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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

Dalahmeh, Sahar S, Alziq, Nancy, Ahrens, Lutz
Environmental pollution 2019 v.247 pp. 155-164
adsorption, biochar, biofilm, filtration, organic matter, perfluorocarbons, sand, sand filters, sulfonates, wastewater, wastewater treatment
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; C4, 6, 8) and one perfluorooctanesulfonamide (FOSA; C8) 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 ΣPFASs 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 ΣPFASs g−1 BC) of short-chain PFCAs (C3-6) and PFSA (C4) was low, whereas the removal efficiency (90–99%) and the adsorption capacity (73–168 ng g−1) was high for C7-C11 PFCAs, C6, C8 PFSAs and FOSA. The relative removal was generally lower for C3-9 PFCAs and C4, C6, C8 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, C4, C6, C8 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, C4, C6 and C8 PFSAs showed 10–30%, 10–50% and 20–30% higher average removal efficiency, respectively, than PFCAs with corresponding perfluoroalkyl 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 C6.