cterial
efficiency, the more complete is this relative protection.

The cost of filtering does not decrease in inverse ratio to the
rate, but at a much slower rate. This is especially true with rates
of more than 5,000,000 or 6,000,000 gal. per acre daily.

In general, a rate of filtration may rationally be selected at which
the value of the possible danger resulting from an increase in rate
is equal to the saving that may be made in cost by its use. This
point must be a matter of individual judgment. The tendency of the
last few years has been to use higher rates, or, in other words, to
cheapen the process and to tolerate a larger proportion of bacteria
in the effluent. The use of auxiliary processes has been favorable
to this, especially the use of chloride of lime, in connection with
either the raw water or the effluent.

[Illustration: ~Figure 10--Rate Million Gallons Per Acre Daily.~]

By the judicious use of this substance, efficiency may be maintained
while using higher rates than would otherwise have been desirable.

The writer believes that there will be many cases where the added
risk of using too high a rate is not worth the relatively small
saving in cost that accompanies it.

~George A. Johnson, Assoc. M. Am. Soc. C. E~.--This paper contains
information of an exceedingly interesting nature. There is
comparatively little difficulty in obtaining accurate figures on the
cost of construction of water purification works, but, with costs of
operation of such works, it is different. The data available in
published reports and papers are usually more or less fragmentary,
and unexplained local conditions with reference to the character of
the raw water, the cost of labor and supplies, and methods of
apportioning these costs, introduce variables so wide as frequently
to render the published figures almost useless for purposes of
comparison.

Mr. Hardy's paper is noteworthy in that it presents certain
relatively new features of slow sand filter operation which have
been only lightly touched on in water purification literature up to
the present time. These refer particularly to means whereby a filter
may be continued in service without removing a portion of the
surface layer of the filter surface itself when the available head
has become exhausted, and to methods whereby washed sand may be
expeditiously and more economically restored to the filter than has
been the case hitherto.

Sand handling is the most i