Is the Treatment Response of Manufactured BMPs to Urban Drainage PM Loads Portable?

Abstract

Hydrodynamic separators (HS) and basins/tanks such as rectangular clarifiers (RC), as unit operations (UOs) for particulate matter (PM) separation, are common smaller-footprint best management practices (BMPs). Test protocols and physical model results for such UOs are often assumed to only be applicable by city, state, or region. Rainfall-runoff, climate, watershed, and PM granulometry are, in part, groups of parameters identified as impacting UO behavior. As a result, UO behavior is often assumed to be nonrepresentative when the UO is ported to different environs. This study examines specific parameters with potential to alter UO behavior: (1) hydrograph unsteadiness; (2) PM particle-size distribution (PSD); (3) PM specific gravity (ρsg); and (4) water temperature. In this study, UO behavior as PM separation efficiency (ßPM) is normalized for each parameter to examine if UO results can be ported across a series of urban environs. To generate normalized ßPM results, computation fluid dynamics (CFD) models of the HS and RC are applied for seven flow rates, three levels of ρsg, and three temperatures over a heterodisperse PSD from 1 to 2, μm. CFD models were validated within 1% of physical model results for measured events. Additionally, these parameters were analyzed with normalized ßPM results to model three hydrographs loading the HS and RC. Results indicate the greatest impact on UO behavior is influent PSD, whereas ρsg and hydrograph characteristics have significant but less pronounced impacts. Temperature has a minor impact. Results demonstrate the normalized ßPM approach for a specific UO accounts for the variability of these common parameters examined, and thus UO behavior can be portable across geographic locations. Results further suggest the potential of a more unified physical model testing protocol for UO certification and the deployment of validated CFD models to support physical testing.