3. Where a siphon chamber is provided, its size is such that the dose of sewage shall be approximately equal to 20 gallons per person; that is, the capacity of the siphon chamber between the discharge and low-water lines is roughly equal to the quantity of sewage discharged in 12 hours. [Illustration: Fig. 23.--One-chamber septic tank--does nothing more than a tight cesspool. Brick construction, heavily plastered inside; size suitable for 180 to 280 gallons of sewage daily (nominally 4 to 7 persons).] A simple one-chamber brick tank suitable for a household discharging 180 to 280 gallons of sewage daily is shown in figure 23. A small two-chamber tank constructed of 24-inch vitrified pipe, suitable for a household discharging about 125 gallons of sewage daily, is shown in figure 24. A typical two-chamber concrete tank is shown in figure 25. Excepting the submerged outlet, all pipes within the tank and built into the masonry are cast-iron soil pipe with cast-iron fittings. Vitrified or concrete sewer pipe and specials are generally used as they are frequently more readily obtainable and a slight saving in first cost may be effected. Cast iron is less liable to be broken in handling or after being set rigidly in masonry, and the joints are more easily made water-tight. The submerged outlet is midway of the depth of liquid in the settling chamber. The inside depth of the siphon chamber is the drawing depth of the siphon plus 1 foot 5 inches. The following table gives the principal dimensions with quantities of materials for four sizes of tank as illustrated in figure 25: _Dimensions and quantities for septic tanks._ --------+--------+--------+-------------------------------------------- |Quantity| | Settling chamber. | of |Capacity| Number | sewage | below +-------+-------+-------+----+-------+---+--- of | in 24 | flow |Length.| Depth.|Width. | W. | X. | Y.| Z. persons.| hours. | line. | | | | | | | --------+--------+--------+-------+-------+-------+----+-------+---+--- | Galls. | Galls. |Ft. In.|Ft. In.|Ft. In.| In.|Ft. In.|In.|In. 5 | 180-280| 240 | 4 0 | 5 0 | 2 0 | 6 | 2 0 | 4 | 6 10 | 320-480| 420 | 5 0 | 5 6 | 2 6 | 6 | 2 3 | 4 | 6 15 | 520-680| 620 | 5 6 | 6 0 | 3 0 | 8 | 2 6 | 5 | 8 20 | 720-960| 860 | 6 0 | 6 6 | 3 6 | 8 | 2 9 | 5 | 8 --------+--------+--------+-------+-------+-------+----+-------+---+---