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The degradation of sewage sludge in single stage and 2 phase anaerobic systems was investigated in bench-scale batch reactors. Average COD reduction efficiency in 2 phase system in this initial phase of the study was 4.5% (30 days HRT) and 3.3% (20 days HRT) higher compared to single stage system. Reducing the HRT from 30 days to 20 days had little influence in VS reduction in the two systems. The average methane content in the biogas was 61.8%, 66.5%, and 64.8% (30 days HRT) in acid reactor, methane reactor and single stage reactor, respectively, and the corresponding values were 57.7%, 66.5% and 64.9% at 20 days HRT. The microbial community analyzed by qPCR using primer/probes specific for orders of Archaea involved in methanogenesis showed that hydrogen and acetate utilizing methanogens were present in all the reactors, but relative abundance of methanogens in the acid reactor was lower compared to the methane and single stage reactors. Reducing the HRT from 30 days to 20 days decreased the relative abundance of methanogens and changed the methanogen community structure.