In this study, we investigated the feasibility of anaerobic sewage treatment at extremely low temperatures (4 °C) using two reactor setups: upflow anaerobic sludge blanket reactors (UASB) without and with (AnMBR_UASB(UF)) a membrane. Both reactors were inoculated with seeds derived from sediments that were putatively acclimatized to low temperatures. A preliminary batch trial showed that treatment is feasible with the removal of carbon coupled to methane and sulphide production. The reactors operated for 180 days at a hydraulic retention time of 3 days. After 40 days acclimation, both systems met the EU chemical oxygen demand (COD) effluent standard (<125.0 mg L⁻¹). Initially, the removal efficiency and methane production rate of the AnMBR were slightly higher than those of the UASB. However, over time, both the performance (COD removal and methane production) and the intrinsic capability of the biomass (expressed as cell specific activity) became similar. The wastewater-fed biomass produced <7.0 fmol_CH4 per cell_methanogen per day at cell densities of observation <1.4 × 10⁶ methanogens per mL. Acetate/formate-fed specific methanogenic activities at 4 °C (<18 fmol CH₄ per cell_methanogen per day) confirmed that acetoclastic methanogenesis is important in both setups and hydrogenotrophic methanogenesis was only unequivocally observed in the UASB reactors. The microbial diversity of the two systems was similar, and interestingly revealed several putatively hydrogenotrophic methanogens (i.e., Methanospirillum, Methanobrevibacter and unassigned Methanomassilococeae). Methanosaeta; the archetypal acetoclastic methanogen was present but not abundant and largely confined to the biofilm. These observations suggest that at 4 °C methane can be produced not only through direct acetoclastic methanogenesis, but also through acetate oxidation coupled with hydrogenotrophic methanogenesis.