. The sanitation of cities. flow flushes them out. With this system,the sewage in dry weather loosens much sus-pended matter during its passage through thesewers, and the storm flow is rendered exceed-ingly heavy with decomposing sewage solidswhich are gathered up from the inverts duringthe first part of a storm. Combined and Intercepting System This system is similar to the foregoing system;in that all of the smaller sewers are combinedsewers; but to obviate the trouble caused by thedry weather, or domestic flow, in the large mains,and prevent the deposits therein of sewage solids,interceptin
. The sanitation of cities. flow flushes them out. With this system,the sewage in dry weather loosens much sus-pended matter during its passage through thesewers, and the storm flow is rendered exceed-ingly heavy with decomposing sewage solidswhich are gathered up from the inverts duringthe first part of a storm. Combined and Intercepting System This system is similar to the foregoing system;in that all of the smaller sewers are combinedsewers; but to obviate the trouble caused by thedry weather, or domestic flow, in the large mains,and prevent the deposits therein of sewage solids,intercepting sewers are provided to collect thedry weather sewage from the smaller sewers, andact to some extent as separate sewers, parallelingthe lines of the large mains. These interceptorstake off the dry weather flow at certain placescalled diversion points, by gravity, as they areusually placed at a lower elevation. Separate System In this, a double system is provided; one sys-tem for the storm water, usually called drainage, 59. 6o ^be Sewerage System and one for the sewage, usually called sanitarysewers. The cost is much greater than the others, butthe system is more efficient and, if possible,should be used rather than either of the others;especially, if the sewage or the storm water, orboth, are to be treated; as their treatment in-volves very different problems, and is much moreeconomical if each is kept separate. The installation of the sewerage system, for agiven city, has usually been completed to a greatextent before the city planner, or replanner, hasbeen consulted. In most of our cities the sewer-age system, like the street system, just grewwithout design and is especially in need of ex-pert study, and complete redesign. The progress that modern cities have made inpopulation, commerce and material prosperity,was not and indeed could not have been foreseen,even a few years ago. In sewer design, the mostthat was usually attempted was the removal ofpolluted waters from the h
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