Models of Wet Basin Design Response with Residence Time Metrics for Presumptive Guidance

Abstract

Wet basins manage hydrologic quantities, particulate matter (PM), and chemicals. Residence time (RT) and hydraulic residence time (HRT) are common metrics to index presumptive water chemistry and PM load response. Models for the response of basin designs were studied. Basin hydrodynamics (based on geometry, inlet layout, and baffles), and PM separation based on influent particle-size distribution (PSD) were indexed with RT and HRT. PM separation and economics were examined for seven basin designs (HRT=0.25–21 days). Scaled designs were patterned geometrically from an existing basin that was oversized based on flows and PSDs. Computational fluid dynamics (CFD) with continuous simulation, surface overflow rate (SOR), and first-order decay models were studied. Basin geometrics, PM separation, and constituent RT were modeled across a representative wet season. The results demonstrate that PM separation is sensitive to influent PSD and basin sizing for 1–5-day HRTs, with less sensitivity to PSD for larger volumes of 5–21-day HRT. Constituent RT (based on PM) varies in the wet season and remains below the HRT. Basin retrofit geometry and inlet layout impact constituent RT and PM separation. HRT alone, from any model tested herein, is not recommended in assessing basin treatment for less than a 5-day HRT, or if the PSD is unknown. For basins with greater than 1-day HRT, transport behavior is approximated reasonably assuming steady flow. Although PM separation is insensitive to HRT beyond a 5-day design, increasing basin size impacts economics. Results indicate that the 14-day basin HRT is not an optimal benchmark for treatment.