A gravity-operated submerged anaerobic membrane bioreactor (SAnMBR) was set up in order to test its principle of operation as an alternative to conventional pumped permeation of the membrane. This operating mode allowed the membrane flux rate to be measured accurately whilst maintaining a constant transmembrane pressure (TMP), and allowed small transient variations in the flux rate to be observed. The reactor was operated at 36°C for a period of 115 days using a nutrient-balanced synthetic substrate with a high suspended solids concentration. Membrane cleaning was in situ by a gas scouring system using recirculation of headspace biogas. With an initial TMP of 7.0 kPa, the membrane flux slowly decreased due to membrane fouling and had not reached a constant value by day 71. The results indicated that the system was still acclimatizing up to 50 days after start-up; but from that point onwards, performance parameters became much more stable. A constant flux of 2.2 L m(-2) h(-1) was achieved over the last 45 days after the TMP was reduced to 2.3 kPa. The stable flux was maintained over this period and the loading raised to 1 g COD L(-1) d(-1) by increasing the influent strength. Under these conditions, the average chemical oxygen demand removal efficiency was 96% and the specific methane potential was 0.31 L CH4 g(-1) COD removed.
Keywords: AnMBR; anaerobic digestion; membrane fouling; sustainable membrane flux; wastewater treatment.