Finding the Sweet Spot: Designing for Wet Weather Flows

The upgrade of two existing wastewater pumping stations owned by the Pequannock, Lincoln Park & Fairfield Sewerage Authority (Two Bridges) demonstrated the challenges faced when attempting to increase wet-weather capacity. Selecting equipment solely to meet wet-weather demands can result in unintended results when the facility operates under typical dry weatherconditions. Though both facilities convey raw sewage from separate sanitary sewer systems, flows during wet weather periods can exceed 5 times the average daily flows. Variable frequency controllers allow the flexibility for pumps to be operated well below their peak flow range. However, these operating points may not be desirable from the point of view of pump efficiency and reliability. Reduced efficiency and internal recirculation cavitation can be associated with operation at flows less than 50% of the pump Best Efficiency Point (BEP).