tank, to attach an elbow to the entering pipe so
that the sewage enters about halfway between the top and bottom of the
tank (see Fig. 72). Similarly, at the outlet or weir an elbow should be
provided because it is not desirable to allow the floating matter of the
surface to be carried onto the bed, and a pipe taking off liquid, open
halfway between top and bottom, will carry away but little of either the
surface scum or bottom sediment. Such a tank must be built of concrete
or masonry or timber, although the latter is not to be recommended
because of its short life. The walls of an ordinary tank may be built 6
inches thick at the top and 12 inches to 18 inches thick at the bottom,
the latter being necessary if the depth is over 8 feet. The tank should
have 6 inches of concrete on the bottom, and the roof may be made of
flagstone or of concrete slabs in which some wire mesh has been buried.

It is not necessary to ventilate this tank, although it is desirable to
have perhaps a foot of air-space between the water level and the roof of
the tank. During the first few months of its operation such a tank is
very likely to smell badly, and, if ventilators are provided, the
presence of the tank will be well known by the odors sent off. After the
tank has been in operation two or three months these odors gradually
disappear, due presumably to the fact that the surface of the water in
the tank has become coated with a thick blanket through which odors
cannot penetrate. On the other hand, there have been a few cases
recorded where the production of gas in a septic tank was so great that
an explosion occurred, tearing off the roof and otherwise doing
considerable damage.

The full plant, therefore, will consist of the settling tank, receiving
the raw sewage from the house and discharging it into a small tank
holding about one hour's flow and containing the automatic syphon
apparatus for intermittent discharge. This dosing tank must provide for
one hour's flow at the maximum rate of flow, and should hold about one
fourth of the total daily flow. Then the ground area, either natural or
artificial, which receives the intermittent discharge from the dosing
tank, completes the installation (see Fig. 73).

[Illustration: FIG. 73.--Plan of sewage disposal for single house with
details of receiving tank.]

_Underdrains._

The question of installing underdrains will arise only in cases where
the ground water, always to be found below