In this study, the variations in system performance and microbial community composition when treating waste activated sludge without prior chemical or physical pretreatment in anaerobic systems is examined. Two lab-scale digesters, namely anaerobic dynamic membrane bioreactor (AnDMBR) and continuously stirred tank reactor (CSTR), were operated and maintained under mesophilic conditions at varying hydraulic retention times (10-25 days). Decreasing the hydraulic retention time (HRT) affected the methane yield (MY) and methane production rate (MPR) proportionally in both systems. The AnDMBR's maximum MY of 252 mL CH4/g-COD and MPR of 0.73 L/L/d were 25% and 34% higher than those obtained using the CSTR, respectively. Moreover, a decrease in HRT was accompanied by accumulation of butyric acid in the CSTR, which caused further performance deterioration. In both systems, AnDMBR's dynamic membrane acted as an added layer preventing methanogen wash-out and retaining the essential microorganisms for stable sludge treatment. Dominance in the microbial composition shifted from a strict acetoclastic to a mixed acetoclastic and hydrogenotrophic methanogenic community and that a decrease in the ration of Bacteroidetes to Firmicutes was associated with the increased MY.