In this study, inert COD production in a crossflow ultrafiltration membrane anaerobic reactor (CUMAR) system treating brewery wastewater, was investigated. The system was operated at a wide range of influent chemical oxygen demand (COD) concentrations and solid retention times (SRT) in order to determine residual non-biodegradable COD produced by biomass in the anaerobic reactor. Previous studies showed that the wastewater had practically no inert soluble GOD. The inert soluble COD in the effluent would, therefore, have been produced in the anaerobic reactor by the biomass. Significant increases in the effluent COD concentration were observed during the operation of the CUMAR system at SRT lower than 100 days. It was also calculated from the results that the soluble microbial products measured in terms of COD were found to be an average of 2.2% of the influent GOD. These microbial products were not removed from the effluent even with extended operation times. Throughout the operation, a maximum of 28.5 kg COD/m(3) day was applied, at which point COD removal efficiencies of no lower than 97% in the anaerobic reactor and 99% in the membrane permeate were achieved during the steady-state operation of the CUMAR system. (C) 2000 Elsevier Science Ltd. All rights reserved.