A Comparative Study of Imhoff Tanks 
353 
This would require^ with steel at 16,000 pounds per square inch, 
8985 6 
^ = 0.56 square inch per unit width. The thickness with 
16,000 
p = 0.0075 would then be 
0.56 74.7 
0.0075 
12 
= 64 inches 
74 ^ 
12 
or 64 inches approximately. 
Assuming a 7-inch wall, the earth pressure would produce a con- 
crete stress 
18 feet X 533.3 
2 
8985.6 pounds , 
^ ' - = 107 pounds per square foot 
compression on the concrete. 
While 7 inches would seem adequate, common practice in this 
type of wall is considerably heavier, so that a 12 inch wall will 
be used throughout. 
Three-quarter inch twisted bar spaced 10 inches and 12 inches 
will be used for horizontal reinforcement, while vertical reinforce- 
ment will be 4 inch rods spaced 18 inches centers. 
Rectangular tank — description. The rectangular tank, as de- 
signed, has a tank, 22 feet by 12 feet internal dimensions, and a 
depth of 22 feet to the bottom of the digestion chamber. The 
walls are straight and vertical throughout their height, reinforced 
inside and outside as calculated above for earth pressure and for 
water pressure. The sedimentation chamber is separated from 
the digestion or sludge chamber by dividing walls 4 inches thick 
of concrete, reinforced by triangle [M] mesh or similar metal 
reinforcement. The sides are supported in the middle of their 
length by buttresses calculated as cantilevers, while the dividing 
wall is supported at the ends by the end walls of the tank and 
in the middle by a wall extending to the level of the slot and 
reinforced by rods to the main wall. 
The tank is so arranged that the sewage flow may be reversed 
in direction through the sedimentation chamber. This is ac- 
complished by a concrete channel 15 inches wide supported on 
brackets and an arrangement of stop planks of 2 inch boards 
properly arranged. Sludge is removed from the bottom of the 
tank by means of a 12 inch cast iron pipe. 
