Food Processing Wastewater Lift Station Retrofit in Green Bay, WI

Scenario

A frozen food processing plant in Green Bay, WI retrofitted its primary wastewater lift station after repeated clogging and seal failures caused by grease-laden effluent and seasonal freezing. The original centrifugal pumps failed every winter due to ice formation in suction piping and insufficient head to overcome backpressure from the municipal force main (which rises 42 ft over 1,200 ft of 8-inch PVC). Key constraints: no shutdown window longer than 8 hours (production runs 24/7), maximum allowable discharge pressure ≤ 75 psi (per municipal utility interface agreement), and requirement to handle suspended solids up to 2.5% by volume without frequent cleaning.

Given Data

• Flow Rate: 215 gpm (design peak flow, including 20% surge factor for batch cleaning cycles)
• Total Dynamic Head: 312 ft (42 ft static + 240 ft friction loss in 1,200 ft of 8" PVC @ 215 gpm + 30 ft safety margin for grease-induced roughness increase, per Hazen-Williams C = 100 recalibration)
• Specific Gravity: 1.03 (measured effluent density from lab analysis of grease/water/solids mix)
• Pump Efficiency: 62% (realistic efficiency for non-clog submersible pump with vortex impeller at low-flow/high-head duty point)

Calculation

Using the same formula:

$$ \text{HP} = \frac{Q \times H \times SG}{3960 \times \eta} $$

Where:

• $Q = 215$ gpm
• $H = 312$ ft
• $SG = 1.03$
• $\eta = 0.62$

$$ \text{HP} = \frac{215 \times 312 \times 1.03}{3960 \times 0.62} = \frac{69{,}290.4}{2455.2} \approx 28.22 \text{ hp} $$

Rounded to two decimal places: 28.22 hp.

Result and Decision

A 30 hp, 460V, stainless-steel submersible non-clog pump (model SC-30-VX) with a 312-ft TDH rating at 215 gpm was installed—validated against manufacturer performance curves showing 62.3% efficiency at that point. The pump included heated cable wrap on discharge piping and a redundant level sensor to prevent dry-run. System commissioning confirmed stable operation at 28.4 hp actual draw (within 0.7% of calculated), meeting both pressure and reliability targets.

Lesson

For non-standard fluids (e.g., grease-laden wastewater), field-measured specific gravity and validated pump efficiency curves—not catalog values—are essential; assuming SG = 1.00 would have underpredicted required HP by 3.1%, risking motor overload and premature failure.