Wet Well Rising During a Storm at a Duplex Lift Station
A duplex submersible lift station serves a 1970s subdivision. The rectangular wet well is 12 ft by 14 ft; lead pump on at 5.0 ft, lag on at 6.5 ft, high-level alarm at 8.0 ft, and the hatch grade sits at 10.0 ft above the wet-well floor. Each pump is rated 900 gpm at 62 ft TDH. During a 3-inch storm the collection system delivers an estimated 2,100 gpm. Both pumps run, but the SCADA trend shows level climbing through 6.5 ft at 14:20; at 14:26 the level reaches 8.0 ft and the alarm calls out. At 14:31 the electrician reports one pump tripped on overload after a grease mat wrapped the impeller. Dry-weather night flow is 0.35 MGD.
1. Calculate the time available before the wet well overflows and state what that number means for your response.
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Volume per foot of depth. 12 ft x 14 ft = 168 ft2, and 168 ft3 x 7.48 = 1,257 gal per foot.
Sanity check on the alarm timing. Lag-on at 6.5 ft to the 8.0 ft alarm is 1.5 ft = 1,885 gal. With both pumps running (1,800 gpm) against 2,100 gpm inflow, net fill is 300 gpm, so 1,885 / 300 = 6.3 minutes - which matches the climb from 14:20 to 14:26.
Time to overflow. From 8.0 ft to the 10.0 ft overflow is 2.0 ft = 336 ft3 = 2,513 gal.
- Both pumps running: 2,513 / 300 = 8.4 minutes.
- With the tripped pump down after 14:31: net 2,100 - 900 = 1,200 gpm, so 2,513 / 1,200 = 2.1 minutes.
Interpretation. You have about two minutes, not twenty. This is an imminent discharge of raw sewage to the surface. The decision is to act first and analyze afterward: mobilize pumping and containment now, and do the forensics in the following days.
2. What are your immediate actions in the first thirty minutes, and what safety issues govern them?
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First thirty minutes, in parallel.
- Call out a vactor or jetter and a portable or bypass pump; start hauling or land-applying if there is no receiving-water pathway you can control quickly.
- Pull the tripped pump and clear the grease mat, then restart - but only after lockout and after confirming the overload came from a mechanical wrap and not a failing motor winding.
- Contain at the surface: sandbags or a berm around the hatch, and block the storm drain route so the discharge cannot reach waters of the state.
- Notify the supervisor and the utility's SSO coordinator, and report to the state agency on its required timeline; open a spill log with estimated volume, duration, location and receiving water.
Safety governs every step. The wet well is a permit-required confined space - no entry for grease removal, continuous H2S and oxygen monitoring for anyone near the hatch, electrical lockout/tagout, traffic control on the road, and a dedicated spotter.
Volume estimate for the report. Every 10 minutes of overflow at roughly 1,200 gpm is about 12,000 gallons. Know that number before you call, because it can drive the public-notification category.
3. Separate the capacity problem from the condition problem, and lay out a fix sequence you could defend to a capital planning committee.
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Capacity versus condition. Installed capacity is 1,800 gpm; the storm delivered 2,100 gpm. That is a 300 gpm (17%) shortfall before any pump failure, so the station is genuinely undersized for the peak - but the grease mat is what turned a surcharge into a spill. Both must be fixed; neither alone is sufficient.
Diagnose before building.
- Meter station inflow for a full wet season to separate base flow, dry-weather infiltration and storm inflow/infiltration. Night flow of 0.35 MGD for a subdivision of this size suggests infiltration is a large share.
- Smoke-test or CCTV the tributary collection system to find footing drains, broken laterals, defective manhole covers and root intrusions.
- Start or tighten a grease control program for commercial contributors upstream.
Fix sequence, cheapest first.
- Operations: high-high alarm on independent telemetry, redundant level sensing, backup generator plus a quick-connect for a portable pump, a spare impeller, and pump maintenance based on run-hours rather than failure.
- Reliability: a third pump, or VFDs so the station trims to inflow instead of start/stop at full speed.
- Capacity: upstream storage or a larger force main if the peak I/I cannot be removed economically - and size it from the metered data, not from a guess.