Biomedical Engineering Reference
In-Depth Information
whether a specific compound is being biodegraded in situ . For example, with carbon-based
stable isotope analysis, the chlorinated ethenes remaining after biodegradation have a higher
13
C: 12 C ratio than the original pollutants due to the biological preference for
12
C bonds,
which are slightly weaker than 13 C bonds (Morrill et al., 2005 ). These tests require laboratory
analyses and cannot be performed easily in the field. Microelectrodes, on the other hand, also
can be used to detect byproducts of bacterial metabolism or the actual products of interest in
the field (Satoh et al., 2003 ).
1.4.5 Other Considerations: Economics and Degradation Kinetics
In cleanup scenarios, the two main concerns are time (time required to meet remediation
goals and/or the duration of site occupation) and cost (covered more thoroughly in Chapter 11).
The time required for cleanup is controlled by the overall degradation kinetics, which in turn are
controlled by the rate of catalysis and pollutant availability. If the rate-limiting step is the
catalysis, then bioaugmentation with either a faster-degrading organism or more organisms will
speed up the degradation, reduce time of cleanup and thus possibly reduce cost. If the site
cannot support a large number of microbes, the bioaugmented population will diminish soon
after inoculation. However, even if the site has to be bioaugmented multiple times, this might
be a cost-efficient solution if it proves to speed site remediation. If, however, the rate-limiting
step is pollutant availability, then no amount of bioaugmentation is going to help - it will, if
anything, only incur cost and frustration and may in some cases increase cleanup time and cost
by plugging wells or aquifers (Vogel, 1996 ). In this case, either the pollutant availability needs to
be increased, such as by surfactants, and then bioaugmentation can be considered, or a
different remediation method needs to be chosen.
The cost of site remediation is related to the level to which the pollutant must be reduced,
which is determined by regulatory standards that vary from place to place. For bioremediation
methods, contaminant removal to very low concentrations can prove problematic. Most
bacteria must be exposed to a certain level of a substrate before the degradation pathways
are induced. If the regulatory levels are lower than the induction levels, the bacteria are not
going to degrade the pollutant unless some momentum exists in the system or other compounds
are inducing the needed enzymes (He and Sanford, 2002 ). One solution is to preinduce the
bioaugmented culture so that the degradation pathways are already activated, or to use bacteria
that constitutively express the degradation pathway, meaning that they express the genes
regardless of the pollutant level.
1.5 BIOAUGMENTATION ISSUES
Despite the apparent simplicity and efficacy of bioaugmentation, this technology remains
controversial due to the inherent complexity of natural systems that do not behave like
laboratory microcosms and the inability to control organisms released into the environment.
While many bioaugmentation experiments in the laboratory show promising results, this
success often does not translate at full scale in the field (Cases and de Lorenzo, 2005 ; Park
et al., 2008 ). Before the late 1990s, bioaugmentation was overlooked due to its unreliable record
(Pritchard, 1992 ; Thompson et al., 2005 ). Bioaugmentation can result in no visible increase in
degradation and increased cost if the full-scale delivery of microorganisms to the site of
interest fails or if there are mixing, localization and bioavailability issues. While bioaugmenta-
tion has become a common treatment for sites contaminated with chlorinated solvents, it has
not fared as well with other pollutants. There are several criteria that must be addressed prior to
bioaugmentation becoming a reliable remediation alternative for a particular pollutant. These
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