160 SEWER DESIGN 



settlement of particles of varying size. Thus, Hazen has 

 found* that in still water particles i mm. in diameter settle at 

 the rate of .33 foot per second, but that particles .1 mm. in 

 diameter reduce this rate to .028 foot per second and those 

 .01 mm. to .005 foot per second. That is, the size of par- 

 ticle, particularly in the case of fine suspended matter, plays 

 an important part in the rate of settling and it is quite pos- 

 sible for the particles to be so small that they will not settle 

 at all. In the separate system the size of the suspended parti- 

 cles is continually changing on account of the organic decom- 

 position of the material and at the lower end of a long line the 

 reduction in size may be so great that a much smaller velocity 

 will suffice to keep the sewer clean than would be needed at the 

 upper end. The relatively low velocities in Table XX shown 

 in the cases of soft earth and clay are undoubtedly due to the 

 small size of the particles involved. Experiments with more 

 finely divided material might show even lower velocities. The 

 question then is not merely what velocities will prevent sedi- 

 mentation, but what are the sizes of particles of suspended 

 matter found in sewage and what velocities are necessary to 

 prevent sedimentation of such particles. 



Since the size of particles carried in suspension in sewers, 

 either for storm water or for domestic wastes, is largely con- 

 jectural, it follows that it is hopeless to attempt to apply any 

 definite law of hydraulics to the problem, but that rather the 

 engineer must be contented to accept the results of experience 

 and secure in new work such velocities as have been found 

 adequate to prevent undesirable subsidence. 



The following required velocities are those suggested by 

 different prominent engineers and have been tabulated by 

 Staley and Pierson: 



Baldwin Latham 2 to 3 ft. per sec. 



Beardmore 2.5 to 3 



Phillips 2.5 to 3 



* Trans. Am. Soc. C. E., Vol. LIII, p. 63. 



