Environmental Engineering Reference
In-Depth Information
Biotreatment processes had been successfully demonstrated for treat-
ment of a wide variety of easily degraded compounds, such as low-molec-
ular-weight fuels and phenols. Strong potential existed for development of
biotreatment processes directed toward contaminant groups traditionally
more difficult to degrade, such as explosives, chlorinated solvents, polycyclic
aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs). All
of these compounds represented a major contaminant problem to the DOD.
This project's objectives enhanced existing funded efforts in DOD pro-
grams, complemented both the EPA and DOD research strategies, and
addressed problems experienced by environmental engineers involved in
Superfund, Resource Conservation and Recovery Act (RCRA) activities, and
international technology exchange programs.
Dr. Jeffrey Talley, P.E., was the project director and was assisted by
Deborah K. Belt.
4.1.1 Chlorinated solvents
Chlorinated solvents entered the environment in massive amounts during
the 1950s, 1960s, and 1970s. These contaminants have migrated through the
subsurface and impacted groundwater at more than 1000 DOD sites. Con-
taminated aquifers can be remediated by removing the solvents in the porous
media of the subsurface. Laboratory and pilot-scale experiments have dem-
onstrated the potential of cosolvent-enhanced
in situ
extraction to remove
dense nonaqueous phase liquids (DNAPLs) in porous media. Although this
method is effective for mass removal, residual amounts of cosolvents and
contaminants are expected to remain at levels that could preclude meeting
regulatory requirements. However, with the bulk of the DNAPLs extracted
in situ
, biotreatment becomes a viable polishing procedure. This was the
emphasis of the work that was conducted by Dr. Guy Sewell, EPA.
In situ
biotreatment may transform the remaining contaminants to non-
hazardous compounds at a rate in excess of the rate of dissolution or dis-
placement. The efficacy of
in situ
bioremediation of chlorinated solvents is
usually limited by transport and mixing considerations, i.e., supplying
excess electron donors in conjunction to the chlorinated solvents at appro-
priate concentrations. The delivery-and-extraction process facilitated the
cosolvency effect and supplied electron donors (cosolvent, ethanol) and elec-
tron acceptors (chlorinated solvent, tetrachloroethylene (PCE)) to the inher-
ent bacteria. The synergism between these abiotic and biotic processes could
minimize problems associated with the individual approaches and lead to
the development of a treatment train approach, which could attenuate or
eliminate the risks posed to human health and the environment by DNAPL
sites.
