Stabilisation and Solidification of Contaminated Soil and Waste Part 1: Introduction to S/S Technology
By Dr Colin Hills, Director, Centre for Contaminated Land Remediation, University of Greenwich
consider long-term performance of stabilised/solidified waste forms.
This first article, investigates the background behind s/s technology and its current status, and draws on two key events by way of reference –the Sydney Tar Ponds remedial operation in Canada in 2010 and the publication of Environment Agency Guidance in 2004.
Introduction The use of cement as a means of turning soil and waste into ‘rock’ is not new, but what has changed in recent years is that the technology can be used within a risk- based framework. Contaminated material can be diverted from landfill, negating the need to import clean soil or geotechnical fill. The risks of using the soils at site can be demonstrably managed, at relatively low cost, and using commonly available binders (4).
Figure 2: In-situ-bucket mixing of cement-slag mix into tar- contaminated soils at the Sydney Tar Ponds
The remediation of the Sydney Tar Ponds in Nova Scotia has been a hugely political and controversial project which has taken years to agree and implement. Before s/s was approved, a comprehensive consultation process involved both Federal and Provincial authorities and the First Nation. New legislation was required and the Sydney Tar Ponds Agency (1) was formed to manage the remedial operation costing $400M.
In 2010, remediation work commenced with cement being applied to stabilise and solidify the Tar Ponds in Nova Scotia, to manage the risks associated with residues resulting from nearly a century of steel production. Sydney Steel was used for the construction of liberty ships, railways and much besides, and its contaminated legacy includes 700,000 tonnes of contaminated sediments found in tar ponds adjacent to Muggan Creek and Sydney Harbour (2).
By 2014 the Sydney Tar Ponds project will have been completed and the contaminated materials will have been solidified with a mixture of Portland cement and steel slag, creating new land for recreational and light industrial use (3). Sydney will be transformed and revitalised.
Scope
In this occasional series of articles, Dr Colin Hills, Director of the Centre for Contaminated Land Remediation at the University of Greenwich, will introduce stabilisation/solidification (s/s) as a risk management strategy. He will explore the binder materials that can be used to treat inorganic and organic contaminants and
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It can be argued that s/s technology extends back some 50 years, having been developed for nuclear waste treatment. Its first application in the UK was in the 1970’s at bespoke facilities treating mixed metal/organic waste streams. However, in the 1980’s a bund collapse at a treatment plant in the English Midlands released inadequately solidified materials. The failure of treated waste to harden was attributed to inadequate process control (particularly in the later years of the plant’s operation) and this emphasised the lack of guidance (at that time) on, for example: waste characterisation, bench and field scale, and compliance and batch testing of new waste streams. The result was that cement-based s/s lost credibility in the UK, and this put back its use by many years. Not so in North America however, where during the 1980’s and 1990’s, approximately 25% of US Superfund remedial actions utilised cement-based s/s. Today s/s remains a Best Available Technology of choice in North America, as illustrated by its recent adoption in Sydney.
In 2004 the Environment Agency published guidance on s/s for treating contaminated soil (5). This was a significant moment for s/s technology in England and Wales as the guidance is built around a risk-based framework and was accompanied by a comprehensive (300-page) science review (6). This enabled decision- making that was directly underpinned by the science base. Consequently, from 2004 the use of s/s increased in the UK, albeit partially encouraged by the rising landfill tax escalator. There are now a considerable number of sites treated by s/s in the UK.
Technology basis Stabilisation/solidification technology utilises separate but complementary processes that can be used for both the in-situ or ex-situ treatment of contaminated soils and industrial waste streams. Both processes rely on the reaction between a binder and the soil (or waste) to reduce the mobility of contaminants:
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