Transport of Escherichia coli in 25 m columns - Transport of Multiple Escherichia Coli Transport in Saturated Porous Media - page 65

experiment, the column was flushed with AGW until the pH and EC at the effluent end of the
column were the same as of AGW. Bacteria influent suspensions were prepared by washing and
centrifuging (4500 rpm) three times in AGW and then diluting 10 times in 0.9 L AGW to obtain
bacteria cell concentrations of 10 8 cells/mL. Experiments were conducted by applying a pulse of
1 L (approximately 0.124 PV) of bacteria influent suspension to the column, followed by
bacteria free AGW. Samples were taken at 4 distances from the inlet of the column, and the
optical density (OD) of each sample was determined by a spectrophotometer (Cecil 1021, Cecil
Instruments Inc, Cambridge, England) at an absorbance of 254 nm. Values were then converted
to cells/mL via a calibration curve between number of cells/mL and corresponding OD 254
obtained from measurements of OD 254 and corresponding number of cells obtain by plating on
chromocult agar (Merck).

Due to the expected long duration of the experiments, bacteria inactivation experiments were
conducted alongside column experiments. This was done by plating in triplicate 0.1 mL of 10 -6
dilution of the influent on chromucult agar (Merck) at 2 hour intervals. All plates were incubated
at 37 °C for at least 18 hours. After each experiment the sand in the column was cleaned by
flushing the column with a pulse of 5 L 1.9 M HCl followed by a pulse of 5 L 1.5 M NaOH. The
column was then flushed with AGW until the pH of the effluent was 6.8-7, and the EC-value was
in the range of 1025 to 1054 S/cm. To assess the possible presence of E. coli in the column
prior to an experiment, samples were taken from all sampling ports and plated on chromocult
agar followed by incubation at 37 o C for 18 hours.

5.2.3 Determination of porosity

The porosity of the entire column was determined by cutting the saturated column into 1 m slices
followed by extrusion of the sand into pre-weighed plastic containers. The sand was then dried in
an oven for 24 hours. Porosities were determined from the mass of the slices, together with
(known) bulk volume of a 1 m slice and the density of quartz sand (2.66 g/cm 3 ).

5.2.4 Sticking efficiency and segment sticking efficiency

Sticking efficiencies over total transport distances ( Α L ) (-) from the influent end of the column to
a sampling port were computed as (Abudalo et al., 2005, Kretzschmar et al., 1997)

2 d M

Α = −

c

ln

L

(5.1)

( )

L

3 1 − Θ L Η M

0 0

where d is the median of the grain size weight distribution (m), Η is the single collector
contact efficiency (-) , Θ is the total porosity of the sand (-),and L is the travel distance (m). The
Tufenkji Elimelech (TE) correlation equation (Tufenkji and Elimelech, 2004a) was used to
compute Η . Thereto, 1055 kg/m 3 was assumed for the bacteria density, while the Hamaker
constant was estimated to be 6.5×10 -21 J (Walker et al., 2004). M is the total number of cells in
the influent and M L is the total number of cells in the effluent (-) at L obtained as (Kretzschzmar
et al., 1997)