The study investigated performance characteristics of super fast membrane bioreactor (MBR) in removing readily biodegradable substrate. MBR experiments were adjusted to extremely high rate operation at steady-state, with all combinations of sludge retention times between 0.5-2.0 d and hydraulic retention times between 0.5-2.0 h. Substrate feeding was adjusted to 200 mg/L and involved a synthetic mixture representing the readily biodegradable COD fraction in domestic sewage. Effluent COD always remained below 25 mg/L with an average value of 15 mg/L under all tested operating conditions. Respirometric analyses indicated complete biodegradation of available substrate so that the effluent COD was essentially composed of soluble microbial products. High rate MBR operation also limited substrate storage below 10 mg/L and generation of proteins and carbohydrates down to 1.5-4.5 mg COD/L. Significant differences in the residual soluble COD levels between permeate and reactor volume showed that effective COD removal relied on biodegradation as well as on membrane filtration, which entrapped soluble microbial products with relatively larger particle sizes. Parallel 16s rRNA experiments based on PCR-DGGE methodology revealed significant changes in the composition of the microbial community as a function of the sludge retention time, an observation with possible implications on different biodegradation kinetics depending on the characteristics of the microbial community. (c) 2012 Elsevier B.V. All rights reserved.