The ability of conventional stormwater best management practices (BMPs) to meet future discharge regulations for per- and polyfluoroalkyl substances (PFAS) is of increasing concern. Geomedia-augmented filtration systems have emerged as an effective approach to enhance retention of trace organic chemicals of concern; however, eventual media exhaustion is inevitable for passive systems treating PFAS-impacted runoff, and future replacement of spent media is a substantial future challenge. Modular rapid filtration systems with granular sorbents could alleviate these challenges while facilitating fast filtration under peak flow conditions and reducing susceptibility to fouling. Sequential filters with black carbon sorbents (BCs) and ion exchange resins (IERs) could show particularly effective performance: initial removal of natural organic matter (NOM) and long-chain PFAS by BC could improve removal of short-chain PFAS by IERs. However, while PFAS removal from groundwater in active adsorption systems has been widely studied, performance under conditions representative of passive stormwater treatment systems (e.g., intermittent, unsaturated, variable flow) is poorly understood. Moreover, a sound understanding of sorption kinetics will be critical to design systems that comprehensively capture PFAS under peak flow design conditions.

This research project will develop tools and knowledge to inform the design of passive rapid stormwater filtration systems incorporating BCs and IERs to achieve comprehensive PFAS removal under variable flow conditions. The approach of this study will involve the following objectives: (1) characterize sorption processes for potential sorbents under environmentally relevant conditions; (2) assess PFAS breakthrough under unsaturated intermittent flow conditions; and (3) adapt existing tools to make screening-level design calculations and perform a life-cycle cost analysis.

The experimental approach combines laboratory batch experiments, bench-scale column experiments in laboratory and field settings, and one-dimensional transport modeling in combination with a life-cycle cost assessment. Batch experiments will be conducted for three BCs and three IERs, and will investigate the impacts of sorbent material properties, particle size, competitive sorption effects, and fouling by NOM on sorption kinetics and capacity. Intermittently-dosed column challenge tests will be conducted to evaluate the effects of variable unsaturated flow conditions on PFAS removal by two IERs and two BCs, and subsequent analogous tests with field-aged media will evaluate the effects of aging and weathering on PFAS retention performance. Existing one-dimensional transport modeling tools will then be adapted to predict PFAS breakthrough times in a representative full-scale system. Results from the screening-level modeling evaluation will be used to inform a life-cycle cost assessment focused on media lifetime, replacement, and regeneration. 

The findings of this research project will lead to an improved understanding of the effects of sorption kinetics, unsaturated intermittent flow conditions, and biological aging on PFAS breakthrough behavior in passive stormwater filtration systems with engineered sorbents. The primary output will be a life-cycle cost assessment for filter media in passive rapid filtration systems, to be used by practitioners for screening-level assessments and design considerations. These findings will provide a technical foundation for the design of passive rapid filtration systems to meet future PFAS discharge regulations at DoD facilities, leading to improved management of AFFF-impacted surface water by preventing PFAS from being transported off-site. (Anticipated Project Completion - 2025)