Bioavailable dissolved organic nitrogen (ABDON) is the component of dissolved organic nitrogen (DON) which supports the growth of algae. Previous research indicates that a membrane bioreactor (MBR) is effective in reducing DON, however, its ability to remove ABDON remains unknown. The present study investigated three full-scale MBRs (membrane type: hydrophilic polyvinylidene fluoride and membrane pore size: 0.04-0.1 mu m) as posttreatment for the removal of ABDON. Results showed that the concentrations of ABDON were not significantly different between influent and effluent at each MBR (p=0.067-0.614, t-test). Analysis of DON molecular composition via ultrahigh resolution mass spectrometry provides supporting evidence that bacterial biomass produced/released ABDON during the biological processes, which would be one of the possible reasons for the low removal efficiency of ABDON (< 0%-14.0%) occurred in the MBR process. Overall, MBRs as posttreatments would have a smaller-than expected impact on primary productivity in receiving waters since a substantial fraction of DON stimulating algal growth cannot be removed by this treatment.