_--The quality of the effluents from all four groups
was satisfactory, and no consistent difference was apparent in favor
of one or another method of treatment. The method pursued with Group
_C_ was entirely insufficient to maintain the capacity indefinitely.
The methods pursued in Groups _B_ and _D_ were both insufficient,
but would have been more effective if a greater depth of sand had
been removed. The costs of treatment of Groups _B_ and _D_ were less
than for Group _A_. It appears, then, that a treatment which would
be more economical than the old method of Group _A_, and would still
maintain the proper capacity, would be one similar to that of Groups
_B_ or _D_, with the removal of a quantity of sand greater than was
done in the case of these two groups, but less than in the old
method.

[Illustration: ~Figure 13--Average Initial Losses of Head for Groups
_A_, _B_, _C_, and _D_ for Successive Runs.~]

At the time the above results were summarized, it was proposed to
proceed with the filter treatment along the lines just mentioned.
The writer did not have an opportunity to study the subsequent
results, as he was transferred to other work. A statement by the
author of any new facts that may have come to light in this
connection would be of interest.

Mention should be made, too, of another expedient that was used to
hasten the restoration of the capacity of a filter, which proved to
be a most useful one. The removal of the scraped sand from a filter
was a matter of a good many hours' work, under the most favorable
conditions. To get the filters quickly into service again, the dirty
sand in a number of them was simply scraped from the surface, heaped
into piles, and left there; then the water was turned in, and the
filter was started again. This was done with some hesitation at
first for fear the presence of the piles of dirty sand might cause
high bacterial counts in the effluents of those filters. No such
effect was observed, however, the counts being entirely normal
throughout. The writer subsequently found the same treatment being
applied as an emergency measure at the Torresdale plant, in
Philadelphia, and, through the courtesy of the Chief Engineer of the
Bureau of Filtration, was furnished with the bacterial counts
through a number of runs made under these conditions, and there,
too, the results were entirely normal.

There was practically no economy in this method, as the sand had
ultimately to be ejec