Biomedical Engineering Reference
In-Depth Information
Contaminant Concentrations
. The contaminant concentration also may be inhibitory to
dechlorinators, and toxicity may be a concern at relatively high dissolved phase concentrations.
For example, Amos et al. (
2007
) measured complete inhibition of reductive dechlorination at
about 540 micromolar (
m
M) PCE (90 mg/L), or roughly half the maximum solubility in water,
though others have demonstrated that dechlorination can proceed at near-solubility PCE
concentrations (Carr et al.,
2000
). The contaminant concentrations also can be too low for
effective reductive dechlorination. Populations of
Dhc
can die off over time at a certian
threshold concentration of total electron acceptors, and this threshold can be above typical
regulatory cleanup levels (Cupples et al.,
2004b
). As a general rule,
Dhc
will die off below a
total VOC concentration of 50 micrograms per liter (
m
g/L), and repeated bioaugmentation
events will be required to sustain complete dechlorination.
Presence of Inhibitory Cocontaminants
. It is also important to evaluate the presence of
cocontaminants that may be inhibitory, such as trichloroethane (TCA) or chloroform. These
compounds can inhibit dechlorination at concentrations that have been observed in chlorinated
ethene plumes (Duhamel et al.,
2002
; Grostern and Edwards,
2006
). Other substances present in
contaminated aquifers also may be inhibitory. For example, Freon from nearby sites may inhibit
TCE dechlorination in commingled plumes (Figgins et al.,
2007
). The presence of potential
inhibitors may not necessarily disqualify a site for bioaugmentation, but it may require
adjustments to the design, or even inclusion of additional bioaugmentation cultures such as
TCA-degrading anaerobic bacteria (e.g., Fung et al.,
2007
).
Sulfate/Sulfide
. Sulfate concentrations also may be inhibitory, although this issue has been a
confusing one. Common guidance is that sulfate levels
>
1,000 mg/L can be problematic
because sulfate-reducing bacteria outcompete dechlorinators for electrons. But sulfate actually
may not be a serious problem, as long as excess electron donor is added (Heimann et al.,
2005
).
Sulfides also can be potent inhibitors. Sulfides that are naturally present, or formed during
sulfate reduction, can be toxic to dechlorinators. However, sulfide toxicity can be alleviated by
precipitating the sulfide into unavailable mineral forms, for example by natural or added iron
(Jeong and Hayes,
2003
). Ferric iron levels may be inhibitory to dechlorination at some sites
(Koenigsberg et al.,
2002
).
Temperature
. In rare cases, temperature also may be an inhibitory factor to consider, although
complete dechlorination has been measured at groundwater temperatures as low as 10 degrees
Celsius (
C), and it still can occur at temperatures up to approximately 40-45
C (Holliger et al.,
1993
; Friis et al.,
2007
).
4.6 IS THE SITE HIGHLY AEROBIC?
The presence of oxygen is fatal to
Dhc
and the other organisms on which they depend.
However,
Dhc
and other obligate anaerobic bacteria are found in most aquifers, even
those considered aerobic, though they often are restricted to anaerobic microsites or low
permeability zones. These reservoirs of indigenous
Dhc
can disperse and colonize the aquifer
after biostimulation, eventually leading to complete dechlorination without bioaugmentation
(e.g., Suthersan et al.,
2002
).
However, there are sites that are so aerobic that they should be considered “functionally
incompetent.” It is likely that
Dhc
strains capable of complete dechlorination will be absent
from highly aerobic sites, or present in low numbers at widely-separated locations, resulting in
unreasonable lag times before effective treatment is achieved. Therefore, bioaugmentation is
recommended for highly aerobic sites, after biostimulation has established a sufficiently
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