Fact sheet: Electrical resistance heating—in situ

From: Public Services and Procurement Canada

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Description

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.

Internet links:

Implementation of the technology

ERH systems may include:

  • Physical and chemical characterization of the soil or sediment (including mapping of the subsurface);
  • Mobilization of equipment (drilling equipment, electrodes material, vapour recovery well material, a steam and vapour collection system, a vapour treatment system, an ERH power supply and control unit, etc.) and construction of temporary facilities and site access considerations;
  • Installation of ERH electrodes (usually constructed on-site in boreholes using conductive materials such as steel tubing and graphite filling);
  • Installation of MPE or SVE extraction wells;
  • Installation of insulated vapour conveyance pipelines;
  • Steam and vapour collection (vacuum) and treatment systems which includes cooling and phase separation equipment;
  • Equipment removal and extraction well decommissioning.

Materials and storage

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.

Waste and Discharges

  • Residuals depend on the water and vapour treatment processes.
  • The water treatment system will generate treated water requiring disposal and will potentially generate non-aqueous phase liquids.
  • The most common air emissions control systems use granular activated carbon or oxidation (with or without a catalyst). Spent granular activated carbon must periodically be transported off site for regeneration or disposal. In the case of catalytic oxidation units, oxidation of chlorinated organic produces acid vapour. This vapour is typically managed with a caustic scrubber. Scrubber water requires periodic neutralization and disposal.

Recommended analyses for detailed characterization

Physical analysis

  • Temperature
  • Soil water content
  • Soil granulometry
  • Soil thermal conductivity
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Measurement of NAPL surface tension under site conditions
  • Presence of non-aqueous phase liquids (NAPLs)

Recommended trials for detailed characterization

Physical trials

  • Vapour survey
  • Evaluation of the radius of influence

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

  • Applicable for residual and dissolved VOCs and SVOCs present in the vadose and saturated zone;
  • Enhances volatilization of light NAPL;
  • Reduces the viscosity of NAPL;
  • Enhances the volatilization of compounds in low-permeability soils.

Applications to sites in northern regions

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.   

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Does not exist
Free Phase
Applies
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Applies

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
Does not apply
Policyclic aromatic hydrocarbons
Applies
Polychlorinated biphenyls
Does not apply

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Electric resistance heating is a short-term technology. Treatment time frames are usually between 6 and 18 months.

Long-term considerations (following remediation work)

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).

Secondary by-products and/or metabolites

  • There is limited potential for in situ by-products; 
  • Biodegradation of certain contaminants such as chlorinated aliphatic hydrocarbons may produce toxic metabolites (such as biological transformation of dichloroethene produces vinyl chloride).

Limitations and Undesirable Effects of the Technology

  • Heating using a three- or six-phase electric current requires the constant addition of water to maintain the electrical conductivity of the soil;
  • Requires high voltage on-site;
  • Soils enriched with organic matter are difficult and costly to treat;
  • Not suitable for inorganic contamination;
  • Electrical resistance heating technology is expensive;
  • In high permeability aquifers, groundwater inflow may inhibit the achievement of an adequate temperature increase;
  • When applied at DNAPL sites, DNAPL may be mobilized due to heat-induced decreases in viscosity and interfacial tension. The potential for downward movement of steam condensate may also be associated with this technology;
  • May have undesirable impacts on nearby buildings, utilities or other enclosed spaces (such as elevated temperatures, high soil vapour concentrations, and potential for soil vapour migration and intrusion into buildings or enclosed spaces);
  • Handling, for example, fuel vapours at levels near the lower explosive limit (LEL) could pose a potential fire/explosion risk;
  • Buried metal objects constitute a safety hazard. The subsurface should be mapped before the heating system is installed;
  • Risks associated with the exposure to high voltages require implementation of engineering controls.

Complementary technologies that improve treatment effectiveness

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.

Required secondary treatments

  • MPE and/or SVE system
  • Treatment of the gas emissions

Application examples

Application examples are available at these addresses:

Performance

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.

Measures to improve sustainability or promote ecological remediation

  • Optimization of heating array placement to reduce energy requirements;
  • Optimization of process equipment related to soil vapour extraction (such as pumps, air treatment systems);
  • Schedule optimization for resource sharing and fewer days of mobilization;
  • Wastewater process optimization to reduce wastes and consumables (activated carbon);
  • Use of heat model for evaluating technology efficiency, particularly in areas with cooler groundwater and elevated hydraulic conductivity, where heat loss is of concern.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

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

Version:
1.2.1