THE AVENUE former coking works located 3 km south of Chesterfield, Derbyshire is a prominent site within the region and also nationally within the remediation sector as it has been dubbed ‘the most contaminated site in Europe’ by more than one commentator.
The 98ha site operated for 36 years as a coking works and had a long and varied industrial history prior to this use. In addition to the production of smokeless fuels the site also processed the by-products from this process and as well as receiving wastes from other National Coal Boards sites in the region.
As such the site possibly lives up to its tag by being complexly contaminated with a range of sulphates, creosote, blended fuel wastes, benzoles, tars, asbestos and spent oxides.
The coking works were closed in 1992 and ownership passed through English Partnerships (now the Homes and Communities Agency – HCA) to East Midlands Development Agency (emda) who is managing the remediation and landscape works which are being funded by the HCA as part of their National Coalfields Regeneration Programme.
VSD – a joint venture consortium comprising DEC (DEME Environmental Contractors – the Belgian based Environmental Contractor), Sita Remediation (a Dutch based thermal desorption specialist) & Volker Stevin (the civil engineering contractor) were awarded the £82million remediation and landscaping contract in July 2009.
This appointment followed previous phases of work including additional site investigation, contamination mapping and contamination treatability trials some of which VSD Avenue carried out in the UK on the site and others which required Transfrontier Shipment Agreements allowing the exporting of contaminated waste which was then treated elsewhere in Europe.
The key objectives of the remediation of the Avenue site are to remove the existing site contamination through on site treatment and to deliver a restored landform which will providesignificant benefits for the local community along with the provision of environmental enhancement and a flood alleviation scheme protecting areas of Chesterfield from future flooding. The restored landform will include a development platform and significant areas of public open space including sports pitches, public footpaths and other public amenity facilities. In order to encourage a diverse range of wildlife, including many protected species, to make the restored site their new home, this new landform was created in harmony with the surrounding environment. The remediation and landscaping package will take over 4 years to deliver.
Due to the size of the site and the varied nature of the made ground, soils and contaminants, the Avenue is a very complex project even before considering the many complex civil engineering issues which the site team has to manage and plan for during and throughout the life of the project. In order to begin remediation work this summer, a significant amount of enabling work had to be completed. As well as works to divert existing services within the site one of the most visible elements of works has been the construction of a large coffer dam and a temporary river crossing in the floodplain area of the River Rother at the northern end of the site.
Currently the Rother runs between a number of artificial lagoons which contain contaminated liquid and solid wastes. The contaminated lagoons are leaking contaminants into the surrounding soils and groundwater and following the installation of the coffer dam and cut off walls later works will allow the lagoons to be emptied and the contents processed. To achieve a more natural-looking path through the newly remediated area, the river will be realigned several times while remediation work is being done to the lagoons. The final alignment includes improved flood defences for the river which has a history of flooding in heavy rains particularly during the winter.
The Avenue project will require the removal and replacement of approximately 2,000,000m3 of material, making it one of the largest remediation projects currently underway in the United Kingdom. Approximately 1,500,000m3 of this material will require little or no treatment and will be replaced on the site in areas where it is immediately suitable for reuse.
Thermal desorption will be used to treat about half of the remaining 422,000m3 of contaminated material, while bioremediation will be used to treat the other half.
All remediation activities, including hauling and storage, will take place on impermeable, structurally sound platforms in order to prevent any further contamination of the site.
Due to the former nature of the site there are a considerable number of very large slabs still present following the demolition works. VSD have been refurbishing a large number of these making them watertight and constructing impermeable bunds around their perimeters and across the slabs to segregate different types of contaminated material during storage. Concrete holding bays for contaminated and processed materials were also built in large numbers. Prior to the treated material being released for reuse on site in accordance with the remediation strategy, this will allow the efficient management of materials before and after processing.
It’s impossible for a single treatment technology to restore the soils and create material that can be recycled on site because of the wide variety of contaminated materials and the variable ground conditions beneath them. VSD’s remediation strategy is based on using the ‘best, most economically viable and practicable technique’ allowing appropriate re-use of processed material within the site dependant upon the sensitivity of the receiving areas. VSD has a wide range of treatment options available because of the breadth of their knowledge and experience. These techniques include; Thermal Desorption, Ex-Situ Bioremediation and Screening & Soil Washing.
These techniques are being supported by screening crushing and grading of materials which are already suitable for re-use on the site.
