Evaluation of a Pretreatment Intake System for Reducing Particulate Loading to a Bioretention Cell

Urbanization and climate change pose substantial challenges to mitigating the risk of flooding and degradation of water quality caused by stormwater runoff from developed areas. Bioretention is a stormwater management practice that addresses these challenges by reducing runoff volume and contaminant transport. However, bioretention systems are prone to physical clogging from suspended particles that reduce infiltration capacity and prevent treatment benefits. Pretreatment of stormwater before entering the bioretention system can alleviate this common issue, yet few studies have focused on the design of pretreatment approaches for bioretention systems. This lack of study is particularly important because most pretreatment systems allow untreated bypass flow directly to the storm sewer. In this case study, we combine field monitoring and hydrologic modeling to evaluate the design of a pretreatment intake structure in Johnston, Iowa. We assess the structure’s effectiveness by comparing inflow concentrations (representing conditions without pretreatment) to outflow concentrations and consider the associated trade-offs for mitigating urban runoff quantity and quality. This multichambered pretreatment intake structure routes runoff through a series of internal weirs and outlets that flow to the bioretention treatment area and allow bypass directly to the storm sewer. Our field-validated results show the pretreatment intake structure removed 37% of suspended particles before they could enter the bioretention basin, thereby reducing clogging and preserving well-established vegetation that enhances performance. Additionally, modeling showed water quality and quantity benefits from a 62% reduction in the mass load of suspended particles and an 80% reduction in peak flow rate directed to the storm sewer. In contrast, trade-offs include a decrease in the amount of water entering the bioretention basin and the potential for subsequent treatment because 60% of flow bypasses directly to the storm sewer. Opportunities to improve the design of pretreatment methods for bioretention systems lie in tailoring the system to site-specific pollutants, reducing bypass flow, and preventing resuspension of settled particles. This case study contributes insights into the benefits, trade-offs, and design considerations of pretreatment for bioretention systems critical to advancing sustainable stormwater management.