BACKGROUND: This study concerned the stoichiometric evaluation of simultaneous nitrification and denitrification in a membrane bioreactor. The evaluation was made to derive relevant mass balance equations for all parameters affecting nitrogen removal and to set the basis for a rational design procedure. RESULTS: The basic stoichiometry was defined to include the effect of oxygen diffusion through microbial flocs by means of half saturation constants in corresponding switching functions. The stoichiometric relationships were used as tools for the design procedure targeting the desired level of nitrogen removal. CONCLUSION: A comparison was made with the outputs associated with a conventional activated sludge system for major design parameters such as denitrification potential, active heterotrophic and autotrophic biomass, oxygen requirements, etc. The proposed rational design for nitrogen removal was tested and validated using model simulation for steady state MBR operation under selected conditions. (C) 2011 Society of Chemical Industry