CHAPTER XV 

 SEWER PLANS 



THE location of the outfall is the prime element among 

 the factors brought together to decide how the mains and laterals 

 of a city shall be arranged. The outfall itself, leading to the 

 place of disposal, is located to agree with the method of disposal 

 chosen, a discussion of which is not here taken up. The outfall 

 may lead to the seashore, to the banks of a stream or lake, to 

 broad farm-lands for irrigation, to a well-adapted area for 

 nitration, or to some low out-of-the-way place for chemical 

 treatment. If the place of disposal is the sea, tides, currents, 

 and winds largely determine the location of the outfall. If 

 onto irrigation-fields, the sewage must be taken wherever suit- 

 able land is available, whether down the valley from the town 

 or on the top of a hill above it. The filtration-area must be 

 chosen where proper soil is to be found; unless the area is arti- 

 ficial, when it can be placed more advantageously as regards 

 distance and grade. If chemical treatment is to be practised, 

 only enough land for the buildings and tanks is necessary, and 

 a location to which the sewage can be led by gravity should be 

 obtained if possible. Thus the position of the outfall is not 

 in all cases to be decided by the topography, but is conditional 

 on the final disposal. However, when the sewage is to be turned 

 into a river or lake, the valley lines to the shore are usually 

 followed. If the combined system is used, provided the sewage 

 has to be treated or led away from the nearest point of the 

 river to a point farther down-stream, it is usual to let the storm- 

 water overflow into the river, while the house-sewage is carried 

 along to the desired point. This is done by automatically 

 arranged outlets. By an ingenious device the height of the 

 overflow-weir is so arranged that the overflow begins to dis- 



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