Long-term operation of a pilot scale anaerobic membrane bioreactor (AnMBR) for the treatment of municipal wastewater under psychrophilic conditions

•Viability of AnMBR at psychrophilic conditions for treating municipal wastewater.•Effluent with high quality (100–120 mg O2/L) at HRT of 7 h.•The membrane permits the biodegradation of slowly biodegradable matter.•SMY depends on the recirculation between the membrane module and the UASB reactor.•Long-term reliability and operability of the AnMBR technology.

The performance of a pilot scale anaerobic membrane bioreactor (AnMBR), comprising an upflow anaerobic sludge blanket (UASB) reactor coupled to an external ultrafiltration membrane treating municipal wastewater at 18 ± 2 °C, was evaluated over three years of stable operation. The reactor was inoculated with a mesophilic inoculum without acclimation. The AnMBR supported a tCOD removal efficiency of 87 ± 1% at hydraulic retention time (HRT) of 7 h, operating at a volumetric loading rate (VLR) of between 2 and 2.5 kg tCOD/m3 d, reaching effluent tCOD concentrations of 100–120 mg/L and BOD5 concentrations of 35–50 mg O2/L. Specific methane yield varied from 0.18 to 0.23 Nm3 CH4/kg CODremoved depending on the recirculation between the membrane module and the UASB reactor. The permeate flow rate, using cycles of 15 s backwash, 7.5 min filtration, and continuous biogas sparging (40–60 m/h), ranged from 10 to 14 Lm2/h with trans-membrane pressure (TMP) values of 400–550 mbar.