ted and washed. The piling up of the sand had
the effect of reducing the effective filtering area by a small
percentage, with a corresponding increase in the actual rate of
filtration, but this was of trifling importance. The great benefit
derived from the method was the saving of time in getting a filter
back into service after scraping, and in this respect it was very
valuable.

~Physical Theory of Purification of Water by Slow Sand Filters~.

The first and most natural conception of the action of a sand filter
is that the removal of impurities is effected by a straining action.
This, of course, is perfectly true as far as it relates to a large
part of the visible impurities. Much of this is gross enough to be
intercepted and held at the surface of the sand. This very straining
action is an accumulative one. After a quantity of suspended matter
thus strained out mats itself on the surface of the sand, it in turn
becomes a strainer, even better adapted than the clean sand surface
which supports it for the removal of suspended matter from the
water.

This, however, cannot explain certain features of the purification
of water by a layer of sand. The removal of color, the reduction of
nitrates, and certain other changes in the organic content of the
water have for a long time been recognized as due to a bio-chemical
action carried on by certain bacteria in the sand. Both the
straining action and this bio-chemical action are not all-sufficient
for the explanation of certain phenomena, and it has been
recognized, too, that sedimentation in the pores of the sand played
a large part in the purification process in those cases in which it
was apparent that the biological agencies were not the chief ones.

In the purification of water containing only insignificant
quantities of suspended matter, but a relatively large amount of
unstable organic matter, it will be conceded at once that the chief
factor in the purification is the nitrification produced by the
bacteria in the upper layers of the sand. On the other hand, the
purification by sand filters of a hypothetical water containing no
organic matter, but only finely-divided mineral matter in
suspension, could take place only by the physical deposition of the
particles upon the sand grains. Between these two extremes lie all
classes of water. In all problems of water purification by
filtration through sand, both these factors--biological action and
sedimentation--play t