Following a comprehensive option study, volume reduction by Soils Washing was selected as the “best practical environmental” treatment option.
The soils washing process is an ex-situ physical treatment involving the mechanical and chemical separation of contaminated particles from unpolluted soil. Treatability testwork, undertaken by Heijmans, confirmed that clean product recovery would be economically viable and that the level of decontamination achievable would satisfy the Environmental Agencies material re-use criteria for the site.
Additional treatment options considered, bio-remediation does not treat metalliferous contamination, whilst encapsulation or sub-surface barriers leave the contamination to be treated and vulnerable to be spread by groundwater fluctuations. Excavation and removal was also an option but had the environmental and cost disbenefit of requiring the transportation of large quantities of hazardous wastes on public roads to distant landfill sites and the subsequent importation of replacement materials.
The main contract was awarded under the provisions of the New Engineering contract (NEC) Option C - target contract with activity schedule. The contract allowed for a ‘risk share’ with the client in the event of a cost over-run, coupled with a ‘reward share’ for any cost savings in relation to the target.
The Project involved the washing of approximately 72,000 tonnes of contaminated soil and enabled the reincorporation of approximately 90% of the treated soil back into the site. Approximately 7,000 tonnes of concentrated residue remained to be disposed to licensed repositories off site.
The soil washing plant was set up on a prepared impermeable surface complete with leachate collection system for washed stockpiles and the ‘dirty’area. All process water, product leachate and surface run-off, was treated and re-cycled within the plant.
The on-site application soils washing resulted in the removal of:
- 4.5 tonnes of Mercury
- 26 tonnes of Zinc
On site treatment saved in excess of 6,000 lorry movements that would of resulted from bulk disposal and reinstatement of clean fill.