Fate and long-term inhibitory impact of ZnO nanoparticles during high-rate anaerobic wastewater treatment

The aim of this study was to evaluate the long-term effect of ZnO nanoparticles (NPs) on the performance of high-rate anaerobic bioreactors. Laboratory-scale upflow anaerobic sludge blanket (UASB) reactors were fed with a mixture of volatile fatty acids and exposed to either low (0.32mgZnL^-1) or high (34.5mgZnL^-1) concentrations of ZnO NPs. Exposure to high NP concentrations caused a rapid and permanent decline in the methane production and the removal of acetate and propionate. In contrast, a gradual and partial inhibitory response was observed in the reactor exposed to low NP concentrations. The long-term effect of the NP exposure was also evident from a decline in the specific methanogenic activity, which was more severe for the acetoclastic compared to the hydrogenotrophic methanogens. ZnO NPs were removed by 62-82% during passage through the UASB reactors. The results taken as a whole indicate that ZnO NPs cause severe inhibition of acetoclastic methanogens. Even sub-ppm levels of the nano-ZnO in the influent had a negative impact on the performance of the UASB reactor due to long-term exposure of methanogens to NPs that accumulated in the sludge bed.