CHAPTER XVIII 

 USE OF FLUSH-TANKS 



THE following paper, read by the author before the American 

 Society of Civil Engineers, in May, 1898, offers a discussion 

 on the suitable use of flush-tanks, their proper capacity, fre- 

 quency of discharge, etc. 



The use of flush-tanks in connection with small pipe sewers, 

 which has been made an integral part of the " Separate System " 

 and generally adopted in systems caring only for house-sewage, 

 is attended with much uncertainty. In such systems it is gen- 

 erally specified that a flush-tank be placed at the head of every 

 lateral, each tank being so regulated as to discharge at least 

 once in 24 hours. The relation between the size of the sewer- 

 pipe and the amount of water used in a flush is not given, nor 

 is the influence of grade discussed. The general law is laid 

 down that all laterals, regardless of size, grade, or contributing 

 population, must be supplied with flush-tanks in order to secure 

 a self-cleansing flow in the laterals and to maintain the integrity 

 of the system. 



The financial burden of such a requirement is evident. 

 As an example, it may be cited that in the plans for the sewerage 

 system of Ithaca, N. Y., in which plans this requirement of flush- 

 tanks was thoroughly complied with, even for the 12 per cent 

 grades, no less than 131 flush- tanks were required in 25.3 miles 

 of sewers, or one for every 1020 feet. The relative importance 

 of the flush- tanks may also be seen by comparing the actual 

 cost of the sewers with the estimated cost of the tanks. The 

 cost of the sewers, viz., the sum of the amounts of the several 

 contracts, was $81,000, and, estimated at $50 each, the flush- 

 tanks would cost $6550, or more than 8 per cent of the cost of 

 the system. It would seem, then, that the cost of flush-tanks 



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