Release of engineered nanoparticles (NPs) to municipal wastewater from industrial and residential sources could impact biological systems in wastewater treatment plants. Methanogenic inhibition can cause failure of anaerobic waste(water) treatment. This study investigated the inhibitory effect of a wide array of inorganic NPs (Ag-0, Al2O3, CeO2, Cu-0, CuO, Fe-0, Fe2O3, Mn2O3, SiO2, TiO2, and ZnO supplied up to 1500 mg L-1) to acetoclastic and hydrogenotrophic methanogenic activity of anaerobic granular sludge. Of all the NPs tested, only Cu-0 and ZnO caused severe methanogenic inhibition. The 50% inhibiting concentrations determined towards acetoclastic and hydrogenotrophic methanogens were 62 and 68 mg L-1 for Cu-0 NP; and 87 and 250 mg L-1 for ZnO NP, respectively. CuO NPs also caused inhibition of acetoclastic methanogens. Cu2+ and Zn2+ salts caused similar levels of inhibition as Cu-0 and ZnO NPs based on equilibrium soluble metal concentrations measured during the assays, suggesting that the toxicity was due to the release of metal ions by NP-corrosion. A commercial dispersant, Dispex, intended to increase NP stability did not affect the inhibitory impact of the NPs. The results taken as a whole suggest that Zn- and Cu-containing NPs can release metal ions that are inhibitory for methanogenesis. (C) 2013 Elsevier B.V. All rights reserved.