Biomedical Engineering Reference
In-Depth Information
5.4.2.2 Implications for Bioaugmentation
Similar to the active recirculation strategy, bioaugmentation is not generally a large
incremental cost for semi-passive systems, because the infrastructure is already in place.
Inoculation in a semi-passive approach is most often performed using the existing electron
donor injection well locations. Generally, only a single inoculation for bioaugmentation is
required after sufficient electron donor injections have been performed to create an appropri-
ate reducing environment for the bacterial culture. Recent studies have shown that semi-passive
bioaugmentation can be successfully applied to sites to achieve complete dechlorination under a
variety of subsurface conditions (Lee et al., 2008 ; Mora et al., 2008 ). If intermittent ground-
water recirculation is used between wells, the potential exists for more uniform culture
distribution transverse to groundwater flow.
5.4.3 Passive Approach
A passive approach refers to a system where the electron donor is emplaced once or on an
infrequent basis (i.e., annually or less) and where no forced gradient is applied. Bioaugmenta-
tion may be conducted either during or following the electron donor addition. Both electron
donor and bacterial transport occur primarily under the influence of ambient groundwater
flow. Example applications include “biobarriers”, where a treatment zone is created perpendic-
ular to groundwater flow (using injection wells, direct push injection points, or trenches) to
reduce the flux of contaminants downgradient and large-scale inundation of source areas or
plumes with a long-lived, or “slow-release,” electron donor with the goal of achieving substan-
tial treatment during the lifetime of the electron donor.
5.4.3.1 Advantages/Disadvantages for Biostimulation
Given that several methods for passive biostimulation/bioaugmentation exist, the advan-
tages and disadvantages of the methods cannot be generalized, but must be considered
individually. Table 5.2 discusses each delivery method relative to several key considerations
for design and implementation. While advantages and disadvantages differ among the passive
approaches, some generalizations can be made for passive approaches relative to semi-passive
and active biostimulation/bioaugmentation strategies. In general, passive approaches require
less O&M and have lower capital costs at small scales, but can have higher capital costs at large
scales.
5.4.3.2 Implications for Bioaugmentation
The means of inoculation in a passive approach can depend largely on the methodology
used for electron donor emplacement. When conventional injection wells or direct-push injec-
tion points are used, bioaugmentation is most often performed using the injection locations.
For trenching, inoculation can be performed using a pipe laid in the trench during installation or
using wells installed in, or immediately downgradient, of the trench. For fracturing applica-
tions, the boreholes used to initiate fractures can be used to inoculate groundwater once
fractures have been installed. Several studies have been performed showing that bioaugmenta-
tion with hydraulic fracturing has significantly better results than standard bioaugmentation,
especially in low permeability subsurface environments (Bjerg et al., 2006 ).
Although it is common to inject electron donors via single-use direct-push points, few cases
of inoculations in direct-push injection points have been documented in the literature. One
reason direct-push points are not used more often may be that they are temporary points
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