Lift Station Wet Well Sizing
Required wet-well storage volume between pump-on and pump-off elevations. Limits motor cycling to protect pump life while keeping detention time short enough to avoid septicity. Single-pump and lead-lag-pump configurations.
Defaults: 200 gpm per pump, 100 gpm peak inflow, 6-min minimum cycle. Δh column is for a 3-ft-diameter wet well as a reference; scale by area for other sizes.
Why a minimum cycle time matters
Motor cycling — the on/off sequence — is the dominant cause of pump-station maintenance. Most submersible sewage pumps in the 5 to 50 hp range are limited to 6 to 10 starts per hour by the manufacturer (Goulds, Flygt, Wilo). More starts than that and the rotor heating during locked-rotor inrush exceeds the cool-down between cycles. Eventually the windings fail.
For a single pump, the worst case is when inflow exactly equals half the pump capacity — pump runs as long as it sits idle, maximum cycles per hour. The single-pump formula V = Qp tc / 4 is derived from this worst-case inflow.
Septicity — the upper bound on V
Sewage held in a wet well goes anaerobic in 30 to 60 minutes at warm temperatures. H₂S production starts immediately, generating odor complaints and corroding the concrete above the waterline. Operating volumes giving HRT > 30 min at average flow are red flags. The fix: pick a smaller pump (cycles more often) and accept lower cycle time, or add aeration.
Duplex (lead/lag) — preferred design
Duplex stations alternate between two equal pumps with each cycle. Effective cycles per pump are halved, so the minimum-volume formula gives smaller V. Both pumps fire together in peak-inflow override conditions. Standard EPA / 10 States Standards require duplex for any station handling raw sewage; triplex (three pumps) for stations where one pump can't handle peak inflow.
Wet well diameter and depth
Wet wells are usually 4 to 8 ft (1.2 to 2.4 m) diameter precast concrete manholes for small stations. Operating depth (between low-level cutoff and high-level alarm) is typically 2 to 4 ft (0.6 to 1.2 m). Total wet well depth includes:
- Pump submergence (~12 in below low level for cooling)
- Operating range (between low and high level)
- Working storage between high level and alarm
- Inflow pipe invert above alarm
- Top slab + access
Reference: Sanks, R.L. (2008). Pumping Station Design, 3rd ed., Butterworth-Heinemann, ch. 12. Great Lakes — Upper Mississippi River Board (2014). Recommended Standards for Wastewater Facilities ("10 States Standards").