Collection Systems and Pumping
Gravity sewers
Gravity collection design relies on self-cleansing velocity. A minimum of about 2 ft/s (0.6 m/s) at design flow keeps solids moving; above roughly 10–15 ft/s, scour, hydrogen sulfide release, and concrete corrosion accelerate. Minimum slopes follow pipe diameter, and manholes are placed at changes in grade, alignment, and size. Infiltration enters through defective joints and groundwater; inflow comes from roof drains, sumps, and illegal connections. Fats, oils, and grease (FOG) and roots are the classic blockage causes.
Pumping
Lift stations use wet wells and dry wells, submersible or suction-lift pumps, and force mains. Total dynamic head is static head plus friction and velocity head; the operating point is where the pump curve crosses the system curve. Affinity laws: flow varies with speed, head with speed squared, and power with speed cubed - doubling speed multiplies power by eight. Cavitation results from insufficient net positive suction head available and shows as noise, vibration, and impeller pitting. Air binding and water hammer are controlled with air-release and air-vacuum valves and surge protection. Trap: assuming a larger pump automatically increases capacity - the system curve governs the operating point.
Safety and reporting
Manholes and wet wells are permit-required confined spaces: test for oxygen, flammables, and hydrogen sulfide before and during entry, ventilate, and post an attendant. Hydrogen sulfide deadens the sense of smell and corrodes concrete; methane is explosive. Sanitary sewer overflows must be reported promptly and corrective action documented. Inspection uses CCTV; cleaning uses jet/vac and rodding tools.
Rule of thumb
If a pump runs at half speed, flow drops to half, head to one quarter, and power to one eighth.