Biomedical Engineering Reference
In-Depth Information
achieved. The design of the EISB system and bioaugmentation also can be made more robust in
order to achieve remediation objectives more quickly. The additional costs of bioaugmentation
must be balanced against the potential for negative outcomes, possibly with additional costs of
their own, if biodegradation is slow to be initiated and stakeholder expectations and objectives
are not met. Stakeholder needs that may require rapid treatment include property transfers,
redevelopment schedules, regulatory pressures and concerns over risks from temporary
accumulations of intermediates, such as vinyl chloride (VC). A more robust design could
include adding more culture at more discrete locations, which can reduce the time to achieve
performance expectations of an EISB system but increase bioaugmentation costs. However,
if the expectations are lower (i.e., time to achieve complete degradation is not considered
critical), then the costs of bioaugmentation may not be warranted.
The remainder of this section discusses the economics of conducting site-specific testing
followed by a discussion of the components and design of a bioaugmentation program.
In addition, this section will address the impact of remedial objectives and design parameters
on the costs and value of bioaugmentation.
11.2.1 Site Specific Testing to Evaluate Bioaugmentation
Testing for the presence and/or activity of microorganisms capable of degrading target
contaminants can help evaluate the likely impacts of bioaugmentation. The presence and/or
activity of dechlorinating bacteria can be determined in several ways. Genetic testing can be
performed on soil and groundwater samples to look for specific organisms (e.g.,
Dhc
)or
functional genes (e.g., vinyl chloride reductive dehalogenases including
vcrA
and
bvcA
) that are
associated with the complete dechlorination of chlorinated ethenes (M¨ller et al.,
2004
;
Krajmalnik-Brown et al.,
2004
). Alternatively, laboratory-scale or
in situ
microcosm treatability
testing can be conducted to evaluate the need for and potential benefits of bioaugmentation
(AFCEE,
2004
). The costs and value of each type of testing are discussed below.
Genetic testing can take several forms, but is most commonly performed using quantitative
polymerase chain reaction (qPCR) analysis by commercial laboratories. This analysis involves
removing microorganisms from soil or groundwater samples (e.g., by filtering groundwater),
rupturing the microorganisms to release their deoxyribonucleic acid (DNA), and adding
“primers” that bind to targeted gene sequences of interest. These primers act as a starting
point for DNA copying enzymes (Taq polymerase) that replicate the target DNA sequence.
A qPCR machine is used to facilitate the exponential replication of the targeted gene sequences
through a thermo-cycling process that incorporates fluorescent dyes and which reads the
fluorescence as it increases using digital optics. The initial number of targeted gene copies in
a sample is quantified by how many PCR cycles are required to cross a threshold level of
fluorescence; samples with high concentrations of the target gene cross the threshold in fewer
cycles while more dilute samples require more cycles (Mackay,
2007
).
Analysis by qPCR is available commercially and can be conducted using groundwater or
sediment from a specific site. Although soil samples can be assayed by qPCR, groundwater is
typically the preferred medium because groundwater samples and sampling techniques are less
expensive. This approach allows testing of a larger subsurface volume than soil samples
and increases the likelihood of detecting key dechlorinating microorganisms. The Strategic
Environmental Research and Development Program (SERDP) Project ER-1561 (project descrip-
tions and documents available at
www.serdp-estcp.org
;
last accessed June 18, 2012) is currently
funding efforts to develop standardized qPCR methods, which should further improve the
reliability and interpretability of these tests (e.g., Ritalahti et al.,
2010
).
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