Wastewater Collection
Collection system questions test practical hydraulics, maintenance judgment, and safety. Most calculation items are velocity, slope, or flow conversions.
Design hydraulics
Gravity sewers must carry solids, not just water. Self-cleansing velocity is about 2 ft/s; design velocities generally stay above that and below roughly 10 ft/s to limit scour and hydrogen sulfide release. Minimum slopes are set by pipe diameter. Know Manning's equation and, critically, its geometry trap: for a circular pipe flowing full, hydraulic radius is diameter divided by four, not diameter.
Infiltration, inflow, and capacity
I/I analysis separates groundwater infiltration (seasonal, steady) from rainfall-driven inflow (spikes). Peak flow design uses peaking factors, and lift stations must handle peak flow with the smallest pump cycling acceptably. Wet-well detention time must stay short enough to prevent septic conditions and odor.
Corrosion and safety
Force mains and long gravity runs generate hydrogen sulfide, which corrodes concrete and creates a confined-space hazard. Know the basic confined-space sequence: pre-entry evaluation, atmospheric testing, ventilation, attendant, rescue plan.
Traps
Unit conversions (cfs to gpm: multiply by 448.8), using slope in percent instead of decimal feet per foot, and assuming a pipe flows full when the design condition is partially full.
Worked example
An 8-inch sewer at 0.005 ft/ft slope, n = 0.013, flowing full. D = 0.667 ft, R = D/4 = 0.1667 ft. Then v = (1.486/0.013)(0.1667)^(2/3)(0.005)^(1/2) = 114.3 × 0.303 × 0.0707 ≈ 2.4 ft/s, which just meets the 2 ft/s self-cleansing rule. Area = 0.7854 × 0.667² = 0.349 ft², so Q = 2.4 × 0.349 ≈ 0.85 cfs, or about 380 gpm.