Dense Nonaqueous Phase Liquids (DNAPLs)Chromium VI
Dense Nonaqueous Phase Liquids (DNAPLs)
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Treatment Technologies
Thermal Processes: In Situ
This page identifies general resources that contain information on the design and implementation of thermal treatment technologies applied in situ. Information on applications of these technologies specific to compounds within a DNAPL chemical class can be found in the class subsections listed to the right. More resources on thermal treatment technologies for a wide range of contaminants can be found in the Thermal Treatment: In Situ pages of Technology Focus.
In situ thermal treatment technologies can be applied to contaminant source zones. The application of heat increases the partitioning of organic chemicals into the vapor or gas phase, where they can be extracted under vacuum. The contaminants may even be destroyed in situ when temperatures are sufficiently high.
Four technologies are grouped under the in situ thermal treatment technologies classification: electrical resistance heating, steam injection and extraction, conductive heating, and vitrification. With the exception of vitrification, all of these treatment technologies rely on the addition of heat to the soil to increase the removal efficiency of volatile and semivolatile contaminants. Vapor extraction is an integral part of these remediation systems to ensure the removal and treatment of mobilized contaminants. Liquid extraction is also used during steam injection, and sometimes with other thermal technologies when groundwater flow rates are high and/or when the contaminant being recovered is semivolatile.
In situ vitrification is unique among the thermal technologies in that the temperatures used will vitrify soil. The stable glass that is formed by vitrification will immobilize any nonvolatile contaminants that are present, including metals and radioactive materials.
Davis (1997) provides a general discussion of the effects of heat on chemical and physical properties of organic contaminants. Vaporization is the main mechanism used in these technologies to enhance the recovery of VOCs. Vapor pressures of organic compounds increase exponentially with temperature, causing significant redistribution to the vapor phase as the subsurface is heated. When a NAPL is present, the combined vapor pressure of the NAPL and water determines the boiling temperature, and co-boiling of the two liquids occurs at temperatures less than the boiling point of water. Thus, by raising the temperature of the subsurface above the co-boiling temperature, NAPL can be vaporized and removed. Continued heating of the subsurface recovers contaminants from the dissolved and adsorbed phases as well.
Increasing the temperature also decreases viscosity, increases solubility, and decreases adsorption, all of which aid in the recovery of VOCs and SVOCs. For some SVOC NAPLs, such as creosote, viscosity reduction may be an important mechanism for increased contaminant recovery (Davis 1997). Hydrolysis may play a role in the destruction of some contaminants (e.g., chlorinated methanes and ethanes) as the soil temperature approaches 100°C; however, the breakdown products may be more recalcitrant than the original contaminants.
Care should be taken in designing thermal treatment systems to ensure that all plumbing, including monitoring wells, are capable of withstanding high heat. In the presence of clay, vadose zone heating by resistivity, conductance, or radio frequency may result in some settlement of the treatment area due to the drying of the clay.
This discussion is taken from 
Engineering Forum Issue Paper: In Situ Treatment Technologies for Contaminated Soil, EPA 542-F-06-013, 2006.
Analysis of Selected Enhancements for Soil Vapor Extraction
U.S. EPA, EPA 542-R-97-007, 246 pp, 1997
This comprehensive survey on soil vapor extraction enhancement technologies includes a chapter on thermal enhancements that covers steam, resistive, and conductive heating. It provides a general discussion on these technologies with several case studies and some dated costing information.
Design: In Situ Thermal Remediation
U.S. Army Corps of Engineers. Interim Unified Facilities Criteria, UFC 3-280-05, 258 pp, 2006
This document provides guidance and background for the appropriate screening and selection of in situ thermal remediation technologies, including steam enhanced extraction/injection, electrical resistivity heating, and thermal conductive heating. The document is intended to help distinguish proper applications of the technology and identify important design, operational, and monitoring issues.
Effects of Thermal Treatments on the Chemical Reactivity of Trichloroethylene
J. Costanza, J. Mulholland, and K. Pennell.
EPA 600-R07-091, 117 pp, 2007
During experiments conducted to investigate abiotic degradation and reaction product formation of TCE when heated, the amounts of TCE degraded were very small at 120°C (0.01%) and 240°C (6.5%); however, a temperature of 420°C converted as much as 20% of the TCE to carbon dioxide and carbon monoxide.
Ground Water Issue: How Heat Can Enhance In-Situ Soil and Aquifer Remediation: Important Chemical Properties and Guidance on Choosing the Appropriate Technique
E. Davis.
U.S. EPA, EPA 540-S-97-502, 18 pp, 1997
Contact: Eva L. Davis, davis.eva@epa.gov
Contains in-depth information on the properties of some common organic contaminants (including DNAPLs) that can affect their movement in and recovery from the subsurface, as well as information on how these properties are affected by temperature. Basic information on which of the heat-based remediation techniques is most appropriate to certain subsurface conditions and certain contaminants is also provided, as well as a comparison of the heat-based techniques to other in situ remediation techniques. Three companion issue papers have been written to provide an explanation of how each of the three general types of thermal processes (steam or hot air injection, electrical heating, and hot water injection) works, as well as preliminary information on the design of a system and some estimates of the expected costs.
In Situ Thermal Treatment Site Profile Database
U.S. EPA.
EPA has developed a Web site to summarize information about field demonstrations and full-scale applications of in situ thermal technologies. The searchable database provides project information consisting of site name, location, chemicals of concern, amount treated, costs (if available), and points of contact.
Return to "Thermal Processes: Ex Situ Introduction"
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Last updated on Monday, November 10, 2008