226 SEWER DESIGN 



is by no means insignificant, but that their use increases the 

 cost of the separate system by nearly one-tenth, besides intro- 

 ducing a permanent charge, both for water used and for intelli- 

 gent care in maintenance. That these annual charges are 

 no bagatelle will be apparent by again referring to the case 

 of Ithaca. Assuming that the tanks required are of a capacity 

 of 150 gallons, a minimum amount, discharging but once a day, 

 the water required is 19,650 gallons a day. Twenty cents per 

 1000 gallons (the amount charged in Ithaca *) is a fair average 

 amount, and at that price the daily charge for water is $3.93, 

 or $1434.45 per year. Adding to this $600 per year as the wages 

 of a mechanic, whose constant attention is found by experience 

 to be necessary in examining and readjusting the tanks, the total 

 annual charge is $2034.45. This, capitalized at 6 per cent, 

 gives $33,908, and, added to the $6550, gives $40,458 as the 

 total expenditure on account of flush-tanks in a sewer system 

 costing for pipe laid $81.000. Surely the item of flush- tanks 

 is an important one, and should be carefully examined, so that 

 if the conditions of the sewer-grade, for example, modify the 

 necessity for tanks, or if the amount of water is a function of 

 the time-interval between flushes, or of the size of the pipe, 

 it may be known in order that the large proportionate cost of 

 flushing may be reduced to what has been found by careful 

 investigation to be an absolute minimum. 



That the requirement given above is felt by present-day engi- 

 neers to be largely in excess of necessity is sufficiently evident 

 from a study of the paper by F. S. Odell, M. Am. Soc. C. E., 

 entitled " The Separate Sewer System without Automatic 

 Flush- tanks," f and the subsequent discussion, in which the 

 author says that at Mt. Vernon, N. Y., no flush-tanks are 

 used, and that, while hand-flushing by means of fire-hose is 

 practised at intervals of six months, even this infrequent flush- 

 ing does not appear necessary, as examination of the sewers 

 invariably shows a very wholesome and satisfactory condition. 



* " Manual of American Water-works," 1897. 

 f Trans. Am. Soc. C. E., Vol. XXXIV, page 223. 



