After the introduction of the septic tank it was seen that ordinary methods of discharging the contents were not desirable.

For instance, in the employment of underground systems of disposal an ordinary constant discharge from the septic tank would give poor results, as the liquid entering the main distributing pipe would be of such small amount that it would escape through the nearest joints, and never reach those farthest away. This would result in giving all the disposal work to a very small area, an amount greater than it could accomplish. In addition, a period of rest following a period of work is necessary in order that the supply of air to the bacteria may be renewed. To solve this difficulty, it was seen that intermittent discharges of the full contents of the discharge chamber were necessary, the interval between successive discharges being a number of hours in duration.

The automatic sewage siphon is the device employed for this purpose. There are numerous varieties on the market, depending for their action on more or less similar principles.

In Fig. A of Plate 49, a very desirable form of sewage siphon is shown, attached to the outlet end of the discharge chamber of a septic tank. Its action is the following, depending upon the confining of air between the liquid standing in the outlet of the siphon and the seal of the large trap: As the liquid rises in the discharge chamber, this confined air becomes constantly more compressed, until the pressure is great enough to blow the water out of the blow-off trap, thereby relieving the air, which is immediately followed by a heavy flow of water from the discharge chamber, of sufficient volume to quickly fill the long vertical arm, and start the siphon into full action, which continues until air enters the siphon from the outlet pipe through the air pipe.

Air enters the air pipe only when the siphon has drawn the liquid in the tank so low that the siphon does not fill the outlet. A quick passage of the contents of the discharge chamber into the siphon is provided for by enlarging the outlet from the tank.