240 SEWER DESIGN 



statements of other engineers, it seems to the author that on 

 grades of less than i per cent automatic flush tanks are an 

 economic necessity, even where water has to be paid for, the 

 added expense of frequent hand-flushing more than offsetting 

 the possible discharge of flush- tanks when not absolutely 

 necessary. 



(4) The volume of water discharged should not be less than 

 40 cubic feet, and the effect of the flush can hardly be expected 

 to reach more than 600 or 800 feet. Below this point accumu- 

 lations may occur which must be removed by hand-flushing and 

 carried on to a point where the sewage-flow has the necessary 

 transporting power. 



(5) On flat lines and where obstructions occur below the 

 influence of the flush-tank, a second flush-tank, placed about 

 800 feet from the first, will be more effective than increasing 

 the first tank to a capacity of three times its original discharge. 



(6) The frequency of discharge should depend on the local 

 conditions, but it is probable that the maximum interval 

 depends on the practical working of the siphon, so that the 

 usual prescription of once in 24 hours is a safe rule. 



(7) If tanks are used on grades greater than i per cent, 

 15 to 20 cubic feet give as good results as larger amounts, with 

 the same rule as to frequency of discharge. 



(8) However, economy is best served, on grades above i 

 per cent, by omitting flush-tanks, and resorting to periodic 

 hand-flushing at such intervals as experience shows to be neces- 

 sary on the different lines. In most cases semi-annual or 

 quarterly flushings, with a hose, are sufficient. 



(9) On grades greater than 3 per cent flush-tanks are unnec- 

 essary, and their installation is a waste of money. 



(10) Hand-flushing should be performed and tanks dis- 

 charged at night, as a flow of even an inch in a sewer offers a 

 large resistance to the flushing action; while with a pipe flowing 

 half full the effect of a flush-tank is scarcely visible. 



