Design of regenerative stormwater biofilters for long term removal of legacy and emerging pollutants

Stormwater biofilters are low-impact development systems that are typically designed to quickly remove stormwater from surfaces for flood control with limited capacity to remove dissolved pollutants. Although certain amendments such as biochar, iron filings, and compost are added to biofilters to increase removal of pollutants, their pollutant removal capacity is unreliable due to various reasons including variable removal capacity of adsorbent media, exhaustion of adsorption sites, and environmental conditions. Adsorbed pollutants accumulate in the biofilter media and reduce the further removal of pollutants in stormwater. Replacement of exhausted biofilter media is cost-prohibitive. An alternate strategy is in-situ regeneration of the biofilters as a non-intrusive method to replenish exhausted biofilter media. This dissertation aims to improve the understanding of the fate and transport of emerging pollutants such as perfluoroalkyl substances (PFAS) and legacy pollutants such as heavy metals and pathogens in subsurface systems and use the improved understanding to develop methods to artificially or naturally regenerate the pollutant removal capacity of filter media to limit their exhaustion rate in stormwater biofilters.The dissertation consists of 5 research chapters. Chapter 2 critically examines the PFAS concentrations in suspended particles or colloids in the environment and shows that the suspended particles can adsorb and transport significant amounts of PFAS in surface water, subsurface soil, and even air. Chapter 3 proves that the fluctuations in groundwater flow can release colloids from PFAS-contaminated aquifer soil, which can carry PFAS. Therefore, removing soil colloids from groundwater samples through filtration or centrifugation can underestimate the PFAS concentration in groundwater. Chapter 4 examines the role of weathering cycles in the transport of PFAS in the subsurface and shows that dry-wet and freeze-thaw cycles can increase the release of colloids and associated PFAS from the subsurface into the groundwater. Chapter 5 demonstrates that in-situ injection of cationic polymers could regenerate exhausted biofilter media and improve their ability to remove PFAS from stormwater without clogging the biofilter. Chapter 6 offers a more natural method for regenerating the pathogen removal capacity of biofilters by utilizing the inherent toxicity of heavy metals to pathogens. Overall, the dissertation improves the understanding of the interactions between pollutants, microorganisms, and natural colloids in the solid-liquid interfaces and the effect of environmental conditions on these interactions in subsurface systems such as stormwater biofilters. The knowledge is useful to design biofilters, which are more efficient in removing legacy and emerging pollutants