In this study, the effect of the aeration rate on simultaneous methanogenesis and denitrification was investigated in a combined system consisting of an oxygen-limited upflow anaerobic sludge blanket (UASB) and aerobic membrane bioreactor (MBR). Different oxygen concentrations in the UASB were achieved by recycling the mixed liquor of the aerobic MBR into the bottom of the UASB. The dissolved oxygen (DO) of the aerobic MBR mixed liquor was determined by the aeration rate in the aerobic MBR. Quantitative analysis of biogas production and composition and total organic carbon (TOC) and total nitrogen (TN) removal performances at different aeration rates in the aerobic MBR were carried out. Results showed that TOC and TN removal performances were improved in the combined system compared to a strictly anaerobic UASB. TOC removal efficiency up to 98% after membrane filtration was achieved. TN removal efficiency was related to the aeration rate in the aerobic MBR, which was limited by the nitrification efficiency at a low aeration rate and improved at aeration rates above 2.5 L/min. Biogas production and composition were also investigated. Results showed that a low DO concentration and degradation of NOx- - N through the denitrification process did not affect the methane production. A slight increase in methane production because of the improvement in the TOC removal rate and decrease of CO2 at a low aeration rate was observed. However, methane production and the ratio decreased at a high aeration rate as a result of more nitrogen and CO2 being produced via the processes of denitrification and oxidation, respectively. In this study, above 50% methane in the biogas and 80% TN and 98% TOC removal efficiencies at the aeration rates between 2.5 and 5.0 L/min in the aerobic MBR could be successfully achieved. On the basis of these results, it was suggested that simultaneous methanogenesis and denitrification could be employed as a bioenergy production technology using a combined UASB and aerobic MBR.