In 9 of the largest representative United States installations, the depth from the flow line to the slot varies from 10 feet 10 inches to 13 feet 6 inches.
Imhoff states, concerning the depth of tanks:
Deep tanks are to be preferred to shallow tanks because in them the decomposition of the sludge is improved. This is so because in the deeper tanks the temperature is maintained more uniformly and because the stirring action of the rising gas bubbles is more intense.
The stirring action of the gas bubbles is desirable as it brings the fresh sludge more quickly under the influence of the active bacterial agents. The greater pressure on the sludge in deep tanks also reduces its moisture content.
Two or more sedimentation chambers are sometimes used over one sludge digestion chamber in order to avoid the depths called for by the sloping sides of a single sedimentation chamber. An objection to multiple-flow chambers is the possibility of interchange of liquid from one chamber to another through the common digestion chamber.
The inlet and outlet devices should be so constructed that the direction of flow in the tank can be reversed in order that the accumulated sludge may be more evenly distributed in the hoppers of the digestion chamber. The sewage should leave the sedimentation chamber over a broad crested weir in order to minimize fluctuations in the level of sewage in the tank. The gases in the digesting sludge are sensitive to slight changes in pressure. A lowering of the level of sewage will release compressed gas and will too violently disturb the sludge in the digestion chamber. Hanging baffles, submerged 12 to 16 inches and projecting 12 inches above the surface of the sewage, should be placed in front of the inlet and outlet, and in long tanks intermediate baffles should be placed to prevent the movement of scum or its escape into the effluent. An Imhoff tank which is operating properly should not have any scum on the surface of the sewage in the sedimentation chamber.
The slot or opening at the bottom of the sedimentation chamber should not be less than 6 inches wide between the lips. Wider slots are preferable, but too wide a slot will involve too much loss of volume in the digestion chamber. One lip of the slot should project at least 3 inches horizontally under the other so as to prevent the return of gases through the sedimentation chamber. A triangular beam may be used as shown in Fig. 161 A. This method of construction is advantageous in increasing the available capacity for sludge storage.
The digestion chamber should be designed to store sludge from 6 to 12 months, the longer storage periods being used for smaller installations. In warm climates a shorter period may be used with success. The amount of sludge that will be accumulated is as uncertain as in other forms of sewage treatment. A widely quoted empirical formula, presented in “Sewage Sludge” by Allen, states:
C = 10.5 PD for combined sewage;
C = 5.25 PD for separate sewage,