The £82m contract, signed in July, was commissioned by the client and site owner the East Midlands Development Agency (emda) and funded by the Government’s Homes and Communities Agency through their National Coalfields Programme.
The 98ha site in Chesterfield, Derbyshire, has a long industrial past; initially home to a coal mine, the site then expanded to include a lime and iron works and in 1952 became the coking works that it remained until it ceased production in 1992.
Alongside the traditional coking works, the commercial by-products of coke production including benzole, creosote and sulphuric acid were also used in the site’s large chemical plant and liquor by-products works, which produced chemicals for industrial use all over the UK.
In addition to these chemically orientated processing systems and infrastructure, the site also had a large railhead, a hazardous waste tip and two contaminated silt lagoons.
Bordered by the river Rother, the Midland Mainline railway and the A61, it is a prominent site in the locality. In 1999 there was a period of demolition; the above ground structures and chemical tanks were emptied and demolished, leaving the below ground contamination problems. From here emda, which is responsible for delivery of the remediation project, went in search of a team of experts to find solutions to the site’s somewhat unusual combination and amount of contaminants.
As preferred contractor VSD Avenue spent the first year (2006) designing and costing its remediation strategy, which included treatability trials on the soil and groundwater and in late 2007 the team, alongside consultants Entec, obtained planning permission for remediation of the works.
Marcus Foweather, VSD Avenue’s project director, said: “Few sites offer challenges such as the ones posed by The Avenue. The range of contaminants including hydrocarbons, heavy metals, cyanides and waste chemical products and the variation of material types they are found in, call for a number of different remediation techniques to be utilised, many of which need to be specifically designed for this site.
“We have spent many years developing and testing the techniques which we will be using so it’s an exciting time for the project – we can now get on site properly and begin the real work!”
Treatment technologies and timescales
The volumes of contamination associated with the project have not previously been seen in the UK – over two million cubic metres of material will have to be excavated, with a significant volume of this having to be processed.
The lagoons alone will require over 200,000m³ of very heavily contaminated sediments to be excavated and processed. To add to the complexity of this process the lagoons were not only built either side of the river Rother, but a waste tip has also been formed above one of the lagoons.
Mr Foweather says: “Detailed design of the working methods here has been challenging, but we are confident that the strict environmental management systems we have put in place will ensure that the lagoons can be safely remediated.
”Due to the toxic nature of the contaminants present in the material from both the lagoons and the waste tip, thermal desorption has been identified as the most appropriate treatment method. This method sees the sediment put through a kiln at temperatures of up to 600°C to remove contaminants, which form vapour that is filtered and treated to remove all trace of the toxins.
The thermal desorption plant will be the largest ever used in the UK and utilises the most advanced ‘off gas’ treatment and filtration systems currently available to ensure compliance with the stringent emission criteria. Due to the high temperatures associated with operating thermal desorption plants, it will operate 24 hours a day for more than two years, only stopping for maintenance.
Mr Foweather adds: “When the contract with emda was signed in July our first job was to start commissioning some of the plant we would require when we started on site. A bespoke thermal desorption treatment plant will be constructed on site and the thermal desorption process will start in late 2010.”
The bioremediation of a further 75,000m³ of hydrocarbon contaminated material will start almost immediately on site using two large aerated bio beds, which will operate for the majority of the project.
Groundwater on the site is contaminated with phenols, thiocyanates, benzene and ammonia; this will also be treated on site using a bespoke water treatment facility which uses a combination of chemical oxidation (by UV light and Fenton’s chemistry) and biological treatment. These techniques are environmentally friendly and the best available for these highly contaminated waters. This will ensure that the contaminated waters that are collected on the site can be discharged safely. Groundwater treatment will commence in spring 2010.
Around 327,000m³ of materials from the former licensed waste tip will require sorting, which will include on-site soil washing. The soil washing process generally allows reuse of 70-85% of the contaminated soil as suitable fill material. The excavation of the waste tip is likely to start in late 2010.
The remediation strategy has been designed to ensure the maximum amount of material can be reused on site: the soil washing, bioremediation and thermal treatments all ensure that the resulting materials can form part of the final landform. This on-site material recycling ensures that there is little requirement for lorry movements to and from the site.
A full environmental impact assessment has been carried out. This not only takes into account the sustainability of the project as a whole, but also ensures strict controls are put in place for each part of the works, to minimise any potential impact on the environment and the local community surrounding the site.
Now that work has started on site, VSD Avenue is able to finalise a programme for works. “We started on site on 9 September and works will initially involve some soil stripping in the low level stocking area and the previous plant area where the chemical plant and coke works were located. We will also be installing protective sheet piles around the northern extent of lagoon two, and a temporary bridge will be installed, allowing us to cross the River Rother giving us access into Lagoon four. Waste tip processing, water treatment and ex-situ bioremediation will follow during 2010,” Mr Foweather explains.
Remediation of the site is expected to take around five years, with completion expected in the middle of 2014. VSD Avenue will then hand the site back to emda with a 23ha development platform, which is expected to be used for a mixed use property scheme in the future. The remaining three quarters of the site will be landscaped by VSD Avenue and returned to the community for various uses including public open spaces, sports pitches and a significant nature reserve.
Mike Fenton, project director for emda, says: “I am pleased to have VSD Avenue on board in delivering this exciting and innovative scheme. The consortium has a wealth of experience in these types of difficult remediation schemes and I am confident they will clean up this contaminated site to the highest possible standards to ensure local residents can benefit from an improved area that will leave a lasting legacy for generations to come.”