BACKGROUND Graphene materials have extensively been applied in several industries due to their enhanced properties. Predictably, these materials also end up in wastewaters generated from industrial sectors, and their effects on biological treatment systems are poorly understood. The aim of the present study was to assess the acute effects of graphene oxide (GO) with three different degrees of reduction [reduced GO (rGO) produced with reduction times of 1, 2, and 4 h] on the methane production of an anaerobic consortium. RESULTS Synthesized rGO materials had a stimulatory effect increasing the maximum methanogenic activity (MMA) up to 14% and 114% when glucose and starch were provided as substrates, respectively, as compared to the MMA achieved in controls incubated in the absence of rGO. Reduction of GO promoted physical-chemical changes in its structure, such as the removal of epoxy groups, which prevented grapping of starch granules by the produced rGO sheets and triggered a better interaction between the latter and microorganisms. Furthermore, this work showed that rGO stimulated starch disintegration into its components, thus accelerating its hydrolysis, which was ultimately reflected in a higher MMA when this particulate polymer was used as substrate. CONCLUSION This study reports for the first time the acute effects of rGO on the methanogenic activity of an anaerobic consortium supplied with both soluble and particulate substrates. This information is relevant to elucidate the effects of this graphene material in anaerobic microorganisms and to improve the performance of anaerobic systems during the treatment of industrial wastewaters. (c) 2020 Society of Chemical Industry