Compared
Gravity sewer vs. force main
A gravity sewer carries wastewater downhill by gravity alone; a force main is a pressurized pipe that a pump pushes wastewater through, used where gravity cannot carry it. Collection systems use gravity where they can and lift stations with force mains where the terrain requires it.
Last reviewed 2026-08-04
| Gravity sewer | Force main | |
|---|---|---|
| How it moves | Gravity, flowing downhill | Pump pressure |
| Pump needed | No | Yes, a lift station |
| Where used | Where terrain allows downhill flow | Where gravity cannot carry it |
| Operator concern | Blockages and slope | Pumps and pressure |
The bottom line
A collection system uses gravity wherever it can, and a lift station with a force main to push wastewater over a rise before gravity flow resumes.
Common questions
- What is a force main?
- A pressurized pipe a pump pushes wastewater through.
- Why not use gravity everywhere?
- Terrain sometimes drops too low, so a pump and force main are needed.
- What pushes a force main?
- A lift station pump.