Determining the minimum sticking efficiency of six environmental Escherichia coli isolates - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 55

4.3.4 Minimum sticking efficiencies

With the fitted power law distributions, we were able to extrapolate retained fractions and
sticking efficiencies to values lower than the ones we determined in the 5 m column of quartz
sand. For the sake of this paper, we assumed that when the retained bacteria fraction was reduced
to 0.001% (5 log units) of the original bacteria mass (total number of cells) flowing into the
column, i.e. F = 10 -5 in Figs. 4.2 a and b , then these retained cells possessed a so-called
minimum sticking efficiency. The choice of a 5 log elimination was arbitrary in the sense that the
total number of cells flowing into the column was much more than 10 5 . However, because we
injected a pulse of a constant concentration with constant velocity, the relation between bacteria
mass and maximum bacteria concentration, along transport distance, was almost linear, assuming
limited dispersion, as was the case in our experiments ( Fig. 4.1 ). Therefore, we interpreted a
bacteria mass reduction of 5 log-units as being equal to a bacteria concentration reduction of 5
log-units. We considered such a concentration reduction to be maximal for environmental
conditions, since maximum E. coli concentrations in waste water are within the 10 4 - 10 6
cells/mL range (Foppen and Schijven, 2006; Baxter and Clark, 1984; Canter and Knox, 1985).
Minimum sticking efficiencies extrapolated in this way ranged from as low as 10 -9 for UCFL-94
to 10 -2 for UCFL-348 in the DI water experiments, and from 10 -6 for UCFL-94 to for UCFL-

≥ 1

348 in the AGW experiments ( Table 4.3 ).

Table 4. 3 Fitted power-law equations between F and Α and extrapolated minimum sticking

efficiency values for a 5 log bacteria mass removal

Bacteria
Strain Fitted power-law

equation of
measured data
(DI)

Extrapolated
minimum sticking
efficiency
(DI)

Fitted power-law
equation of measured
data
(AGW)

Extrapolated
minimum sticking
efficiency
(AGW)

UCFL -71 8.50E-08 4.70E-02

0.7 3.49

F = 0.89 Α F = 0.43 Α

i i i i

UCFL -94 1.93E-09 6.06E-06

0.55 0.97

F = 0.62 Α F = 1.15 Α

i i i i

UCFL-131 3.45E-06 9.44E-03

0.92 1.63

F = 1.06 Α F = 0.02 Α

i i i i

UCFL-167 6.65E-07 1.73E-01

0.83 3.42

F = 1.34 Α F = 0.004 Α

i i i i

UCFL-263 6.81E-07 3.39E-02

0.82 1.77

F = 1.14 Α F = 0.004 Α

i i i i

UCFL-348 1.48E-02 - ≥ 1

3.6

F = 38.5 Α

i i

4.4 Discussion

Our results showed that overall, in both DI and AGW, for all E. coli strains we used, the
computed lower values of Α were in the same order of magnitude (10 -3 -10 -2 ). However, for the
DI experiments, the fraction of E. coli mass that had passed the column without being retained