For the purposes of the remediation and landscaping works the site has been divided into 5 main zones. It is the presence of contaminants and the geotechnical nature of the material in a given area that dictate the primary remediation technique used in this zoning.
Excavation begins with the identification and separation of all material containing Visible Free Product (VFP) by trained operatives. Any material containing VFP is deemed unsuitable for reuse on site even if it can be shown to satisfy the reuse criteria following chemical analysis and must go for processing by the appropriate remediation technique.
Following trials undertaken by VSD it has been shown that the waste tip consists of a variable mixture of soils, waste and colliery spoil and the material in the plant area consist of made ground containing a large proportion of colliery spoil. These materials due to their granular nature are ideal for soil washing techniques. Soil washing as its name implies is a fast physicochemical process involving agitating the soils in water with detergents and flocculants to remove the contaminated materials leaving a reusable sand and gravel. Various non-washable materials such as wood, metals, plastics, and other harmful matter will be removed from the soil before washing. Directed for immediate re-use (after chemical testing), further treatment, or off-site disposal, these separated wastes are then sorted again.
Following the soil washing the silt fraction (75mm) is suitable for reuse across the site and may be used directly or following crushing to a specific grading.
A number of treatment trials have been undertaken by emda and verified by CL:AIRE confirming that thermal treatment is the only technique that can remediate the worst of the contaminated material on site cost effectively.
This material contains gross contamination by tars and oils as well as phenols and thiocyanates.
This material is mainly contained in the lagoon areas but also includes a proportion of materials from the other zones on site. These trials have proven that Thermal Desorption is the only viable method that ensures the treatment of the very heavily hydrocarbon contaminated soils to achieve the strict reuse criteria on the site.
It is possible to process 25 tonnes per hour with the Thermal Desorption plant at the Avenue, which makes use of both indirect and direct thermal processing methods. Soil contaminants are heated to temperatures ranging from 500 to 650 degrees Celsius by a main process that is highly variable. Before being destroyed in the oxidizer, where temperatures exceed 1000oC, soil contaminants are vaporised using this high-temperature heating. Activated carbon filters, gas scrubbers, lime scrubbers, and dust removal are all used to clean and treat these “off gases.”
VSD designed and built the thermal desorption plant to meet all of the project’s requirements while also being exceptionally efficient. The cutting-edge plant also complies with all agreed upon UK regulations on emissions. Unless there is an emergency, the Thermal Desorption plant will run 24 hours a day, 7 days a week for at least two years after it is operationally started processing materials. Natural gas from a nearby medium-pressure gas main is the primary energy source for the desorption process.
In order to obtain additional chemical testing and results before the soil can be released for reincorporation into the main earthworks, it is kept in storage bins after treatment. The removal of water from the soils during the thermal process can reduce the volume of the material by up to 30 percent .
It is possible to repurpose the cleaned soil in high-risk areas due to the thermal process’ exceptional ability to remove contaminants.
The Avenue is the largest Thermal Desorption project to be undertaken on site to date in the UK and is using the largest plant ever operated over here.
Trials by DEC and others between 2000–2004 have proved that some of the contaminated material has the potential for treatment by Bioremediation. The biodegradation of contaminants that are water soluble is achieved through the use of microorganisms and oxygen in bioremediation. Additionally, as the material is aerated, some volatile compounds will partially volatilize, and contaminants will leach into the drainage layer. To ensure efficient bioremediation the optimum oxygen and moisture contents of the spoil must be maintained along with sufficient oxygen availability.
Two bio-beds will be constructed on the Avenue, one in each of two different locations. Both bio-beds will consist of a number of winrows or biopiles situated upon a low permeability liner and appropriate drainage and may use forced aeration to provide oxygen. The bio-beds will be capable of treating soils with a range of different levels of hydrocarbon contamination. The treatment times range from 4–16 weeks dependant upon the degree to which the soil is contaminated.
After treatment and testing, the material can be reused in the development area and in open spaces away from sensitive receptors, such as the Rother River, because bioremediation transforms contaminants into less harmful compounds.
Even though it’s been vacant for a long time, The Avenue has long been a popular tourist destination. Even now it retains a high profile with the local residents keeping a close eye on the remediation works as well as the wider remediation industry following progress keenly. Our team at VSD is honoured to be working on one of “Europe’s most polluted sites” in order to build a platform that the community can use in the future.
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