4.4.2.4   Reduction of Mobility, Toxicity, or Volume through
Treatment -- Alternative 2

This evaluation criterion addresses the statutory preference for
selecting remedial actions that employ treatment technologies
that permanently and significantly reduce the toxicity, mobility,
or volume of hazardous substances.  This preference is satisfied
when treatment reduces the principal threats at a site through
destruction of toxic contaminants, reduction of the total mass of
toxic contaminants, irreversible reduction in contaminant
mobility, or reduction of the total volume of contaminated media. 
Dechlorination and HTTD systems reduce the toxicity and volume of
contaminated material by thermally desorbing contaminants from
the waste matrix in the processor and concentrating them in oils
or aqueous streams in the condensation systems.  The oils or
aqueous streams are then dechlorinated.  Alternately,
dechlorination reagents may be sprayed on the contaminated
materials before they enter the HTTD system.  The PCBs are
dechlorinated in the HTTD system, thereby reducing the toxicity
of the PCBs.

The ability of HTTD and dechlorination to reduce the toxicity or
volume of contamination at a site was assessed based on the
following considerations:

          Reduction in toxicity of contaminated material
          Irreversibility of treatment
          Reduction in volume of contaminated materials

Each of these considerations is discussed below.

Reduction in Toxicity of Contaminated Material

HTTD does not reduce the toxicity of PCBs.  Dechlorination
reduces the toxicity of PCBs or other halogenated phenols.  At
the WBD site, SoilTech used APEG to dechlorinate PCBs in the ATP. 
During dechlorination, polyethylene glycol monomethyl ethers
substitute for chlorine atoms on the biphenyl ring, resulting in
a less toxic compound.  The results of the SITE demonstration
indicate that the APEG dechlorinated most of the PCBs in the ATP,
with some PCBs leaving the system in the treated solids
(43 ūg/kg), stack gas (23 ūg/dscm), and treated condensed water
(37 ūg/L).  No bioassay tests were conducted during the SITE
demonstration; however, studies have indicated that APEG reduces
the toxicity of PCBs (EPA 1985).  Likewise, BCD reduces the
toxicity of PCBs by substituting hydrogen atoms for chlorine
atoms on the PCB molecule.

Irreversibility of Treatment

The permanence of APEG and BCD reactions were not studied during
the SITE demonstration; however, the dehalogenation process is
believed to be irreversible.

Reduction in Volume of Contaminated Materials

The reduction in the volume of contaminated materials may vary
depending on the type of HTTD system used and the dechlorination
process used.  At the WBD site, only 600 pounds of oil remaining
in the vapor recovery system at the end of the remediation
project required off-site disposal because the contaminated soil
contained a low level of hydrocarbons and the oil from the vapor
recovery system was used as carrier oil for the APEG reagents. 
Much of the carrier oil is cracked and reduced to lighter
hydrocarbons in the ATP.  The amount of oil condensed in the ATP
vapor recovery system is proportional to the amount of
hydrocarbons in the contaminated materials (EPA 1993c).

During dechlorination, the condensed oils from the HTTD system
are dechlorinated, and the resulting oil is treated off site. 
Therefore, the contaminated material volume reduction achieved by
this alternative depends on the volume reduction achieved by the
HTTD system.