In search of a sustainable alternative to dumping at sea or disposal on land, a Scandinavian consortium blended contaminated sediment with a special mix of binders to produce a safe construction material for use in ports and harbours.
Stricter regulations have reduced the use of hazardous chemicals and heavy metals in industrial activities, but their legacy lives on in the environment, notably in polluted soils and sediments. One sector where they present a particular headache is in the shipping and port industry, where dredging routinely turns up sediment contaminated with the likes of carcinogenic PCBs, TBT, cadmium, lead and mercury. Port owners are caught between constraints on dumping sediment at sea, the cheap but polluting option, and removing it to be treated for landfill, an expensive alternative.
Enter a recent EUREKA project, STABCON, in which a Swedish-Norwegian consortium -- of research bodies, binder manufacturers, port authorities and design consultants -- sought to adapt the 'stabilisation and solidification' method to treat polluted sediments and other dredged material commonly found in Scandinavia.
Having worked together on an earlier study into the potential of the stabilisation and solidification technique in Sweden for the country's environment protection agency, the project participants teamed up to test the method in a pilot study and draw up guidelines for ports.
A cost-effective solution
Led by Merox, a subsidiary of Swedish steelmaker Svenskt Stål AB (SSAB), they first compared the three alternatives for handling sediments -- dumping, solidification and stabilisation, and dredging and disposing on land -- from a sustainability perspective. Stabilisation and solidification proved to be a sustainable and cost-effective solution. Contaminated sediments are mixed, on site, with products that bind it to create a solid material that contains the hazardous substances.
As well as being more environmentally friendly than dumping and cheaper than landfilling, "this method offers a number of additional benefits," explained Göran Holm, R&D director of the Swedish Geotechnical Institute, one of the project partners. "It reduces the demand for natural resources, such as blasted rock; and by treating the sediments in situ and using them in port areas, the need for transport is reduced, along with the associated health risks."
Supported by funding from EUREKA member countries, the project partners conducted tests in a pilot project to identify the most suitable binder composition and ideal mixing procedure for a variety of contaminants and sediment types. Researchers observed the behaviour of the treated sediment for leakage, permeability, strength and durability. The binder they used was a mixture of cement and a Merox product, Merit 5000, a derivative from the steel-making process. The slag is able to bind heavy metals chemically at the same time as it cures.
Putting it to the test
The final step of the project translated the results into a report and guidelines for port authorities, to enable them to assess options for using stabilisation and solidification and select the best binder for their local conditions, while providing design principles for using treated sediments in harbour structures, such as paved areas, loading zones and buildings. The STABCON test site was the Swedish port of Oxelösund, itself a partner in the project. The port wanted to build a new harbour area, and needed to remove contaminated sediment while at the same time respecting Sweden's strict environmental regulations.
Its aim was to dredge a section of harbour and treat the sediment for use in the new land area. The team dredged about 500 cubic metres of soft sediment, and strengthened it with a mix of cement and Merit 5000. They placed the composition on gravel and sand, and studied its properties, taking samples and conducting laboratory tests for leakage, including in nearby waters. The results were impressive. Once stabilised, there was no degradation from a chemical point of view, and no physical damage either.
The new material also passed the test for durability. "We are pretty confident that it will last for the long term," said Therese Stark, a research and development engineer at Merox. "The main thing is to keep the sediments in as natural an environment as possible, which you can't do if you take them away to deposit elsewhere. We are trying to keep them as they were in the ocean." Key to the outcome of the project was a close working relationship between partners and the expertise that each brought to the table.
An expanding market
"The work was very successful. We had a very strong consortium," reflected Göran Holm. "It included manufacturers of binders, like Merox and Cementa, research organisations, consultancies that took charge of design, a harbour with polluted sediment and contractors that performed the test." Working in symbiosis, the research bodies gained expertise in the technical and environmental area, and in support for decision-making, while the industrial partners gained insight into how their products could be used to greater effect: "We all have a lot more knowledge about how to treat sediments with proportions of binders for different pollutants," says Therese Stark of Merox.
The project has already translated into new contracts for the participants. A number of ports in Sweden have taken a keen interest in the cost-effective 'stabilisation and solidification' method, and many are undertaking tests with their own sediments, consulting STABCON partners for their know-how. Meanwhile, a more extensive R&D project has been launched for the whole Baltic Sea region, thanks partly to the work achieved by STABCON.
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