000 gallons of diesel and associated air emissions.

Whilst the primary aim of the project was to remediate the site by waste minimisation methods, it is important to consider the amount of energy consumed in this approach. In this case, soil washing consumed approximately 30% less energy than digging and dumping. The basis of the difference is mostly due to the relative energy consumptions of the on-site processing plant compared to road haulage, showing that materials recovery can be energy efficient as well as attracting more obvious environmental benefits.

Spatial uncertainty in pollutant movement

Dr Fred Worrall of the University of Durham has obtained funding from the Engineering and Physical Sciences Research Council (EPSRC) to explore the variability of parameters controlling pollutant movement through contaminated sites, as a means of enhancing risk assessment and so improving risk management.

One of the dominant chemical controls on the transport of organic pollutants to groundwaters is adsorption, or the attraction between the pollutant and other particles in the subsurface, which retards pollutant movement. The adsorption of organic pollutants is largely controlled by the presence of naturally occurring organic matter. Adsorption coefficients (Koc), which are normalised to organic matter, have been reported for a range of organic pollutants as constant values and are used extensively in fate and risk models.

Orders of magnitude

Yet the Koc value for any one compound is far from constant and values can vary by orders of magnitude. If we are to model pollutant transport from contaminated sites then we must understand, and hence be able to predict, the variability in adsorption parameters. Moreover, this prediction must be a practical possibility for consultants and contractors who work in the field.

The project will be carried out over a three-year period and will use samples from several CLAIRE sites. The results, it is hoped, will: improve site investigation methodologies; develop risk-based management and remediation of contaminated sites based on a greater understanding of uncertainty and variability in pollutant movement to controlled waters; give added value to existing regulatory transport models; and deliver a user-friendly technology validated against well-characterised field sites.

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