Collection Systems and Pumping
Lift stations
Wet well volume, pump capacity, and inflow set the cycling. Size the wet well so a pump starts no more than roughly 6–12 times per hour, and check that inflow plus storage handles peak events. A pump runs where its curve crosses the system curve (static lift plus friction). Traps: reading the pump curve at the wrong speed or impeller trim; ignoring available NPSH, which leads to cavitation; and deadheading or running a pump far off its best efficiency point.
Flow
Sanitary sewers are designed with a peaking factor; minimum self-cleansing velocity is about 2 ft/s at design flow, and force mains typically run 2–4 ft/s to keep solids moving. Flow is measured with weirs and flumes at the plant and with area-velocity meters in sewers.
Infiltration and inflow
Infiltration is groundwater entering through defects such as cracked pipe, joints, and manholes. Inflow is direct stormwater - roof leaders, yard drains, and unsealed covers. Both are extraneous flow, quantified with night-flow studies and rainfall comparisons, and located with smoke testing, dye testing, and CCTV.
SSOs and CSOs
Sanitary sewer overflows are releases from separate sanitary sewers and must be reported per state rules, often at a 1,000-gallon threshold plus immediate notice for any discharge reaching surface water. Combined sewer overflows are addressed through the nine minimum controls and a long-term control plan.
Worked example
A wet well 12 ft by 8 ft with 3 ft of drawdown holds 12 × 8 × 3 = 288 ft³ × 7.48 = 2,154 gallons. A 300 gpm pump against 100 gpm inflow removes a net 200 gpm, emptying the well in about 10.8 minutes.