CFD study of multiphase flow in aerated grit tank

Abstract

Grit removal is an important step in a municipal wastewater treatment plant to remove fine particles from wastewater. One of the prominent technologies for grit removal is an aerated grit tank (AGT), which uses air diffusers to create spiral flow that allows heavier particles to accelerate and settle to the bottom of the tank. The performance of an AGT largely depends on induced flow patterns and hydrodynamic forces. The current study investigated an industrial-scale AGT using computational fluid dynamics modelling and simulations. For the simulated tank, industrial trials were conducted to measure grit quantity and particle size distribution in the influent and effluent of the tank. The measured data were used to validate the CFD model and then the validated model was used to investigate the effect operating conditions and modifications in tank geometry on flow patterns and grit capture efficiency. The objective of this parametric investigation was aimed to improve grit removal efficiency of the simulated AGT. The predictions suggested only 6.6 % change in the grit capture efficiency of the tank when air flowrate was either increased by 50 % from the current setpoint (230 Nm3/hr). Simulation of a peak wet weather wastewater flowrate (= 986 L/s) resulted in ∼50 % drop in grit capture compared to dry weather wastewater flowrate (= 460 L/s). Among the simulated tank modifications to improve grit removal during peak wet weather events, increase in tank length and width did not provide significant improvement. However, modification in tank base by relocating the screw conveyor recess from its current position (midway along the width of the tank) to a location adjacent to the wall resulted in 34 % increase in the capture efficiency of smaller particles in a size range of 100–150 μm.