One of the UK's largest insitu remidiation schemes has just finished at a former gasworks site in Yorkshire. Jon Freeman reports.
For over 100 years a town gasworks north of Leeds in Yorkshire released large volumes of liquid coal tar into a 20m thick alluvial aquifer that provides base flow to a nearby major water course.
Delineation works by remediation contractor Celtic identified contaminant sources including underground tanks up to 5m deep as well as coal tar impacted alluvium to 15m below ground level.
After an evaluation of the risks to human health and controlled waters both beneath the 5ha site and downgradient, Celtic developed a remedial strategy focused on maximising treatment and recycling of material on site. This made it one of the UK's largest insitu treatment remediation schemes.
The strategy struck a balance between risk mitigation to the site redevelopment and maximising the treatment and recycling of contaminated material by using multiple technologies.
This ensured the retention of an estimated 128,000t of material on site that would previously have been sent to landfill; a solution that would have incurred additional costs of more than £2M.
Celtic's main innovation was using high vacuum extraction (HVE) to recover coal tar from the alluvium. This limited mass excavation works and significantly lessened the project's environmental impact by reducing lorry movements, as well as limiting the local release of volatile organic compounds and disturbance to the underlying aquifer.
Celtic operated two of its Atex HVE systems at the site. Each has an oil sealed vacuum pump capable of extracting 760m 3/hour. These were connected through effluent separation vessels to 87 treatment wells via a series of computer controlled manifolds. In late 2004 the treatment wells were installed 1020m below ground - depending on the extent of the coal tar contamination level - and fitted with 50mm extraction lances.
The technology recovers organic contaminants by applying a vacuum to the subsurface sufficient to induce the flow of liquids through the extraction lances, such as groundwater and free-phase oils and soil gas. In turn, the HVE treatment unit directs the contaminated effluent streams to a variety of recovery or destruction processes.
The recovered soil vapour was passed through a catalytic oxidiser, which destroyed hydrocarbon contaminants by oxidation to water vapour and CO 2 in a self-sustaining reaction at 3000ºC.
Continuous data logging of the vapour stream from day one provided a powerful tool for remediation validation and reassured the regulators of the remediation's reliability.
Liquid phase effluent was diverted to a water treatment plant. Challenges arose from the recovery of complex emulsified fluids high in coal tar NAPL (non-aqueous phase liquid).
Celtic used several stages of oilwater separation to recover the gross oil. The recovered coal tar was NAPL-segregated for reuse as a supplementary power station fuel. Site workers further treated the groundwater effluent through sand filters and granular activated carbon before discharge to sewer, under consent from the local water authority.
The systems were controlled through a computer display that allowed site engineers to optimise the remediation programme by focusing on the wells that recovered the greatest contaminant mass.
Following commissioning in February 2005, the systems ran 24 hours a day, seven days a week with weekly shutdowns for maintenance checks.
Operators achieved maximum rates of contaminant recovery through July, August and September 2005. By October contaminant recovery rates began to decline significantly.
In November rebound monitoring was done to demonstrate that further operation of the system would not recover contaminant mass cost effectively. This data satisfied the regulator that sufficient remediation had been achieved and Celtic decommissioned the system in December.
Final tallies showed the contractor had achieved the recovery of 67m 3 of liquid coal tar, the destruction of 8.7t of hydrocarbons in the vapour phase and the extraction of 590kg from the dissolved phase in groundwater. This represents a significant reduction in risk to the down gradient receptors.
This combination of on-site treatment technologies and a risk-based remedial design provided a costeffective remedial solution for a heavily contaminated site.
Jon Freeman is operations manager at Celtic.