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Thermal Treatment: In Situ
Many different methods and combinations of techniques can be used to apply heat to polluted soil and/or groundwater in situ. The heat can destroy or volatilize organic chemicals. As the chemicals change into gases, their mobility increases, and the gases can be extracted via collection wells for capture and cleanup in an ex situ treatment unit. Thermal methods can be particularly useful for dense or light nonaqueous phase liquids (DNAPLs or LNAPLs). Heat can be introduced to the subsurface by electrical resistance heating, radio frequency heating, dynamic underground stripping, thermal conduction, or injection of hot water, hot air, or steam.
The main advantage of in situ thermal methods is that they allow soil to be treated without being excavated and transported, resulting in significant cost savings; however, in situ treatment generally requires longer time periods than ex situ treatment, and there is less certainty about the uniformity of treatment because of the variability in soil and aquifer characteristics and because the efficacy of the process is more difficult to verify.
ELECTRICAL RESISTANCE HEATING uses arrays of electrodes installed around a central neutral electrode to create a concentrated flow of current toward the central point. Resistance to flow in the soils generates heat greater than 100ºC, producing steam and readily mobile contaminants that are recovered via vacuum extraction and processed at the surface. Electrical resistance heating is an extremely rapid form of remediation with case studies of effective treatment of soil and groundwater in less than 40 days. Three-phase heating and six-phase soil heating are varieties of this technology.
INJECTION OF HOT AIR can volatilize organic contaminants (e.g., fuel hydrocarbons) in soils or sediments. With deeper subsurface applications, hot air is introduced at high pressure through wells or soil fractures. In surface soils, hot air is usually applied in combination with soil mixing or tilling, either in situ or ex situ.
INJECTION OF HOT WATER via injection wells heats the soil and ground water and enhances contaminant release. Hot water injection also displaces fluids (including LNAPL and DNAPL free product) and decreases contaminant viscosity in the subsurface to accelerate remediation through enhanced recovery.
INJECTION OF STEAM heats the soil and groundwater and enhances the release of contaminants from the soil matrix by decreasing viscosity and accelerating volatilization. Steam injection may also destroy some contaminants. As steam is injected through a series of wells within and around a source area, the steam zone grows radially around each injection well. The steam front drives the contamination to a system of ground-water pumping wells in the saturated zone and soil vapor extraction wells in the vadose zone.
RADIO FREQUENCY HEATING is an in situ process that uses electromagnetic energy to heat soil and enhance soil vapor extraction. The technique heats a discrete volume of soil using rows of vertical electrodes embedded in soil or other media. Heated soil volumes are bounded by two rows of ground electrodes with energy applied to a third row midway between the ground rows. The three rows act as a buried triplate capacitor. When energy is applied to the electrode array, heating begins at the top center and proceeds vertically downward and laterally outward through the soil volume. The technique can heat soils to over 300ºC.
THERMAL CONDUCTION (also referred to as electrical conductive heating or in situ thermal desorption) supplies heat to the soil through steel wells or with a blanket that covers the ground surface. As the polluted area is heated, the contaminants are destroyed or evaporated. Steel wells are used when the polluted soil is deep. The blanket is used where the polluted soil is shallow. Typically, a carrier gas or vacuum system transports the volatilized water and organics to a treatment system.
VITRIFICATION uses an electric current to melt contaminated soil at elevated temperatures (1,600 to 2,000ºC or 2,900 to 3,650ºF). Upon cooling, the vitrification product is a chemically stable, leach-resistant, glass and crystalline material similar to obsidian or basalt rock. The high temperature component of the process destroys or removes organic materials. Radionuclides and heavy metals are retained within the vitrified product. Vitrification can be conducted in situ or ex situ.
A Citizen's Guide to in Situ Thermal Treatment
EPA 542-F-12-013, 2012
The Citizen's Guide series summarize cleanup methods used at Superfund and other sites. Each two-page fact sheet answers six questions about the cleanup method: 1) What is it? 2) How does it work? 3) How long will it take? 4) Is it safe? 5) How might it affect me? and 6) Why use it?
Critical Evaluation of State-of-the-Art In Situ Thermal Treatment Technologies for DNAPL Source Zone Treatment
J.T. Kingston, P.R. Dahlen, P.C. Johnson, E. Foote, and S. Williams.
ESTCP Project ER-0314, 1,272 pp, 2010
The performance of thermal technologies for DNAPL source zone remediation was assessed with particular emphasis on post-treatment groundwater quality and mass discharge (i.e., mass flux). Documents from 182 applications were collected and reviewedó87 electrical resistance heating, 46 steam-based heating, 26 conductive heating, and 23 other heating technology applicationsóconducted between 1988 and 2007, with attention to the site geologic settings, chemicals treated, design parameters, operating conditions, and performance metrics. The results of the study are summarized in a set of spreadsheet-based summary tables linking this information to five generalized geologic scenarios. The Summary Tables identify generalized scenarios that can be used to anticipate the likely performance of thermal-based DNAPL treatment technologies at a site. Another product of this work, 'State-of-the-Practice Overview of the Use of In Situ Thermal Technologies for NAPL Source Zone Cleanup,' condenses the 1,000-plus pages of this report into an 86-page primer prepared for a program manager audience. State-of-the-Practice Overview; ESTCP Cost & Performance Report
The purpose of this paper is to convey useful information gained from approximately 10 years of development and deployment of in situ thermal treatment (ISTT) technologies. This paper is the result of a series of in-depth interviews with U.S. EPA Remedial Project Managers (RPMs) and On-Scene Coordinators (OSCs) and with ISTT vendors whose experience extends beyond federal response action sites to include state-regulated cleanups and Brownfields/voluntary cleanups, as well as international projects. While the focus is on federally funded cleanup sites, many of the lessons learned will be of interest to RPMs and OSCs who are overseeing potentially responsible party (PRP)-lead cleanups.
Remediation Technologies Screening Matrix and Reference Guide, Version 4.0.
Federal Remediation Technologies Roundtable.