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A combination of onsite treatment and a riskbased remedial design is providing a costeffective solution for a heavily contaminated site, claims remediation contractor Celtic.

Restrictions on high organic content waste disposal, introduced this month by the Waste Acceptance Criteria and the recent Landfill Regulations, make it important to maximise onsite material retention.

This was the aim of ground and groundwater remediation at a 5ha former gasworks in Yorkshire, north-east of Leeds, that includes insitu treatment of coal tar by high vacuum extraction (HVE).

A century of town gas production and waste generation at the site presents a range of obstacles to be overcome before client SecondSite Property Holdings can redevelop it for commercial use.

As well as old gasworks buildings, a large number of below-ground structures were found, including tar tanks up to 5m deep, liquor separators used for high ammonia content water, and basements.

Early evaluation, including a site investigation and a desk study risk assessment, uncovered a history of ad-hoc waste disposal practices. Their legacy was heavily tarred demolition rubble and spent oxide within the fill coupled with significant losses of liquid coal tar to the alluvium beneath the site.

Remedial contractor Celtic needs to strike a balance between risk mitigation for the proposed development and maximising treatment and recycling of contaminated material on site.

The company says it uses multiple technologies to deliver maximum cost benefit to the client. Phase one of the clean-up, completed last autumn, dealt with shallower contamination to about 2m below ground. This included detailed screening, where the material's fate was sealed at the point of excavation, supported by chemical validation as well as exsitu bio-physical treatment.

For phase two, the company is using insitu HVE for deeper contamination down to 7m. The strategy should see onsite retention of 128,000t of material that would previously have gone to landfill.

In phase one main contractor VHE Construction excavated 47,300m3 of material which was screened at the excavation face, and about 14,800m3 containing spent oxide (or classified as heavily tarred) was scheduled for off-site disposal as hazardous waste.

Site workers identified a further 4,400m 3 as suitable for on-site biophysical treatment to reduce organic contaminant concentrations to levels suitable for on-site reuse.

The remaining 28,100m 3 of material was either uncontaminated demolition rubble or contained cement-bonded asbestos board, leading to it being screened and stockpiled. Asbestos-containing mat -erial was visually screened, hand picked and laboratory tested to demonstrate asbestos absence before being used as backfill in the deepest onsite excavations.

Celtic operations manager Jon Freeman says: 'Since completion of this phase of works, we have developed the Evocem range of stabilisation techniques. In future, these will enable even spent oxide and other heavily contaminated materials to be treated and potentially reused on-site to agreed risk-based standards.

'This is a significant step forward in the development of our Smart remediation service where remedial designs are focused on maximising retention and recycling of materials onsite using a variety of remediation technologies.' The deeper phase two work involves the most significant contaminant mass within the natural ground. This is associated with the groundwater smear zone with areas of mobile free-phase hydrocarbons identified beneath the footprints of historical tar-containing plant.

Celtic is targeting the hydro arbon contaminants in the ground ater smear zone 3m to 7m deep using HVE remediation plant it claims is 'industry defining'. Freeman says this is because it is capable of extracting soil vapours at concentrations exceeding explosive limits.

'This technology recovers organic contaminants by applying a vacuum to the subsurface sufficient to induce the flow of liquids, such as groundwater and free-phase oils, as well as soil gas to extraction pipework. In turn this directs the contaminated ef' uent streams to a variety of recovery or destruction processes, ' he says.

Celtic is operating two of its Atex HVE sys ems at the site. Containerised for easy trans ortation, they comprise an oil-sealed vacuum pump con cted through sand and water/oil separation vessels to 87 treatment wells via a series of computer-controlled manifolds.

Operators can control the systems through a computer graphics display allowing system optimisation on a site-speci' c basis. Safety of operation is secured with a series of interlocks.

The vacuum systems extract contaminants as free-phase hydrocarbons as well as in dissolved and vapour phases. Separation vessels then isolate the effluent streams which are treated as appropriate.

Catalytic oxidation destroys vapour phase contaminants and continuous data logging of the vapour stream done from day one provides a tool for remediation validation.

Particular challenges at the Yorkshire site arise from the recovery of complex emulsified fluids high in coal tar NAPL for on-site treatment.

A specialist water treatment plant optimises the recovery of tars and minimises the need for carbon polishing to clean the water before foul sewer discharge.

Work on the deeper contamination is due for completion towards the end of this year.

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