Cytotoxicity mitigation in drinking water upon chlorination: Enhancing aged biological activated carbon filters with micro- and nanobubbles.

It has been demonstrated that the cytotoxicity reduction of chlorinated water by aged biological activated carbon (BAC) pretreatment declines due to decreased biosorption capacity and inactive aerobic metabolism. The potential enhancement of micro- and nanobubble (MNB) introduction on cytotoxicity mitigation of chlorinated water after 15-year BAC filter treatment and the underlying mechanisms were investigated. Results showed that MNB introduction could promote the cytotoxicity reduction from <10% to ∼23% and 41% without and with pre-ozonation, respectively, predominantly due to mitigation in nonvolatile fraction of disinfection byproducts (DBPs). Using Orbitrap mass spectrometry analysis, MNB-assisted BAC filter was effective in the abatement of aromatic nitrogenous compounds including lignin/carboxylic-rich aromatic moiety-, tannin-, and aromatic structure-like substances. Combining suspect screening and machine learning prioritization, structures of 72 higher-carbon (carbon number≥3) DBPs were tentatively identified. Enhanced mitigation of precursors for aromatic nitrogenous DBPs, especially heterocycles with likely functional groups of pyridine, pyrrole, benzoxazole, and carbazole, was achieved in MNB-assisted BAC. MNB introduction could alleviate the oxygen deficiency in aged biofilm, probably due to increased porosity and enhanced mass transfer. This, in turn, altered extracellular polymeric substance composition and microbial community, which could further expand biosorption sites and restore aerobic metabolism. Therefore, precursors for aromatic nitrogenous DBPs with high cytotoxic potencies could be more effectively abated in MNB-assisted BAC. This study provides insights into MNB-involved enhancement in the performance of aged BAC filters.