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J. Goldshmid, D. Zohar, Y. Argaman and Y. Kott
The changes in filtration efficiency are much in line with the Shulze-Hardy laws, which
describe the effect of addition of electrolytes on the stability of lyophobic colloids.
Following this line of reasoning, the increase in filtration efficiency measured at lower pH
is also explainable by the increase in hydrogen ion concentration of the solution. To
demonstrate that it is the change in hydrogen ion concentration that increases efficiency
and not the addition of sodium cations as part of the buffer, the effect of pH was
checked twice, at low and high conductives. It is evident from Fig. 4 that the effect of
sodium ion concentration is small compared with that of the pH. An interesting byline is
that an increase in conductivity caused an increase in filtration efficiency at high pH, and
a decrease of efficiency at low pH. This can be explained as follows: at high pH the
concentration of hydrogen ions is so low that addition of sodium cations decreased the
zeta potential a little and increased filtration efficiency. At low pH the effect is different.
The pH of the bacterial surface at low ionic strength is about 0.4 pH units lower than the
medium, while at high ionic strength it approaches the medium pH (6). Thus an increase
in conductivity decreases the actual pH of the bacterial surface and decreases filtration
efficiency.
Addition of oxidation pond effluents to tap water decreased filtration efficiency in
proportion to the proportion added. Fig. 6 shows that to achieve the same filtration
efficiency in suspensions containing sewage plant effluents as was achieved in tap water,
much higher concentrations of the same cation were required. When the sewage effluent
was dialysed after centrifugation and filtration, filtration efficiency was the same as
measured, for tap water. It seems that the decrease in filtration efficiency is caused by the
presence of macromolecules which are polyanionic colloids. Several possible mechanisms
for the effect of these macromolecules are now being examined.
The reversibility of the phenomena was tested by changing the medium during the run.
As can be seen from Fig. 7 the change from 0.01 M MgCl 2 solution and from tap water to
distilled water caused a large increase in bacteria in the effluent, before reaching a
constant level. The phenomenon is reversible and changing solution concentration can
dislodge already filtered bacteria.
CONCLUSIONS
1. An increase in cation concentration and valence increases filtration efficiency of
coliform bacteria through dune sand. The effect is similar to that predicted from
Shulze-Hardy laws regarding coagulation of colloids.
2. In the presence of oxidation pond effluents more cations are required to achieve the
same filtration efficiency.
3. Bacterial balance showed that bacteria stay alive even after being attached to sand
particles.
4. No agglomeration of bacteria was observed in the concentration range tested in these
experiments.
5. A decrease of pH from 9.3 to 3.9 increased filtration efficiency.
6. The effect of cation concentration on filtration efficiency is reversible. A decrease in
cation concentration of the same valence will filter out bacteria which had already been
removed.
J. Goldshmid, D. Zohar, Y. Argaman and Y. Kott
The changes in filtration efficiency are much in line with the Shulze-Hardy laws, which
describe the effect of addition of electrolytes on the stability of lyophobic colloids.
Following this line of reasoning, the increase in filtration efficiency measured at lower pH
is also explainable by the increase in hydrogen ion concentration of the solution. To
demonstrate that it is the change in hydrogen ion concentration that increases efficiency
and not the addition of sodium cations as part of the buffer, the effect of pH was
checked twice, at low and high conductives. It is evident from Fig. 4 that the effect of
sodium ion concentration is small compared with that of the pH. An interesting byline is
that an increase in conductivity caused an increase in filtration efficiency at high pH, and
a decrease of efficiency at low pH. This can be explained as follows: at high pH the
concentration of hydrogen ions is so low that addition of sodium cations decreased the
zeta potential a little and increased filtration efficiency. At low pH the effect is different.
The pH of the bacterial surface at low ionic strength is about 0.4 pH units lower than the
medium, while at high ionic strength it approaches the medium pH (6). Thus an increase
in conductivity decreases the actual pH of the bacterial surface and decreases filtration
efficiency.
Addition of oxidation pond effluents to tap water decreased filtration efficiency in
proportion to the proportion added. Fig. 6 shows that to achieve the same filtration
efficiency in suspensions containing sewage plant effluents as was achieved in tap water,
much higher concentrations of the same cation were required. When the sewage effluent
was dialysed after centrifugation and filtration, filtration efficiency was the same as
measured, for tap water. It seems that the decrease in filtration efficiency is caused by the
presence of macromolecules which are polyanionic colloids. Several possible mechanisms
for the effect of these macromolecules are now being examined.
The reversibility of the phenomena was tested by changing the medium during the run.
As can be seen from Fig. 7 the change from 0.01 M MgCl 2 solution and from tap water to
distilled water caused a large increase in bacteria in the effluent, before reaching a
constant level. The phenomenon is reversible and changing solution concentration can
dislodge already filtered bacteria.
CONCLUSIONS
1. An increase in cation concentration and valence increases filtration efficiency of
coliform bacteria through dune sand. The effect is similar to that predicted from
Shulze-Hardy laws regarding coagulation of colloids.
2. In the presence of oxidation pond effluents more cations are required to achieve the
same filtration efficiency.
3. Bacterial balance showed that bacteria stay alive even after being attached to sand
particles.
4. No agglomeration of bacteria was observed in the concentration range tested in these
experiments.
5. A decrease of pH from 9.3 to 3.9 increased filtration efficiency.
6. The effect of cation concentration on filtration efficiency is reversible. A decrease in
cation concentration of the same valence will filter out bacteria which had already been
removed.