TM 5-814-3/AFM 88-11, Volume III
Assume a grit chamber width of 2 feet for flows less than 1 million gallons per day, and 2 to 4 feet for flows
between 1 and 2 million gallons per day. For flows greater than 2 million gallons per day, divide the area
calculated above by the assumed width to obtain the length of the channel. At a flow velocity of 1.0 foot per
second, the depth of flow can be determined by equation 10-2:
D = depth of flow (ft);
L = length of channel (ft);
V = flow velocity (fps);
X = settling rate (sec/ft).
The rectangular cross-section channel will be used in treatment plants with capacity less than 2 million gallons
per day. The parabolic channel is costly and best suited to larger plants. Square grit chambers are sized as
rectangular cross-section channel chambers; horizontal area is the basis of design. These units are best suited
to flows greater than 2 million gallons per day.
(b) Channel size. In channel grit chambers, the length will be designed to be 50 percent longer
than theoretically required to allow for turbulence and outlet distribution. The floor of the chamber will be
far enough below the weir crest to make allowance for the accumulation of about 2.5 cubic feet of grit every
10 days per million gallons of wastewater. This depth allowance will be not less than 2 inches. The channels
will be as narrow as possible without causing serious submergence of the crown of the inlet sewer. The
effluent channel will be designed so that objectionable shooting velocities are not produced and submergence
of the weir crest by tail water will not exceed permissible limits.
c. Aerated grit chamber. When wastewater flows into an aerated grit chamber, the grit will settle at
rates dependent on the size, specific gravity, and the velocity of roll in the tank. The variable rate of air
diffusion is a method of velocity control which can be easily adjustable to different field conditions. The
following design criteria are to be used:
(1) Air rates. The air system should be designed to provide 8 cubic feet per minute per foot of grit
chamber length. The design should allow the air rate to be controlled over a range.
(2) Detention time. The chamber should be designed to have a detention time of 3 minutes at the
maximum flow rate.
(3) Geometry. The inlet and outlet should be placed to prevent short circuiting in the chamber. In addi
tion, the inlet should introduce the wastewater directly into the circulation pattern caused by the air. The
outlet should be at a right angle to the inlet with a baffle. A length to width ratio of 4:1 should be used.
d. Quantity of grit. The following design values should be used to provide storage for the collected grit.
These values, along with the anticipated frequency of grit removal, will determine the storage volume
(1) Combined sewer system. For a sewer system carrying both stormwater and domestic wastewater,
storage for 30 cubic feet of grit per million gallons of flow should be provided.
(2) Separate sewer system. Storage for 10 cubic feet of grit per million gallons of flow should be
e. Disposal of grit. Impervious surfaces with drains will be provided as grit-handling areas. If the grit is
to be transported, the conveying equipment must be designed to minimize loss of material. Suitable drainage
facilities must be provided for a screenings-collection platform and for storage areas. Grit disposal will be
in a sanitary landfill.