TM 5-814-3/AFM 88-11, Volume III
(1) Inlets and outlets. Inlets to rectangular tanks will be designed so as to prevent channeling of
wastewater in the tank. Submerged ports, uniformly spaced in the inlet channel, are an effective means of
securing distribution without deposition or channeling. Outlet overflow weirs used in rectangular tanks will
be of the adjustable type, and serrated weirs are preferred over straight ones. Overflow weirs will be used
in most cases.
(2) Collection and removal of scum and sludge. Means for the collection and removal of scum and
sludge are required for all settling tanks. The removal of scum from the tank will take place immediately
ahead of the outlet weirs, and the equipment may be automatic or manual in operation. Provisions will be
made so that the scum may be discharged to a separate well or sump so that it can be either sent to the
digester or disposed of separately. Rectangular tanks will be provided with scum troughs with the crest about
1 inch above maximum water surface elevation. For small installations (less than 1.0 million gallons per day),
hand-tilt troughs consisting of a horizontal, slotted pipe that can be rotated by a lever or screw will be used.
Proven mechanical scum removal devices such as chain-and-flight types may be used for larger installations.
To minimize the accumulation of sludge film on the sides of the sludge hoppers, a side slope of at least 1
vertical to 1 horizontal will be used. Separate sludge wells, into which sludge is deposited from the sludge
hoppers and from which the sludge is pumped, are preferable to direct pump connections with the hoppers.
b. Circular tanks
. Circular tank diameters
25 to 150 feet. Side-water
minimum, and tank floors are deeper at the center. Flocculator-clarifiers, gaining wide acceptance in recent
years, require much greater depths to accommodate sludge collection mechanisms. Adjustable overflow weirs
(V-notch type) will extend around the entire periphery of the tank. Scum baffles, extending down to 6 inches
below water surface, will be provided ahead of the overflow weir; and the distance between scum collection
troughs will not exceed 75 feet along the periphery of the clarifier. A circular sludge-removal mechanism with
peripheral speeds of 5 to 8 feet per minute will be provided for sludge collection at the center of the tank.
Figure 11-4 illustrates a typical circular clairfier.