Collection Systems and Influent Flow
System layout
A sanitary collection system moves flow by gravity wherever possible. Sewers are designed to maintain a self-cleansing velocity of about 2 ft/s minimum while avoiding scour; velocities above roughly 10 ft/s erode pipe and damage structures. Slope, material, and roughness (Manning's equation) set capacity. Where depth is insufficient, lift stations and force mains take over.
Infiltration and inflow
Infiltration is groundwater seeping through defects. Inflow is direct stormwater from roof drains, sumps, and manhole covers. Both dilute influent and can overload the plant in storms. Find them with smoke testing (smoke blown into laterals reveals illegal connections), dye testing, and CCTV inspection. Peak wet-weather flow can be several times dry-weather flow - always check whether a question asks for average or peak.
Lift stations and maintenance
Wet wells store flow so pumps cycle instead of running continuously; usable storage between lead and lag setpoints controls the cycle. Avoid short-cycling. Force mains need air-release and vacuum-relief valves at high points. Septic odors are controlled by chemical addition, covers, or biofilters. Grease (FOG) causes blockages and pump fouling, so grease control programs matter upstream.
Traps
Confusing infiltration with inflow; assuming a pump runs continuously; computing wet-well volume from total depth instead of usable depth between setpoints.
Worked example
A wet well is 8 ft × 10 ft with 4 ft between pump-on and pump-off levels. Usable volume = 320 ft³ × 7.48 = 2,394 gal. With 300 gpm inflow and a 600 gpm pump, net pump rate is 300 gpm, so the pump runs about 2,394/300 ≈ 8 minutes and rests about 8 minutes - roughly 3.75 cycles per hour.