Effects of metal oxide nanoparticles (TiO2, Al2O3, SiO2 and ZnO) on waste activated sludge anaerobic digestion

► The impact of metal oxide nanoparticles (TiO2, Al2O3, SiO2 and ZnO) on sludge digestion has never been reported. ► The generation of methane was only affected by higher dosages of nano-ZnO. ► The released Zn2+ from nano-ZnO was a main reason for its inhibitory effect on methane generation. ► The steps of sludge hydrolysis, acidification and methanation were inhibited by nano-ZnO. ► Nano-ZnO decreased the activities of protease, acetate kinase and F420.