Biology Reference
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6.2.3 Porous media
The porous media comprised of 99.1% pure quartz sand (Kristall-quartz sand, Dorsilit,
Germany) with sizes ranging from 180 to 500 m, while the median of the grain size weight
distribution was 356 m. With the median of the grain size weight, straining was excluded as
a possible retention mechanism in the column set up: assuming a bacteria equivalent
spherical diameter of 1.5 m, the ratio of colloid and grain diameter was 0.004, which was
well below the ratio (0.007) for which straining was observed by Bradford et al. (2007) for
carboxyl latex microspheres with a diameter of 1.1 mm suspended in solutions with ionic
strengths up to 31mM (the ionic strength of the solutions we used was 4.7 mmol/L only).
Total porosity was determined gravimetrically to be 0.38. Prior to the experiments, the sand
was rinsed sequentially with acetone, hexane and concentrated HCl, followed by repeated
rinsing with de-mineralized water until the electrical conductivity was below 3
µ
S/cm. This
was done to remove impurities.
6.2.4 Column experiments
To study the transport of the six
E. coli
strains isolated from the Kampala springs, column
experiments were conducted in artificial groundwater (AGW) prepared as described above.
The column consisted of a straight tube of 1.5 m transparent acrylic glass (Perspex) with an
inner diameter of 9 cm, and with five sampling ports placed at 20-40 cm intervals along the
tube. A stainless steel grid for supporting the sand was placed at the bottom of the tube. The
column was gently filled with the clean quartz sand under saturated conditions, while the
sides of the column were continuously tapped during filling, to avoid layering or trapping of
air. The column was connected both at the funnel shaped effluent end and influent end with
two Masterflex pumps (Console Drive Barnant Company Barrington Illinois, USA) via teflon
tubes, and the pumps were adjusted to a mean fluid approach velocity of 1.16x10
-4
m/s. Prior
to a column experiment, the column was flushed for 18 hours with AGW to arrive at stable
fluid conditions inside the column. Bacteria influent suspensions were prepared by washing
and centrifuging at 4600 rpm for 10 minutes three times in AGW. Bacteria cell
concentrations of the influent suspension were approximately 10
9
cells/mL. Experiments
were conducted by applying a pulse of 0.3 PV (approximately 1.1L) of bacteria influent
suspension to the column, followed by bacteria free AGW. Samples were taken at 5 distances
from the column inlet except for the experiment with SL20 where samples were taken at 4
sampling distances. Samples were diluted 3 times in AGW and the optical density was
measured at 254 nm using a spectrophotometer (Cecil 1021; Cecil Instruments Inc.,
Cambridge, England). Bacteria inactivation was assessed in all experiments by plating
samples of the influent at 30 minutes intervals during the entire experiment. All plates were
incubated at 37 °C for 24 hours. After each experiment, to clean the sand in the column, and
to prepare for the next experiment, a pulse of 0.5 L 1.9 M HCl followed by a pulse of 0.5 L
1.5 M NaOH was flushed through the column, followed by flushing with AGW water until
the electrical conductivity and pH of the effluent was equal to that of the AGW.
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