MIL-HDBK-1005/9A
2.5.1.2
Uncontaminated Wastes. Segregate uncontaminated
cooling waters and similar discharges from municipal and
commercial establishments from polluted wastes and discharged to
storm sewers or natural watercourses. Check with the local
Engineering Field Division (EFD) or Engineering Field Activity
(EFA) to determine if a discharge permit is required for such
uncontaminated waters.
2.5.1.3
Hazardous and Toxic Wastes. Exclude or pretreat
wastes that can create a fire or explosion hazard, endanger
lives, impair hydraulic capacity, cause corrosion, or carry
toxic elements in sufficient quantities to impair downstream
treatment processes. Refer to WEF MOP No. 3.
2.5.2
Gravity Sewers.
The design for gravity sewers is as
follows.
2.5.2.1
Design Flows. Design sewers for peak flow, except
design main interceptor to treatment facility for 125 percent of
peak flow.
2.5.2.2
Flow Formula. Use the Manning formula (refer to WEF
MOP No. FD-5, Gravity Sanitary Sewer Design and Construction) to
design sewers to flow full, without surcharge, under peak flow
(equivalent to design for flow at 0.8 depth under peak flow with
friction factor constant). Use friction factor "n" of 0.013 for
most smooth wall pipes; for corrugated wall pipes, use "n" of
0.025.
2.5.2.3
Velocity Constraints. Design pipe slopes to insure a
velocity of at least 2.5 feet per second (fps) (0.76 meters per
second [m/s]) when pipe is flowing full at peak flow. Velocity
should not exceed 10 fps (3.05 m/s) for any flow in design
range. Velocities as low as 2.0 fps (0.6 m/s) flowing fully are
permitted where appreciable cost benefits can be realized.
2.5.2.4
Maintenance of Energy Gradient. Design to maintain
the energy gradient when the diameter of the sewer changes. Set
0.8 depth point of each pipe at the same elevation to
approximate this requirement.
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