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 soils. Ex situ processes, however, require excavation of soils leading to increased costs and engineering for equipment, possible permitting, and materials handling worker safety issues.

Thermal treatments offer quick cleanup times but are typically the most costly treatment group. This difference, however, is less in ex situ applications than in in situ applications. Cost is driven by energy and equipment costs and is both capital and O&M-intensive.

Thermal processes use heat to increase the volatility (separation); burn, decompose, or detonate (destruction); or melt (immobilization) the contaminants. Separation technologies include thermal desorption and hot gas decontamination. Destruction technologies include incineration, open burn/open detonation, and pyrolysis. Vitrification immobilizes inorganics and destroys some organics.

Separation technologies will have an off-gas stream requiring treatment. Destruction techniques typically have a solid residue (ash) and possibly a liquid residue (from the air pollution control equipment) that will require treatment or disposal. If the treatment is conducted on-site, the ash may be suitable for use as clean fill, or may be placed in an on-site monofill. If the material is shipped off-site for treatment, it will typically be disposed of in a landfill that may require pretreatment prior to disposal. It should be noted that for separation and destruction techniques, the residual that requires treatment or disposal is a much smaller volume than the original. Vitrification processes usually produce a slag of decreased volume compared to untreated soil because they drive off moisture and eliminate air spaces. A possible exception can occur if large quantities of fluxing agent are required to reduce the melting point of the contaminated soil.

Available ex situ thermal treatment technologies include hot gas decontamination, incineration, open burn/open detonation, pyrolysis, and thermal desorption (high and low). These technologies are discussed in Section4. Completed ex situ thermal treatment projects for soil, sediment, bedrock and sludge are shown in Table 3-9 and additional information on completed demonstration projects are shown on the FRTR Web Site.