Biomedical Engineering Reference
In-Depth Information
Figure 1.8. Conceptual diagram of a DNAPL-contaminated site (USEPA,
2007
).
and Gossett,
1989
). It has been shown that the organisms performing the dechlorination are
using the CAH as an electron acceptor that is able to sustain growth of these organisms
(Holliger et al.,
1999
). Presently, there are only a few known organisms that can degrade
CAHs, including strains of
Dehalococcoides, Sulfospirillum
(formerly
Dehalospirillum
)
,
Desulfitobacterium
and
Dehalobacter
spp. (Damborsky,
1999
). Thus far, the only microbes
that have been found to degrade chlorinated ethenes all the way to ethene are members of the
group
Dehalococcoides
. If these organisms are not present at the site to be bioremediated,
bioaugmentation might be a benefit. Thus, in areas polluted with CAHs, it is clearly beneficial
to either verify the presence of
Dehalococcoides
or to consider bioaugmentation with these
organisms (ESTCP,
2005
; Rahm et al.,
2006
; Lee et al.,
2008
).
Dehalococcoides
can be found in
many polluted areas, but their absence has been correlated with CAH degradation stalling
before conversion to ethene (Hendrickson et al.,
2002
). Further details regarding
Dehalococ-
coides
and chlorinated solvent biodegradation are provided in Chapter 2.
Naturally, there are several limitations to the use of such cultures, besides the limitations
already detailed above for bioaugmentation in general. First, these organisms perform best
under anoxic conditions and have a low tolerance for oxygen (ESTCP,
2005
). They degrade the
CAHs by reductive dechlorination, and thus should be kept under favorable redox conditions
and with appropriate electron donors, such as lactic acid or another organic substrate. Judging
from the number of inocula available and the number of sites in which they were applied, the
use of these cultures has been a tremendous commercial success. Part of the intelligent
application includes manipulation of the environment in order to induce hydrogen production
under anaerobic conditions. This manipulation also aids the naturally occurring
Dehalococ-
coides
, which were found in 21 out of 24 sites examined (Hendrickson et al.,
2002
).
There have been a considerable number of field-scale studies of chlorinated solvent
degradation, with varying levels of success. For example, bioaugmentation using an enriched
culture from a different contaminated site was clearly demonstrated to increase the rate and
extent of biodegradation at a CAH-contaminated site (Semprini et al.,
2007
). Similarly,
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