4 SEWER DESIGN 



chemical examination goes, the real danger in sewage comes 

 from definite disease-germs which are only found in sewage 

 proper, and therefore the latter is the dangerous material. It 

 is further said that the sewers should not in any case be 

 made to take the place of street-sweeping carts, and that if 

 the streets are kept clean, as they should be, the wash- 

 water from them will not be foul, and will require no special 

 care. 



State Boards of Health have gradually acquired the right to 

 regulate the disposal of sewage, and it is their usual practice 

 to insist upon small pipe-sewers for house- wastes only and to 

 disregard storm-sewage altogether. .This indicates that the 

 current scientific belief is that the real danger to health does 

 not come from street wash, even if it does decompose and 

 appear offensive. 



2. In the matter of keeping the sewers clean, the large 

 sewer, it is said, has the advantage over the small, both in that 

 it can accumulate a larger amount of sewage for flushing pur- 

 poses, which from the greater hydraulic radius will have a greater 

 velocity and scouring power for the same grade, and in that, 

 while ordinary obstructions will be cared for by flushing, there 

 will be times when excessive deposits will occur which must be 

 removed by hand, and then a sewer large enough for a man to 

 enter can be cleaned at a much less expense than the small 

 pipe which must be opened from the surface or cleaned by rods 

 worked from the manholes. 



To this it is answered that experience shows that flushing 

 by automatic flush-tanks is sufficient to keep the smallest sewer 

 constantly clean, and that stoppages in the pipes are of rare 

 occurrence and easily removed. On the other hand, a large 

 sewer, in which there is a variable flow, allows floating matter 

 carried along in large volumes to be deposited later on the walls 

 of the sewer, clinging in a slimy layer to the uneven brick sur- 

 faces; when the amount of sewage becomes less, this matter, 

 in the warmth and darkness, generates noxious gases and fosters 

 the development of bacteria. These micro-organisms when 



