198 



SEWER DESIGN 



the sewage from Waltham, Newtonville, and other places 

 which formerly had sewer systems discharging directly into the 

 river. Similarly, in New Jersey along the Passaic River and 

 In New York, along the Bronx River, are sewers which act as 

 interceptors for villages and cities instead of for lines of sewers 

 all within one city. The first system, i.e., the perpendicular, 

 can always be designed so that, when the necessity occurs, the 

 interceptor may be put in with the elevations of the mains 

 properly adjusted. This large sewer is usually expensive to 

 .build, being in the lowest ground, often below the stream-level, 

 in gravel or soft mud. Besides preserving the stream from 

 pollution, this system has the further advantage of allowing 



FIG. 50. 



all the sewage to be brought to one point for pumping in case 

 this is necessary, so that one large pump can take the place of 

 several small ones. 



(C) Zone system. In case the sewage has to be pumped 

 it may happen that a large part of the contributing territory 

 is high enough so that the sewage from that part will flow to 

 the outfall by gravity, and in this case the sewers may be 

 arranged to form the Zone system, that is, a double intercept- 

 ing system. An intercepting sewer is laid nearly following a 

 contour so that it may discharge all the sewage from the land 

 above it to the outfall by gravity, while the second interceptor 

 collects only that part of the sewage which would in any case 

 have to be pumped (Fig. 50). The advantages are the reduced 



