Grit Chamber Modeling: Computational fluid dynamics modeling of flow into the aerated grit chamber of a water reclamation plant
A water reclamation plant serving a population of around 1.3 million people in a service area of 142.4 square miles had been, through its history, experiencing non-uniform distribution of grit among its tanks. In particular, more grit was making it to two of its tanks, nos. 11 and 12. The problem of uneven distribution of grit persisted even after the original 12 tanks were replaced by 6 aerated tanks in 1997. To counter the problem of grit overloading in the 6th grit tank (formerly tanks 11 and 12), the inlet to the tanks has to be throttled, which gives rise to loss of capacity of the grit chamber. Since the problem persisted after replacement of the grit tanks with aerated tanks, the need to study the hydrodynamics of the water distribution system became apparent.
Figure 1: Velocity and shear velocity distribution along feeding tunnel and distributor.
Computational fluid dynamics simulations of the flow going into the aerated grit tank were prepared using FLOW-3D. It was observed that for higher pumping rates (Q ≥ 225 mgd) the flow in the distribution system became unsymmetrical with relatively lower velocities on the southern side of the distribution system (where the 6th grit tank is located). Because a lower velocity of water entering the grit chamber produces higher grit deposition, the results gives an initial idea about the origin of the problem. A continuous input of suspended sediment at the chamber inlet tunnel is being investigated with the help of FLOW-3D for different pumping rates and the results show that asymmetry in the sediment diverted to chamber 6 is more pronounced as flow discharge increases.
Figure 2: Bedload and suspended sediment distribution in the grit chambers.
Acknowledgments: This material was provided by Yovanni A. Cataño-Lopera, Research Associate at University of Illinois at Urbana-Champaign. Yovanni is a runner-up of the 3rd Flow Science 30th Anniversary Simulation Contest. For visualization as well as post-processing of the simulation data, Tecplot 360 by Tecplot was used.