Ecologia Environmental Solutions won Most innovative remediation method for in-situ soil heating technology at a former petrol station
Total UK appointed Ecologia to carry out a remediation assessment of an in-situ soil heating technology at a typical decommissioned petrol station with high groundwater vulnerability. It was impacted by VOCs such as benzene, ethylbenzene, toluene and xylenes (BTEX) and total petroleum hydrocarbons (TPH), mostly with C<21.
The contamination was confined within the unsaturated chalk strata l0m below ground level (mbgl). In-situ radio frequency heating (ISRFH) electrodes were used in a triangular array between 3 and 6mbgl together with eight multilevel SVE extraction wells (screened depth 2.5-5mbgl and 5.5-75mbgl). Hydrocarbon contamination at depth meant additional remediation was required to reduce potential future environmental liabilities.
In-situ radio frequency heating
ISRFH is used to excite polar molecules present within the soil profile, generating heat like a microwave does. ISRFH uses a frequency (13.56 MHz) with a wavelength of 22m, so has the capacity to penetrate the soil more than microwaves.
ISRFH is effective in dealing with volatile contamination in tight, unsaturated soils which cannot be efficiently heated using steam due to mass transfer limitations. It also reduces the potential for uncontrolled contaminant mobilisation generally associated with steam injection in impermeable/fissured soils, and is less susceptible to soil moisture than resistive heating (three-phase or six-phase) so the system performance is unlikely to collapse when soil dries.
Due to the good penetration of radiowaves into the soil, ISRFH produces a much smoother soil temperature gradient than the extreme gradient generally associated with conductive soil heating with simple heating coils/rods.
ISRFH energy is delivered into surrounding soils by electrodes 3-4m apart at predetermined, discrete depths. It can be coupled with a soil vapour extraction (SVE), or a multi phase extraction (MPE) system to extract the volatile organic compounds (VOCs) and semi-volatile contaminants that are mobilised by heat from the ISRFH system. This can also be easily coupled with resistive heating to heat both unsaturated and saturated soils for less energy, using the same piece of equipment.
The data from the remediation process demonstrated that combining ISRFH in-situ heating with SVE would:
- Significantly increase the removal rate of volatile and semi-volatile contaminants from unsaturated chalk
- Significantly improve the final soil contaminant concentrations
- Significantly reduce treatment times without excessive energy costs.
Soil retains heat well, so once a predetermined temperature has been reached the energy intensive ISRFH can be turned off, while the SVE continues to operate at a much improved extraction rate. This greatly reduces the treatment time, and therefore energy demand.
"It is more energy efficient than steam." Jonathan Atkinson
"A project with potential for widespread replication." Clive Boyle
