SEWER PLANS 201 



part is built up, so that there is little possibility of the section 

 growing and needing larger sewers, while the part on the out- 

 skirts which will grow is near the pumping-stations or disposal 

 works and can be served at comparatively small cost and with- 

 out interference with the rest of the system. In the other 

 systems, as the outskirts away from the outfall grow, the whole 

 system must be increased, the intermediate lines being designed 

 to carry off only the amount first considered. 



The ideal topography for the radial system would be a 

 gently sloping conical hill with the city at the apex. Such an 

 ideal condition is really never found, so that the practical use of 

 this system is limited to level areas for the drainage of which 

 pumping will have to be resorted to by any system. At Berlin 

 the outfall sewers leave the city in four different directions and 

 the sewage is lifted by pumps and discharged onto the several 

 irrigation fields which constitute the celebrated sewage farm of 

 Berlin. 



Many combinations of these systems occur, and the out- 

 lines given are to be considered only as guides to judgment in 

 the individual case. The arrangement and combination must 

 be adjusted to the topographical conditions. In the intercept- 

 ing system, if there are a number of subordinate mains and 

 one of those farthest up-stream is low, it follows that the inter- 

 ceptor in order to take the sewage from this and still have a 

 grade down-stream must at the last contributing main reach 

 a point much lower than otherwise necessary. 



It is also often possible to take up a few hundred feet of the 

 original subordinate main and, by relaying on a lighter grade, 

 be able to raise the intercepting sewer throughout its entire 

 length and thereby considerably reduce its cost. A small 

 auxiliary pumping-station may sometimes be introduced to 

 care for the sewage from the single low main and so reduce the 

 entire cost of construction. In the new intercepting sewer 

 for Chicago, where the depth is determined both by the depth 

 of the present mains and by the requirements of grade, tunnel- 

 work is resorted to as being cheaper than open cut, and large 



