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 pipe diameter alone.
- Lift-station firm capacity is the capacity available with the largest pump out of service, not the sum of all installed pumps.
- Wet-well active volume controls pump cycling and must be coordinated with inflow, pump rate, storage, odor, 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 includes wastewater collection systems, including lift stations, sewer networks, infiltration, inflow, smoke testing, maintenance, and odor control. That scope is broader than a 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 the wastewater generated by homes, businesses, and industries. Infiltration is groundwater entering through defects such as cracked pipe, poor joints, and manholes. Inflow is stormwater entering quickly through roof drains, yard drains, cross connections, missing cleanout caps, or leaky manhole covers. Infiltration may persist after a storm; inflow often spikes during rainfall. PE questions often ask which investigation or fix matches the hydrograph response.
Collection-System Elements
| Element | Main check | Common failure signal |
|---|---|---|
| Gravity sewer | Manning capacity, slope, depth, velocity | Surcharging, deposits, overflows |
| Manhole | Access, drop control, 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 program | Cleaning, CCTV, smoke testing, root control | Repeated blockages or wet-weather spikes |
Gravity Sewer Checks
Use Manning flow for open-channel gravity sewers. Full-pipe capacity can be checked quickly when the pipe is flowing full but not pressurized. Partly full flow requires hydraulic radius and area at the actual depth. Many design standards seek enough velocity at typical flow to limit solids deposition, but exact criteria are jurisdiction-specific. On the exam, use the velocity or slope criterion given in the problem statement, and remember that a larger pipe can reduce velocity at low flow.
Lift-Station Workflow
- Build the design inflow: base sanitary flow plus infiltration, inflow, peaking factors, and any industrial contribution.
- Check firm pumping capacity with the largest pump unavailable if the problem asks for reliability.
- Use the pump and force-main system curve to confirm actual operating flow, not just nameplate capacity.
- Size or check wet-well active volume for cycling: the drawdown time and fill time both count.
- Check high-water storage, emergency power, bypass pumping, alarms, and odor or corrosion risk if the scenario is operational.
For one pump cycling between lead-on and lead-off levels, drawdown time is V / (Qpump - Qin), and fill time is V / Qin, where V is active volume. The total cycle time is the sum. If inflow approaches pump capacity, drawdown takes longer and the station has little reserve. If active volume is too small, starts per hour increase and motor life suffers. If active volume is too large, wastewater detention can increase odors and septicity.
Force Mains and Air
A force main is a pressure pipe. Its headloss is part of the pump system curve, and its high points can trap air. Air pockets reduce area, increase headloss, and can cause unstable pump operation. Air-release and air-vacuum valves may be needed at high points, depending on the profile and transient risk. Minimum velocity may be required to keep solids moving, while excessive velocity increases headloss and surge risk.
Troubleshooting should match symptoms to causes. A wet-weather flow spike suggests inflow sources and smoke testing. A slow recession after rainfall suggests infiltration and CCTV or manhole inspection. High pump discharge pressure with low flow may indicate a closed valve, blocked force main, air binding, or a system curve that has steepened. Odor and corrosion point toward detention, sulfide generation, ventilation, chemical dosing, or cleaning. On the PE exam, the best operational answer usually addresses the cause without violating hydraulic capacity.
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?