CHAPTER XVII 



FLUSHING 



NOTWITHSTANDING the fact that of late years the grades 

 and sizes of sewers have been more carefully determined and 

 more accurately proportioned to the work required of them, 

 and that they are now so built that the scouring and suspending 

 power of the running sewage at no time gets below a prede- 

 termined minimum, yet accumulations of silt and filth often 

 occur which must be cared for by some special means. There 

 are two ways by which such deposits may be removed, by 

 flushing or working out the obstruction with a strong flow of 

 water, and by scraping or dragging it out with a suitably 

 designed hoe or scraper. 



The water for flushing may be obtained in several ways. 

 Where the topography admits of it, water from some stream 

 may be introduced at the upper parts of the system and dis- 

 charged into the same stream at a lower level; in the case of 

 a seaside city the high tide may be allowed to enter the sewer 

 and flow out at some point where the tide is lower; a reservoir 

 may be filled at high tide, and discharged after the tide has 

 fallen; rain-water, waste water from baths, factories, etc., 

 may be accumulated for a time, and then discharged into the 

 sewer; the public water-supply may be used; or, finally, the 

 sewage itself may be dammed up and made to act as flush- 

 water. In planning the flushing arrangements, it must be borne 

 in mind that a quiet flow of sewage or water, however large, 

 is of little effect in removing obstructions once formed, and 

 that to be effective the flush-wave must be sudden, of large 

 volume, and introduced within a short distance of the obstruc- 

 tion. This wave-action, in all cases except where the stream- 

 flow is always sufficient to fill the pipes, must be formed by 



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