Rechargeable stormwater biofilters: In situ regeneration of PFAS removal capacity by using a cationic polymer, polydiallyldimethylammonium chloride

Conventional stormwater biofilter media such as sand and compost have limited capacity to remove PFAS due to the exhaustion of attachment sites with pollutants and other stormwater constituents. Replacing exhausted filter media is expensive whereas in situ regeneration of their adsorption capacity can be cost-effective. This study demonstrates that in situ application of cationic polymers such as polydiallyldimethylammonium chloride (PDADMAC), a drinking water coagulant, could regenerate the filter media capacity to remove anionic PFAS. The benefit of PDADMAC increased with a decrease in PFAS carbon chain length: the removal of PFBA increased by 9.9 +/- 1.8 times while the removal of PFOA only increased by 0.3 +/- 0.03 times. The result indicates the appli-cation of the cationic polymer can help remove short-chain PFAS, the ones that are most difficult to remove by typical organic amendments such as activated carbon and biochar. Despite being used as coagulants, the addition of PDADMAC did not negatively affect the clogging rate of the biofilters in the presence of suspended sediments. Surprisingly, biofilters exposed to PDADMAC required 35.3% more suspended sediments than the biofilters without PDADMAC to reduce the hydraulic conductivity below half of the initial value. PDADMAC application reduced the release of colloids from the biofilter media during intermittent flow, thereby decreasing the potential of the colloid-facilitated release of adsorbed PFAS. The adsorbed PDADMAC in the biofilter was not degraded, indicating that applied PDADMAC could last long and continue to remove PFAS in the biofilter. Thus, in situ application of cationic polymers can help regenerate the PFAS removal capacity of conventional biofilters.