BACKGROUND: A laboratory-scale membrane aeration bioreactor was employed to treat synthetic ammonium-rich wastewater to yield an appropriate NH4+/NO2- ratio for anaerobic ammonium oxidation (ANAMMOX). The main objectives of this study were to steadily obtain 50% partial nitrification in batch experiments, to evaluate the effects of aeration and to identify the dominant bacterial community of the biofilm for partial nitrification. RESULTS: Some of the ammonium in the synthetic wastewater was partially nitrified. A suitable NH4+/NO2- ratio (1:1 to 1:1.3) for the ANAAMOX process was obtained after 24h. The dissolved oxygen (DO) level in the treated water was very low (below 0.6 mg L-1). Both the appropriate NH4+/NO2- ratio and the low DO level make this bioreactor an ideal pretreatment system for ANAMMOX. In addition, a molecular biotechnology method was applied to prove that the ammonia-oxidizing bacteria dominated the biofilm. CONCLUSION: This system achieved surprising cost savings in the aeration process compared with traditional aeration systems. The combination of this system with the subsequent ANANMOX process has great potential as a favorable short-cut in the treatment of ammonium-rich wastewater. (c) 2007 Society of Chemical Industry.