ling | 38 | 24 | 4 | 14 | 29
left from: | filtering| 2.4 | 3.0 | 11.2 | 8.2 | 2.8
| both | 0.90 | 0.79 | 0.44 | 1.12 | 0.81
================+==========+========+========+========+========+========

The fluctuation in the efficiency of the plant as a whole by seasons
is greater with the turbidity than with the bacteria. During the
winter the effluent contains 3% of the turbidity of the raw water,
and in summer only 0.3 per cent. Most of this difference is
represented by the increased efficiency of the filters in summer,
and only a little of it by the increased efficiency of settling.
With bacteria, on the other hand, the seasonal fluctuation of the
plant as a whole is comparatively small, but the settling and
storage processes are much more efficient in summer than in winter,
the filters being apparently less efficient. The writer believes
that they are only apparently less efficient, and not really so, the
explanation being that some bacteria always grow in the under-drains
and lower parts of the filter, and are washed away by the effluent.
The average number of bacteria in summer in the settled water is 160
per cu. cm. and in the filtered water 18. These are very low
numbers. It is the writer's view that nearly all of these 18
represent under-drain bacteria, and practically bear no relation to
those in the applied water, and, if this view is correct, the number
of bacteria actually passing through the various processes is at all
times less than the figures indicate. In the warmer part of the year
the difference is a wide one, and the hygienic efficiency of the
process is much greater than is indicated by the gross numbers of
bacteria.

The reduction of the typhoid death rate has not been as great with
the change in water supply as was the case at Lawrence, Albany, and
other cities, apparently because the Potomac water before it was
filtered was not the cause of a large part of the typhoid fever.

The sewage pollution of the Potomac is much less than that of the
Merrimac and the Hudson, and it is perhaps not surprising that this
relatively small amount of pollution was less potent in causing
typhoid fever than the greater pollution of rivers draining more
densely populated areas.

The method of replacing the washed sand hydraulically seems to have
worked better than could have been reasonably anticipated, and the
writer believes that this was