Can a stepwise steady flow computational fluid dynamics model reproduce unsteady particulate matter separation for common unit operations?

Computational fluid dynamics (CFD) is emerging as a model for resolving the fate of particulate matter (PM) by unit operations subject to rainfall-runoff loadings. However, compared to steady flow CFD models, there are greater computational requirements for unsteady hydrodynamics and PM loading models. Therefore this study examines if integrating a stepwise steady flow CFD model can reproduce PM separation by common unit operations loaded by unsteady flow and PM loadings, thereby reducing computational effort. Utilizing monitored unit operation data from unsteady events as a metric, this study compares the two CFD modeling approaches for a hydrodynamic separator (HS), a primary clarifier (PC) tank, and a volumetric clarifying filtration system (VCF). Results indicate that while unsteady CFD models reproduce PM separation of each unit operation, stepwise steady CFD models result in significant deviation for HS and PC models as compared to monitored data; overestimating the physical size requirements of each unit required to reproduce monitored PM separation results. In contrast, the stepwise steady flow approach reproduces PM separation by the VCF, a combined gravitational sedimentation and media filtration unit operation that provides attenuation of turbulent energy and flow velocity.