There are three types of apparatus used to raise sewage to the street sewer, each of which has certain features to recommend it. When the volume of the sewage to be removed is large and the height to be raised is small, a centrifugal pump will give very satisfactory results. This type of pump can be driven by belting or may be operated by an electric motor directly attached to the pump shaft. By means of a float and an automatic switch an electric driven pump can be made to operate automatically, starting when the tank is filled with sewage and stopping when it is empty. The manner of installing a centrifugal pump and tank is shown in Fig. 49. With this type of ejector an ordinary steel tank is used that may be either open or closed. The pump should be set below the level of the receiving tank, so it will remain full of water and not require priming. If placed above the level of the tank a primer will be necessary to start the pump, and this so complicates the apparatus that it is more difficult to fit up to work automatically. Where the sewage is coarse and full of solid matter, as is likely to be the case in slaughter houses or factories, a centrifugal pump will give the best results. It has few working parts to get out of order, and no parts that can choke up and thus render the pump temporarily useless; for any substance, even coal or bricks, that passes through the inlet port can easily be discharged from the outlet. Speed is an important factor in the capacity of centrifugal pumps; increasing the speed increases the capacity and also the height to which it will raise sewage, while decreasing the speed will reduce considerably the volume of sewage and the height it will be raised. The following table gives the sizes of pulleys required for pumps of different sizes, the diameters of discharge pipes, capacities per minute and revolutions per minute required to raise sewage to given heights.

Fig. 49

Table XI - Capacities Of Centrifugal Pumps

The operation of the apparatus shown in Fig. 49 is as follows: Sewage enters the sump a through the house drain b; as the tank fills the sewage raises the float c, and thus by means of the chain, pulleys and weight w, depresses the lever d until it reaches a certain point when contact is made that completes an electric circuit connected to the electric motor e. The current thus automatically turned on operates the electric motor that drives the pump p and thus ejects the sewage from the tank through the pipe e to the sewer s. As the water line in the tank lowers, the float falls until it reaches a certain level near the bottom, when the automatic switch opens, thus breaking the electric circuit and stopping the pump.