efficient and economical administration, with thorough record of all
details, should furnish a groundwork for the careful consideration
of the question stated above.

The writer, using as a text some of the ideas given in the paper,
but more particularly some of those becoming prevalent elsewhere,
desires to discuss methods and costs of operation, especially in
relation to sand handling; and to offer suggestions looking toward
greater efficiency, as well as economy, in carrying out the standard
and well-tried methods.

_Theory of Slow Sand Filtration._--First, what is the process of slow
sand filtration? The answer to this question involves many factors, some
of which are even yet but imperfectly understood. In the early history
of filtration, at the time of the construction of the London filters,
only the straining capacity of the sand bed, to remove gross particles,
was known. Later, when the organic contents of water had become better
understood, the chemical or oxidizing powers of the process were
recognized as performing an important part. Finally, co-existent with
the discovery of the so-called "germ theory of disease," a study of the
bacterial action of filters resulted in the recognition of its
importance. It is now universally thought that each of these factors
performs its useful function; that the size of the sand, the amount of
organic matter remaining on the surface of the bed, the turbidity of the
applied water, and the bacterial content of the influent, are some of
the things on which depends the determination of the relative importance
of each.

[Footnote 1: _Transactions_, Am. Soc. C. E., Vol. XLVI, p. 258.]

Engineers have been taught to believe, by the German school of
thought, that the film of organic matter on the surface of the sand
plays a very important role in filtration. This _Schmutzdecke_, as
it is called, has been considered so precious that stress has been
placed on treating it with great care. It was not to be wholly
removed at the time of cleaning, and it was not to be walked on, or
indented, or in any other way consolidated or destroyed. In fact, in
some cases, the wasting of the first water after cleaning has been
advocated, for the reason that not a sufficient amount of this
organic film would be left on top of the sand to begin the
filtration process properly immediately after the cleaning.

In late years, however, there has been a tendency to depart from
this fundamental doc