The influence of selected geometric bioreactor parameters on the performance of continuous-flow-type low-temperature biogas production from biomass by anaerobic digestion was studied to determine the optimal geometric parameters of the digester. A continuous-mode two-stage bioreactor was applied to produce biogas by anaerobic digestion using model dairy wastewater sludge as substrate. The Monod approach was used to find the optimal diameter of the two cylinder-separated stages of the reactor that maximizes the amount of biogas produced per unit of time. Total biogas production derived from the theoretically optimized reactor in the calculation model was 1.6 times higher than that derived for the experimental bioreactor. The methane fraction in biogas increased from 64.5% to 71.2% after optimization, whereas the carbon dioxide fraction in biogas decreased from 34.5% to 27.8%. The optimization of the intermediate cylinder of the digester significantly increased total biogas production (by up to 160%) in comparison with the output noted before optimization. (C) 2014 Elsevier Ltd. All rights reserved.