Biomedical Engineering Reference
In-Depth Information
needed for remediation. This time period may be lengthy, on the order of years. One strategy
that has proven effective is adjustment of the pH of the environment targeted for remediation
to favor strain KC. Also important is delivery of the organism itself. Tests with aquifer solids
from Schoolcraft, Michigan, indicated that a minimum concentration of cells required for
solids colonization and CT degradation activity was 3
10
4
cells/mL groundwater at pH 8
(S. Hashsham, Michigan State University, East Lansing, MI, personal communication). The
requirement for such a “minimum invasion force” is similar to the concept of minimum
infectious dose in the field of public health microbiology, but in this case, gives a target for
minimal cell delivery to the treatment zone. Achieving this target depends upon factors
affecting cell transport, while long-term maintenance of cells and their CT-degradation activity
depends upon the properties of the indigenous microflora and chemical delivery. These issues
are discussed in the following sections.
9.6.1 Inoculation and Transport
Biomass injected during an inoculation event is transported with the fluid and deposited by
adhesion to solid particles. Breakthrough of biomass during cell inoculation is therefore
retarded relative to the transport of an ideal tracer. Several authors (Harvey, 1991; Martin
et al.,
1992
; Rijnaarts et al.,
1996a
,
b
) have applied clean-bed filtration theory to describe
deposition of bacteria in porous media, and modeled this process in terms of collector
efficiency. The percentage of biomass that attaches to solid particles is a function of the
collision efficiency and a site-specific “blocking factor”. These parameters in turn depend upon
the solid matrix characteristics, the ionic strength of the solution, the cell physiology and the cell
concentration. Radabaugh (
1998
) used clean-bed filtration theory to determine the basic
filtration parameters for strain KC in one-dimensional columns containing solids from the
Schoolcraft site. These solids become saturated at a biomass concentration of 3
10
7
cfu/g.
Once this concentration was achieved, 100% breakthrough of biomass occurred in the liquid
phase. Vidal-Gavilan (
2000
) successfully used the transport parameters measured by Rada-
baugh (
1998
) to model the distribution of strain KC following inoculation in a three-
dimensional (3-D) model filled with Schoolcraft aquifer solids.
In addition to adherence, other factors also influence transport of
P. stutzeri
KC, including a
tendency to flocculate under nutrient-limited conditions and chemotactic motility in response to
nitrate gradients. Flocculent cells are transported poorly, and may necessitate a reinoculation
(Dybas et al.,
2002
), but effective colonization still may be possible if nitrate is present in the
background groundwater. Witt et al. (
1999
) demonstrated use of strain KC for remediation of a
0.5 meter (m) static sediment column filled with of CT- and nitrate-contaminated groundwater.
Strain KC cells and acetate were added to the middle of the column. Migration of cells away
from the point of inoculation was monitored, along with concentrations of nitrate and CT
throughout the column. Introduction of excess acetate quickly caused nitrate-limited conditions
at the center of the column, and formation of a nitrate gradient. The nitrate gradient triggered a
chemotactic response. As strain KC migrated away from the center of the column and towards
the ends of the column, nitrate and CT levels fell. The column was remediated without flow of
groundwater.
In the field-scale inoculations performed to date (Dybas et al.,
1998
,
2002
), nitrate was
present in the background groundwater. Addition of high levels of acetate along with strain KC
likely facilitated a chemotactic response and more rapid colonization of the targeted treatment
zones.
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