A lab-scale (6.2 L) anaerobic membrane bioreactor combined with a tubular, cross-flow, PVDF ultrafiltration membrane was developed and operated to assess the long-term fouling behavior of a cyclically operated anaerobic membrane bioreactor (AnMBR). The AnMBR was operated at 35 +/- 1 degrees C for 200 days with a synthetic influent of 501 mg center dot L-1 COD to mimic municipal wastewater. The system exhibited high treatment performance with an average COD removal efficiency of 86.5 +/- 6.4% (n = 20) and an average permeate COD concentration of 63.9 +/- 31.1 mg center dot L-1. A clear permeate with an average turbidity of 0.6 +/- 0.2 NTU, was achieved. Permeate TN and TP concentrations were 22.7 +/- 5.1 mg center dot L-1 and 6.9 +/- 2.0 mg center dot L-1 corresponding to removal efficiencies of 20.6% and 49.3%, respectively, likely due to membrane rejection of particulate, colloidal, and organic fractions. A stable membrane flux of 4.3 L.m(-2).h(-1) (LMH) was maintained for 183 days without gas-lift, gas sparge, or chemical cleaning. Cyclical operation with frequent relaxation (60 s for every 30 min of the permeate production run) and periodic permeate backwash (15 s for every 186 min) maintained stable membrane operation with an average TMP of 0.25 bar and a fouling rate of 0.007 kPa/h for the entire operating period. The comparison revealed frequent backwashing and relaxation is a sustainable strategy for operation of the AnMBR.