Biomedical Engineering Reference
In-Depth Information
Ethene, Ethane or Carbon Dioxide (CO
2
) Formation
. If the environmental conditions are
favorable and competent microbial strains are present, reductive dechlorination can proceed
through
cis
-DCE and VC to ethene or ethane. The presence of significant ethene generally is
conclusive evidence for complete microbial dechlorination (Major et al.,
2002
; Bradley and
Chapelle,
2010
). Although trace levels of ethene may occur naturally or as a result of relatively
inefficient cometabolic processes (Bradley and Chapelle,
2010
), if ethene or ethane represent a
significant fraction (
>
10%) of the total volatile organic compounds (VOCs), bioaugmentation
should not be needed.
At other sites, reductive dechlorination may not proceed all the way to ethene, but the
resulting byproducts (
cis
-DCE and VC) may be further degraded to CO
2
under aerobic
conditions. Aerobic biodegradation of these partial degradation products can occur in aerobic
groundwaters downgradient of the anaerobic zone, or may occur in an aerobic vadose zone if
the compounds are volatilized. Aerobic biodegradation of VC may occur even if the dissolved
oxygen (DO) concentration is far below typical detection limits of roughly 1 milligram per liter
(mg/L), in environments that typically would be considered “anaerobic” (Gossett,
2010
). If such
aerobic polishing has been observed and can be relied upon to protect potential receptors, even
if the production of
cis
-DCE and VC is increased as a result of biostimulation, then bioaug-
mentation should not be needed.
Contaminant Concentrations
. Contaminant data need to be viewed with some caution. Mass
balances from chlorinated solvent sites are rarely, if ever, complete. There are several reasons
for the difficulty in determining an accurate mass balance. In particular, some of the later
products (VC and ethene) can be quickly degraded under aerobic conditions, and probably
under anaerobic conditions as well (Davis et al.,
2008
; Bradley and Chapelle,
1998
; Klier et al.,
1999
). These byproducts also can be volatilized during sampling, handling or analysis.
Also, abiotic degradation can lead to rapid losses of chloroethenes (Lee and Batchelor,
2002
), and can be an important degradation mechanism (Ferrey et al.,
2004
). However, abiotic
degradation is rarely evaluated; the key intermediate (acetylene) can be quantified from the
analyses used for ethene and ethane, but acetylene may be degraded so quickly that it is not
a reliable indicator (AFCEE et al.,
2008
). Guidance on recognizing and quantifying abiotic
degradation could significantly improve mass balances and degradation rate estimates.
4.5 ARE THE SITE CONDITIONS INHIBITORY?
If contaminant data indicate that reductive dechlorination is not occurring or is not
complete, it may be due to process-specific inhibitory conditions that are sometimes overlooked
when screening technologies. An exhaustive investigation of potential inhibitory factors is not
needed at this stage, but a few common problems can be evaluated by examining the existing
site data. In some cases, a brief screening of conditions that often inhibit reductive dechlorinat-
ing bacteria can be enough to reject
in situ
bioremediation from further consideration. Some of
the most common inhibitory parameters include:
pH
. The most common site-specific issue is pH. Bacteria capable of dechlorinating DCE and
VC are sensitive to even mildly acidic conditions. They are at least partially inhibited below pH
6.0, and a pH below 5.5 is clearly a concern (Vainberg et al.,
2006
; Fogel et al.,
2009
). Even if the
pH is favorable before biostimulation, acidification during fermentation of added electron
donors can cause pH values to decrease below 5.5, at least temporarily inhibiting reductive
dechlorination. Understanding the buffering capacity of an aquifer, therefore, can be as
important as knowing the current pH and alkalinity. On the upper end, the pH should not be
greater than 8.0 for dechlorinators to function effectively.
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