Modeling Annual Particulate Matter Separation and Washout by Unit Operations with CFD

Abstract

Computational fluid dynamics (CFD) is a tool to simulate the coupled hydrodynamics and particulate matter (PM) fate in unit operations (UOs) subject to transient rainfall-runoff events. Most frequently, CFD is applied for the relatively rapid analysis of steady flows (CFD)s. More recently, an event-based stepwise steady flow CFD model (CFD)ss was developed to predict particulate matter (PM) separation with reduced computational overhead compared with unsteady CFD modeling (CFD)us. This study developed an event-based (CFD)ss model to evaluate longer-term UO performance. The (CFD)ss models simulated PM fate (separation and washout) in a rectangular clarifier (RC) and a screened hydrodynamic separator (SHS) for a representative year of runoff (2007) in North Florida. Washout and separation for all unsteady events were integrated into a time domain continuous simulation model. The (CFD)ss model was validated with measured PM separation and washout data from full-scale physical models. The annual PM separation for the RC was 40% with no washout and 38% with washout. Annual PM separation for the SHS was 37% with no washout and 27% with washout. The annual SHS washout profile indicated maintenance cleanout at the start and end of the wet season. Coupling (CFD)ss with continuous simulation represents a tool extending UO evaluations from an unsteady event-basis to an annual basis. Simulating all runoff events in 2007 required 14 h of computational time for the RC and SHS. Implications from these results indicate that longer-term prediction of UO separation/washout is viable with (CFD)ss coupled with a continuous simulation model such as the Storm Water Management Model (SWMM), and can serve as a tool for UO testing/certification.