SEWER PLANS 



199 



amount of water to be pumped, the decreased probability of 

 flooding the lowest part of the city, and, in the case of land- 

 disposal, the possibility of using the sewage at the place of 

 treatment at different levels, as is done in England at some of 

 the irrigation-areas. In this case, as in " B," the sewers leading 

 into the interceptors may be arranged according to the per- 

 pendicular system or to the fan system. 



One of the best examples of the Zone system may be seen 

 in the city of London, England, where three different zones 

 have been arranged to be drained by a low-level sewer, a mid- 



FIG. 51. 



die-level sewer, and a high-level sewer. The sewage from the 

 low-level district has to be pumped three times, that from the 

 middle-level only once, and that from the high-level sewer not 

 at all. 



It should be noted that the possibility of making use of the 

 zone system economically depends upon the cost of the high- 

 level intercepting sewer being less than the capitalized cost of 

 pumping the sewage not so intercepted. It is easily possible 

 for a high-level intercepting sewer to cost so much for construc- 

 tion as to far exceed the expense of handling the relatively small 

 amount of liquid by pumps or otherwise. 



