Display omitted •Biochar had a positive influence on digestion of sludge and orange peels waste.•Eubacterial populations were more sensitive than archaeal to biochar addition.•Biochar provides higher surface area favoured adhesion and growth of microorganisms.•For batch systems biochar leads to a decrease in lag phase and an increase in gas yields.•Under semicontinuous regimen an enhancement of methane yield was observed. In this study, the effects on process performance and changes in microbial populations with the addition of biochar to the anaerobic digestion of sludge and orange peels were evaluated. Biochar had a positive influence in batch digestions, leading to a decrease in the lag phase and an increase in methane yields; this was even more evident for citrus peel wastes, which reached an increase of approximately 33% when 10 g L−1 of biochar was added and 56% when 30 g L−1 was added. Particle size analysis performed for the experiments shows greater surface area available in biochar systems for biomass immobilization. Analysis of the microbial communities by means of 16S rRNA gene pyrosequencing shows that the biochar addition favoured the electro-active microorganisms consortia creating a synthrophic metabolism between eubacterial and archaeal populations, which resulted in an improvement of the anaerobic digestion performance. The codigestion of the mixture under a semicontinuous regimen showed an improvement in methane yields of approximately 60% and at hydraulic retention times of 30–20 days (reaching methane production values above 500 L CH4 kg VS−1 at an OLR of 1.49 kg VS m−3 d−1). The enhancement observe in biochar supplemented fermentations may be explained by the adsorption of inhibitors and the relatively high surface area favoured the adhesion and growth of microorganisms.