The results of two pilot studies of an immersed membrane bioreactor (MBR) treating fresh water and saline sewage in Hong Kong are presented. The objectives were to demonstrate suitability of the MBR technology to the treatment of Hong Kong sewage and its ability to achieve total nitrogen removal effectively. When operated in nitrification/denitrification mode, the MBR was able to achieve 98% BOD removal, 90-93% COD removal and 82-84% total nitrogen removal with a HRT of 6.8 hours and 300% internal flow recirculation. Very low effluent NH4+-N levels were observed throughout the study suggesting complete nitrification. The MBR was able to achieve full denitrification utilizing organic matter in the raw sewage as a carbon source. The nitrogen removal capacity of the MBR was limited by nitrogen loadings rather than the biological activity in the reactor. The results did not indicate any significant differences in treatment performances with fresh water and saline sewage except that a higher frequency of membrane cleaning was required for the latter.