Multicomponent diffusion and reaction of electrolytes within poly(vinyl-alcohol) hydrogels (PVA) spheres was modelled by the Stefan-Maxwell approach. The model may be transferred to other problems. Denitrification of drinking water in PVA pellets by reduction with the aid of hydrogen and Pd/Cu catalyst particles, which were immobilized inside the pellet, was taken as an example. The differential-algebraic system of model equations was solved by a finite difference approach. Results of various calculations will be presented.