Both Pumps Running and the Level Still Climbing
A duplex lift station with two 500-gpm pumps serves about 1,200 homes. On a dry Friday afternoon SCADA shows both pumps running continuously and the wet well - 10 ft inside diameter, 12 ft deep, high-level alarm at 9 ft - rising 0.8 ft in 10 minutes. The level is now at 7.5 ft. Pump run-time hours were normal yesterday, and it has not rained in nine days. The station discharges through 4,000 ft of 8-inch force main to a gravity interceptor. The next operator is not on shift for two hours, and the maintenance yard is 25 minutes away.
1. Compute the inflow implied by the level rise and decide whether the problem is on the collection side or the pump side. What three checks do you make first?
Read the worked answer
Wet well area: 0.785 x 10² = 78.5 ft². Rise volume: 78.5 x 0.8 = 62.8 ft³ x 7.48 = 470 gallons in 10 minutes = 47 gpm net accumulation.
Implied inflow: both pumps running at nameplate is 1,000 gpm, plus the 47 gpm accumulating = about 1,050 gpm. For 1,200 homes that is roughly 0.9 gpm per home in dry weather - several times a realistic peak and utterly implausible nine days after rain.
Inference: the pumps are not delivering 500 gpm each. Either the inflow is enormous (a broken water main or a hidden I/I source) or, far more likely, one or both pumps are degraded - plugged impeller, worn wear rings, a partially closed discharge valve, a failed check valve recirculating flow back into the well, or air binding. The run-time hours were normal yesterday, which points to a change today.
First three checks:
- Discharge pressure and motor amps against normal readings - low pressure with normal amps suggests a valve or blockage; low amps suggests the pump is not loaded, i.e., moving little or nothing.
- Valve position and check valves - physically confirm suction and discharge valves are open and listen for reverse flow through a failed check.
- Drawdown test - run one pump alone and time the level drop; that gives the actual delivered capacity of each pump directly.
Also stick the well to confirm the SCADA level is real; a failed transducer reads whatever it wants.
2. If nothing changes, how long until it overtops, and what is your pre-overflow plan?
Read the worked answer
Time available: from 7.5 ft to the 12-ft rim is 4.5 ft. At 0.8 ft per 10 minutes (0.08 ft/min) that is about 56 minutes - call it 45 minutes to be safe, because if a pump trips or degrades further the rate accelerates, and if the well is already at the high-level alarm at 9 ft the alarm has been telling you the margin is gone.
Plan, executed now rather than after the spill:
- Call out maintenance immediately and tell them what you have: both pumps running, apparent capacity loss, level at 7.5 ft, roughly 45 minutes of margin.
- Mobilize a vacuum truck or portable bypass pump to the wet well before it is needed. Discharge must go somewhere legal - a downstream manhole, not a ditch.
- Notify the supervisor and the downstream treatment plant; if the interceptor can take pumped flow at a downstream manhole, plan the bypass route.
- Stage containment - berms, absorbent, and a pump at the likely overflow point; identify the overland path and keep it out of storm drains and surface water.
- Stop or reduce inflow if possible - check for a broken water main feeding the station and shut it if identified.
- Document - level trend, times, calls placed, actions taken. That record is the difference between a managed incident and an enforcement finding.
3. It does spill, roughly 3,000 gallons over the wet well pad and into a storm drain. What do you do, and what does your reporting look like?
Read the worked answer
Immediate, in order: stop or contain the release (pump the well down, plug or berm the storm drain inlet if it can still be done safely), recover what you can with a vac truck, and stop the discharge from reaching surface water. Do not wash it down - dilution is a violation, not a fix.
Then:
- Notify the state regulatory agency within the required timeframe - many states require a call within 2 to 24 hours for any sanitary sewer overflow, and a spill reaching a storm drain or water of the state is treated more seriously than one contained on site. Call, do not email and hope.
- Notify the local health department and downstream users if there is a plausible public contact or drinking water intake.
- Post signage at the affected area and keep the public out until it is cleaned and disinfected.
- Report the volume and duration with the basis for your estimate - 3,000 gallons from pump-down records and level change is defensible; a guess is not. Say clearly if the number is an estimate and how you derived it.
- Follow up in writing with a root cause: pump capacity loss, missed preventive maintenance, alarm response time. The written report is where the utility either demonstrates a system or demonstrates a pattern.
The judgement item: the prohibition is on the discharge, not on the paperwork. Report even if the volume seems small and even if no one saw it - undisclosed overflows discovered later are treated far worse than self-reported ones.