BACKGROUND: a diffusion and reaction model was developed for a hollow fiber membrane biofilm reactor (MBfR) to control nitric oxide (NO) emissions. in the MBfR, waste gases containing biodegradable compounds pass through the lumen of microporous hydrophobic hollow fiber membranes. Soluble, biodegradable compounds diffuse through the membrane pores and partition into a biofilm attached to the membranes where they are biodegraded. the membranes serve as a support for the microbial population and provide a large surface area for mass transfer. a dynamic model was developed for the MBfR which assumed biodegradation via Monod kinetics and constant biofilm thickness and density. Themodel was validated using experimental data from a study of NO removal (100 ppm) from a combustion gas mixture in a bench-scale MBfR with an acclimatized nitrifying population. RESULTS: NO gas was treated in a bench-scale MBfR at varying liquid recirculation velocities of 0.8 to 2.0 cm s-1. the gas residence time (τ ), calculated as themembrane lumen volume divided by the gas flow rate, was 1.9 s. NO removal efficiency for synthetic combustion gas ranged between 68% and 73% at room temperature (20 °C). CONCLUSION: the MBfR shows promise for treatment of waste gases from combustion processes. the model predicted an optimal liquid recirculation velocity of 1.5 cms-1 for NO removal, which is in good agreement with experimental data. Sensitivity experiments with the numeric model indicated that removal was a strong function of the biofilm density and the Monodmaximum specific growth rate (μmax).