n in Table 29 show but
little difference in the bacterial contents of the effluents from
the four groups of filters. All are entirely satisfactory, and the
differences in favor of one method or another are small. In looking
for possible differences in the quality of the effluents from the
four groups, it was thought that such differences might be most
apparent at a time when the entire plant was working under the most
adverse conditions. The bacterial counts, therefore, were summarized
for the period from December 23d, 1907, to January 6th, 1908,
inclusive, following a period of high turbidity and high bacteria in
the raw water, with results as follows:

Group....... _A_ _B_ _C_ _D_
Maximum..... 204 178 189 206
Minimum..... 61 45 62 57
Average..... 120 107 104 155

The following is a summary of the turbidity results for a similar
period:

Group....... _A_ _B_ _C_ _D_
Maximum..... 10.8 11.7 8.7 9.3
Minimum..... 6.7 4.7 6.2 5.7
Average..... 8.7 8.3 7.2 7.9

These numbers, though high, do not show any significant differences.
All the averages for each group are less than the lowest maximum,
and all are greater than the highest minimum, and therefore vary
less than do the individual filters, from other causes, within the
different groups.

_Future Capacity of the Filters._--An indication of the dangers
which might affect the future capacity of the filters was shown in
the above discussion of the present capacity. A more effective way
of showing this was obtained by a study of the initial resistances
or losses of head in the four groups. A filter kept in ideal
condition would show no increase in this initial loss of head from
one run to the next. If there is such an increase, it means that at
some future time measures more heroic than ordinarily used would be
necessary to restore the proper capacity.

The average initial losses of head for the different groups are
plotted on the diagram, Figure 13. Group _A_ shows an initial loss of
head, increasing gradually but slightly during more than two years
of service. In Group _B_ the initial loss of head increased in a
manner similar to that in Group _A,_ up to the time of the beginning
of these experiments; after which the increase becomes more rapid.
Groups _C_ and _D_ show conditions generally similar to Group _B_,
with some variations which are self-explanatory.

_Conclusions.