Grit Removal from Wastewater Using Secondary Currents in Open-Channel Flow around Bends

A grit removal process that exploits the phenomenon of secondary currents induced in open-channel flow as water moves around a bend has been investigated by using both physical and computational fluid dynamics (CFD) modeling approaches. The flow field was first validated against a case study by solving the full Reynolds-averaged Navier-Stokes equations and Reynolds stress turbulence models with a finite-volume method. The air-water interaction at the free surface was simulated by using a volume of fluid multiphase model. Discrete phase modeling was then used to calibrate the hydrodynamic interaction with grit particles (varying in size from 63 to 2,000 mu m) observed in a physical model with a 30 degrees bend, with respect to particle diameter, flow rate, shape factor, and injection location. This was followed by detailed parametric investigations using the calibrated model at different flow rates, angle of bend, and radius of curvature that showed individual particle trajectories (and consequent grit removal efficiency of the process) for different particle sizes. The CFD model was also used to compare different configurations of receiving sump whereby different outflows and baffles were compared. Overall, the study shows that a very slightly curved channel or straight channel is, in fact, the optimum solution, because the secondary currents in curved channels act to uplift particles away from the sump, thus reducing grit removal efficiency. DOI: 10.1061/(ASCE)EE.1943-7870.0000400. (C) 2011 American Society of Civil Engineers.