SEWER PLANS 



197 



the perpendicular system (Fig. 48).* The mains follow down 

 the beds of the separate valleys with laterals running from the 

 ridge-lines between. There are as many mains as there are 

 subordinate valleys, the grades are the best possible, and the 

 sections of the sewers are small. Wherever a flat area is adjacent 

 to the stream, and sewers from higher land must cross this to 

 reach the water, it is possible that in heavy rains the low land 

 may be flooded from the gorged sewers; this, however, is a 

 question of the design and can be avoided. 



(B) Intercepting system. If the stream is not large enough 

 for satisfactory dilution and the sewage has all to be carried 



Int 



nq5e 



ver 



FIG. 49. 



to a sing'e point for treatment, or if the river- water is used for 

 domestic purposes, so that the sewage has to be carried down- 

 stream below the intake of the river-water, then the ends of 

 the mains of system " A " are picked up by an intercepting 

 sewer, the combination making the intercepting system, Fig. 49. 

 This is sometimes an after-thought (as in Milwaukee and 

 Chicago), in which case the different elevations of the main 

 ends makes the construction of the intercepting sewer very 

 difficult. Sometimes this intercepting sewer may be designed 

 on such a scale as to pick up the ends of outfall sewers from 

 separate villages or cities. Thus along the Charles River, 

 just outside of Boston, is an intercepting sewer which takes 



* Report of the National Board of Health, 1881, page 117 et seq. 



