The aim of this work is to evaluate the performance of a submerged membrane bioreactor that operated continuously for 200 days and to assess biomass activity and membrane fouling under various operating conditions. Furthermore, a method for the characterisation of biomass activity is developed based on its separation into various fractions. Respirometry and Activated Sludge Models are used as biological process to identify active biomass and species of activated sludge. However, the equations describing the active biomass are developed. The submerged membrane bioreactor system operated at the solids retention times of 40 and 60 days with organic loading rate of 0.5635 kg/COD/m−3/day−1 and chemical oxygen demand/nitrogen ratio of 3.5. Activated sludge is generated in a system fed with soluble and easily biodegradable substrate [ethanol/sodium acetate (1:1)], this synthetic wastewater, containing no hardly biodegradable organic or inorganic particulate matter, has led to the generation of a sludge constituted essentially of two fractions: active biomass and cell debris.