Potential of biochar filters for onsite wastewater treatment: Effects of active and inactive biofilms on adsorption of per- and polyfluoroalkyl substances in laboratory column experiments

This study investigated the potential of biochar filters as a replacement for, or complement to, sand filters for removal of per- and polyfluoroalkyl substances (PFASs) from wastewater in on-site wastewater treatment systems (OWTSs). Concentrations and removal of nine perfluoroalkyl carboxylates (PFCAs; C3-11) and three perfluoroalkane sulfonates (PFSAs; C-4,C- 6,C- 8) and one perfluorooctanesulfonamide (FOSA; C-8) were investigated over 22 weeks in four treatments with column filters: biochar (BC) without biofilm (BC-no-biofilm), biochar with active biofilm (BC-active-biofilm), biochar with inactive biofilm (BC-inactive-biofilm) and sand with active biofilm (Sand-active-biofilm). The filters were operated under hydraulic loading (50 L m(-2) day(-1)) to mimic the loading rate in on-site filtration beds. The initial concentrations of the Sigma PFASs in the influent were in the range of 1500-4900 ng L-1. In BC-no-biofilm, the removal efficiency (20-60%) and adsorption capacity (0-88 ng EPFASs g(-1) BC) of short-chain PFCAs (C3-6) and PFSA (C-4) was low, whereas the removal efficiency (90-99%) and the adsorption capacity (73-168 ng g(-1)) was high for C-7-C-11 PFCAs, C-6, C-8 PFSAs and FOSA. The relative removal was generally lower for C3-9 PFCAs and C-4, C-6, C-8 PFSAs using BC-active-biofilm and BC-inactive-biofilm compared with BC-no-biofilm. This can be explained by the presence of biofilm and solids in BC-active-biofilm and the presence of wastewater solids in BC-inactive-biofilm, which decreased the availability and number of adsorption sites for PFASs compared with BC-no-biofilm. On the other hand, inactivation of the biofilm resulted in lower removal efficiencies for C5-11 PFCAs, C-4, (C)6, C-8 PFSAs and FOSA, probably because the biofilm degraded organic matter and thus increased the availability and number of adsorption sites compared with BC-inactive-biofilm. Sand-active-biofilm showed poor removal (0-70%) for all PFASs except FOSA (90%) and its adsorption capacity was low (0.0-7.5 ng g(-1)). In general, for all biochar treatments, shorter-chain PFASs were more resistant to removal than longer-chain PFASs. In addition, C-4, C-6 and C-8 PFSAs showed 10-30%, 10-50% and 20-30% higher average removal efficiency, respectively, than PFCAs with corresponding perfiuoroalkyl chain length. In conclusion, biochar is a promising filter medium for removal of PFASs in OWTSs, especially for PFASs with a perfluorocarbon chain longer than C-6. 2019 Elsevier Ltd. All rights reserved.