Wet well sizing for lift stations: the cycle-time method explained

The two opposing constraints

Wet well volume between pump-on and pump-off levels has to satisfy two competing requirements:

1. Maximum cycle time — the pump can only start so many times per hour without overheating the motor or wearing out the contactor. The standard limits are 6 starts/hour per pump (10-States §42.32) for fractional and small horsepower, 4 starts/hour for larger units (>20 hp), some manufacturers spec 3 starts/hour for soft-starter or VFD-controlled units. 2. Maximum detention time — wastewater septicity. Solids settle, anaerobic conditions begin, hydrogen sulfide generates. 10-States §42.34 recommends 30 minutes maximum detention at average daily flow, with 60 minutes as an absolute upper bound.

Wet well volume too small → pumps cycle too fast → motor failures. Wet well volume too large → wastewater sits too long → odor + corrosion.

The cycle-time method finds the volume that satisfies the first constraint at the *worst inflow case*, then checks the detention against the second.

The worst inflow case (math)

For a single-pump station with pump rate Qp and incoming flow Qi, the cycle time is:

T_cycle = V / (Q_p − Q_i) + V / Q_i
        = V · Q_p / [Q_i · (Q_p − Q_i)]

Where:

• V = active wet well volume (gallons) between start and stop levels
• Q_p = pump rate (gpm)
• Q_i = inflow rate (gpm)
• T_cycle = total cycle time (minutes)

Differentiating w.r.t. Qi and setting to zero shows the cycle time is minimum when Qi = Q_p / 2. At that worst-case inflow:

T_min = 4 · V / Q_p

This rearranges to a clean sizing formula:

V_min = T_max · Q_p / 4

Where T_max is the maximum allowable cycle time (1 / max-starts-per-hour, in minutes; so 6 starts/hour → 10 min cycle).

Worked example: small lift station

Design parameters:

• Average daily flow: 50,000 gpd ≈ 35 gpm
• Peak design flow: 175 gpm (peaking factor ~5 typical for small stations)
• Pump rate (each pump): 200 gpm
• Max starts per pump: 6 per hour → T_max = 10 minutes

Active volume:

V_min = 10 · 200 / 4 = 500 gallons

That's the minimum between pump start and pump stop floats.

Detention check at average flow:

Detention = 500 / 35 = 14.3 minutes

Well under the 30-minute upper limit. Good.

Detention check at peak flow:

Detention = 500 / 175 = 2.9 minutes

Also fine.

Duplex station with alternation

Two pumps alternating on each cycle effectively doubles the allowed starts because each pump only sees half the cycles. For the same Q_p:

V_min_duplex = T_max · Q_p / 8

In our example, V_min drops to 250 gallons. But check whether the smaller volume still meets detention requirements at low-flow hours — sometimes the duplex calculation gives a volume so small that hydraulic detention at minimum flow becomes the binding constraint.

10-States §42.32 recommends sizing for the worst case (single pump operating) even with duplex stations, because pump #2 might be out for maintenance.

Triplex and beyond

For triplex with proper alternation and step-up logic (1 pump → 2 pumps → 3 pumps as inflow rises), each pump's starts-per-hour drops further. Standard practice: design active volume for single-pump duty, use larger pumps for parallel staging at peak inflow.

Some designers split the wet well into multiple stages with separate float bands per pump. This works but complicates control logic and SCADA. The simpler approach is one active volume designed for single-pump worst-case, with the additional pumps staged by inflow level rather than separate float bands.

VFD-controlled stations

A VFD-controlled lift station modulates pump speed to match inflow continuously, rather than cycling on/off. Active volume can be smaller — often 5-10× smaller than a constant-speed equivalent — because the pump rarely fully stops.

The trade-off:

• VFD adds capital + harmonic mitigation costs
• Below ~30% speed the pump may not solids-pass (impeller flow patterns break down)
• Septicity reasserts itself: even with a moving pump you need adequate turnover to prevent settling at low-flow hours

10-States §42.34 still applies — the wet well should not hold any particular slug of wastewater longer than 30 minutes at average flow. Don't shrink the wet well volume below this even if cycle time would allow.

Real-world pitfalls

Phantom floats. Operators in the field sometimes "fix" cycling by raising the stop float, shrinking the active volume further. This makes the problem worse — fewer gallons per cycle means more cycles per gallon. The right fix is to raise the *start* float or replace the pump with a lower-flow unit.

Wedge-shaped wet wells. Volume between floats isn't always linear with elevation. A round wet well with a 45° sloped floor has more volume at the bottom than the top. Confirm volume-vs-elevation with actual geometry before sizing.

Pump curve sliding. As the wet well draws down, suction static head increases (more lift), pump head drops, and pump flow rises. The "200 gpm pump" might actually average 180 gpm over the active range. Compute the average flow over the active range, not the nameplate value, when applying the formula.

Solids loading. Grit and rags affect the effective volume. The active volume should be measured to the float locations as designed, with a clean wet well — but the maintenance plan should include regular cleanouts because accumulated solids reduce effective volume over time.

Quick check

| Step | Number | |---|---| | Design pump rate (each) | Qp gpm | | Max starts per hour | starts/hr | | Max cycle time | Tmax = 60 / starts | | Min active volume | V = Tmax · Qp / 4 | | Detention at ADF | V / Qavg in minutes — must be < 30 | | Detention at peak | V / Qpeak in minutes — must be < 30 |

If both detention checks are well under 30 minutes and cycle time hits 10+ minutes at the worst inflow, the wet well is right-sized.

How the calculator handles it

The Wet Well Sizing tool walks you through inflow, pump rate, station configuration (simplex / duplex / triplex / quadplex), and starts-per-hour limits, then returns:

• Required active volume (gallons + cubic feet)
• Cycle time at worst-case inflow
• Detention time at average flow
• Detention time at peak flow
• Pass/fail vs. 10-States §42.32 + §42.34

Open the Wet Well Sizing calculator when you're ready to size yours.

References

• Great Lakes — Upper Mississippi River Board of State and Provincial Public Health and Environmental Managers. *Recommended Standards for Wastewater Facilities ("Ten-States Standards"),* latest edition — §42 Pumping Stations.
• Hydraulic Institute. *ANSI/HI 9.8 — Rotodynamic Pumps for Pump Intake Design.*
• WEF / ASCE. *Design of Wastewater Treatment Plants,* MOP 8 — pump stations chapter.