BACKGROUNDIn this study an anaerobic fluidized bed membrane bioreactor (AFMBR) is added as a second stage to an operating anaerobic baffled bioreactor (ABR) to form a staged anaerobic baffled-fluidized membrane bioreactor (SAB-FMBR) operating at ambient temperature. RESULTSThe ABR first stage was operated at 3h hydraulic retention time (HRT) and was followed by the AFMBR operated at 0.93 to 1.16h HRT. The SAB-AFMBR system was fed with complex synthetic wastewater with COD averaging 250mgL(-1). GAC fluidization increased the critical flux of the AFMBR due to fouling reduction associated with the cleaning action of the GAC particles on membrane surfaces. However, sustainable flux for long-term operation of the AFMBR was lower than the critical flux determined in short-term tests. Overall COD removal by the SAB-FMBR system when operating at a sustainable flux (8-9Lm(-2)h(-1)) was 96% with effluent COD of about 11mgL(-1) and suspended solids concentration near zero. The electrical energy required for system operation in the AFMBR was 0.0087 kWh m(-3), much less than that of an anaerobic MBR using biogas sparging to control membrane fouling. CONCLUSIONThe AFMBR was applied successfully as a second stage polishing system for an ABR treating complex synthetic wastewater at a temperature of 25 degrees C. (c) 2014 Society of Chemical Industry