USE OF FLUSH-TANKS 227 



In the discussion very little positive evidence is given, but the 

 experiences recorded go chiefly to show that while automatic 

 flush-tanks do not in themselves make the separate system 

 practicable, there is, nevertheless, a need, under certain con- 

 ditions, for flushing, those conditions being as yet not fully 

 determined. 



The questions, answers to which are essential for an intel- 

 ligent disposal of flush- tanks on a sewer system, are four, viz.: 



1. What is the relation, if any, between the grade of the 

 sewer and the necessity for automatic flush-tanks? 



2. Assuming a need for automatic tanks, how does the 

 grade of the sewer affect the amount of water required, and 

 what is the proper amount to be used? 



3. How often should tanks be discharged? 



4. What effect does the substitution of a 6-inch for an 8-inch 

 lateral have on the necessity for tanks and on the amount of 

 water to be used? 



Before attempting to answer these questions, it will be well 

 to look at the subject broadly, and consider the hydraulic 

 problem involved. Sewage is water carrying in suspension 

 less than i part in 1000 of solid matter, and sewers are supposed 

 to be so laid that the resulting velocity of flow is sufficient 

 to keep this solid matter in suspension. This suspending 

 and scouring power probably depends on the velocity, and 

 on the depth, of the sewage stream, and if either gets below 

 a certain point, sedimentation will follow and a deposit take 

 place. It is generally stated that a velocity of about 2^ feet 

 per second is required; but the effect of depth is neglected. 

 At the lower end of a 6-inch lateral the depth and velocity are 

 assumed to be sufficient to prevent this sedimentation, but as 

 the contributing population grows less toward the upper end, 

 the depth and velocity decrease and the transporting power of 

 the stream falls so low as to allow the solid matter, brought 

 into the sewer by the house-drains, to become stranded. This 

 deposit increases by gradual accumulation until the sewer is 

 blocked, until the head from the backed-up sewage is sufficient 



