Wastewater Collection Systems
Moving sewage by gravity
Collection systems must move solids as well as water. Gravity sewers are laid on a slope that produces scouring velocity - commonly a minimum of 2 ft/s at design flow to keep solids suspended, and no more than about 10 ft/s to limit abrasion and hydrogen sulfide release. Manning's equation governs: velocity rises with slope and hydraulic radius and falls with roughness (n ≈ 0.013 for new PVC).
What every operator should recognize
- Infiltration is groundwater entering through defects; inflow is direct stormwater from downspouts, sumps, and manhole covers. Inflow spikes after storms; infiltration is chronic.
- Lift stations use wet wells, pumps, and force mains. Check air release valves, sags, and grease; a plugged vent or stuck check valve can cause overflows.
- FOG, roots, debris, and collapsed pipe cut capacity; CCTV, smoke testing, and dye testing find the defects.
- H2S forms in slow, deep, septic sewers, corrodes concrete, and is lethal - ventilation and chemical addition help.
- Design flows use a peaking factor, and I/I reduction is often the cheapest capacity gain.
Traps
Questions swap slope and velocity, or ask which flow arrives first after a rainstorm (inflow). Also remember that the slope needed to reach 2 ft/s rises as pipe size falls - an 8-inch sewer wants roughly 0.4%.
Worked example
For 2 ft/s in an 8-inch pipe flowing full: n = 0.013, R = 0.167 ft, so V = (1.486/0.013)(0.167)^(2/3)S^0.5 ≈ 34.6 S^0.5. Setting V = 2 gives S ≈ 0.0033, about 0.33%; design at 0.4%.