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4.3.2 Variation in sticking efficiencies
In DI, for all E. coli strains, Α values in the top segments were highest (0.103-0.352; Table 4.1 ) ,
and reduced with distance. Lowest Α values (10 -5 -0.06) were determined for the two most
distant column segments (between 2.33 and 4.83 m from the top of the column). Negative values
of
Α were computed for the third and fourth segments for UFCL-71 in DI.
Table 4.1: Results of
Α obtained in DI and AGW at distances from top of the column.
Distance
from top of
column to
end of
segment
Segment
Thickness
(m)
DI Water
UCFL-71 UCFL-94 UCFL-131 UCFL-167 UCFL-263 UCFL-348
AGW
UCFL-71 UCFL-94 UCFL-131 UCFL-167 UCFL-263 UCFL-348
0.13
0.13
0.352
0.103
0.140
n.d.
0.326
0.252
n.d
1.034
5.527
4.017
5.954
9.470
0.33
0.20
0.174
0.079
0.260
0.136
0.092
0.218
n.d
0.002
0.017
1.191
0.745
b.d.
0.83
0.50
-0.030
0.021
0.080
0.024
0.013
0.148
n.d
0.002
0.023
0.669
0.007
b.d.
1.33
0.50
-0.037
0.005
0.100
0.070
0.001
0.185
0.719*
0.002
0.047
0.139
0.126
b.d.
2.33
1.00
0.041
0.015
0.042
0.055
0.003
0.217
0.145
0.022
0.020
b.d
0.203
b.d.
3.33
1.00
0.003
0.027
0.020
0.090
0.032
0.168
0.019
0.020
0.283
b.d
b.d
b.d.
4.83
1.50
0.003
0.025
0.037
0.062
1.E-05
0.061
0.085
0.006
b.d
b.d
b.d
b.d.
* Value is for segment 0 -1.33 m, n.d., No data; b.d., Below detection limit
In these cases, the breakthrough at the end of the column segment was higher than at the
beginning of the column segment. Although this phenomenon could possibly be due to release of
attached cells, we were more inclined to attribute the negative values to inaccuracies in the
method to determine the E. coli concentration (plating on Chromocult): in case of little
attachment, the length of the column segment should be such that significant differences in E.
coli influent and effluent mass can be measured in order to avoid negative values of
Α.
In AGW (artificial groundwater), Α values in the top segments were highest (1.034 - 9.470),
and reduced with distance. We did not consider sticking efficiency values in excess of 1 to be
very strange. The presence of cell surface organelles like flagella and pili that extend beyond the
cell surface may cause underestimation of the aspect ratio used in the computation of the single-
collector contact efficiency (Shellenberger and Logan, 2002; Morrow et al. 2005; Paramonova et
al. 2006). For many strains (UCFL-131, UCFL-167, UCFL-263, and UCFL-348), in AGW
removal of E. coli was complete before the final sampling port was reached. In case of UCFL-
348, removal was already complete within a traveled distance of 13-33 cm from the column
inlet. For UCFL-71, E. coli breakthrough in sampling ports at 0.13, 0.33, and 0.83 m was not
measured, and therefore, Α was calculated for the segment 0-1.33 m ( Α = 0.719 in Table 4.1 ) .
Also in DI, negative Α values were computed for UCFL-71, suggesting either a complex
mechanism of release of attached cells or inaccuracies of the plating method, as was discussed
 
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