The anaerobic digestion of outdoor cultivated Rhizoclonium biomass was investigated in this study. The influence of applying mechanical and biological pre-treatment methods prior to the biomass digestion on the overall methane yields was examined. The results show that the application of a combined biomass blending (<0.1 mm length) and an enzymatic pre-treatment improved the methane yields by >20% compared to use of a mechanical size reduction method alone. The methane yields from Rhizoclonium biomass were however observed to be considerably lower than those of other algae species from the literature. (C) 2012 Elsevier Ltd. All rights reserved.