A number of remediation techniques are being used to tackle the industrial legacy of the Wingerworth coking plant, near Chesterfield, including technology rarely used in the UK. Gemma Goldfingle investigates.
The Avenue coking plant has long been an integral part of Wingerworth, Derbyshire, first as its lifeblood, employing hundreds of village residents, and since its closure in the early 1990s as a universal eyesore.
The site has a long industrial heritage.
Originally a 19th century coal mine, when supplies ran out the site was reclaimed as a coking works, producing a smokeless fuel by carbonising coal. In a process that was sustainable for its day, the site also used waste from the coking process to make chemicals for industry.
However, its method of clean-up was far less sustainable.
“When the coking plant closed down, people just downed tools and left.
There was no clear up. All materials, regardless of how hazardous, were left on site,” explains East Midlands Development Agency (EMDA) project director Mike Fenton.
The substances produced on site included benzene, tar and sulphuric acid, so when materials were piled up when production stopped, an ugly legacy was left behind, and EMDA was charged with the clean up operation of what has been described as one of “the worst contamination site in Europe”.
Nearly 17 years after the plant’s closure, EMDA has secured £170M from English Partnerships’ National Coalfields Programme to invest in the latest remediation technology.
“We have asbestos, PAHs [polycyclic aromatic hydrocarbons], cyanide, arsenic and, our biggest challenge, a tremendous amount of hydrocarbons.”
A quarter of the 98ha site will be transformed into an area for future development and the remainder will be landscaped and sculpted into a place of natural beauty.
One of the reasons the project has been so long in the planning is the severity of the pollution.
“The chemical residue is immense,” says Fenton. “We have asbestos, PAHs [polycyclic aromatic hydrocarbons], cyanide, arsenic and, our biggest challenge, a tremendous amount of hydrocarbons.”
Extensive site investigation and chemical testing revealed that 2.1M.m3 of surface material would have to be removed or remediated - an unprecedented amount in the UK.
The worst of it lies in two toxic lagoons either side of the River Rother.
A waste tip containing 250,000m³ of chemical residues from the coking process has been dumped on top of the lagoons.
There is evidence of water pollution in the river in contravention of the Environmental Protection Act 1990.
Despite the huge regenerative effect the project will have on the small community of Wingerworth, EMDA insists the work is not driven by regeneration, but these environmental concerns.
Contractor VSD Avenue - a joint venture comprising Volker Stevin, Dutch firm Deme Environmental Contractors and French contaminated land specialist Sita Remediation - is tasked with removing the mountains of polluted materials.
The first challenge was to find an alternative to sending this vast amount of material to landfill.
More than 29 remediation options were trialled, with only one technology succeeding in cleaning up the most severe pollution: the very lengthy, not to mention costly, process of thermal desorption.
Relatively novel in the UK, thermal desorption uses intense heat to remove contaminants.
Heavily polluted sediment and soil is heated to temperatures in excess of 600ºC.
The heat breaks down the hydrocarbons into water and CO2 and converts the contaminants from solids to gases, which are collected and intensely treated.
The soil comes out of the process hot but clean, and can be re-used anywhere on the site.
Despite the novelty of the technology, all three members of the VSD Avenue JV have used thermal desorption before.
“This is one of the most complex civil engineering remediation schemes in Europe.Thermal desorption was the only available option to clean up the levels of contamination especially in the silt lagoons on site,” says VSD Avenue project director Marcus Foweather.
“Micro-organisms are encouraged to thrive in the soil and chomp down the hydrocarbons. We’re using Wingerworth bugs as they are more effective at breaking down local soil,”
The thermal desorption plant, designed specifically for The Avenue project, will take 13 months to build and will be ready for action in late 2010.
The state-of-the-art 150m by 100m plant will be able to process more than 25t of material an hour.
To clean the huge amounts of toxic material on this site the plant will operate 24 hours a day for more than two years, only stopping for maintenance.
As constant gas heating to 600ºC can produce a significant amount of emissions, the plant will use the most sophisticated filtration systems available to ensure remediation meets emissions criteria.
Sludge material from the site, including from the toxic lagoons, will be transferred to the desorption plant and mixed with a granular material to form a homogenous mix.
All feed material needs to be the same consistency if the plant is to operate efficiently.
Material with a high moisture content is also to be avoided.
Once inside the plant, solid materials will be separated using dust filters, while carbon filters and scrubbers will do the same with gases.
When this process is complete chemical tests will be carried out to ensure the material is suitable for reuse. If not, the process will be repeated.
Thermal desorption is an expensive process, so it will only be used to remediate the most toxic materials.
Separating the different levels of contaminants is therefore key to keeping costs down.
Excavators will sort the material and transfer it to a central processing area where it will be passed over a conveyor belt. The material will then be sorted manually by particle size, as hydrocarbons generally attach to finer grains.
Tests are then carried out by what the VSD team calls “sentencing managers” - site managers who determine the fate of the given sample, and which remediation technique will be used.
Material not destined for the thermal desorption plant will be remediated using soil washing and bio-remediation.
Soil washing works by mixing soil with water in the equivalent of a large washing machine.
The friction between the soil particles and the machine releases the contaminants, which can then be filtered and disposed of safely. Around 70% to 85% of material can be reused after washing.
Bioremediation will utilise two large aerated bio beds on site, with a local species of micro-organism being used to literally eat any contamination in the soil.
“We’re using Wingerworth bugs as they are readily available and generally indigenous species are more effective at breaking down local soil,” explains Foweather.
“Micro-organisms are encouraged to thrive in the soil and chomp down the hydrocarbons.”
Groundwater on the site is also deeply contaminated with phenols, thiocyanates, benzene and ammonia.
Early in the project EMDA decided to build a water treatment plant on site, which would be safer and more cost-effective than taking the water to an external waste treatment facility.
Once sieved and screened, dredged material will undergo both biological and chemical oxidisation treatment methods to remove metals, destroy pollutants such as cyanides and filter out harmful chemicals commonly found in tars and creosote.
After treatment the water will flow through a weir where the flow rate will be monitored and samples collected automatically by a sampling device.
These samples will be analysed twice a week to make sure the water treatment works are working properly, and samples will also be tested independently by Yorkshire Water.
Contractors have been on site since October to prepare for remediation to begin this Spring.
Soil stripping is under way, and material is being stockpiled ready for bio-remediation.
Remediation will take four years and is due for completion mid 2014.