Leachate Arising from the Land Remediation of a Former Steelworks
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The following are extracts from the paper by: A.J. MacARTHUR* AND P.J. YOUNG**.
*ENVIROS CONSULTING, Resource and Risk Management, Edinburgh, UK
** ENVIROS CONSULTING, Corporate Group, Shrewsbury, UK
The steelworks at Ravenscraig, Lanarkshire closed in 1992 after 35 years of steel making. Management of the environmental legacy of the 400-hectare site to enable future redevelopment posed a significant challenge to the owner. The Best Practical Environmental Option was to construct a complex engineered secure containment facility (SCF) to take the 1.2 million m3 of contaminated materials identified on site. The land remediation and SCF was completed in 1998 and this paper describes the contaminants contained within the SCF, the quality of the subsequent leachate and its treatment in a purpose designed treatment plant.
INTRODUCTION
Ravenscraig was a fully integrated iron and steelworks, which included ore stockyards, sinter plant, lime plant, coke ovens, a power station and rolling mills as well as blast furnaces and BOS (Basic Oxygen Steelmaking) furnaces. Contamination was identified in ten principal areas contained within or on made ground comprising slag, clay or construction hardcore. These areas were of three main types, raw material stockyards, operational areas and waste or by-product stockpiles or lagoons. The raw material stockyards contaminants included arsenic and aromatic compounds; the operational areas contained oils, aromatic compounds, toxic metals and inorganic compounds; and the waste product areas comprised filter cakes, sludges and lagoon dredgings containing oils, aromatic compounds, toxic metals and inorganic compounds. Also included in the remediation works was the disposal within the SCF of asbestos from the plant demolition and significant quantities of complexed cyanide residues from the coke oven gas distribution network. These required special handling and disposal procedures within the SCF and influenced details of the SCF engineering design.
Discussion
Steelworks sites present a considerable environmental threat with extensive areas of contaminated land including waste stockpiles that require an intelligent remediation strategy in order to restore the land. At Ravenscraig remediation was required to a standard suitable for facilitating future development and at the same time to dispose of the waste material in such a manner as to minimise the long-term liability.
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At Ravenscraig these materials were classified as special wastes and with the variety of contaminants the best practicable environment option at that time was to dispose of the material in an on site landfill. This meant that there was a potentially significant long-term liability associated with leachate generation and quality.
The laboratory results for the assessment of the waste material clearly demonstrate the gross contamination of the material that could potentially lead to a difficult leachate in terms of handling and treatability. However, this paper has also shown through the frequent laboratory analyses on the leachate that it is relatively simple to manage and that the quality is improving with age.
This is largely due to the robust and innovative engineering design of the secure containment facility that has effectively minimised the ingress of water and, through the buffer layer, maintained the alkalinity of the waste mass thus stabilising the metals and preventing their subsequent leaching. Similarly, the nature of the waste is predominately soils which on engineered compaction would naturally inhibit the passage of water through the waste mass. It should also be noted that much of the contamination was within made ground which had a degree of permeability that comprised areas of reworked clay and granular material such as slag and colliery spoil. However, by controlled disposal methods, including proper compaction, the permeability of the waste can be minimised thus also contributing to the reduction of leachate generation.
The experience at Ravenscraig may prove to be a precursor of similar future landfills influenced by changing legislation driven by the EU Landfill Directive. With the withdrawal of co-disposal, future landfills are more likely to contain toxic contaminants without the range of biodegradable organic waste materials that has been the case historically. Ravenscraig’s SCF is effectively inert in a biodegradable sense, in that the organic materials are dominated by mineral oils and derivates which are stable in the long term in soils. As such the long term trends now being discerned in the leachate quality from Ravenscraig are thought provoking for those charged with designing and managing future special waste facilities which will behave very differently from the mixed waste landfills in operation at the present.
CONCLUSIONS
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In conclusion, the wastes arising from grossly contaminated land may not necessarily lead to the generation of difficult leachate when landfilled, providing that a clear understanding of the nature of the waste is obtained at the design stage, as well as the potential chemical interactions within a waste mass. With this information an optimal engineering design of the facility can be produced to minimise the environmental risk and the long-term liability.
This paper has provided the composition of the contamination found at the former Ravenscraig steelworks and following disposal within an engineered facility the resultant characterisation of the leachate produced. This demonstrates the reduction in risk from leachate by adhering to a competent remediation strategy that has allowed informal public access and aided the successful planning application for redeveloping the site for a mixed-use new community.