From: Public Services and Procurement Canada
In situ electrical resistance heating (ERH) uses relatively large amounts of energy to heat the subsurface, stripping volatile contaminants out of the soil and vaporizing groundwater and non-aqueous phase liquids. Heating is accomplished by imposing a voltage difference in the subsurface using electrodes. Electrical resistance heating systems use alternating current, typically either three-phase or six-phase current. Water is added near the electrodes during the process to prevent drying and loss of conductivity.
Vaporized groundwater and contaminants are extracted by a soil vapour extraction system (SVE), multiphase extraction system (MPE) or other collection system. Electrical resistance heating technology enhances the volatilization of volatile and semi-volatile organic compounds (VOCs and SVOCs) in low-permeability soils such as clayey soils and bedrock. In the case of clays, if sufficiently high temperatures are achieved, the clay can desiccate and crack, increasing the soil permeability. Soil heating also facilitates the migration of non-aqueous phase liquid (NAPL) by lowering its viscosity, density, and the extent to which it is adsorbed to soil particles. During the heating process, hydrolysis and pyrolysis may occur. Hydrolysis is a reaction with water at low temperature (from 60 °C to 80 °C) forming simpler compounds, while pyrolysis is a high-temperature decomposition.
Overall, depending on the temperature of the system, electrical resistance heating is effective for the treatment of some pesticides and fuels.
ERH systems may include:
Extraction wells (MPE or SVE) contain significant quantities of water. The above-ground treatment system must include provisions for air-liquid separation, vapour condensation, vapour, condensate and liquid cooling, non-aqueous liquid phase separation, water treatment and vapour treatment. The water treatment system can consist of cooling towers and carbon adsorption units. Vapour treatment systems are commonly comprised of combustion (thermal oxidation, catalytic oxidation) or filtration/sorption (activated carbon, biofiltration) units.
Process equipment is generally provided by one of a limited number of specialized remediation subcontractors.
Sufficient storage space is required to house the ERH unit as well as the power supplies and the water and vapour collection and treatment units.
Electrical resistance heating systems may not be appropriate for remote sites without access to an adequate electrical supply.
Extreme cold increases the energy input and cost required to heat soil to the treatment temperature. However, because deep soil temperatures are relatively constant over the course of the year, ERH may still be applied effectively in northern environments, particularly for sites in which the targeted contamination is located well below the ground surface.
Northern systems require climate-appropriate design, including consideration of deep frost, permafrost, seasonal changes in saturation and air permeability, fuel supply or sorbent removal capabilities. Even with an electrical supply, electrical resistance heating can be expensive or not feasible due to the high power requirements, high electricity consumption and long operation periods which may be required due to difficult geology and heat loss.
Notes:
Electric resistance heating is a short-term technology. Treatment time frames are usually between 6 and 18 months.
Soil heating can affect geotechnical properties of soils and care should be taken when treating soils around or under buildings or infrastructure. However, ERH has not been observed to adversely affect geotechnical properties (USACE, 2014).
Electrical resistance heating may be combined with in situ biological technologies (such as biostimulation, bioaugmentation, and bioventilation) when the operational temperature is less than 40 °C. Elevated temperatures may increase biodegradation rates; there is some evidence that microbial recolonization of aquifers occurs rapidly following thermal treatment.
ERH is sometimes combined with steam injection to target high-permeability zones with high water flow rates.
Application examples are available at these addresses:
During a pilot study at the Missouri Electric Works Superfund Site (Cape Girardeau, Missouri, U.S.A.) this technology eliminated 99.9% of the PCB contamination. The FRTR (2002) estimated that 18,200 metric tons of contaminated soil could be treated by electrical resistance heating technology in approximately 9 months.
Unavailable for this fact sheet
Composed by : Mahaut Ricciardi-Rigault, M.Sc., MCEBR
Updated by : Josée Thibodeau, M.Sc, National Research Council
Updated Date : March 1, 2008
Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée
Updated Date : March 31, 2018
