The treatment and disposal of excess sludge represents a problem of growing importance of wastewater treatment plants all over the world. Anaerobic digestion is the most common method for sludge stabilization resulting in biogas production at the same time. In the present study, in pilot-scale experiments, process performance and biogas production of a cascade of two methanogenic continuously stirred tank reactors (CSTR) connected in series, was compared to a conventional one-step CSTR reactor treating sewage sludge. Retention times between 12.3 and 19.7 days were examined for both systems. Results showed that the serial configuration could improve biogas production by 9.5-40.1%. Biogas selectivity estimated 0.49 +/- 0.06 m(biogas)(3)/kg TVSdegraded for the cascade and 0.44 +/- 0.02 m(biogas)(3)/kg TVSdegraded for the one-step process. Both systems contained low VFA concentrations in the effluent. In terms of volatile suspended solids reduction, values ranged between 31.5% and 33.8% for the one-step process and between 36.2% and 40.7% for the cascade. The study thus suggests that the process performance and biogas production from sewage sludge can be optimized through serial digestion. (C) 2012 Elsevier Ltd. All rights reserved.