TM 5-814-3/AFM 88-11, Volume III
Sedimentation tanks designed for chemical addition applications will utilize the overflow rates stipulated in
table 11-4 regardless of the design plant size.
(1) Design considerations.
(a) Detention period. Detention time is commonly specified as 2.5 hours for primary tanks
serving all types of plants except when preceding an activated sludge system, where detention time is
specified as 1.5 hours. Selection of optimum detention time will depend on the tank depth and the overflow
rate. For those military installations where the contributing population is largely non-resident, the detention
period to be used in design of primary settling tanks is 2 hours, based on the average hourly rate for the 8-
hour period when the maximum number of personnel will be contributing to sewage flow.
(b) Weir rate. The overflow loading on weirs will not exceed 5,000 gallons per day per lineal foot
for plants designed for less than 0.1 million gallons per day, or 10,000 gallons per day per lineal foot for
plants designed between 0.1 and 1.0 million gallons per day. Weir loading for plants designed for flows of
more than 1.0 million gallons per day may be higher, but must not exceed 12,000 gallons per day per lineal
foot. When pumping is required, the pump capacity will be related to tank design to avoid excessive weir
Sedimentation design features.
Inlets to a settling tank will be designed to dissipate the inlet velocity, to distribute the flow uniformly, and
to prevent short circuiting. The inlet and outlet channels will be designed for a minimum velocity of 2 feet
per second at the average flow rate and will have corners filleted to prevent deposition and collection of
solids. The guidelines shown in table 11--2 will be used for designing the depths of settling tanks:
Limit the use of circular clarifiers to applications greater than 25 feet in diameter. Where space permits, at
least two units will be provided except as modified by paragraph 11-2b.
a. Rectangular tanks. The minimum length of flow from inlet to outlet of a rectangular tank will be 10
feet in order to prevent short circuiting of flow in the tank. In existing installations, tank length-to-width ratio
varies between 3:1 and 5:1. Tanks will be designed with a minimum depth of 7 feet except final tanks in
activated sludge plants, which will be designed with a 9-foot minimum depth. Figure 11-3 illustrates a typical
rectangular sedimentation tank.