Wastewater Collection
Wastewater Collection
This domain tests gravity sewers, lift stations, inspection, and maintenance. Questions cover slope and velocity, capacity, infiltration/inflow, corrosion, and safety at manholes and wet wells.
Core rules and concepts
- Design for a self-cleansing velocity near 2 ft/s at average flow and avoid exceeding about 10 ft/s, which scours and damages pipe.
- Minimum grades: commonly 0.4% for 8-inch, 0.28% for 10-inch, and 0.22% for 12-inch, but confirm against local standards.
- Use Manning's equation: v = (1.486/n) R^(2/3) S^(1/2), with R = D/4 for a full circular pipe.
- Infiltration is groundwater entering through defects; inflow is direct stormwater connection. Both consume treatment capacity.
- H2S forms in slow, septic sewers and corrodes concrete; control with ventilation, chemical addition, or line rehabilitation.
Worked example
8-inch sewer, n = 0.013, slope = 0.004 ft/ft. Full-pipe R = D/4 = (8/12)/4 = 0.1667 ft. R^(2/3) = 0.302; S^(1/2) = 0.0632. v = (1.486/0.013) × 0.302 × 0.0632 = 2.18 ft/s. That meets the 2 ft/s target. If the grade were 0.002 ft/ft, velocity would fall to about 1.54 ft/s, so the line would need more grade, a smaller pipe, or scheduled flushing.
Traps
Entering percent slope as 0.4 instead of 0.004; using pipe diameter as hydraulic radius; assuming full flow when the sewer runs at half depth; and entering a manhole without gas testing or fall protection.