Pilot moving-bed biofilm reactors (MBBRs), fed on primary settled wastewater, were used in order to study organic carbon removal and nitrification. Nitrate uptake rate (NUR) tests were performed by feeding sodium acetate and potassium nitrate to a bench-scale moving-bed batch biofilm reactor. In both experiments the same polyethylene biofilm carriers were used. Both particulate and filtered COD removal rates appear to be proportional to the corresponding loading rates. Particulate COD removal is the net effect of adsorption onto and release from the biofilm surface. Filtered COD removal is the sum of the influent filtered COD removal and the removal of hydrolysed colloidal COD. Filtered COD removal rates could not be evaluated with a kinetic expression because back-diffusion from biofilm is not always negligible. Nitrification tests, performed at oxygen limiting conditions, show that the reaction rate was nearly first order with respect to dissolved oxygen due to liquid film diffusion. Denitrification batch tests showed denitrification rates very close to other reported data. Since the process proved reliable and easy-to-operate, it is suitable for application to small WWTPs, either in designing new plants or in upgrading existing overloaded activated sludge systems.