Wastewater Collection
Gravity sewers and the numbers behind them
Collection systems are designed so flow moves fast enough to keep solids suspended and slow enough not to scour the pipe. A common design range is about 2 ft/s minimum for self-cleansing and 10-15 ft/s maximum. Flow is estimated with Manning's equation, but exam items usually test slope arithmetic and concepts. Slope = fall ÷ run, expressed either as ft/ft or as a percent; 0.005 ft/ft is 0.5%.
System components
Know manholes (access, ventilation, change of direction), drop manholes, lift stations with wet wells and pumps, force mains, and the difference between sanitary and combined systems. Infiltration is groundwater entering through defects; inflow is stormwater entering through direct connections such as roof drains and area drains. Both add clear water that consumes capacity and raises treatment cost. Grease, roots, and debris cause blockages; hydrogen sulfide causes corrosion and odor and is a serious safety hazard.
Traps
Mixing percent slope with ft/ft; forgetting that velocity rises with slope and with hydraulic radius; assuming a lift station removes the need for downstream gravity capacity; and confusing infiltration with inflow. Also watch questions on minimum cover, bedding, and air or water tightness testing.
Worked example
A 400-ft sewer run drops 2.0 ft. Slope = 2.0 ÷ 400 = 0.005 ft/ft, or 0.5%, a normal minimum for an 8-inch line. If the same drop occurred over 200 ft, slope would double to 1.0%, raising velocity.