different methods of treatment are shown in a general way in Table
29 for days of service and millions of gallons filtered per run.
This element by itself is decidedly in favor of the deep scrapings,
and least in favor of the repeated rakings.

A clearer conception of the capacities of the filters under these
different conditions may be obtained from the four diagrams, Figure
12, showing, for the four different groups, the average number of
days of service of the successive runs. The diagram for Group _A_
shows that the variations in the period of service of the filters
scraped each time to clean sand follow a more or less definite curve
from year to year. For the period covered by this curve, the
tendency seems to be toward a slight decrease in capacity from year
to year, as shown by the lower average maximum and minimum in the
second year than in the first. Group _B_ shows a sudden decrease in
capacity following the first light scrapings and, since that time, a
low but quite constant capacity. Group _C_ shows a constantly
decreasing capacity with successive rakings. The only significance
attaching to the curve after the first raking is the prohibitively
low capacity indicated, and the ineffectiveness of the measures
taken to restore the capacity after the sixth raking. Group _D_,
after the first raking, shows a prohibitively low and constantly
decreasing capacity. The diagrams for _C_ and _D_ indicate a
dangerous reduction in capacity if long persisted in. The method
followed with Group _C_ may be dismissed with the statement that it
is entirely insufficient, and would be of use only in the rarest
emergencies.

As far as the question of capacity is concerned, these diagrams indicate
that a filter in normal condition may safely be raked once. It is
believed that the constantly decreasing capacity shown in Group _D_ is
not due so much to the rakings as to the small quantities of sand
removed at the alternate scrapings, and therefore it would not be proper
to condemn this method of treatment without a further trial in which
this defect was remedied. This view seems to be supported by the results
of Group _B_. The low but approximately constant capacity there shown
would undoubtedly have been higher if a greater depth of sand had been
removed each time.

[Illustration: ~Figure 12--Average Number of Days of Service of
Successive Runs for Groups _A_, _B_, _C_, and _D_.~]

_Quality of the Effluent._--The averages give