A Comparative Study of Imhoff Tanks 349 
Design of Tanks 
Size of sedimentation and sludge chambers. 15,620 gallon ca- 
1 5 620 
pacity is equivalent to ^ 2085 cubic feet (7.48 = the 
number of liquid gallons in 1 cubic foot). This is the required 
capacity of the sedimentation chamber. 
For the assumed population of 2500 and a storage capacity 
for six months’ sludge (required on account of winter storage 
of sludge) and a rate of 0.0035 cubic foot per capita per day, 
there will be required 2500 X 0.0035 X 6 X 30 = 1575 cubic feet, 
or 42 per cent of the total volume. With a storage capacity in 
the sedimentation chamber of 2610 cubic feet (calculated on the 
basis of two and one-half hours detention period) this would be 
36.5 per cent of the total volume. 
In view of the small size and necessary small depth of the 
tanks, this is the ratio of capacities of the sedimentation and 
sludge digestion chambers, though larger than usually specified, 
the ratio for the Proctor Creek plant in Atlanta, Georgia, being 
25 per cent. 
Stresses in tank walls. In calculating the stresses, for the 
purpose of detailed design, two kinds will develop. 
When the tank is empty, earth pressure will act, upon the walls 
inward, producing bending moments in one direction at the middle 
of one side of the rectangular tanks and in the opposite direc- 
tion at the ends. In the circular tank compression will result in 
the entire ring. 
As an alternative loading, the case may be considered where 
the tank is full of water, and because of slippage, earth contrac- 
tion and bad construction, the backing has been removed far 
enough for flexure of the walls to take place, to develop flexural 
stresses of reverse signs in the rectangular tank and tensile stresses 
in the circular tank. 
The sloping bottoms may be considered as any tank founda- 
tion and designed accordingly. At the points of connection with 
the vertical walls, stresses will be developed which are uncertain 
in amount and character, but will be provided for by connecting 
reinforcing steel. 
The walls will be considered as slabs resting upon two vertical 
beams, and loaded uniformly along a horizontal strip. The two 
vertical beams may be either the end walls or intermediate beams 
