Anaerobic submerged membrane bioreactor (AnSMBR) treating municipal wastewater at ambient temperature: Operation and potential use for agricultural irrigation

•Temperature was the main parameter affecting the process performance.•In summer at VLR of 5 kg COD/m3d and HRT of 10 h the effluent quality was 150 mg O2/L.•The biogas production was negligible during the winter at temperatures below 15 °C.•Rapid drop of temperature in cold countries hinders the sludge acclimation.•The permeate complies with the Spanish law for agricultural irrigation. A 496 L pilot scale anaerobic submerged membrane bioreactor (AnSMBR) for the treatment of municipal wastewater was evaluated during a year of stable operation at ambient (28–10 °C) temperature, and inoculated with mesophilic inoculum. The temperature was the main parameter affecting the process performance. The chemical oxygen demand (COD) of the effluent was around 150 mg O2/L in the summer period, operating with a volumetric loading rate (VLR) of 5 kg COD/m3 d and hydraulic retention time (HRT) of 8–10 h, with a specific methane production between 0.09 and 0.14 Nm3/kg CODremoved. However, during the winter season, an important increase of effluent COD was observed, and therefore the VLR was decreased to values around 1 kg COD/m3 d in order to recover the quality of the effluent. Biogas production was negligible in this period. The effluent complies with the parameters stipulated by Spanish law regarding the use of treated wastewater for agricultural irrigation.