148
J. Goldshmid, D. Zohar, Y. Argaman and Y. Kott
to sand when it is positively charged due to adsorption of cations. Oulman et al (9)
studied removal of coliform bacteria by filtration through diatomaceous earth. They
found that filtration efficiency increased markedly when the diatomaceous earth was
covered by trivalent cations, due to the positive zeta potential of the filtration medium.
Anderson and Meadows (1) claim that distilled water removes bacteria from sand better
than sea water. Tschapek and Garbosky (11) found that bacteria adsorption to sand
depends mainly on pH and zeta potential, and addition of electrolytes increased
adsorption. They also found that bacterial adsorption to sand is reversible and changes
with change in pH and electrolyte concentration.
EXPERIMENTAL
The bacterial filtration experiments were carried out in a 3.9 cm diameter Perspex
pipe. The pipe was filled with dune sand to the height of 10 cm and the sand compressed
by tapping on the pipe wall. A metal screen at the bottom of the pipe prevented the sand
from leaving it. The bacterial suspension was pumped from a glass container through a
Tygon tubing which ended 1-2 cm above the center of the sand column at a constant rate
of 1 ml/min.
After the suspension flow rate was fixed, a sample was taken to determine the MPN of
the suspension according to Standard Methods (2). Samples of the entering suspension
were taken at constant time intervals. The suspension leaving the sand column was
continuously.collected throughout the experiment for E. coli MPN determination. At the
end of the experiment the sand was taken out of the column, dried on a filter paper and
washed with tap water to determine the number of bacteria adhering to it (7).
The sand used was taken from the area designated for the infiltration ponds. Its
effective size was 0.12 mm and the uniformity coefficient 1.80. The sand was boiled in
concentrated hydrochloric acid for 2 hours and washed with distilled water. This
treatment was repeated three times. Following the acid treatment, the sand was washed
with distilled water until the entering and leaving water was of the same pH and
conductivity. The sand was then dried and sterilized in an oven at 180°C for four hours.
In the experiments using sewage plant effluents, natural dune sand that was not
washed in boiling hydrochloric acid was used.
The bacteria used in the experiments were a laboratory strain of Escherichia coli B.
The bacterial suspension was prepared from an agar slant culture after a growth period of
18 hours, during the logarithmic growth period. The bacteria were then washed ten times
in triple-distilled water and the desired bacterial concentration was prepared by dilution
with the test solution.
Wastewater effluent for the experimental work was taken from the Technion
experimental oxidation pond (30 days detention time). The effluent was centrifuged at
31,000 G for two hours and was later filtered through a 0.45μ membrane filter.
RESULTS
Fig. 1 compares filtration efficiency of bacteria in three different media. Filtration
efficiency is higher when the medium is tap water than distilled water. When
triple-distilled water was used as the medium, MPN of the effluent was practically
J. Goldshmid, D. Zohar, Y. Argaman and Y. Kott
to sand when it is positively charged due to adsorption of cations. Oulman et al (9)
studied removal of coliform bacteria by filtration through diatomaceous earth. They
found that filtration efficiency increased markedly when the diatomaceous earth was
covered by trivalent cations, due to the positive zeta potential of the filtration medium.
Anderson and Meadows (1) claim that distilled water removes bacteria from sand better
than sea water. Tschapek and Garbosky (11) found that bacteria adsorption to sand
depends mainly on pH and zeta potential, and addition of electrolytes increased
adsorption. They also found that bacterial adsorption to sand is reversible and changes
with change in pH and electrolyte concentration.
EXPERIMENTAL
The bacterial filtration experiments were carried out in a 3.9 cm diameter Perspex
pipe. The pipe was filled with dune sand to the height of 10 cm and the sand compressed
by tapping on the pipe wall. A metal screen at the bottom of the pipe prevented the sand
from leaving it. The bacterial suspension was pumped from a glass container through a
Tygon tubing which ended 1-2 cm above the center of the sand column at a constant rate
of 1 ml/min.
After the suspension flow rate was fixed, a sample was taken to determine the MPN of
the suspension according to Standard Methods (2). Samples of the entering suspension
were taken at constant time intervals. The suspension leaving the sand column was
continuously.collected throughout the experiment for E. coli MPN determination. At the
end of the experiment the sand was taken out of the column, dried on a filter paper and
washed with tap water to determine the number of bacteria adhering to it (7).
The sand used was taken from the area designated for the infiltration ponds. Its
effective size was 0.12 mm and the uniformity coefficient 1.80. The sand was boiled in
concentrated hydrochloric acid for 2 hours and washed with distilled water. This
treatment was repeated three times. Following the acid treatment, the sand was washed
with distilled water until the entering and leaving water was of the same pH and
conductivity. The sand was then dried and sterilized in an oven at 180°C for four hours.
In the experiments using sewage plant effluents, natural dune sand that was not
washed in boiling hydrochloric acid was used.
The bacteria used in the experiments were a laboratory strain of Escherichia coli B.
The bacterial suspension was prepared from an agar slant culture after a growth period of
18 hours, during the logarithmic growth period. The bacteria were then washed ten times
in triple-distilled water and the desired bacterial concentration was prepared by dilution
with the test solution.
Wastewater effluent for the experimental work was taken from the Technion
experimental oxidation pond (30 days detention time). The effluent was centrifuged at
31,000 G for two hours and was later filtered through a 0.45μ membrane filter.
RESULTS
Fig. 1 compares filtration efficiency of bacteria in three different media. Filtration
efficiency is higher when the medium is tap water than distilled water. When
triple-distilled water was used as the medium, MPN of the effluent was practically