A Physical Model Study of the North Shore Wastewater Treatment Plant Pump Station

Large scale water pumps with bell mouth intakes have been broadly used by municipal wastewater services to move sewage to wastewater treatment plants. Swirling of the flow when entering the suction bell of the pump intake can cause free-surface and/or sub-surface vortices, resulting in poor pump operation. In order to properly design, the wet well of sewage pump station both physical and numerical models are used to analyze the flow condition entering the pump intake and the associated flow pattern and potential vortex formation. In cooperation with WSP Canada Ltd., a physical modelling study of the First Narrows Sewage Pumping Station was conducted by the Ryerson research team at Ryerson University’s Centre of Urban Innovation Laboratory. During the physical model testing, uneven flow distributions including vortices were observed at the intake chamber under three pump-working conditions. To achieve an even flow distribution with minimal vortices, alternative slot designs at the entrance of the chamber were analyzed.

Additionally, a tapered design of the suction bell intake was tested for potential vortex formation. The results showed that a reduced area of the entrance slot could distribute the inflow evenly in the chamber. Moreover, no vortex formation around the tapered suction bell was found under (a) a low flow condition of 10 l/s at a water level -5.10 m below datum and (b) a cleaning cycle scenario of 16 l/s at a water level -4.9 m below the datum. However, there was observed water rotations at the backwall side. For tight and intense water rotations, it might cause vortex formation. This study has provided design changes that can smooth the flow and reduce vortices at the bell mouth intakes of the pump intake chamber.