Environmental Engineering Reference
In-Depth Information
rates for DNAPLs and their significant residual mass lead to both very large
plumes for treatment and extensive treatment times. These designed treat-
ment times for chlorinated solvent sites can range from decades for active
systems to centuries for natural attenuation systems.
Combining bioremediation with enhanced source removal, however,
could be a mechanism for obtaining complete site restoration. With the bulk
of the residual phase removed by enhanced source removal,
in situ
biotreat-
ment could transform the remaining contaminants to nonhazardous com-
pounds at a rate in excess of dissolution.
5.1.5 Flask to field
Although the solvent flushing and bioremediation technologies have been
shown in laboratory and limited field evaluations to enhance the remediation
of contaminated aquifers, there has been no previous attempt to couple these
technologies in the field. There are a number of factors that have not been
fully evaluated for both the individual and coupled technologies. The high
degree of spatial variability in the subsurface environment, the difficulty in
obtaining
in situ
mixing of the remedial fluid, the changes in the hydraulic
properties of the system as the DNAPL is removed, and the effects of high
concentrations of remedial solvent mixtures on the microbial ecology are
major factors that must be accounted for in the system design to ensure the
success of implementation.
Beginning in the late 1990s, Environmental Protection Agency (EPA)
researchers began evaluating the effects of potential cosolvents and their
breakdown products on the reductive dechlorination process (Gibson and
Sewell, 1992; Gibson et al., 1994). Observations from the Hill AFB source
control trials also indicated that native biological transformations were
occurring during NAPL source removals. These early observations, labora-
tory experiments, and field tests supported the development of a process
conceptual model and testable pilot system.
The Solvent Extraction Residual Biotreatment (SERB) technology is a
treatment train approach that combines both cosolvent extraction and
in situ
bioremediation to obtain complete site restoration. Advantages of this tech-
nology are:
•
Rapid removal of large masses of DNAPL in a very short time
•
In situ
treatment train technique
•
Cosolvent injection/extraction overcomes transport and mixing limita-
tions of standard bioremediation infiltration and injection techniques
•
DNAPL mass removal reduces toxicity to microbes for bioremediation
•
Continued removal of dissolved contaminants following cessation of
pumping
•
Combination of an active process, solvent extraction, with an essen-
tially passive process, residual biotreatment
