The market for graphene-based products, such as graphene oxide (GO), is projected to reach nearly $675 million by 2020, hence it is expected that large quantities of graphene-based wastes will be generated by then. Wastewater treatment plants will be one of the ultimate repositories for these wastes. Efficient waste treatment relies heavily on the functions of diverse microbial communities. Therefore, systematic investigation of any potential toxic effects of GO in wastewater microbial communities is essential to determine the potential adverse effects and the fate of these nanomaterials in the environment. In the present study, we investigate the acute toxicity, i.e. short-term and high load, effect of GO on the microbial functions related to the biological wastewater treatment process. The results showed that toxic effects of GO on microbial communities were dose dependent, especially in concentrations between 50 and 300 mg/L. Bacterial metabolic activity, bacterial viability, and biological removal of nutrients, such as organics, nitrogen and phosphorus, were significantly impacted by the presence of GO in the activated sludge. Furthermore, the presence of GO deteriorated the final effluent quality by increasing the water turbidity and reducing the sludge dewaterability. Microscopic techniques confirmed penetration and accumulation of GO inside the activated sludge floc matrix. Results demonstrated that the interaction of GO with wastewater produced significant amount of reactive oxygen species (ROS), which could be one of the responsible mechanisms for the toxic effect of GO. (C) 2013 Elsevier B.V. All rights reserved.