Accumulations of solids must be cleaned for safety reasons, among others
by Hydraulic Institute
November 12, 2019

How is cleaning conducted for wastewater and stormwater wet wells?

Organic solids accumulations in wet wells will become septic. This causes odors, increases corrosion and releases hazardous gasses. The design of a solids-bearing wet well must both provide for proper approach flow to the pumps and prevent the accumulation of sediments and surface scum in the sump. The goal is to minimize horizontal surfaces in the wet well anywhere but directly within the influence of the pump inlets, thereby directing solids to where they may be removed by the pumping equipment. Vertical or steeply sloped sides ease the transition from upstream conduits or channels to pump inlets.

Trench-type wet wells are designed to provide for cleaning with the periodic operation of the pumping equipment using a special procedure. The standard ANSI/HI 9.8 Rotodynamic Pumps for Pump Intake Design provides guidance on the geometry necessary to induce scouring velocities during cleaning. Experience has shown that trench-type wet wells with an ogee transition between the entrance conduit and the trench floor provides optimum geometry for efficient cleaning operations.

Image 1. Open trench-type wet-wellImage 1. Open trench-type wet-well hydraulic jump (Image courtesy of Hydraulic Institute)

Trench-type wet wells for solids-bearing liquids can be quickly cleaned by choosing a time when the inflow is about half of the capacity of the last pump. If that pump, operating at full speed, takes more than about a minute to lower the liquid level to the middle of the trench, two pumps can be activated. The liquid flowing down the ramp reaches supercritical velocity and forms a hydraulic jump (see Image 1) that, taking all solids with it, moves to the last pump.

The hydraulic jump should move from the toe of the ramp to the last pump in no more than 30 seconds, because operation at low intake submergence is severe service for the pump. As the hydraulic jump passes under each upstream pump inlet, the pump loses prime and should be stopped, and will need to be reprimed prior to the next start.

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