Lift Station Outage: Time to Overflow and Pump-and-Haul Loads

ScopeThe question is how many minutes until it overflows into basements and streets, and whether the portable generator gets there first. This calculator treats the powerless station as a tank: everything between the high-level alarm and the lowest point where sewage escapes -- a wet-well overflow, a manhole rim, or the lowest basement drain -- is emergency storage, and the gravity sewer below that elevation adds to it as it surcharges, at 0.0408 gallons per foot per square inch of diameter. An 8 ft diameter wet well with 3 ft of freeboard to the lowest basement holds 1,128 gal, and 1,200 ft of 8 in sewer adds 3,133 gal, so three-quarters of the 4,261 gal of storage is in the pipe; at 100 gpm inflow that is 42.6 minutes. The Ten States Standards require the storage to cover detection plus the transport and connection of portable generating equipment. A crew that needs 20 minutes to be alerted, 60 to drive, and 25 to hook up loses by more than an hour, and about 6,240 gallons backs up into the lowest property on the line. When no generator is coming, an 8-hour outage with 3,000 gal vacuum trucks needs 15 loads net of storage. Use the wet-weather inflow for the event, not the dry-weather average. The sewer is taken as full-pipe storage ignoring slope, so its figure is an upper bound. It does not size the generator, the pumps, or the wet well's working volume, or address spill reporting. The Ten States Standards, the state regulatory agency, and the utility's emergency response plan govern.

Formula or tablewet well = plan area x (overflow elevation - high-alarm elevation) x 1728/231 gal; sewer = 3 pi d_in^2 / 231 (= 0.0408 d^2) gal/ft x length; time to overflow = storage / inflow; response = detection + travel + hookup; shortfall = inflow x response - storage when positive; haul loads = ceil((inflow x outage - storage) / truck gallons).

Source and editionGreat Lakes-Upper Mississippi River Board, Recommended Standards for Wastewater Facilities (Ten States Standards), Sections 47.1, 47.2, and 47.433, by name, as the emergency-storage requirement; the arithmetic is a mass balance. The state regulatory agency and the utility's emergency response plan govern.

Free accessThe Ten States Standards are free to read; the requirement is named, not reproduced, and the storage and time are a mass balance on the user's dimensions and inflow.

What governsEstimate. Operator of record and primacy agency govern.

API field nameswet_well_shape, wet_well_diameter_ft, wet_well_length_ft, wet_well_width_ft, storage_depth_ft, sewer_length_ft, sewer_diameter_in, inflow_gpm, detection_min, travel_min, hookup_min, outage_hr, truck_capacity_gal, wet_well_gal, sewer_gal, storage_gal, time_to_overflow_min, response_min, shortfall_gal, truck_loads

• Wet-weather inflow the inflow during the storm that caused the outage, not the dry-weather averageTen States Standards
• Full-pipe sewer storage the gravity line below the overflow elevation taken full, ignoring slope -- an upper boundmass balance
• Lowest escape point governs the wet-well overflow, a manhole rim, or the lowest basement drain on the line, whichever is lowestTen States Standards Section 47