Terra Vac won Most innovative remediation method for groundwater decontamination at the BB Remediation Innovation Awards using Six-Phase Heating, Ecclesfield, Sheffield.
A former tool manufacturing site at Ecclesfield, Sheffield, acquired by Taylor Wimpey in 2007 for residential development, was the location for the first application of Electric Restive Heating (Six Phase Heating) in the UK.
The site is bordered on all sides by high density residential properties. Soils analysis revealed two distinct hotspots of chlorinated hydrocarbons (Plume 1 and Plume 2) with contaminants identified as TCE and VC and a total of 3,500m3 of soil requiring treatment to 7m bgl (4m below groundwater).
About SPH
SPH was developed by Terra Vac Corp and the Battelle Memorial Institute under the auspices of the US Department of Energy to develop a technique to address persistent ‘source' areas in former military establishments in the US.
High-voltage electricity is introduced into the impacted soils and groundwater using electrodes. Electric current heats the soils resistively, causing groundwater to boil, which is then extracted as steam together with the volatised contaminants.
Technology selection
An evaluation of the remedial options established that although SPH was not the cheapest solutions (although still significantly cheaper than disposal to landfill), it would bring other major advantages, notably: a low level of local disruption thanks to the in-situ, below ground process; a reduced remediation timescale (estimated at 12-20 weeks); low risk of cost escalation due to the ability to accurately target contaminated layers and vary the heating of electrical elements; and the thoroughness of the process.
Product recovery
Over the 20-week period, the equivalent of 1,662 litres of TCE was recovered from the sub-soils. This represents approximately 70 times the original required site-specific target levels. Groundwater concentrations showed a dramatic net reduction in TCE concentration of in excess of 99%.
Sixteen validation boreholes were installed by independent consultants to verify the results. In addition, separate soil samples were recovered to confirm soil moisture had returned to normal and there were no signs of ‘heave' or desiccation.
The project was completed on budget and the site handed over to the developer three months ahead of schedule.
Genuine novelty
This project is based on technology that has been successfully used in the US, but these projects have typically been large expanses of uniform geology without the challenges faced at the Sheffield site. In addition, the design of the electrode arrays was modified significantly to deal with both the local geology and the degree of contamination posed by the site.
The use and success of SPH in such a challenging environment means that former industrial sites, previously discounted for remediation because of geology, local environment, stringent targets etc, can be revisited and reassessed for development.
"A very real advance in the application of new technology.... Truly a new technology" Phil Crowcroft
