Biomedical Engineering Reference
In-Depth Information
transformation activity for up to 6-7 weeks when stored as cell suspensions and sustained 70%
of their activity for up to 21 days (Taylor et al.,
1993
; Shah et al.,
1996
).
8.4.1.2 Column Studies with
Methylosinus trichosporium
OB3b
Cells having favorable attachment characteristics (i.e., they attach readily and are slow to
detach) are desirable for developing a biofilter of resting cells for aerobic cometabolism
(Hanna and Taylor,
1996
). The attachment/detachment of a rosette-dominated form of
Methy-
losinus trichosporium
OB3b and the longevity of the resting cells' ability to transform TCE
was evaluated in small 1 centimeter (cm)
10 cm columns packed with a quartz-sand (Hanna
and Taylor,
1996
) (Table
8.3
). Because the chemical composition of the solution affects
the attachment/detachment, a medium containing phosphate buffer at pH 7 was employed.
Initial attachment resulted in a cell concentration of 7 to 8
10
8
cells/g of dry sand.
Including1.0 millimolar (mM) magnesium chloride (MgCl
2
), 100 micromolar (
m
M) ferrous
sulfate (FeSO
4
) and 0.025% agar resulted in an increase in the attached cell concentration
to 1.5
10
9
cell/g dry sand. These additions increased the time to reach 50% detachment
from 5 days to ~ 45 days. About 34% of the cells were retained on the biofilter for about
15 weeks. Studies of TCE cometabolism showed weekly pulses of 250
m
g/L of TCE were
transformed to below the drinking water standard of 5
m
g/L for a period of 8 weeks. These tests
demonstrated that a biofilter could be operated for up to 8 weeks before needing replenish-
ment, and the replenishment was needed primarily because the sMMO enzymes were slowly
inactivated over time.
Tompson et al. (
1994
) also evaluated the addition of
Methylosinus trichosporium
OB3b
expressing sMMO for its resting cell cometabolic transformation potential (Table
8.3
). A 10-cm
thick biofilter was created by injecting bacteria into a sand pack through five wells. A steady
flow of groundwater at a velocity of 1.5 cm/hour (h) was established through the filter and TCE
was added as 4 mg/L pulse input every 13 days. TCE cometabolism was found to be complete
from 0 to 5 days and then became increasingly limited from 5 to 15 days. The results of the test
were simulated using an advective-dispersive transport model that included Michaelis-Menton
kinetics and a limited transformation capacity model for TCE transformation. Based on
simulation fits to the observed breakthrough of TCE at the downstream monitoring wells,
the transformation capacity was estimated to be 0.30 g TCE per g cells. The authors indicated
that this capacity would not limit the application of this technology in the field.
8.4.1.3 Field Study: Resting Cell Biofilter Using
Methylosinus
trichosporium
OB3b
The field test conducted at the Chico Municipal Airport, California, involved injection of
about 5.4 kilograms (kg) (dry weight) of cells suspended in 1,800 L of groundwater, to achieve a
cell injection concentration of 5.4
10
9
cells/mL (Duba et al.,
1996
) (Table
8.4
). The cells were
delivered to the site as a cold paste and diluted into site groundwater and injected at a rate of
3.8 L/min for about 8 h, followed by the addition of groundwater without cells to help spread
the cells further into the formation. At the end of the injection process, the injection well was
turned into an extraction well and groundwater was extracted for 30 h at a rate of 3.8 L/min,
followed by a rate of 2 L/min of 38 days.
Measurements of the suspended cells in the extracted groundwater indicated that the
bioaugmentation culture became attached to the aquifer solids. As shown in Figure
8.3
,
background concentrations of TCE in the groundwater of 425
m
g/L decreased to less than
10
m
g/L during the first 50 h of extraction and then gradually increased to the background level
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