The start-up of mesophilic anaerobic digestion from waste activated sludge with internal inoculum was carried out in a laboratory-scale completely mixed reactor. The microbial community shift during the start-up period was monitored by molecular techniques. The self-degradation period, when the digester was operated in batch mode, consists of two characteristic phases, acidogenic (A) phase and methanogenic (M) phase. During the A phase, Volatile Fatty Acid (VFA) accumulation and a low level of methane production were observed. On the other hand, during the M phase, rapid VFA degradation and an increase in methane production were observed. Clone analysis results suggest that after self-degradation, Methanosarcina and Methanoregula were predominant at day 30, while the former was detected at low level (4%) and the latter was not detected at day 0. The population dynamics of the archaeal community were investigated by Real-time PCR. During the transition from A phase to M phase, the digester showed a significant increase by one order in the Methanosarcina rRNA gene. A significant positive correlation between the population of Methanosarcina and methane production rate was observed.
