What minimum flow velocity must pressurized sewers maintain?

• A. 1 foot per second
• B. 2 feet per second
• C. 3 feet per second
• D. 4 feet per second

Answer: (B)

Maintaining a minimum flow velocity in pressurized sewers is about keeping solids moving so they don’t settle and form deposits. This idea, often called self-cleansing velocity, ensures the sewer isn’t gradually clogged by grit, grease, and other solids.

The commonly recommended minimum is 2 feet per second. At this speed, the flow has enough shear to suspend typical solids and scour the pipe walls, reducing the chance of buildup that could narrow the pipe or cause blockages. If the velocity drops below this level, solids tend to settle, leading to deposits that require cleaning and can cause sewer backups.

Choosing 2 ft/s as the lower bound is a balance: it is fast enough to keep debris in suspension, but not so fast that it wastes energy or causes excessive wear and erosion. Speeds around 1 ft/s are generally too slow for reliable self-cleansing, while speeds much higher than 2 ft/s increase energy losses and can bring other issues without significant additional benefit for everyday transport.