Ex situ thermal treatment generally involves the destruction or removal of contaminants through exposure to high temperature in treatment cells, combustion chambers, or other means used to contain the contaminated media during the remediation process. The main advantage of ex situ treatments is that they generally require shorter time periods, and there is more certainty about the uniformity of treatment because of the ability to screen, homogenize, and continuously mix the contaminated media; however, ex situ processes require excavation of soils, which increases costs and engineering for equipment, permitting, and materials handling worker safety issues.

Thermal processes use heat to separate, destroy, or immobilize contaminants. Thermal desorption and hot gas decontamination are separation technologies. Pyrolysis and conventional incineration destroy the contaminants. Vitrification destroys or separates organics and immobilizes some inorganics.

Incineration is a heat-based technology that has been used for many years to burn and destroy contaminated materials. Because it is considered to be a conventional rather than an innovative technology, its treatment here is limited to information listed under "Additional Resources."

EX SITU THERMAL DESORPTION involves the application of heat to excavated wastes to volatilize organic contaminants and water. Typically, a carrier gas or vacuum system transports the volatilized water and organics to a treatment system, such as a thermal oxidation or recovery unit. Based on the operating temperature of the desorber, thermal desorption processes can be categorized as either high-temperature thermal desorption (320 to 560ºC or 600 to 1,000ºF) or low-temperature thermal desorption (90 to 320ºC or 200 to 600ºF).

HOT GAS DECONTAMINATION involves raising the temperature of contaminated solid material or equipment to 260ºC (500ºF) for a specified period of time. The gas effluent from the material is treated in an afterburner system to destroy all volatilized contaminants. This method will permit reuse or disposal of scrap as nonhazardous material.

PLASMA HIGH-TEMPERATURE RECOVERY uses a thermal treatment process applied to solids and soils that purges contaminants as metal fumes and organic vapors. The vapors can be burned as fuel, and the metals can be recovered and recycled.

PYROLYSIS is defined as chemical decomposition induced in organic materials by heat in the absence of oxygen. Pyrolysis typically occurs under pressure and at operating temperatures above 430ºC (800ºF). The pyrolysis gases require further treatment. The target contaminant groups for pyrolysis are SVOCs and pesticides. The process is applicable for the separation of organics from refinery wastes, coal tar wastes, wood-treating wastes, creosote-contaminated soils, hydrocarbon-contaminated soils, mixed (radioactive and hazardous) wastes, synthetic rubber processing wastes, and paint waste.

THERMAL OFF-GAS TREATMENT is one of several approaches that can be used to cleanse the off-gases generated from primary treatment technologies, such as air stripping and soil vapor extraction. In addition to the established thermal treatments, organic contaminants in gaseous form can be destroyed using innovative or emerging technologies, such as alkali bed reactors.

VITRIFICATION technology 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 most heavy metals are retained within the vitrified product. Vitrification can be conducted in situ or ex situ.

Community Guide to Thermal Desorption
EPA 542-F-21-019, 2021

The Community Guide series (formerly Citizen's Guides) is a set of two-page fact sheets describing cleanup methods used at Superfund and other hazardous waste cleanup sites. Each guide answers six questions about the 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? 6) Why use it?

Overview of Thermal Desorption Technology
R.J. Feeney, P.J. Nicotri, and D.S. Janke. NFESC-CR-98-008-ENV, NTIS: ADA352083, 32 pp, 1998.

This report summarizes a presentation on thermal desorption designs, identifies contaminants for which thermal desorption is known or believed to be effective, and presents generalized cost data for a range of project sizes.

Remediation Technologies Screening Matrix and Reference Guide, Version 4.0
Federal Remediation Technologies Roundtable.

Thermal Treatment of Hydrocarbon-Impacted Soils: A Review of Technology Innovation for Sustainable Remediation
Vidonish, J.E., K. Zygourakis, C.A. Masiello, G. Sabadell, and P.J.J. Alvarez.
Engineering 2(4):426-437(2016)

The authors review thermal treatment technologies for hydrocarbon-contaminated soils, assess their potential environmental impacts, and propose frameworks for sustainable and low-impact deployment based on a holistic consideration of energy and water requirements, ecosystem ecology, and soil science. The review covers thermal desorption in situ and ex situ, smoldering, incineration, pyrolysis, vitrification, radio-frequency heating/microwave heating, hot air injection, and steam injection. Selecting an appropriate thermal treatment depends on the contamination scenario (including the type of hydrocarbons present) and on site-specific considerations such as soil properties, water availability, and the heat sensitivity of contaminated soils.