38 
BULLETIN" 57, U. S. DEPARTMENT OF AGBICULTUBE. 
This tank is suited to flat ground where outlet fall is difficult to obtain, 
as will be noted by the difference in elevation between the floors of the 
two chambers. For satisfactory operation a small septic tank should 
not be built of smaller size than for six nersons. 
LOCATION AND CONSTRUCTION. 
The septic tank, although air-tight and supposedly water-tight, 
should be located as far from the house and the well or spring as con- 
venience and local surroundings will permit, thus reducing the danger 
of pollution or nuisance in case of leakage or improper operation of 
the system. 
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Fig. 30. — Double chamber septic tank for family of six people, suitable to conditions where outlet fall is 
difficult to obtain. 
The sewer from the house should be of vitrified sewer pipe, usually 
of 4 inches size, with tightly cemented joints, and should be laid to 
a grade not less than 9 inches per 100 feet. Where the fall from 
the house to the tank is excessive, it is a good idea to lay the last 100 
feet of tile to the minimum grade to break up entrance velocity. 
It is assumed that the farmer has a working knowledge of small con- 
crete structures. 1 The septic tank should be constructed as nearly 
water-tight as possible, preferably of concrete. The walls should be 
6 or 8 mches thick, the floor 4 to 6 inches thick, and the roof about 
6 inches thick and reenforced. Some means should be provided at 
the bottom to facilitate the cleaning out of settled sludge. Either 
the floor may be sloped toward the inlet end for this purpose or a pipe 
1 U. S. Dept. Agr., Farmers' Bui. 461. 
