e to do this, since all three processes described are
interfered with if the solids taken out by sedimentation are allowed to
be deposited either upon the surface of the ground, giving rise to odors
as well as to objectionable appearances, or onto the surface of the sand
beds, which they clog up, or in the three-inch tile drain, which may be
filled in a short time.

It has been further found by experience that if these sedimentation
tanks are made large, really larger than necessary for sedimentation, in
some way a large proportion of the matter accumulating in the tank will
disappear, so that the amount of sediment to be taken out of the tank is
not as large as might be expected. In fact it is usual for such tanks to
run one or two years without cleaning, although the amount of solids
shown by chemical analysis to have been removed from the sewage would
fill the tank twice over.

It has been found that a tank, in order to do successful work in
separating solids and in eliminating as much as possible of the
sediment, needs to be of a capacity to equal about one day's flow of the
sewage, and this is a good basis for computation. Here, again, the fact
that the sewage from a single house is considerably fresher than the
sewage from a city must be remembered, since, while many cities build
tanks holding only one third or one fourth of their daily flow with good
results, in the case of a single house this is not possible, and the
tanks, if built at all, ought to hold at least the full day's flow. Ten
persons, at 25 gallons each, furnish 250 gallons per day or 33 cubic
feet. The tank, then, must be large enough to hold this volume, and
suitable proportions generally require that the tank be at least 5 times
as long as wide. A certain allowance must always be made for deposit in
the bottom and for the accumulation of scum on the top, so that an extra
foot or more of depth is desirable. The tank, then, to furnish the
required 33 feet, might be made 3 feet wide, 3 feet deep, and 5 feet
long, and probably in no case would a tank much smaller than this be
used.

[Illustration: FIG 72.--Section of a septic tank with syphon chamber.]

There are two or three details of tank construction which may be
suggested, although almost any kind of tank will answer the purpose. It
is desirable in order that the surface scum may not be disturbed, and in
order that the inflowing sewage may distribute itself as uniformly as
possible across the