A Pioneering Remediation Technology

In this article, Geoff Jennings, Systems Manager - Busch UK, discusses a recent pioneering innovation made in soil remediation technology by AIG Engineering Group at the UKAEA site in Harwell Oxfordshire.

From the industrial revolution right up to the middle of the twentieth century, the level of potentially harmful waste dumped into the ground has left us with a costly legacy.

It is not often you hear the words ‘fortunate’ and ‘legislation’ used in the same sentence, but the fact that there are ever-stricter laws covering the decontamination of the remains of our industrial past is indeed fortunate. Society can not only redress the ignorant approach that our ancestors made to disposal of industrial waste – but also utilise these brownfield sites. And political pressure for developing such sites here in the UK has never been greater. In recent years, it has been estimated that, England alone, available brownfield sites account for an area equivalent in size to the West Midlands conurbation.

When looking at the options for these sites – finding a cost effective means of removing contaminants is essential.

Traditional methods of cleaning up contaminated land vary from the ex-situ methods that involve the excavation of the contaminated soil – often referred to as “dig and dump” – to the in-situ methods that remove the contamination from the surrounding soil whilst it is in place.

Soil vapour extraction (SVE)

Soil vapour extraction (SVE) is a quick and effective in-situ process for soil remediation where the contamination is removed by carrying it out through a medium such as air. Once extracted, the vapours are separated into liquids and vapours with each stream being treated according to the nature of the particular contaminant. This process is suitable for removing a variety of contaminants that have a high vapour pressure or a low boiling point compared to water, such as chlorinated solvents.

SVE can handle high concentrations of contaminant, including contaminants in the form of a non-aqueous phase liquid (NAPL).

Albeit successful, it was discovered that SVE could be improved by the addition of heat into the area surrounding the extraction well.

A UK First

In what is believed to be a UK first, AIG Engineering Group (AIGE) has successfully applied a new thermally-enhanced soil vapour extraction process to clean-up contaminated land at the UKAEA Harwell site in Oxfordshire.

The Western Storage Area (WSA) at UKAEA Harwell covers an area of 1ha. It was originally licensed for the disposal of various chemical wastes including chlorinated solvents and other organic chemicals which were poured into a number of pits excavated into the underlying chalk.

In disposing of these wastes in this manner, over time, chemical contamination has occurred in the unsaturated zone beneath the pit area and groundwater in the chalk aquifer. During 2004 the pits were excavated to just below the original depth and the contents disposed of.

As part of the development of the remediation technology needed, AIGE worked closely with vacuum specialist Busch UK to develop a robust and portable extraction plant that would be capable of dealing with a variety of site conditions.

UKAEA has an ongoing programme of restoration at the site and AIGE were commissioned to provide remediation of the unsaturated zone beneath the original pits.

The chalk at the WSA site is a heterogeneous fractured rock with a dual porosity. The rock matrix itself has a relatively low effective permeability but high porosity (25%) and retains contaminated pore water – which would act as a long-term residual source if left untreated.

The Process

During the treatment, an in-situ conductive thermal process was used to heat the target zone to around the boiling point of water (100°C).

This heat mobilises contaminates by vaporising them and a vacuum system draws them into the SVE well, along with ground water. Once extracted, the two components are separated and dealt with accordingly.

This process of conductive heating and vacuum was applied to depths of between 6m and 18m using an array of vertical heaters nested around the SVE well.

The selection of the vacuum system was crucial to the project’s success, particularly as it needed to meet the ATEX regulations for explosive atmospheres and deal with harsh working conditions.

Busch UK was chosen to supply a vacuum system packaged inside a six metre-long container that could be easily moved around the various sites.

Simon Dockerty, process and systems manager with AIGE, explained: “This thermal process is far more aggressive than conventional soil vapour extraction methods and as such is highly effective at removing contaminants; they are in fact vaporised by the heat.

”This, coupled with the high level of vacuum supplied by the Busch vacuum system, means that it can significantly speed up decontamination of a site, making it much more cost effective.

The new thermally enhanced SVE process has proven suitable for a range of volatile and semi-volatile contaminates (not all at UKAEA Harwell) that include: DNAPLs, PCBs, Dioxins, LNAPLs, Pesticides, Chlorinated Solvents, Tars, PAHs and Explosives Residues.

Busch UK

Busch (UK) Ltd was established in 1971 and supplies a comprehensive range of vacuum pumps, low-pressure compressors and systems, which are designed and built to meet the individual needs of its customers. The Telford base also features a full service and support centre.

With more than 1,800 employees working in 48 manufacturing and operations facilities located around the world, Busch Vacuum Pumps & Systems was founded in Germany in 1963 by Dr-Ing Karl and Mrs Ayhan Busch.

AIG Engineering

AIG Engineering Group is sole provider of this innovative technology in the UK and can provide case studies from both the UK and the US.

The company is owned by AIG Global Real Estate – Part of the AIG Inc Group of companies.