Wastewater Collection
System layout
Gravity sewers carry flow to lift stations and the plant. Minimum self-cleansing velocity is about 2 ft/s at design flow; maximum is about 10 ft/s to limit scour and hydrogen sulfide. Slope and velocity are linked by Manning equation, so check both: a line can have a slope yet still be too flat for the design flow.
Infiltration, inflow, and SSOs
Infiltration is groundwater entering through defects; inflow is storm water from direct connections. Both drive wet-weather overflows. Sanitary sewer overflows must be contained, cleaned up, and reported to Georgia EPD with documentation. Grease and roots are the leading blockage causes; CCTV, smoke testing, and dye testing locate defects.
Lift stations
Wet well volume sets pump cycling; short cycling damages motors. Know lead/lag and alternation, check valves, and emergency storage or standby power. A pump running but not moving flow usually means a clogged impeller or closed valve.
Worked example
A 400-ft run drops 2.0 ft. Slope = 2.0 / 400 = 0.005 ft/ft, or 0.5%. Percent grade equals ft/ft x 100, so 0.005 is 0.5%, not 5%. At that slope a correctly sized line should still produce at least 2 ft/s; if Manning gives less, the line is undersized or too flat.
Traps
Mixing percent grade with ft/ft is the most common math error. Manholes still need ventilation and gas testing. Private laterals are a real source of I/I.