TM 5-814-3/AFM 88-11, Volume III
10-4.
Grit chambers.
a. Purpose. The primary purpose of grit chambers is to protect pumps and other mechanical equipment.
They may not be required if surface runoff is excluded from the sanitary sewer system; however; current
policy is to include grit chambers for equipment protection regardless of the nature of the sewer system.
Silting occurs through improper joints, broken manholes, and other openings in the system even without
contributing surface runoff. Grit chambers will be located ahead of pumps and comminuting devices. Coarse
bar racks will be placed ahead of mechanically cleaned grit-removal facilities. There are two types of grit
chambers: horizontal-flow and aerated. The first attempts at controlling the wastewater velocity so that grit
would settle out were the use of horizontal-flow chambers designed to maintain the velocity as close to 7 feet
per second as practical. This velocity will carry most of the smaller organic particles through the chamber and
will tend to resuspend those that settle but will allow the heavier inorganic grit to settle out. In recent years,
the aerated grit chamber has been more widely used because introducing oxygen into the wastewater early
in the treatment process is beneficial and there is minimal head loss through the chamber; however, the
increased operational and energy costs must be included in evaluating this option.
b. Horizontal-flow grit chamber.
(1) Design basis. Grit chambers will be designed for a controlled velocity of 1 foot per second (at the
average rate of flow) in order to prevent settling of organic solids (at low rates of flow) and scouring (at high
rates of flow). The velocities at these conditions will not vary more than 10 percent from the design velocity.
A sample design is shown in appendix C.
(2) Velocity control. Control of velocity within a grit chamber will be provided by a control section
paced by a weir, a Parshall flume, or a Venturi flume.
(a) The weir will be either a proportional weir, Parshall flume or Venturi flume. Appendix C
contains formulations and tables for design parameters applicable to such flow-control devices.
(b) A Parshall flume is effective in controlling the velocity through a grit chamber within
reasonable limits if the width of the flume throat is narrow enough to cause wide variations in the depth of
water for the expected range of flow rates. Appendix C includes formulations and parameters of design for
Parshall flumes. One advantage of Parshall flume control is that it can serve for both metering and velocity
control.
(c) A Venturi flume of rectangular cross-section is effective for the control of velocity in a
rectangular grit chamber. The design of the effluent channel and other structures below the flume must be
such that the head loss between the grit chamber and the effluent channel is not less than one-third the
difference in elevations of the upstream crest and flume floor. Formulation and related design parameters for
Venturi flume control are presented in appendix C.
(3) Design factors. Grit chambers will be designed for a controlled velocity of approximately 1 foot
per second and a detention period of 45 seconds. The design of grit chambers and flow-control devices is
discussed and illustrated in appendix C.
(a) Horizontal surface area. To size a grit chamber with rectangular cross-section, first
determine the horizontal surface area using equation 10-1:
A = QX,
(eq 10-1)
where:
A = horizontal surface area (sq ft);
Q = flow rate (cfs);
X = settling rate of grit particle (sec per ft).
(Assume X = 16.7 sec/ft for all military installations.)
10-8
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