581: TRANSACTIONS OF THE A?,IERICAN INSTITUTE. 



of water actually used in New York has exceeded ninety gallons per day, 

 for every person in the city, while the quantitj' it is possible to use, for 

 drinking and cooking, is less than ten per cent, of this amount. Conse- 

 quently we find that, in addition to the ordinary house sewage proper, 

 vast quantities of water, as it conies from the supply pipes, are rushing 

 into the sewers, bearing comparatively slight impregnation of sewage mat- 

 ter. It is also observed, in corroboration, that the periods of maximum 

 daily sewage flow correspond with those of water supply ; about fifty per 

 cent, of the daily flow being between 9 A. M. and 5 P. M. 



In addition to these conditions of ordinary discharge the sewers must be 

 able to dispose of rain-fall, ordinar}- and extraordinary. 



From the varied character of surface in all cities, with gardens, parks, 

 areas, and other retarding parts, it follows that the rapidity of rain-fall or 

 storm-fall discharge is much impeded, so that it rarely happens that sixty 

 per cent, of the fall enters the sewers within six hours of the commence- 

 ment, in closely built districts, while suburban districts have given forty 

 per cent, within twenty-four hours. Nor does the entire rainfall enter the 

 sewers, by more than twenty-five per cent. 



• London observations in 1852, show that " a rainfall of half an inch in three 

 hours, took twelve hours in discharge, that is to say, twelve hours elapsed 

 from the commencement of the rain before the flow of the sewer resumed 

 its ordinary level. In a second case, a rainfall of I. II inches in about an 

 hour, with an addition of 0.33 inch in the next two hours, being nearly an 

 inch and a half in three hours, occupied in discharge fifteen and three- 

 quarter hours from the commencement of the rain." 



Observations made for the Brooklyn sewerage, show that "in a period 

 of seven years, but three days occur in which the rainfall in four hours is 

 as high as one inch, and but three days in which the whole rain during 

 twenty-four hours was as much as two inches." 



This condition of storm-flow, so much exceeds the ordinary house-sewage 

 flow, that adequate provision for the first, fully provides for all other con- 

 ditions of service, and among hydraulic engineers, a basis one-inch storm- 

 fall per hour, has been adopted for trunk sewers, which receive the supply 

 of lateral systems, the basis for the latter being two inches fall per hour. 

 ,From what has been experimentally shown under this head, it is evide^it 

 that such a basis of computation and arrangement is in abundant excess of 

 all contingencies ; and as this basis is the key to a very radical defect in 

 sewerage systems in general, it deserves special attention, as will more 

 fully appear, in I'emarks on the principles of arrangement. 



The nature of sewage flow, has an important connection with its pro- 

 cesses of flow, and is shown by analysis of its contents. 



As may be expected from the various imperfections of sewers, which 

 have been analyzed, the results vary largely in dissolved and suspended 

 matter; the proportions of solid matter vary, in cases, from 1 in 142, 1 in 

 830, and 1 in 420, to 1 in 1,000 ; advocates of utilization having assumed 

 1 in 600, as a reliable mean. The results confirm the deductions from the 

 remarks on house-sewage as to the entire fluidity of the flow, demonstrating 

 the facili-ty of removal, while it is found in some instances that the sewer 

 discharge may be actually purer and more wholesome than wells, main- 

 tained at public e.xpense for pi\blic use, in more than one modern city. 



