General Consideration

Mechanical ejectment of sewage is resorted to in cases where the street sewer is above the level of the area to be drained. This condition, however, is only found in the sub-basement floors of tall city buildings, underground public toilet rooms and underground passenger stations.

A system of mechanical ejectment consists of a gravity drainage system to a receiving tank or sump located in a water-tight pit at the lowest part of the drainage system, and a pump or compressed air ejector to raise the sewage and discharge it into the street sewer.

Systems of piping for sub-sewer drainage are the same as for gravity discharge systems, except that no main drain trap is required in sub-sewer drainage systems. Vent stacks from the sub-sewer system in tall buildings may connect to the vent stacks of the gravity system, and thus save the expense of extending them separately through the roof. In subway stations and underground toilet rooms the vent pipe should extend above the roof of the building.

The fresh air inlet should be run, and the outlet located in the same manner as for gravity systems. The point where it intersects the house drain, however, varies with the method of sewage ejectment. When pumps are used to discharge the sewage the fresh air inlet should connect to the top of the receiving tank, where, besides serving as a fresh air inlet, it also serves as a vent to the tank when filling, and admits air when the pump is operating. When an ejector operated by compressed air is used the fresh air inlet should intersect the main house drain on the house side of the receiving pump.