Wastewater Collection
Gravity sewer hydraulics
Design velocity has a floor and a ceiling. About 2 feet per second is the self-cleansing minimum that keeps solids moving; roughly 10 to 15 feet per second is the maximum before scour and hydrogen sulfide problems grow. Slope is expressed in ft/ft or percent, and the two are easy to convert. Know that flatter grades mean lower velocity and more deposition.
What gets into the pipe
Infiltration is groundwater entering through defects; inflow is storm water, roof drains, and similar direct connections. Both add clear water that overloads the plant during wet weather. Smoke testing, dye testing, and closed-circuit television inspection are the standard tools for finding them, and rehabilitation follows.
Lift stations and force mains
Pumps are sized against the inflow, and wet wells cycle between start and stop levels. Force mains go septic and generate hydrogen sulfide, which causes odor and corrodes concrete; air release and vacuum relief valves protect the main from surge and water hammer.
Safety
Sewers, manholes, and wet wells are permit-required confined spaces. Test before entry: oxygen between 19.5 and 23.5 percent, combustible gas below 10 percent of the lower explosive limit, hydrogen sulfide and carbon monoxide below the site's limits (commonly 10 ppm and 35 ppm). Never enter without continuous monitoring, an attendant, and a rescue plan.
Traps
Reversing minimum and maximum velocity, confusing infiltration with inflow, and treating a manhole as an ordinary space because the opening is small.
Worked example
A 400-foot run drops 2 feet. Slope = 2 / 400 = 0.005 ft/ft, or 0.5 percent. If the resulting velocity falls under 2 ft/s, solids will settle and the line needs flushing or regrading.