Treatment of impacted stormwater at its source is considered as a viable strategy to prevent downstream impacted surface waters and sediments; however, the utility of stormwater treatment systems such as biofilters is often limited due to a low per- and poly-fluoroalkyl substance (PFAS) removal capacity of filter media used in stormwater treatment systems. Moreover, biofilter media get exhausted quickly during high flow conditions and in the presence of high concentrations of suspended particles and dissolved organic carbon (DOC) typically present in stormwater. The conventional passive design of biofilters is not adequate to remove PFAS under these conditions.

This project aims to develop passive-active modular stormwater treatment units that can consistently remove PFAS at varying flows and complex stormwater mixtures. The modular units will utilize two in situ methods to alter the surface charge of filter media and dramatically increase their capacity to ionic and short-chain PFAS that are typically most difficult to remove. Both methods include application of either direct current at low potential (1.5 volts) through granular activated carbon (GAC) or cationic polymer such as polydiallyldimethylammonium chloride (polyDDA) on any conventional biofilters installed in DoD sites. The specific objectives of this project include:

• examining changes in PFAS removal capacity of biofilter after in situ modification polyDDA,
• examining the degradation and impacts of polyDDA on the biofilter microbiome,
• quantifying the clogging rate of cationic-modified biofilters with and without sediment trap units,
  evaluating the removal of PFAS in electrostatically-charged GAC embedded in rolled carbon cloth, and
• examining the improvement in PFAS removal capacity in variable flow conditions within a complex stormwater matrix.

The project will examine PFAS removal from stormwater containing suspended particles and DOC in under variable flow conditions using modular units. The modular units consist of: (1) a sediment trap unit to capture PFAS-impacted suspended sediments typically present in the first-flush flow, (2) electrostatically charged (< 1.5 V) GAC embedded in rolled carbon cloth, and (3) polyDDA modified biochar or GAC-amended biofilters. Controlled laboratory column experiments will be conducted to quantify the optimum amount of polyDDA needed to maximize PFAS removal and minimize unintended consequences such as clogging and alteration of biofilter microbiome responsible for long-term degradation of other legacy chemicals of concern. Each unit will be tested independently for a series of PFAS at various conditions to optimize the design and improve the mechanistic understanding. All modules will then be connected in series and tested for the removal PFAS present in aqueous film-forming foam (AFFF) from a complex stormwater matrix.

This project will help develop passive-active modular units that can be tailored to site-specific needs, deployed or retrofitted at most sites, and remain operational for a long period without the need for frequent maintenance. The active unit of using low voltage direct current to alter charge can be automatically activated only during rainfall, which will remove PFAS during peak flow. The passive unit with polyDDA-modified biofilters can consistently remove PFAS and its sorption capacity can be regenerated in situ, thereby preventing the need to replace exhausted media. The sediment trap units will remove PFAS associated with particles present in the first flush and minimize clogging, thereby lowering the maintenance needs. The innovative biofilter design can be easily modified to treat various other chemicals. The modular units are designed to be installed at existing DoD sites with limited disturbance, permit efficient and safe replacement or enhancement of each unit, limit clogging, and function over the long term with minimal maintenance, thereby lowering the overall design and maintenance costs. The results will help develop innovative full-scale stormwater treatment technologies that can be retrofitted to treat and reuse stormwater. (Anticipated Project Completion - 2026)