Wastewater Collection: Gravity Sewers, Lift Stations, and Overflow Prevention
Wastewater Collection tests gravity sewers, force mains, lift stations, manholes, inspection and cleaning, and overflow prevention. Hawaii's high groundwater, corrosive gases, tropical storms, and coastal discharges make infiltration/inflow, hydrogen sulfide corrosion, and emergency response recurring themes. Expect questions on hydraulic capacity, pump station operation, confined-space safety, and sanitary sewer overflow (SSO) response.
Core rules and concepts
- Gravity flow. Slope, Manning roughness, and pipe diameter set capacity. Maintain self-cleansing velocity, generally about 2 ft/s minimum; excessive velocity can scour and damage pipe.
- Lift stations and force mains. Wet-well level controls pump cycling. Know pump rate, inflow, drawdown volume, and cycle time. Force mains need air-release or vacuum-relief protection and surge control.
- Infiltration/inflow (I/I). High groundwater enters through defects; stormwater enters through illegal connections, manhole covers, and broken pipe. Use CCTV, smoke testing, dye testing, and flow monitoring.
- Confined space. Wet wells and manholes can contain hydrogen sulfide, methane, and low oxygen. Monitor, ventilate, use a permit, and have rescue arrangements before entry.
- SSO response. Stop the spill, contain it, notify regulators, sample, clean up, and document. Fats, oils, and grease (FOG) are a major cause.
Worked example
An 8 ft × 8 ft wet well has 2 ft of drawdown. Volume = 8 × 8 × 2 = 128 ft³ × 7.48 = 957 gal. The pump delivers 250 gpm and inflow is 100 gpm, so net removal is 150 gpm. Pumping time = 957/150 = 6.4 min. Off time = 957/100 = 9.6 min. Total cycle ≈ 16 min. Using 250 gpm instead of net 150 gpm gives the wrong answer.
Traps that cost marks
Forgetting to subtract inflow from pump rate; using diameter as radius in pipe volume; confusing percent slope with ft/ft; entering a wet well without confined-space controls; and treating H2S only as an odor rather than a corrosion and safety hazard.
Key points
- Gravity sewers need self-cleansing velocity, about 2 ft/s minimum.
- Pump cycle time uses net pumping rate, not total pump capacity.
- H2S causes corrosion and is a confined-space hazard.
- SSO response requires containment, notification, sampling, and documentation.