Lift Station Wet Well Rising During Wet Weather
A duplex lift station serves a 1,200-home subdivision. Each pump is rated 350 gpm; the 12-foot-diameter wet well normally cycles one pump at a time. During a 3-inch rain, inflow reaches an estimated 520 gpm, and one pump has tripped on overload and cannot be reset. The wet well level is 3.0 feet below the overflow point and rising about 0.2 feet per minute. A manhole 400 feet upstream is surcharging, and a portable gas meter at the wet-well hatch reads 35 ppm H2S. The standby generator starts but the transfer switch must be manually operated, and the electrician is 25 minutes away. The collection system has a known infiltration problem, and a sanitary sewer overflow would discharge to a ditch that leads to a creek 0.4 mile away.
1. Calculate the approximate time to overflow and explain what the calculation does and does not tell you.
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The wet well is 12 feet in diameter, so the surface area is about 113 square feet. A rise of 0.2 feet per minute means the stored volume is increasing by about 22.6 cubic feet per minute, or about 170 gallons per minute. That matches the flow balance: 520 gpm inflow minus 350 gpm from the one operating pump equals 170 gpm net. At 3.0 feet of remaining depth, the time to overflow is about 3.0 divided by 0.2, or 15 minutes.
The calculation tells you the situation is immediate; it does not tell you the overflow will wait 15 minutes. The rise rate may increase if inflow grows, and the pump may trip or lose capacity. It also does not account for the upstream manhole already surcharging, which means sewage may reach the ditch before the wet well overflows. Treat 15 minutes as an upper-bound planning number, not a safe cushion.
2. List your immediate actions and notifications in priority order.
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First protect public health and the environment: call for additional operations staff and the electrician, and start the manual transfer to the generator if it is safe and the procedure is authorized. Try to reset the tripped pump only after checking the overload cause; repeated resetting can destroy the motor. If the pump cannot be restored, call a portable-pump or vacuum-truck contractor immediately, because 15 minutes is not enough time to rely on the electrician alone.
Second, notify the utility supervisor, the state, and the local health department as required by the sanitary sewer overflow procedures. Post warning signs at the ditch and creek, and keep the public away. Third, contain and disinfect any discharge if equipment is available, and document times, levels, and volumes. Do not enter the wet well; the 35 ppm H2S reading at the hatch is already a serious hazard. A confined-space entry to clear a pump is not an acceptable emergency shortcut. The priority is restoring pumping capacity and controlling the surface discharge.
3. The electrician arrives and wants to enter the wet well to check the tripped pump. How do you respond, and what would a safe entry require?
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I would stop the entry. The space is a permit-required confined space, the wet well is rising, and the hatch reading is 35 ppm H2S. That reading is above the odor threshold, can rapidly deaden the sense of smell, and would be expected to rise if sludge is disturbed. The space has not been tested at multiple depths, ventilated, or isolated. An electrician's urgency does not override the entry permit.
A safe entry would require a written permit, lockout/tagout of pumps and any electrical source, continuous ventilation, testing from outside at multiple levels for oxygen, flammable gas, H2S, and carbon monoxide, an attendant outside, a harness and tripod, a retrieval line, and a rescue plan with a team capable of rescue. The fire department is 20 minutes away, which is too long for an unconscious entrant; the utility would need a standby rescue capability or a non-entry solution. The better course is to use a portable pump, vacuum truck, or external bypass and repair the pump from outside whenever possible.