High-Level Alarm at the Ash Street Lift Station During a Storm
Rain began at 21:00. Ash Street lift station serves 1,900 connections with a 6-ft diameter wet well, 11 ft from low water to overflow, and two constant-speed 350 gpm pumps on alternation. Floats sit at 4 ft (lead), 5.5 ft (lag), and a high-level alarm at 7.5 ft. Dry-weather inflow is 180 gpm. At 23:30 the SCADA high-level alarm comes in. Only one pump is running; drawdown checks show it is pumping normally, but the lag pump never starts. The wet well has been rising at close to 1 ft per minute. The station is 400 ft from a creek, and the last overflow was four years ago. Storm flow is estimated at 550 to 650 gpm.
1. How much time do you actually have, and what is the priority order of your first actions?
Read the worked answer
Do the storage math first, because it sets the whole response.
- Wet well area = 3.1416 x 3 ft x 3 ft = 28.3 sq ft.
- One pump at 350 gpm against 600 gpm inflow leaves 250 gpm surplus, which is 250 / 448.8 = 33.4 cfm.
- Rise rate = 33.4 / 28.3 = 1.18 ft/min, which matches the observed 1 ft/min.
- From the 7.5 ft alarm to the 11 ft overflow is 3.5 ft, so roughly 3 minutes.
Priority order:
- Get capacity on the station now: call out the vac truck and portable pumps, and have the crew attempt a manual start of the lag pump at the panel. A manual start may work even if the automatic float signal does not.
- Notify: supervisor, the collection system lead, and downstream treatment plant, so they know a surge and possible spill are coming.
- Stage containment and spill response near the creek while the crew works, so response is already in place if the well tops out.
The wrong first move is troubleshooting wiring from the SCADA screen while the well fills.
2. What are the credible reasons the lag pump did not start, and how do you check them under time pressure?
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Split the failure into signal, control power, and pump side, and check them in that order because it is fastest.
- Signal: lag float stuck, fouled with grease, or transducer drift. Confirm by watching the float state in the panel and by jumping the lag call at the terminal block, which immediately tests every downstream component.
- Control power and logic: tripped breaker, blown fuse, failed alternator or contactor, hand-off-auto switch left in HAND for the other pump so the alternation logic never calls the lag. Check pilot lights and the overload relay.
- Pump side: plugged impeller or suction, closed discharge valve, failed check valve, or a pump that is running but deadheaded so the flow meter reads nothing. Feel and listen at the pump, then read the discharge pressure gauge rather than trusting SCADA alone.
A pump that runs but moves no water is a common storm failure and is easy to misread as a pump that is not running.
3. Beyond fixing the pump, what does a 1 ft/min rise during a storm tell you about the system, and what would you propose?
Read the worked answer
A dry-weather inflow of 180 gpm jumping to 600 gpm is more than three times the base flow, and most of that is infiltration and inflow, not sewage. The station is under-sized for the wet-weather peaking factor it actually sees.
Near term:
- Review the emergency response plan and spill reporting obligations now, not during the event; know the state notification timeline and who signs off.
- Add portable pumping capacity or a contract for a standby pump, and set the alarm to call out at a lower level so you are not starting with 3 minutes of margin.
Capital and program:
- Smoke-test and televise the laterals and mains tributary to the station to find direct inflow connections (sump pumps, roof drains, broken lateral seals) and rehab manholes and joints for infiltration.
- Evaluate firm capacity: at 650 gpm peak, two 350 gpm pumps are barely adequate and only if both run. A third pump or wet well storage buys the reliability the current design lacks.
- Use the flow data to justify the work with a peaking factor and gallons-per-inch-mile of infiltration, which is what funding applications ask for.