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 titaniumdioxide (TiO

2

) NPs showsevere damage to many model microbes. However

it is still unknown whether the long-term (

e.g

.

100 d) presence of TiO

2

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 TiO

2

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 TiO

2

NPs does not affect methane production fromsludge fermentation. The analysis of fluorescence

in situ

hybridization indicates that these concentrations of TiO

2

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 tomethane production such as protease

acetate kinase

and coenzyme F

420

are unchanged by the long-term presence of 6 and 150 mg·(g TSS)

-1

of TiO

2

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 TiO

2

NPs on sludge fermentation.