Wet-Weather Overflow Risk at a Crippled Pump Station
A 12-inch gravity sewer carries about 1.4 MGD dry-weather flow to a pump station with a 15-foot by 20-foot wet well and three 900 gpm pumps, one of which has its motor out for repair. During a 2-inch-per-hour storm, both available pumps are running continuously, the wet well has risen 0.9 feet in 10 minutes and is now 3.5 feet below the overflow elevation, and a hydrogen sulfide reading at the wet well hatch is 45 ppm. Upstream, a 300-foot reach of sewer has a heavy grease mat and roots at two joints, and the station's 10-inch force main has a history of air-binding at its high point. The spill response plan requires notifying the state within two hours of any discharge and the public within 24 hours where required. Rain is forecast for six more hours. A vacuum truck and a portable 6-inch diesel pump rated 1,200 gpm at 40 feet of head are 30 minutes away, along with 500 feet of layflat hose.
1. Calculate the inflow reaching the station and the time before overflow, state your assumptions, and give the order of actions.
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Wet well area is 15 x 20 = 300 square feet. A rise of 0.9 feet in 10 minutes is 270 cubic feet, or about 2,020 gallons, so the accumulation rate is about 202 gpm.
Inflow is pumping plus accumulation: 900 plus 900 equals 1,800 gpm, plus about 200, giving roughly 2,000 gpm. Treat this as a minimum. Assumptions that can make it worse: the pumps may not be delivering their rated 900 gpm against actual force main head, especially with air-binding at the high point, and the rain is forecast to continue, so inflow is probably still rising.
Time to overflow: 3.5 feet of freeboard times 300 square feet is 1,050 cubic feet, or about 7,850 gallons. At 202 gpm that is roughly 39 minutes, and less if the storm intensifies.
Order of actions:
- Call the crew, supervisor, and on-call manager immediately; state the 39-minute estimate.
- Start the portable pump and set layflat to an approved discharge point as soon as it arrives, and pre-position containment and sandbags to protect storm drains and nearby structures.
- Continue both station pumps, and have someone at the force main high point to manage air binding.
- Monitor H2S and keep people out of the wet well.
- Only consider upstream flow restriction (temporary plugging or diversion) if it will not back up into homes; a basement backup trades one problem for a worse one.
- When the storm passes, clean the grease and root reach.
2. The hatch reading is 45 ppm H2S. Can staff open the hatch or enter the wet well? What controls apply?
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No entry, and opening the hatch should be treated as a controlled operation. At 45 ppm, the reading is above OSHA's 20 ppm ceiling and far above the NIOSH 10 ppm 10-minute ceiling. It is below the 100 ppm IDLH only narrowly in appearance, because the reading at the hatch is likely diluted relative to the wet well headspace. Worse, H2S causes olfactory fatigue within minutes, so a worker who stops smelling it may wrongly conclude it is gone. Effects at this level include eye and respiratory irritation and headache, and the gas is heavier than air, so it can travel along the ground into basements and vaults in the area.
Controls:
- Post the area, keep the public back, and monitor the perimeter and nearby structures.
- Do not enter without a written permit, full-face supplied-air or SCBA, a calibrated four-gas monitor with H2S, a standby attendant with retrieval, and continuous forced ventilation with retesting before entry.
- Never use a sniff test, and never reach into the well to retrieve a dropped instrument.
- If a pump must be serviced, do it from outside the well or after ventilation brings readings well below the ceiling, and re-test at the start of every shift.
- Record all readings and times in the incident log.
3. If a discharge occurs, what does your notification and documentation process look like, and what volume estimate would you use?
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Notification: any discharge from the collection system, whether to a storm drain, ditch, street, or surface water, is reportable. Call the state spill hotline as soon as the discharge is discovered and no later than the two-hour requirement; notify the local health department, the downstream receiving-water users, and the public within 24 hours if the plan or the state requires it. Report what you know at the time (location, receiving water, estimated volume, cause, actions underway) and call back with updates rather than waiting for perfect information.
Volume estimate: use the wet well geometry. At 300 square feet, one foot of depth is 300 cubic feet, or about 2,244 gallons, so a 3-foot overflow is roughly 6,700 gallons. If the overflow ran for 20 minutes at the 200 gpm deficit, that is about 4,000 gallons. Report which method you used and note that it is an estimate.
Documentation: an incident log with times of discovery, notification, and control; names and phone numbers of everyone contacted; rainfall records; pump runtimes and wet well levels; photos; sampling results from the receiving water if required; and the corrective action list. The same log supports the post-event review, the root-cause analysis on the force main air-binding and the grease reach, and any regulatory follow-up.