The feasibility of nitrite conversion by immobilized Nitrobacter agilis in an air-lift loop bioreactor cascade was examined. Especially the effects of substrate and product inhibition on the performance of the reactor series were studied. A significant improvement in the degree of substrate conversion was observed with the reactor series as compared to a single vessel at the same overall residence time. The experiments were simulated by a dynamic model that estimates biomass concentration profiles across gel beads in time, incorporating effects of diffusion limitation and substrate and product inhibition. The trend of the model estimates agrees well with the experiments.