Wastewater Collection Systems and Lift Stations
What the candidate must know
Collection systems convey wastewater by gravity where possible, using lift stations where topography requires. Know gravity sewer design concepts (slope, velocity, capacity), the difference between sanitary and storm sewers, and the causes of sanitary sewer overflows (SSOs).
Concepts to master
- Self-cleansing velocity and the problems of too-low (solids deposition) and too-high (scouring, hydrogen sulfide) velocities.
- Infiltration and inflow (I/I) and their effect on treatment plant flows.
- Lift station components: wet well, pumps, controls, force main, and emergency storage/power.
- Hydrogen sulfide generation, odor, and corrosion.
- SSO reporting requirements and prevention through inspection and maintenance.
- Fat, oil, and grease (FOG) control programs.
Traps in questions
Questions often test whether you know that too steep a slope is also a problem (H2S and corrosion), or that I/I is clean storm/groundwater entering the system - not wastewater. Another trap is confusing a force main with a gravity main.
Worked example / rule of thumb
Rule of thumb: gravity sewers are designed to flow at a self-cleansing velocity; if velocity is too low, solids settle. Lift stations should have emergency storage and backup power because pump failure leads quickly to overflows.