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Pore size is one of the most important properties in the successful operation of membrane-based bioprocesses for the treatment of municipal wastewater. The characteristics of two anaerobic membrane bioreactors (AnMBRs), one with a hollow fiber membrane of 0.4 μm pore size (AnMBR1), and the other with a membrane of 0.05 μm pore size (AnMBR2) were investigated for the treatment of real municipal wastewater at room temperature (25 °C) under varied hydraulic retention times (HRTs). Process performance was evaluated in terms of organic removal efficiency, biogas production and membrane filtration behaviours during a long-term continuous operation. Both AnMBRs showed good organic removal performance with COD and BOD removal efficiencies of around 89% and 93%, respectively. High energy recovery potential was achieved, with the biogas yield ranging between 0.20 and 0.26 L-gas/g-COD rem and a methane content of approximately 75%. The differences in the membrane filtration behaviours in the two AnMBRs included different permeate flux and total filtration resistance (R t). In the AnMBR with a 0.4 μm pore size membrane, an average R t of 1.08 × 10^12 m−1 was obtained even when the permeate flux was a high 0.274 m/day, while a higher average R t of 1.51 × 10^12 m−1 was observed in the AnMBR with 0.05 μm pore size membrane even when the flux was a low 0.148 m/day. The off-line membrane cleaning strategy used for AnMBR1 indicated that the membrane restoration efficiency was 90.2%. Unlabelled Image • Two AnMBRs with different pore size membranes treated municipal wastewater. • Average COD and BOD removal efficiencies reached 89% and 93%. • Biogas yield was 0.20–0.26 L-gas/g-COD rem with CH 4 content of 75%. • Membrane with 0.4 μm pore size was more appropriate than 0.05 μm. • Membrane restoration efficiency of 90.2% was achieved by the off-line cleaning. [ABSTRACT FROM AUTHOR]