The influence of the addition of granular activated carbon (GAC) on the anaerobic treatment of wastewater with a mixture of ethanol and 1-ethoxy-2-propanol (E2P), which are typical pollutants from the flexographic/printing industry, was evaluated. For this purpose, the reactor performance and microbial community in two continuous stirred tank reactors (CSTRs), i.e. control and GAC supplemented, were assessed. Both reactors, seeded with anaerobic granular biomass, showed similar global performance (RE > 93%), except after organic load shocks and after E2P addition. GAC supplementation substantially enhanced the response of the biological system to organic load shocks and reduced the acclimation period for E2P degradation. This improvement was found to be related to the effect of GAC on the microbial community which, in turn, is related to an increase in the degradation rate of intermediates (acetone and isopropanol). The microbial community was analysed through denaturing gradient gel electrophoresis and high-throughput sequencing. Both methods showed an evolution from the inoculum to the final time point and considerable enrichment in microorganisms involved in direct interspecies electron transfer, such as Geobacter species in the final biomass samples, especially in the biomass attached to the GAC.