Lift & Pump StationsAdvanced 14 min read

Lift & Pump Stations: Advanced Pump Station Design I – Wet Well Sizing & Capacity/Redundancy Design

The full mathematical derivation of the wet well minimum cycle time formula, a worked active-volume sizing example, multi-pump staging, firm capacity/redundancy design, and vortex/submergence fundamentals for Class 4 pump station design.

Every lower-level guide in this series has told you, in plain language, that a wet well has to be "big enough" to avoid short-cycling without being so big that wastewater goes septic. At Class 4, that rule of thumb becomes an actual equation — one you can derive from first principles, verify with calculus, and use to size a real wet well. This guide builds that equation from the ground up, then applies it alongside the redundancy and vortex-prevention criteria that round out true pump station design.

From Rule of Thumb to Design Equation: The Wet Well Cycle Time Formula

Define

V

as the active (cycling) volume of the wet well — the volume between the pump-on and pump-off elevations, in gallons —

Q_{in}

as the wastewater inflow rate to the wet well (gpm), and

Q_{out}

as the pump's discharge rate (gpm). A single pump's cycle consists of a fill period while it's off and a pump-dow

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