8.7 Lift Station Configurations, Preventive Maintenance, Wet Well Cleaning, Troubleshooting & Emergency Bypass Pumping
Key Takeaways
- Low motor amperage with low discharge pressure means an unloaded pump, while high amperage means a bound pump, and high discharge pressure with low flow means a downstream obstruction.
- Air accumulating at force main high points without air release valves is a classic cause of a station that gradually stops keeping up.
- Wet wells are cleaned from outside with a vacuum truck, and any entry requires a full permit-required confined space entry because hydrogen sulfide and oxygen deficiency have killed entrants and rescuers alike.
- Reduced atmospheric pressure at Colorado elevations lowers the maximum practical suction lift, so bypass pumps that prime easily near sea level may not at 8,000 feet.
- Testing the high-level alarm and verifying that pump alternation is actually rotating duty are routine checks, because a standby pump that never runs is not a standby.
Keeping the station running and getting flow around it when it does not
WPI's collection outline lists "rehabilitate and repair collection system: lift station (wet wells, fittings, and piping)", "inspect equipment or monitor operating conditions, meters, and gauges to determine load requirements and detect malfunctions of the lift station", "perform preventative maintenance including repair, replacement, and installation of equipment: motors, pumps, valves, engines, generators, level sensing equipment", "clean and maintain tanks (wet wells, chemical, holding)", and "operate pumping equipment during emergency bypass operations."
Station configurations and their maintenance implications
| Configuration | Description | Maintenance character |
|---|---|---|
| Submersible | Pump and motor submerged in the wet well, removed on guide rails | No dry well to enter; pump lifted for service; the wet well is a permit-required confined space |
| Dry pit / dry well | Pumps in an adjacent dry chamber, suction piped from the wet well | Pumps accessible while running; the dry well is itself a confined space in most designs |
| Wet pit / dry pit combination | Vertical pumps with motors above and wet-end below | Long shafts and intermediate bearings |
| Suction lift (self-priming) | Pumps at grade drawing from the wet well | Easiest access; priming and suction-line air leaks are the recurring problem |
| Vacuum and pressure (grinder) systems | Small-diameter collection with grinder pumps or vacuum valves | Many distributed units; valve and grinder maintenance dominates |
Routine preventive maintenance
Daily or per visit: record runtime hours and starts for each pump, wet well level behavior, discharge pressure, motor amperage on all three legs, and any alarms since the last visit. Look and listen. Note odor, unusual noise, vibration, and any sign of surcharge above the normal high-water mark.
Weekly to monthly: exercise the standby pump and confirm alternation is actually rotating duty; test the high-level alarm by lifting the float and confirming the alarm reaches the answering point; inspect and clean float switches and level transducers of grease and rags; check the ventilation fan operates; verify the standby generator exercises and the transfer switch operates.
Quarterly to annually: pull and inspect pumps for wear, rag accumulation, and impeller clearance; inspect check valves and exercise isolation valves; service seals and packing; lubricate bearings per the schedule; megger motor insulation; inspect the wet well interior for corrosion, benching condition, and structural deterioration; clean the wet well.
Wet well cleaning is a job of its own. Grease mats float on the surface and harden; grit settles in corners the pumps cannot reach and gradually reduces the working volume; rags rope together and blind the pump intake. Cleaning uses a vacuum truck to remove the accumulation, followed by washdown. Never enter to clean without a permit-required confined space entry with atmospheric testing, forced ventilation, an attendant, and retrieval equipment. Hydrogen sulfide and oxygen deficiency in a wet well have killed operators and, repeatedly, the would-be rescuers who followed them in.
Diagnosing a station that is not keeping up
Work the list in order rather than jumping to the pump:
- Is this a flow problem or a pump problem? Compare current inflow with historical. A wet-weather inflow spike is an I&I problem, not a pump failure.
- Amperage. Low amperage with low discharge pressure means the pump is not loaded: a blocked or air-bound suction, a lost prime, a broken shaft, or a worn impeller. High amperage means the pump is bound: rags, a seized bearing, or a closed discharge valve.
- Discharge pressure. High discharge pressure with low flow means an obstruction downstream — a partly closed valve, a failed check valve seated shut, or a blocked force main.
- Check valve behavior. A check valve stuck open lets flow return between pump starts, causing short cycling and false level readings.
- Air in the force main. High points without air release valves accumulate gas from septic wastewater and progressively reduce capacity. This is a classic cause of a station that "used to keep up."
- Impeller wear and clearance. Adjustable wear plates on solids-handling pumps are reset to specification; excessive clearance destroys capacity.
- Level control error. A drifting transducer changes the working volume and masquerades as a capacity problem.
Emergency bypass pumping
Bypass pumping keeps flow moving when a station is down, a force main is being repaired, or a gravity segment is being rehabilitated. It is an explicit WPI job task and is planned in advance rather than improvised.
Elements of a bypass plan:
- Sizing for the expected peak flow, not the average, with standby capacity.
- Suction access — a manhole or wet well opening that can be reached and will not collapse under the suction hose.
- Pump selection. Diesel or electric trash pumps or vacuum-assisted self-priming units capable of passing solids. Suction lift is limited by atmospheric pressure and by elevation, which matters in Colorado: at 8,000 feet the maximum practical suction lift is meaningfully less than at sea level, so a pump that primes easily on the Front Range may not at altitude.
- Discharge routing to a downstream manhole with hose or pipe secured against movement, protected at road crossings by ramps or by boring, and restrained at the discharge so it cannot whip.
- Containment and spill response materials staged in case a hose fails.
- Continuous attendance or remote monitoring with alarms, plus fuel management for extended operation.
- Notification. Any bypass that results in a discharge is a sanitary sewer overflow and is reported to CDPHE and, where applicable, under the CDPS permit's 24-hour reporting condition.
- Documentation — start and stop times, flows, personnel, and any spillage.
A lift station pump shows low motor amperage together with low discharge pressure. What does this combination indicate?
Why does bypass pumping at high Colorado elevations require particular attention to suction lift?
What is the correct procedure for cleaning accumulated grease and grit from a lift station wet well?