Wet Well Alarm During a Rainstorm
A duplex submersible pump station serves a 400-home subdivision with two 300 gpm pumps on alternating lead. The 10-foot-diameter wet well has a 6-foot operating band; the lag pump starts at 7 feet and the high-level alarm at 8.5 feet. In dry weather, pumps run about 4 minutes every 20 minutes and the level cycles between 3 and 5 feet. After 1.8 inches of rain in six hours, both pumps run continuously, the level reaches 8.6 feet, and the alarm calls out at 11 p.m. Manholes two blocks upstream are holding sewage near the rim; none has overflowed yet. The 4,000-foot, 8-inch force main discharges to a gravity interceptor, and the treatment plant reports flow at 2.2 times the dry-weather average. Pump amp draw reads normal. A resident complained of sewage odor at a manhole the previous week.
1. Both pumps are running continuously with normal amps. What does that tell you, and how do you determine whether this is inflow/infiltration or a downstream restriction?
Read the worked answer
What normal amps tell you: the pumps are running and are not obviously jammed, but amperage alone does not prove capacity. On a centrifugal pump, a partially blocked discharge raises head and reduces flow, and the amp reading can move in either direction depending on where the pump sits on its curve. Compare amps to the pump curve and, if installed, check discharge pressure and any flow indication.
Do the level-rise math. A 10-foot-diameter well has an area of 78.5 square feet, so 1 foot of depth is about 587 gallons.
- If the level rises 0.2 ft in 10 minutes with both pumps running, that is 117 gallons in 10 minutes, or about 12 gpm.
- Inflow is then roughly 600 + 12 = 612 gpm, about 0.88 MGD, against a station capacity of 600 gpm.
The decisive evidence is downstream. The treatment plant is seeing 2.2 times dry-weather flow, so the water is reaching the plant. A blocked force main would not deliver that flow. Combined with rainfall of 1.8 inches and upstream manholes backing up because the downstream system is at capacity, this is a wet-weather inflow and infiltration event, not a blockage.
Still verify cheaply: send a crew to check the downstream interceptor and the nearest manholes for a grease mat or debris, and confirm the force main air valve is working. A blockage found at midnight is a cheap fix; assuming I/I when it is a blockage is an overflow.
2. What are your priorities tonight, in order?
Read the worked answer
- Keep both pumps running and confirm they are actually moving water. Check for ragging, confirm the check valves are seated properly, and confirm neither pump is air-locked. Do not cycle pumps off to 'rest' them during a high-level event.
- Protect public health first. The manholes at the rim are one rain burst from overflowing. Barricade and sign them, keep vehicle and foot traffic off, and check for any basement backups in the immediate area.
- Reduce inflow if you can. Crews can check for illegal connections, catch basins tied into the sanitary system, and sump pumps or downspouts discharging to the sanitary sewer in the affected blocks, and can advise residents to stop discharging clean water.
- Prepare for a discharge. Have portable pumps, vacuum trucks, and containment on call. If an overflow occurs, notify the state regulator within the timeframe your permit requires and document the volume, duration, and receiving water. Do not wait for daylight if sewage reaches a water of the Commonwealth.
- Notify the treatment plant of the elevated flow so it can protect its own process, and warn them the flow will be high and dilute.
- Document everything in real time: levels, times, pump run status, rainfall, and actions taken. Post-event reconstruction from memory always looks worse.
The odor complaint from the prior week matters too: it may be early evidence of a developing grease or solids problem in the force main, which the rain event has now compounded.
3. After the storm, what is your investigation plan, and where do you look first?
Read the worked answer
Establish the numbers before spending money.
- Review the station's SCADA flow and run-time records against rainfall for the last 12 months to quantify how much wet-weather flow you actually get and how fast it arrives.
- Install temporary area flow monitors in the major sub-basins to split the response into its fastest and slowest components.
Then look, in order of cost:
- Manhole inspections in the areas that respond fastest, which usually indicates direct inflow, not infiltration.
- Smoke testing to find roof leaders, yard drains, and broken laterals connected to the sanitary sewer.
- Dye testing private laterals and foundation drains in the suspect blocks. In a 400-home subdivision, private inflow is usually the largest single source.
- CCTV the mains that respond slowly and consistently, which points to infiltration through joints, cracks, and root intrusions.
- Rehabilitate in priority order: remove illegal connections first (cheap and immediate), then line or grout the worst infiltration reaches.
Also address the capacity question. The station has 600 gpm of capacity and 3,500 gallons of usable wet well storage. If the peak wet-weather inflow is fundamentally above the pump capacity, no amount of I/I work entirely removes the risk; evaluate a larger wet well, a third pump, or high-level emergency storage. Track the specific peak inflow number from this event as the design benchmark.