In this study, the impact of different types of biochar and biochar ratios on the anaerobic digestion of citrus peel waste was investigated. Citrus peel has an inhibitory effect on anaerobic digestion. The presence of biochar had two effects: a reduction in the length of the lag phase and greater production of methane relative to citrus peel waste only incubations. The microbial lag phases decreased with increase in citrus peel to biochar ratios, with 2:1 having the longest lag phase of 9.4 days and 1:3, the shortest, with the value of 7.5 days. The cumulative methane production in incubations containing biochar and citrus peel ranged from 163.9 to 186.8 ml CH4 g VS (1), while citrus peel only produced 165.9 ml CH4 g VS (1). Examination of the biochar material revealed colonies of putative methanogens. The synergy of D-limonene adsorption and microbial immobilization by biochar appears to improve the performance of anaerobic digestion. (C) 2016 Elsevier Ltd. All rights reserved.