Due to the large-scale production and wide applications, many nanoparticles(NPs) enter wastewater treatment plants and accumulate in activated sludge. It is reported that titanium dioxide(Ti O2) NPs show severe damage to many model microbes. However, it is still unknown whether the long-term(e.g., 100 d) presence of Ti O2 NPs would affect the performance of sludge fermentation. In this study, long-term exposure experiments(105 d)were conducted to investigate the potential risk of Ti O2 NPs to sludge fermentation system. It is found that the presence of environmentally relevant [6 mg·(g TSS)-1] and higher [150 mg·(g TSS)-1] concentrations of Ti O2 NPs does not affect methane production from sludge fermentation. The analysis of fluorescence in situ hybridization indicates that these concentrations of Ti O2 NPs present marginal influences on abundances of bacteria and methanogenic archaea in sludge fermentation system. The viability of sludge microorganisms and activities of key enzymes related to methane production such as protease, acetate kinase, and coenzyme F420 are unchanged by the long-term presence of 6 and 150 mg·(g TSS)-1of Ti O2 NPs. Further investigations reveal that the insolubility of NPs and the protection role of sludge extracellular polymeric substances are the main reasons for the marginal influence of Ti O2 NPs on sludge fermentation.
