ertainly removed by the application of coagulant to the raw water
during the occasional periods when its character is such as to
require it.

These general propositions were understood by those responsible for
the original design of the plant, as is shown by the author's
quotations. These experiments, however, were necessary in order to
demonstrate and bring home the conditions to those who thought
differently, and who believed that full purification could be
obtained by filtration alone, or by double filtration, without
recourse to the occasional use of coagulant.

The experiments briefly summarized in Table 20 are of the greatest
interest and importance. Six small filters, otherwise alike and like
the large filters, all received the same raw water and were operated
at different rates to determine the effect of rate on efficiency.

That the experimental results from the filter operating at the same
rate as the large filters were on the whole somewhat inferior to
those from the large filters for approximately the same period, may
be attributed to the fact that the experimental filter was new while
the large filters had been in service for some time and had thereby
gained in efficiency. The greatest difference was in the _coli_
results in Table 20, where it is shown that 24% of the 10-cu. cm.
effluent samples from the experimental filter contained _coli_, in
comparison with only from 1 to 3% of such samples from the main
filters.

The results from the experimental filter operating at a rate of
1,000,000 gal. per acre daily may fairly be excluded, as the
effluent probably contained more under-drain bacteria in proportion
than filters operated at higher rates. The number of bacteria in the
filter operating at a 3,000,000-gal. rate were 1.7% of those in the
applied water; for the filter operating twice as fast, the
percentage was 2.4; and, for the one operating more than ten times
as fast, was only 3.0; thus indicating a surprisingly small increase
in the number of bacteria with increase in rate.

Further and more detailed study by the writer of the unpublished
individual results, briefly summarized in Table 20, confirms the
substantial accuracy of the comparison based on the average figures
as stated in that table.

It must be kept in mind, in considering these results, that the
number of bacteria in each case is made up of two parts, namely,
those coming through the filter--which number is presumably greater
as t