In gravity sewer systems, the wastewater flows primarily due to

• A. Pressure
• B. Elevation difference
• C. Pumping
• D. Temperature

Answer: (B)

Gravity sewer systems move wastewater downhill because of the elevation difference along the line. The higher upstream point has more gravitational potential energy, and as wastewater flows toward the lower downstream end, gravity converts that potential energy into flow energy, pushing the liquid through the pipe. This is why the main idea is the slope and elevation change, not any external pressure. Pressure isn’t the driving force in gravity sewers, since these systems are designed to carry flow under atmospheric conditions without active pumping along the mainline. Pumps only come into play when you need to move wastewater uphill or over an obstacle. Temperature doesn’t create the flow, though it can slightly affect viscosity; the movement itself is governed by gravity from elevation differences.