The minimum scouring velocity normally used for sanitary collection lines is

• A. 1.0 ft/sec
• B. 2.0 ft/sec
• C. 5.0 ft/sec
• D. 10.0 ft/sec

Answer: (B)

Scouring velocity is the minimum flow speed needed to keep solids in suspension and prevent sediment buildup in sanitary collection lines. For gravity sewers carrying typical domestic wastewater, this velocity is about 2 ft/sec, which provides enough energy to move coarse particles without wasting excessive head or causing erosion. If flow is slower, like 1 ft/sec, sediments can settle and form deposits that lead to blockages; much higher speeds (5 or 10 ft/sec) would waste energy and increase the risk of pipe wear. So 2.0 ft/sec is the standard minimum to maintain a self-cleaning sewer.