MBRs necessitate two sources of aeration: one for scouring the membranes (coarse bubble, CB) and the other for the biomass requirements (usually fine bubble, FB). In the framework of a in-depth study of a full scale municipal MBR plant (10 000 p.e.) where CB and FB are in the same tank (Biosep), mass balances were used to determine the OTR of the different combinations of CB+FB at different air flows. A two year monitoring was performed with on-line COD influent measurements and a SCADA system provides an overall information on the plant operation. The different OTRs configurations were calculated before and after the modification of the number of diffusers. Results are close to those obtained with the conventional methods used in the presence of sludge and the calculations do not require any reagent or specific conditions. This methodology allows the prediction of the maximum treatment capacity especially for total nitrogen. Moreover, the calculated OTRs could help the operators to monitor the oxygen transfer performance and assess the fouling status of the diffusers.