A Comparative Study of Imhoff Tanks 
347 
average flow, then, this means a capacity in the settling chamber 
of 15,620 gallons or a flowing- through time for maximum flow of 
or one hour and twenty minutes. 
11,715 
For a considerable period of time after the inception of the 
disposal process, it is probable that the period of detention will 
be much longer than the above figures. 
In designing tanks, care must be taken to see that the flow is 
evenly distributed, that no '^dead’^ places or places of low veloc- 
ity occur. 
In old-style, single-story, sedimentation tanks, the velocity 
was an important factor in design, as affecting the deposition of 
material. The effect of excessive velocity in the two-story tank 
is minimized by the fact that the solid part or sludge is removed 
from the disturbing effect of velocities on the liquid. 
The velocities of flow in Imhoff tanks operated in the Emscher 
District have been studied, and Mr. Chas. Saville, formerly of 
the Emschergenossenschaft, recommends 0.20 of an inch per 
second, with 0.50 inch per second as a maximum, for satisfactory 
results. 
Volume of sedimentation chamber. Knowing the value of av- 
erage and maximum flows and the best detention period deter- 
mines the volume or capacity of the settling chamber and the 
arrangement of baffles in the chamber will be such as to keep the 
velocity and mixing in proper condition. 
Volume of sludge chamber. The next step in design is to de- 
termine the capacity of the sludge or digestion chamber. 
The capacity of the sludge chamber will depend upon the 
character and amount of suspended matter. In the deeper tanks, 
the sludge, being under greater pressure, is much more compact 
than in the case of the shallow tank. The patentees of the tank 
recommend that the sludge chamber for shallow tanks be made 
50 per. cent greater than in the deep tank. 
In large tanks some sludge may be drawn off every month, 
but in the smaller tanks the sludge should be kept nearly to the 
slots. Not all of the sludge should be removed at any one time. 
At the end of summer, the sludge is usually drawn off, to receive 
the winter’s accumulation of sludge. The usual storage capacity 
is six months. In the plant at Winters, California, the sludge- 
ohamber capacity was calculated on the basis of 0.007 cubic foot 
of sludge per capita per day. 
