Computational Fluid Dynamics (CFD) Modeling of Flow into the Aerated Grit Chamber of the MWRD's North Side Water Reclamation Plant, Illinois

The North Side Water Reclamation Plant (NSWRP) is one of the seven water reclamation plants owned and operated by the Metropolitan Water Reclamation District of Greater Chicago (MWRGC) and serves a population of around 1.3 million people in a service area of 142.4 square miles. Through its history the plant has experienced non-uniform distribution of grit between the tanks 1–10 and 11 & 12. To counter the problem of grit overloading in the 6^th (southernmost) grit tank, the inlet to the tank has to be throttled which on the other hand gives rise to loss of capacity of the grit chamber. Computational Fluid Dynamics (CFD) simulations of the flow and the flow coupled with sediment transport; going into the aerated grit tank were done using the CFD modeling code, 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 and discharges on the southern side of the distribution system, especially in distribution channels 11 and 12. Similarly the sediment deposition distribution showed a clear shift towards channels 11 and 12 with increase in discharge, this goes to show the tendency of the sediment to move along the southernmost channels. A possible mechanism for explaining the non-uniform grit deposition in the aerated grit chambers has been discussed and it has been backed by the generation of a secondary current at the bend and the Bulle Effect.