Urban Particle Capture in Bioretention Media. II: Theory and Model Development

As a novel stormwater best-management practice, using a soil/sand/mulch mixture to capture urban pollutants while promoting infiltration, the unique media composition renders bioretention significantly different from conventional sand filters. In this work, a three-layer model is presented to describe particulate capture in bioretention media employing parameter calibration and presensitivity analysis. Since the fine size of bioretention media strictly limits the particulate penetration distance, the media column is modeled as a pristine zone (bottom), a working zone (middle), and a cake zone (top). Mechanisms of both depth filtration and cake filtration are examined through mass balances, which show that both are significant. The developed resistance of each layer due to solid deposition was also estimated. Experimental data for different media/TSS-type combinations of selected experimental trials were used in parameter calibration. The calibrated model successfully predicted the effluent TSS and media hydraulic conductivity of subsequent trials with appropriate boundary and initial conditions as input. A weighted combination of calibrated parameters from different TSS types also agreed well with media behavior for treating a complex TSS mixture. The results of media replacement (top removal and refill) simulation also reasonably fit experimental data. Using proper assumptions, a long-term scenario analysis for permeability reduction was performed for a field bioretention facility. Based on modeling results, this study recommends a shallow bioretention media depth design, an annual or biannual field inspection schedule, and periodic media replacement maintenance.