Long-term exposure to nano-TiO interferes with microbial metabolism and electron behavior to influence wastewater nitrogen removal and associated NO emission.

The extensive use of nano-TiO has caused concerns regarding their potential environmental risks. However, the stress responses and self-recovery potential of nitrogen removal and greenhouse gas NO emissions after long-term nano-TiO exposure have seldom been addressed yet. This study explored the long-term effects of nano-TiO on biological nitrogen transformations in a sequencing batch reactor at four levels (1, 10, 25, and 50 mg/L), and the reactor's self-recovery potential was assessed. The results showed that nano-TiO exhibited a dose-dependent inhibitory effect on the removal efficiencies of ammonia nitrogen and total nitrogen, whereas NO emissions unexpectedly increased. The promoted NO emissions were probably due to the inhibition of denitrification processes, including the reduction of the denitrifying-related NO reductase activity and the abundance of the denitrifying bacteria Flavobacterium. The inhibition of carbon source metabolism, the inefficient electron transfer efficiency, and the electronic competition between the denitrifying enzymes would be in charge of the deterioration of denitrification performance. After the withdrawal of nano-TiO from the influent, the nitrogen transformation efficiencies and the NO emissions of activated sludge recovered entirely within 30 days, possibly attributed to the insensitive bacteria survival and the microbial community diversity. Overall, this study will promote the current understanding of the stress responses and the self-recovery potential of BNR systems to nanoparticle exposure.