18.3 Sewer Collection, Lift Stations, and Force Mains
Key Takeaways
- Collection-system flow is sanitary base flow plus infiltration and inflow; wet-weather response often controls lift-station and overflow questions.
- Gravity sewer checks usually require Manning capacity, slope, depth ratio, and velocity reasonableness rather than diameter alone.
- Lift-station firm capacity is the capacity with the largest pump out of service, not the sum of all installed pumps.
- Wet-well active volume controls pump cycling and must balance inflow, pump rate, starts per hour, and detention time.
- Force mains are pressure conduits, so headloss, air release, surge, corrosion, and minimum velocity all matter.
Collection Systems from Pipe to Pump Station
The April 2024 WRE specification covers wastewater collection systems, including lift stations, sewer networks, infiltration, inflow, smoke testing, maintenance, and odor control. That scope is broader than the Manning equation. A gravity sewer, wet well, pump, and force main behave as one system during wet weather, power loss, and maintenance.
Sanitary flow is wastewater from homes, businesses, and industry. Infiltration is groundwater entering through defects such as cracked pipe, bad joints, and leaking manholes; it persists for hours or days after rain and rises with the seasonal water table. Inflow is stormwater entering quickly through roof drains, yard drains, cross connections, missing cleanout caps, or leaky manhole covers; it spikes during the storm and recedes fast. PE questions frequently ask which investigation or repair matches the hydrograph response, so memorize the two timing signatures.
Collection-System Elements
| Element | Main check | Failure signal |
|---|---|---|
| Gravity sewer | Manning capacity, slope, depth, velocity | Surcharging, deposits, overflows |
| Manhole | Access, drop, leakage, corrosion | I/I, odor, hydrogen-sulfide damage |
| Lift station | Firm capacity, wet-well volume, controls | Excess cycling, high-level alarms |
| Force main | Headloss, pressure class, air, surge | High pump head, air binding, breaks |
| Maintenance | Cleaning, CCTV, smoke testing, root control | Repeated blockages, wet-weather spikes |
Design flow typically equals peak sanitary flow plus infiltration plus inflow. If sanitary base is 0.90 MGD, a peaking factor of 2.5 gives 2.25 MGD peak sanitary, and adding 0.35 MGD I/I yields 2.60 MGD that the conveyance must handle.
Gravity Sewer and Lift-Station Workflow
Use Manning's equation, V = (1.49/n) R^(2/3) S^(1/2) in U.S. units, for gravity sewers. Full-pipe capacity is quick to check when the pipe flows full but unpressurized; partly full flow needs the hydraulic radius and area at the actual depth ratio (d/D). Many standards seek a minimum velocity at typical flow (often near 2 ft/s) to limit solids deposition, but exact criteria are jurisdiction-specific. On the exam, use the velocity or slope criterion given, and remember that an oversized pipe can drop velocity below the scour value at low flow.
Lift-station steps:
- Build design inflow: base sanitary flow plus infiltration, inflow, peaking, and industrial contribution.
- Check firm capacity with the largest pump out of service when reliability is asked.
- Use the pump-and-force-main system curve to confirm the actual operating flow, not nameplate.
- Size or check wet-well active volume for cycling: drawdown and fill time both count.
- Check high-water storage, emergency power, bypass pumping, alarms, and odor or corrosion if operational.
For one pump cycling between lead-on and lead-off levels, drawdown time = V / (Qpump - Qin), fill time = V / Qin, and cycle time = drawdown + fill. The minimum cycle time for a maximum starts-per-hour limit Z is T = 60/Z minutes. The active volume that worst-cases the cycle occurs when Qin = Qpump/2; designers often size to that point. If inflow approaches pump capacity, drawdown stretches and reserve shrinks; if active volume is too small, starts per hour climb and motors overheat; if it is too large, detention rises and septicity and odor follow.
Force Mains, Air, and Troubleshooting
A force main is a pressure pipe whose headloss is part of the pump system curve. Its high points trap air; air pockets cut effective area, raise headloss, and destabilize pumping. Air-release valves and air-vacuum valves may be required at high points depending on profile and transient risk. A minimum velocity (frequently cited near 2 ft/s for raw wastewater) keeps solids in suspension, while excessive velocity drives up headloss and surge. Ductile iron, PVC, and HDPE all carry force mains, but the crown is also the prime spot for hydrogen-sulfide-driven crown corrosion in any partially full or turbulent section.
Match symptoms to causes rather than swapping parts:
| Symptom | Likely cause | Targeted action |
|---|---|---|
| Wet-weather flow spike | Inflow sources | Smoke testing, manhole cover repair |
| Slow recession after rain | Infiltration | CCTV, joint and manhole sealing |
| High discharge pressure, low flow | Closed valve, blockage, air binding | Verify valve, inspect force main |
| Frequent starts, dry weather | Wet well too small or controls off | Resize active volume, reset floats |
| Odor and concrete spalling | Sulfide generation, low velocity | Ventilation, chemical dosing, cleaning |
A gravity-sewer overflow during dry weather points to a blockage or pump failure, not I/I. A surcharge only during storms points to I/I and the conveyance peaking factor. On the PE exam, the best operational answer addresses the cause without violating hydraulic capacity: you never solve an I/I problem by enlarging a pump if the receiving treatment plant cannot take the wet-weather load, and you never claim a smoke test will fix steady seasonal infiltration. Smoke testing finds rapid inflow paths (roof leaders, cross connections, uncapped cleanouts); CCTV and dyed-water testing find slow infiltration through cracks and joints.
Matching the diagnostic tool to the hydrograph timing is itself a common exam distinction, so keep the inflow-versus-infiltration signatures separate when you choose a remedy.
A lift station has two identical pumps rated at 700 gpm each. The design peak flow is 0.90 MGD of sanitary flow plus 0.35 MGD of infiltration and inflow. If firm capacity is required with one pump out of service, which statement is correct?
A single-pump wet well receives 250 gpm. The pump discharges 900 gpm when running. To limit starts to 6 per hour, use a minimum cycle time of 10 minutes. What active wet-well volume is required between pump-on and pump-off levels?