Correct installation of the disposal field is of great importance for
proper functioning of the septic tank. Therefore, the width, depth,
and spacing of the tile trenches must be carefully selected. Line of
4-inch, open- jointed, agricultural drain tile laid in shallow trenches
are ordinarily used. Perforated fiber drain pipes also may be used and
are obtainable in 4-foot lengths.
A distribution box with an inlet for the effluent sewer and an outlet
for each individual run of disposal tile is the best means of dividing
the flow. The outlet serving a large or double disposal field may be
alternately opened and closed by means of a sewage switch that permits
half the disposal field to work and rest alternately several weeks. A
switch is especially helpful in tight soils but should not be provided
unless proper maintenance is assured, so that a portion of the disposal
field will not be left to handle the entire load of the system for an
indefinite period. There are many variations of boxes, but figure 8
shows a practical type.
[Illustration: Figure 8.--Typical distribution box.]