Performance of simultaneous organic and nutrient removal in a pilot scale anaerobic–anoxic–oxic membrane bioreactor system treating municipal wastewater with a high nutrient mass ratio

• Treatment of high ratio of TP: TN: COD municipal wastewater with an A2O-MBR.
• System operated at different conditions of aerobic HRT and MLR ratio.
• F/M ratio at a constant SRT, is independent of HRT, is dependent on SRT and feed COD.
• Treatment performance of the A2O-MBR was not affected by aerobic HRT variation.
• Efficient organic and nutrient removal was achieved simultaneously.

A pilot-scale anaerobic–anoxic–oxic membrane bioreactor (A2O-MBR) was applied to treat Shiraz municipal wastewater with high ratio of total phosphorous (TP): total nitrogen (TN): chemical oxygen demand (COD) (3.4: 21.4: 100) and remarkable nitrate concentration. Various operating conditions were tested to investigate nutrient and organic matter removal as well as membrane capacity and biomass development. The pilot system was operated at a constant sludge retention time of 60 d. During different runs, the biomass concentration increased with a reduction of HRT while the F/M ratio was stable around an average of 0.087 gCOD g−1MLSS d−1. Results also showed that the treatment performance of A2O-MBR system was extremely stable and was not affected by aerobic hydraulic retention time (HRT) changes (12, 10, 8, 6 h). COD reduction reached over 95% and nitrification was completed. The maximum efficiency of TN and TP removal was 86.2% and 55.9%, respectively, which occurred with an overall HRT of 12 h, external and internal mixed liquor recycle rate of 200%. Membrane capacity measurements showed an average flux of 5.5 L m−2 h−1 with a mean transmembrane pressure of 30 kPa under periodical air backwashing without any chemical cleaning over three months.