Computational Fluid Dynamics Analysis of a Vortex Grit Removal System

A vortex grit removal system (two tanks rated at 70 MGD each) was installed at East Bay Municipal Utility District (EBMUD) in its Main Wastewater Treatment Plant in 2006 to improve grit removal from influent wastewater. Although use of the system has made some reductions in energy use and odors at the plant, independent performance testing concluded that the specific grit removal efficiencies were not being met. EBMUD retained AECOM to develop a computational fluid dynamics (CFD) model to analyze the vortex grit system, and to evaluate design modifications that would potentially improve grit removal. The CFD model included a particle tracking component to assess grit travel through the system. Numerous design and operational modifications were tested and evaluated based on in-tank velocity and turbulence, relative grit removal, and short-circuiting potential. Based on model results, AECOM recommended removal of several vortex grit tank components, including an internal influent baffle, the cover of the grit hopper and an in-tank propeller. AECOM also recommended that both vortex grit units be operated during dry weather, resulting in average flow rates of approximately 35 MGD per unit. After the modifications were implemented, a repeat performance test was completed. Preliminary test results indicate that removal efficiency has increased for 50-mesh, 70-mesh and 100-mesh grit.