Wastewater Force Main Design
A wastewater force main carries pressurized sewage from a lift station to a downstream gravity sewer or treatment plant. Force mains are sized for the design peak flow at velocities that keep solids suspended (≥2 ft/s scour) without exceeding ~8 ft/s where erosion accelerates.
Target industries
Municipal wastewater Sanitary district Industrial pretreatmentRecommended calculation methods
- Hazen-Williams (C ≈ 130 for new ductile iron)
- Darcy-Weisbach (for low-temperature service)
Key design factors
- Minimum scour velocity: 2 ft/s (Ten States §44.4)
- Maximum velocity for erosion: 8 ft/s
- C-factor degradation over service life — design with C=120 for grit-laden flow
- Air release and combination valves at high points
- Surge analysis required for runs > 1,000 ft
Common pitfalls
- Underdesigning for peak flow only — sub-scour at low flow
- Ignoring water hammer from pump-trip events
- Skipping NPSH analysis at the lift station suction