Discussion by Ebba Lund, Denmark
It should be pointed out that the work was not done in sand dunes, but in a small laboratory
model. When the filtration efficiency using distilled water is compared with the results obtained with
the different salts applied (as shown in figs 2 and 3) it is seen that they lie within the same range in a
number of cases. It would be of interest to know how the salt solutions were made and what were the
conductivities of the distilled water and tap water. In the discussion and in Fig. 6 it is taken for
granted that only the cations affect the filtration. The body of the experiments do support this
hypothesis; it would have been better to define solutions in terms of ionic activities in the true
physical-chemical sense before making such emphatic statements. The filtering efficiency is shown to
depend very much on the pH. At different pH values it is stated "that an increase in conductivity
caused an increase in filtration efficiency at high pH, and a decrease of efficiency at low pH." This
statement does not agree with the values of fig. 4. Is there an error in this figure? The practical
importance of the pH-effect cannot be very great as it does not seem possible to work at such pH
levels.
Fig. 5 shows results that are very significant. It is shown that with sewage treatment effluent the
point is soon reached where no bacteria are retained in the filter. Thus, the filter is soon saturated and
therefore might be useless as a filter. If this applies to real life situations the serious situation would be
reached when large quantities of effluent filtered through sand filters of unknown filtering capacity
might lead to equally inefficient results. What are the authors' views on the practical implications of
their work on this point? In fig. 6 it is shown that it takes much higher salt concentrations to increase
filtration efficiency when treatment plant effluent is run through than when distilled water is used and
finally that the returned bacteria can easily be washed off the filter when the salt concentration is
decreased. What is the practical significance of these observations?
Reply
The salt solutions were made dissolving analytical grade crystals in triple distilled water. The
conductivities of distilled and triple distilled water were practically the same and equal to 1.5
micromho/cm. The conductivity of tap water was about 1000 micromho/cm It was, therefore, concluded that distilled water contained minute quantities of polyvalent cations, which markedly affect
the filtration efficiency while not detected by conductivity measurements.
There is a typographical error in Figure 4: the conductivities of the solutions at pH 3.9 should be
3.0 x 10 and 1.4 x 10
2 micromho/cm.
With regard to the practical implications of the results shown in Fig. 5 it should be noted that the
laboratory filter was only 10 cm. deep and was loaded by a highly concentrated bacterial suspension.
Under field conditions, with considerably deeper soil strata and lower loading rates, saturation and
bacterial breakthrough may be reached after a much longer period.
157
It should be pointed out that the work was not done in sand dunes, but in a small laboratory
model. When the filtration efficiency using distilled water is compared with the results obtained with
the different salts applied (as shown in figs 2 and 3) it is seen that they lie within the same range in a
number of cases. It would be of interest to know how the salt solutions were made and what were the
conductivities of the distilled water and tap water. In the discussion and in Fig. 6 it is taken for
granted that only the cations affect the filtration. The body of the experiments do support this
hypothesis; it would have been better to define solutions in terms of ionic activities in the true
physical-chemical sense before making such emphatic statements. The filtering efficiency is shown to
depend very much on the pH. At different pH values it is stated "that an increase in conductivity
caused an increase in filtration efficiency at high pH, and a decrease of efficiency at low pH." This
statement does not agree with the values of fig. 4. Is there an error in this figure? The practical
importance of the pH-effect cannot be very great as it does not seem possible to work at such pH
levels.
Fig. 5 shows results that are very significant. It is shown that with sewage treatment effluent the
point is soon reached where no bacteria are retained in the filter. Thus, the filter is soon saturated and
therefore might be useless as a filter. If this applies to real life situations the serious situation would be
reached when large quantities of effluent filtered through sand filters of unknown filtering capacity
might lead to equally inefficient results. What are the authors' views on the practical implications of
their work on this point? In fig. 6 it is shown that it takes much higher salt concentrations to increase
filtration efficiency when treatment plant effluent is run through than when distilled water is used and
finally that the returned bacteria can easily be washed off the filter when the salt concentration is
decreased. What is the practical significance of these observations?
Reply
The salt solutions were made dissolving analytical grade crystals in triple distilled water. The
conductivities of distilled and triple distilled water were practically the same and equal to 1.5
micromho/cm. The conductivity of tap water was about 1000 micromho/cm It was, therefore, concluded that distilled water contained minute quantities of polyvalent cations, which markedly affect
the filtration efficiency while not detected by conductivity measurements.
There is a typographical error in Figure 4: the conductivities of the solutions at pH 3.9 should be
3.0 x 10 and 1.4 x 10
2 micromho/cm.
With regard to the practical implications of the results shown in Fig. 5 it should be noted that the
laboratory filter was only 10 cm. deep and was loaded by a highly concentrated bacterial suspension.
Under field conditions, with considerably deeper soil strata and lower loading rates, saturation and
bacterial breakthrough may be reached after a much longer period.
157